reduce false positives of tail-expr-drop-order from consumed values

take 2

open up coroutines

tweak the wordings

the lint works up until 2021

We were missing one case, for ADTs, which was
causing `Result` to yield incorrect results.

only include field spans with significant types

deduplicate and eliminate field spans

switch to emit spans to impl Drops

Co-authored-by: Niko Matsakis <nikomat@amazon.com>

collect drops instead of taking liveness diff

apply some suggestions and add explantory notes

small fix on the cache

let the query recurse through coroutine

new suggestion format with extracted variable name

fine-tune the drop span and messages

bugfix on runtime borrows

tweak message wording

filter out ecosystem types earlier

apply suggestions

clippy

check lint level at session level

further restrict applicability of the lint

translate bid into nop for stable mir

detect cycle in type structure

compiler/rustc_borrowck/src/dataflow.rs

@@ -641,6 +641,7 @@ impl<'tcx> rustc_mir_dataflow::Analysis<'tcx> for Borrows<'_, 'tcx> {
             | mir::StatementKind::Coverage(..)
             | mir::StatementKind::Intrinsic(..)
             | mir::StatementKind::ConstEvalCounter
+            | mir::StatementKind::BackwardIncompatibleDropHint { .. }
             | mir::StatementKind::Nop => {}
         }
     }

compiler/rustc_borrowck/src/lib.rs

@@ -652,6 +652,8 @@ impl<'a, 'tcx> ResultsVisitor<'a, 'tcx, Borrowck<'a, 'tcx>> for MirBorrowckCtxt<
             | StatementKind::Coverage(..)
             // These do not actually affect borrowck
             | StatementKind::ConstEvalCounter
+            // This do not affect borrowck
+            | StatementKind::BackwardIncompatibleDropHint { .. }
             | StatementKind::StorageLive(..) => {}
             StatementKind::StorageDead(local) => {
                 self.access_place(

compiler/rustc_borrowck/src/polonius/loan_invalidations.rs

@@ -88,6 +88,7 @@ impl<'a, 'tcx> Visitor<'tcx> for LoanInvalidationsGenerator<'a, 'tcx> {
             | StatementKind::Nop
             | StatementKind::Retag { .. }
             | StatementKind::Deinit(..)
+            | StatementKind::BackwardIncompatibleDropHint { .. }
             | StatementKind::SetDiscriminant { .. } => {
                 bug!("Statement not allowed in this MIR phase")
             }

compiler/rustc_borrowck/src/type_check/mod.rs

@@ -1252,6 +1252,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
             | StatementKind::Coverage(..)
             | StatementKind::ConstEvalCounter
             | StatementKind::PlaceMention(..)
+            | StatementKind::BackwardIncompatibleDropHint { .. }
             | StatementKind::Nop => {}
             StatementKind::Deinit(..) | StatementKind::SetDiscriminant { .. } => {
                 bug!("Statement not allowed in this MIR phase")

compiler/rustc_codegen_cranelift/src/base.rs

@@ -924,6 +924,7 @@ fn codegen_stmt<'tcx>(
         | StatementKind::FakeRead(..)
         | StatementKind::Retag { .. }
         | StatementKind::PlaceMention(..)
+        | StatementKind::BackwardIncompatibleDropHint { .. }
         | StatementKind::AscribeUserType(..) => {}
 
         StatementKind::Coverage { .. } => unreachable!(),

compiler/rustc_codegen_cranelift/src/constant.rs

@@ -583,6 +583,7 @@ pub(crate) fn mir_operand_get_const_val<'tcx>(
                         | StatementKind::PlaceMention(..)
                         | StatementKind::Coverage(_)
                         | StatementKind::ConstEvalCounter
+                        | StatementKind::BackwardIncompatibleDropHint { .. }
                         | StatementKind::Nop => {}
                     }
                 }

compiler/rustc_codegen_ssa/src/mir/statement.rs

@@ -92,6 +92,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
             | mir::StatementKind::AscribeUserType(..)
             | mir::StatementKind::ConstEvalCounter
             | mir::StatementKind::PlaceMention(..)
+            | mir::StatementKind::BackwardIncompatibleDropHint { .. }
             | mir::StatementKind::Nop => {}
         }
     }

compiler/rustc_const_eval/src/check_consts/check.rs

@@ -609,6 +609,7 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
             | StatementKind::Coverage(..)
             | StatementKind::Intrinsic(..)
             | StatementKind::ConstEvalCounter
+            | StatementKind::BackwardIncompatibleDropHint { .. }
             | StatementKind::Nop => {}
         }
     }

compiler/rustc_const_eval/src/interpret/step.rs

@@ -143,6 +143,9 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
             // Defined to do nothing. These are added by optimization passes, to avoid changing the
             // size of MIR constantly.
             Nop => {}
+
+            // Only used for temporary lifetime lints
+            BackwardIncompatibleDropHint { .. } => {}
         }
 
         interp_ok(())

compiler/rustc_hir_analysis/src/check/region.rs

@@ -17,6 +17,7 @@ use rustc_index::Idx;
 use rustc_middle::bug;
 use rustc_middle::middle::region::*;
 use rustc_middle::ty::TyCtxt;
+use rustc_session::lint;
 use rustc_span::source_map;
 use tracing::debug;
 
@@ -167,8 +168,23 @@ fn resolve_block<'tcx>(visitor: &mut RegionResolutionVisitor<'tcx>, blk: &'tcx h
             }
         }
         if let Some(tail_expr) = blk.expr {
-            if blk.span.edition().at_least_rust_2024() {
+            let local_id = tail_expr.hir_id.local_id;
-                visitor.terminating_scopes.insert(tail_expr.hir_id.local_id);
+            let edition = blk.span.edition();
+            if edition.at_least_rust_2024() {
+                visitor.terminating_scopes.insert(local_id);
+            } else if !visitor
+                .tcx
+                .lints_that_dont_need_to_run(())
+                .contains(&lint::LintId::of(lint::builtin::TAIL_EXPR_DROP_ORDER))
+            {
+                // If this temporary scope will be changing once the codebase adopts Rust 2024,
+                // and we are linting about possible semantic changes that would result,
+                // then record this node-id in the field `backwards_incompatible_scope`
+                // for future reference.
+                visitor
+                    .scope_tree
+                    .backwards_incompatible_scope
+                    .insert(local_id, Scope { id: local_id, data: ScopeData::Node });
             }
             visitor.visit_expr(tail_expr);
         }

compiler/rustc_index/src/bit_set.rs

@@ -460,6 +460,10 @@ impl<T: Idx> ChunkedBitSet<T> {
         self.chunks.iter().map(|chunk| chunk.count()).sum()
     }
 
+    pub fn is_empty(&self) -> bool {
+        self.chunks.iter().all(|chunk| matches!(chunk, Chunk::Zeros(..) | Chunk::Ones(0)))
+    }
+
     /// Returns `true` if `self` contains `elem`.
     #[inline]
     pub fn contains(&self, elem: T) -> bool {
@@ -668,12 +672,140 @@ impl<T: Idx> BitRelations<ChunkedBitSet<T>> for ChunkedBitSet<T> {
         changed
     }
 
-    fn subtract(&mut self, _other: &ChunkedBitSet<T>) -> bool {
+    fn subtract(&mut self, other: &ChunkedBitSet<T>) -> bool {
-        unimplemented!("implement if/when necessary");
+        assert_eq!(self.domain_size, other.domain_size);
+        debug_assert_eq!(self.chunks.len(), other.chunks.len());
+
+        let mut changed = false;
+        for (mut self_chunk, other_chunk) in self.chunks.iter_mut().zip(other.chunks.iter()) {
+            match (&mut self_chunk, &other_chunk) {
+                (Zeros(..), _) | (_, Zeros(..)) => {}
+                (
+                    Ones(self_chunk_domain_size) | Mixed(self_chunk_domain_size, _, _),
+                    Ones(other_chunk_domain_size),
+                ) => {
+                    debug_assert_eq!(self_chunk_domain_size, other_chunk_domain_size);
+                    changed = true;
+                    *self_chunk = Zeros(*self_chunk_domain_size);
+                }
+                (
+                    Ones(self_chunk_domain_size),
+                    Mixed(other_chunk_domain_size, other_chunk_count, other_chunk_words),
+                ) => {
+                    debug_assert_eq!(self_chunk_domain_size, other_chunk_domain_size);
+                    changed = true;
+                    let num_words = num_words(*self_chunk_domain_size as usize);
+                    debug_assert!(num_words > 0 && num_words <= CHUNK_WORDS);
+                    let mut tail_mask =
+                        1 << (*other_chunk_domain_size - ((num_words - 1) * WORD_BITS) as u16) - 1;
+                    let mut self_chunk_words = **other_chunk_words;
+                    for word in self_chunk_words[0..num_words].iter_mut().rev() {
+                        *word = !*word & tail_mask;
+                        tail_mask = u64::MAX;
+                    }
+                    let self_chunk_count = *self_chunk_domain_size - *other_chunk_count;
+                    debug_assert_eq!(
+                        self_chunk_count,
+                        self_chunk_words[0..num_words]
+                            .iter()
+                            .map(|w| w.count_ones() as ChunkSize)
+                            .sum()
+                    );
+                    *self_chunk =
+                        Mixed(*self_chunk_domain_size, self_chunk_count, Rc::new(self_chunk_words));
+                }
+                (
+                    Mixed(
+                        self_chunk_domain_size,
+                        ref mut self_chunk_count,
+                        ref mut self_chunk_words,
+                    ),
+                    Mixed(_other_chunk_domain_size, _other_chunk_count, other_chunk_words),
+                ) => {
+                    // See [`<Self as BitRelations<ChunkedBitSet<T>>>::union`] for the explanation
+                    let op = |a: u64, b: u64| a & !b;
+                    let num_words = num_words(*self_chunk_domain_size as usize);
+                    if bitwise_changes(
+                        &self_chunk_words[0..num_words],
+                        &other_chunk_words[0..num_words],
+                        op,
+                    ) {
+                        let self_chunk_words = Rc::make_mut(self_chunk_words);
+                        let has_changed = bitwise(
+                            &mut self_chunk_words[0..num_words],
+                            &other_chunk_words[0..num_words],
+                            op,
+                        );
+                        debug_assert!(has_changed);
+                        *self_chunk_count = self_chunk_words[0..num_words]
+                            .iter()
+                            .map(|w| w.count_ones() as ChunkSize)
+                            .sum();
+                        if *self_chunk_count == 0 {
+                            *self_chunk = Zeros(*self_chunk_domain_size);
+                        }
+                        changed = true;
+                    }
+                }
+            }
+        }
+        changed
     }
 
-    fn intersect(&mut self, _other: &ChunkedBitSet<T>) -> bool {
+    fn intersect(&mut self, other: &ChunkedBitSet<T>) -> bool {
-        unimplemented!("implement if/when necessary");
+        assert_eq!(self.domain_size, other.domain_size);
+        debug_assert_eq!(self.chunks.len(), other.chunks.len());
+
+        let mut changed = false;
+        for (mut self_chunk, other_chunk) in self.chunks.iter_mut().zip(other.chunks.iter()) {
+            match (&mut self_chunk, &other_chunk) {
+                (Zeros(..), _) | (_, Ones(..)) => {}
+                (
+                    Ones(self_chunk_domain_size),
+                    Zeros(other_chunk_domain_size) | Mixed(other_chunk_domain_size, ..),
+                )
+                | (Mixed(self_chunk_domain_size, ..), Zeros(other_chunk_domain_size)) => {
+                    debug_assert_eq!(self_chunk_domain_size, other_chunk_domain_size);
+                    changed = true;
+                    *self_chunk = other_chunk.clone();
+                }
+                (
+                    Mixed(
+                        self_chunk_domain_size,
+                        ref mut self_chunk_count,
+                        ref mut self_chunk_words,
+                    ),
+                    Mixed(_other_chunk_domain_size, _other_chunk_count, other_chunk_words),
+                ) => {
+                    // See [`<Self as BitRelations<ChunkedBitSet<T>>>::union`] for the explanation
+                    let op = |a, b| a & b;
+                    let num_words = num_words(*self_chunk_domain_size as usize);
+                    if bitwise_changes(
+                        &self_chunk_words[0..num_words],
+                        &other_chunk_words[0..num_words],
+                        op,
+                    ) {
+                        let self_chunk_words = Rc::make_mut(self_chunk_words);
+                        let has_changed = bitwise(
+                            &mut self_chunk_words[0..num_words],
+                            &other_chunk_words[0..num_words],
+                            op,
+                        );
+                        debug_assert!(has_changed);
+                        *self_chunk_count = self_chunk_words[0..num_words]
+                            .iter()
+                            .map(|w| w.count_ones() as ChunkSize)
+                            .sum();
+                        if *self_chunk_count == 0 {
+                            *self_chunk = Zeros(*self_chunk_domain_size);
+                        }
+                        changed = true;
+                    }
+                }
+            }
+        }
+
+        changed
     }
 }
 

compiler/rustc_lint/messages.ftl

@@ -772,9 +772,6 @@ lint_suspicious_double_ref_clone =
 lint_suspicious_double_ref_deref =
     using `.deref()` on a double reference, which returns `{$ty}` instead of dereferencing the inner type
 
-lint_tail_expr_drop_order = these values and local bindings have significant drop implementation that will have a different drop order from that of Edition 2021
-    .label = these values have significant drop implementation and will observe changes in drop order under Edition 2024
-
 lint_trailing_semi_macro = trailing semicolon in macro used in expression position
     .note1 = macro invocations at the end of a block are treated as expressions
     .note2 = to ignore the value produced by the macro, add a semicolon after the invocation of `{$name}`

compiler/rustc_lint/src/lib.rs

@@ -75,7 +75,6 @@ mod redundant_semicolon;
 mod reference_casting;
 mod shadowed_into_iter;
 mod static_mut_refs;
-mod tail_expr_drop_order;
 mod traits;
 mod types;
 mod unit_bindings;
@@ -116,7 +115,6 @@ use rustc_middle::ty::TyCtxt;
 use shadowed_into_iter::ShadowedIntoIter;
 pub use shadowed_into_iter::{ARRAY_INTO_ITER, BOXED_SLICE_INTO_ITER};
 use static_mut_refs::*;
-use tail_expr_drop_order::TailExprDropOrder;
 use traits::*;
 use types::*;
 use unit_bindings::*;
@@ -240,7 +238,6 @@ late_lint_methods!(
             AsyncFnInTrait: AsyncFnInTrait,
             NonLocalDefinitions: NonLocalDefinitions::default(),
             ImplTraitOvercaptures: ImplTraitOvercaptures,
-            TailExprDropOrder: TailExprDropOrder,
             IfLetRescope: IfLetRescope::default(),
             StaticMutRefs: StaticMutRefs,
             UnqualifiedLocalImports: UnqualifiedLocalImports,

compiler/rustc_lint/src/tail_expr_drop_order.rs

@@ -1,308 +0,0 @@
-use std::mem::swap;
-
-use rustc_ast::UnOp;
-use rustc_hir::def::Res;
-use rustc_hir::intravisit::{self, Visitor};
-use rustc_hir::{self as hir, Block, Expr, ExprKind, LetStmt, Pat, PatKind, QPath, StmtKind};
-use rustc_macros::LintDiagnostic;
-use rustc_middle::ty;
-use rustc_session::lint::FutureIncompatibilityReason;
-use rustc_session::{declare_lint, declare_lint_pass};
-use rustc_span::Span;
-use rustc_span::edition::Edition;
-
-use crate::{LateContext, LateLintPass};
-
-declare_lint! {
-    /// The `tail_expr_drop_order` lint looks for those values generated at the tail expression location,
-    /// that runs a custom `Drop` destructor.
-    /// Some of them may be dropped earlier in Edition 2024 that they used to in Edition 2021 and prior.
-    /// This lint detects those cases and provides you information on those values and their custom destructor implementations.
-    /// Your discretion on this information is required.
-    ///
-    /// ### Example
-    /// ```rust,edition2021
-    /// #![warn(tail_expr_drop_order)]
-    /// struct Droppy(i32);
-    /// impl Droppy {
-    ///     fn get(&self) -> i32 {
-    ///         self.0
-    ///     }
-    /// }
-    /// impl Drop for Droppy {
-    ///     fn drop(&mut self) {
-    ///         // This is a custom destructor and it induces side-effects that is observable
-    ///         // especially when the drop order at a tail expression changes.
-    ///         println!("loud drop {}", self.0);
-    ///     }
-    /// }
-    /// fn edition_2021() -> i32 {
-    ///     let another_droppy = Droppy(0);
-    ///     Droppy(1).get()
-    /// }
-    /// fn main() {
-    ///     edition_2021();
-    /// }
-    /// ```
-    ///
-    /// {{produces}}
-    ///
-    /// ### Explanation
-    ///
-    /// In tail expression of blocks or function bodies,
-    /// values of type with significant `Drop` implementation has an ill-specified drop order
-    /// before Edition 2024 so that they are dropped only after dropping local variables.
-    /// Edition 2024 introduces a new rule with drop orders for them,
-    /// so that they are dropped first before dropping local variables.
-    ///
-    /// A significant `Drop::drop` destructor here refers to an explicit, arbitrary
-    /// implementation of the `Drop` trait on the type, with exceptions including `Vec`,
-    /// `Box`, `Rc`, `BTreeMap` and `HashMap` that are marked by the compiler otherwise
-    /// so long that the generic types have no significant destructor recursively.
-    /// In other words, a type has a significant drop destructor when it has a `Drop` implementation
-    /// or its destructor invokes a significant destructor on a type.
-    /// Since we cannot completely reason about the change by just inspecting the existence of
-    /// a significant destructor, this lint remains only a suggestion and is set to `allow` by default.
-    ///
-    /// This lint only points out the issue with `Droppy`, which will be dropped before `another_droppy`
-    /// does in Edition 2024.
-    /// No fix will be proposed by this lint.
-    /// However, the most probable fix is to hoist `Droppy` into its own local variable binding.
-    /// ```rust
-    /// struct Droppy(i32);
-    /// impl Droppy {
-    ///     fn get(&self) -> i32 {
-    ///         self.0
-    ///     }
-    /// }
-    /// fn edition_2024() -> i32 {
-    ///     let value = Droppy(0);
-    ///     let another_droppy = Droppy(1);
-    ///     value.get()
-    /// }
-    /// ```
-    pub TAIL_EXPR_DROP_ORDER,
-    Allow,
-    "Detect and warn on significant change in drop order in tail expression location",
-    @future_incompatible = FutureIncompatibleInfo {
-        reason: FutureIncompatibilityReason::EditionSemanticsChange(Edition::Edition2024),
-        reference: "issue #123739 <https://github.com/rust-lang/rust/issues/123739>",
-    };
-}
-
-declare_lint_pass!(TailExprDropOrder => [TAIL_EXPR_DROP_ORDER]);
-
-impl TailExprDropOrder {
-    fn check_fn_or_closure<'tcx>(
-        cx: &LateContext<'tcx>,
-        fn_kind: hir::intravisit::FnKind<'tcx>,
-        body: &'tcx hir::Body<'tcx>,
-        def_id: rustc_span::def_id::LocalDefId,
-    ) {
-        let mut locals = vec![];
-        if matches!(fn_kind, hir::intravisit::FnKind::Closure) {
-            for &capture in cx.tcx.closure_captures(def_id) {
-                if matches!(capture.info.capture_kind, ty::UpvarCapture::ByValue)
-                    && capture.place.ty().has_significant_drop(cx.tcx, cx.typing_env())
-                {
-                    locals.push(capture.var_ident.span);
-                }
-            }
-        }
-        for param in body.params {
-            if cx
-                .typeck_results()
-                .node_type(param.hir_id)
-                .has_significant_drop(cx.tcx, cx.typing_env())
-            {
-                locals.push(param.span);
-            }
-        }
-        if let hir::ExprKind::Block(block, _) = body.value.kind {
-            LintVisitor { cx, locals }.check_block_inner(block);
-        } else {
-            LintTailExpr { cx, locals: &locals, is_root_tail_expr: true }.visit_expr(body.value);
-        }
-    }
-}
-
-impl<'tcx> LateLintPass<'tcx> for TailExprDropOrder {
-    fn check_fn(
-        &mut self,
-        cx: &LateContext<'tcx>,
-        fn_kind: hir::intravisit::FnKind<'tcx>,
-        _: &'tcx hir::FnDecl<'tcx>,
-        body: &'tcx hir::Body<'tcx>,
-        _: Span,
-        def_id: rustc_span::def_id::LocalDefId,
-    ) {
-        if !body.value.span.edition().at_least_rust_2024() {
-            Self::check_fn_or_closure(cx, fn_kind, body, def_id);
-        }
-    }
-}
-
-struct LintVisitor<'a, 'tcx> {
-    cx: &'a LateContext<'tcx>,
-    // We only record locals that have significant drops
-    locals: Vec<Span>,
-}
-
-struct LocalCollector<'a, 'tcx> {
-    cx: &'a LateContext<'tcx>,
-    locals: &'a mut Vec<Span>,
-}
-
-impl<'a, 'tcx> Visitor<'tcx> for LocalCollector<'a, 'tcx> {
-    type Result = ();
-    fn visit_pat(&mut self, pat: &'tcx Pat<'tcx>) {
-        if let PatKind::Binding(_binding_mode, id, ident, pat) = pat.kind {
-            let ty = self.cx.typeck_results().node_type(id);
-            if ty.has_significant_drop(self.cx.tcx, self.cx.typing_env()) {
-                self.locals.push(ident.span);
-            }
-            if let Some(pat) = pat {
-                self.visit_pat(pat);
-            }
-        } else {
-            intravisit::walk_pat(self, pat);
-        }
-    }
-}
-
-impl<'a, 'tcx> Visitor<'tcx> for LintVisitor<'a, 'tcx> {
-    fn visit_block(&mut self, block: &'tcx Block<'tcx>) {
-        let mut locals = <_>::default();
-        swap(&mut locals, &mut self.locals);
-        self.check_block_inner(block);
-        swap(&mut locals, &mut self.locals);
-    }
-    fn visit_local(&mut self, local: &'tcx LetStmt<'tcx>) {
-        LocalCollector { cx: self.cx, locals: &mut self.locals }.visit_local(local);
-    }
-}
-
-impl<'a, 'tcx> LintVisitor<'a, 'tcx> {
-    fn check_block_inner(&mut self, block: &Block<'tcx>) {
-        if block.span.at_least_rust_2024() {
-            // We only lint up to Edition 2021
-            return;
-        }
-        let Some(tail_expr) = block.expr else { return };
-        for stmt in block.stmts {
-            match stmt.kind {
-                StmtKind::Let(let_stmt) => self.visit_local(let_stmt),
-                StmtKind::Item(_) => {}
-                StmtKind::Expr(e) | StmtKind::Semi(e) => self.visit_expr(e),
-            }
-        }
-        if self.locals.is_empty() {
-            return;
-        }
-        LintTailExpr { cx: self.cx, locals: &self.locals, is_root_tail_expr: true }
-            .visit_expr(tail_expr);
-    }
-}
-
-struct LintTailExpr<'a, 'tcx> {
-    cx: &'a LateContext<'tcx>,
-    is_root_tail_expr: bool,
-    locals: &'a [Span],
-}
-
-impl<'a, 'tcx> LintTailExpr<'a, 'tcx> {
-    fn expr_eventually_point_into_local(mut expr: &Expr<'tcx>) -> bool {
-        loop {
-            match expr.kind {
-                ExprKind::Index(access, _, _) | ExprKind::Field(access, _) => expr = access,
-                ExprKind::AddrOf(_, _, referee) | ExprKind::Unary(UnOp::Deref, referee) => {
-                    expr = referee
-                }
-                ExprKind::Path(_)
-                    if let ExprKind::Path(QPath::Resolved(_, path)) = expr.kind
-                        && let [local, ..] = path.segments
-                        && let Res::Local(_) = local.res =>
-                {
-                    return true;
-                }
-                _ => return false,
-            }
-        }
-    }
-
-    fn expr_generates_nonlocal_droppy_value(&self, expr: &Expr<'tcx>) -> bool {
-        if Self::expr_eventually_point_into_local(expr) {
-            return false;
-        }
-        self.cx
-            .typeck_results()
-            .expr_ty(expr)
-            .has_significant_drop(self.cx.tcx, self.cx.typing_env())
-    }
-}
-
-impl<'a, 'tcx> Visitor<'tcx> for LintTailExpr<'a, 'tcx> {
-    fn visit_expr(&mut self, expr: &'tcx Expr<'tcx>) {
-        if self.is_root_tail_expr {
-            self.is_root_tail_expr = false;
-        } else if self.expr_generates_nonlocal_droppy_value(expr) {
-            self.cx.tcx.emit_node_span_lint(
-                TAIL_EXPR_DROP_ORDER,
-                expr.hir_id,
-                expr.span,
-                TailExprDropOrderLint { spans: self.locals.to_vec() },
-            );
-            return;
-        }
-        match expr.kind {
-            ExprKind::Match(scrutinee, _, _) => self.visit_expr(scrutinee),
-
-            ExprKind::ConstBlock(_)
-            | ExprKind::Array(_)
-            | ExprKind::Break(_, _)
-            | ExprKind::Continue(_)
-            | ExprKind::Ret(_)
-            | ExprKind::Become(_)
-            | ExprKind::Yield(_, _)
-            | ExprKind::InlineAsm(_)
-            | ExprKind::If(_, _, _)
-            | ExprKind::Loop(_, _, _, _)
-            | ExprKind::Closure(_)
-            | ExprKind::DropTemps(_)
-            | ExprKind::OffsetOf(_, _)
-            | ExprKind::Assign(_, _, _)
-            | ExprKind::AssignOp(_, _, _)
-            | ExprKind::Lit(_)
-            | ExprKind::Err(_) => {}
-
-            ExprKind::MethodCall(_, _, _, _)
-            | ExprKind::Call(_, _)
-            | ExprKind::Type(_, _)
-            | ExprKind::Tup(_)
-            | ExprKind::Binary(_, _, _)
-            | ExprKind::Unary(_, _)
-            | ExprKind::Path(_)
-            | ExprKind::Let(_)
-            | ExprKind::Cast(_, _)
-            | ExprKind::Field(_, _)
-            | ExprKind::Index(_, _, _)
-            | ExprKind::AddrOf(_, _, _)
-            | ExprKind::Struct(_, _, _)
-            | ExprKind::Repeat(_, _) => intravisit::walk_expr(self, expr),
-
-            ExprKind::Block(_, _) => {
-                // We do not lint further because the drop order stays the same inside the block
-            }
-        }
-    }
-    fn visit_block(&mut self, block: &'tcx Block<'tcx>) {
-        LintVisitor { cx: self.cx, locals: <_>::default() }.check_block_inner(block);
-    }
-}
-
-#[derive(LintDiagnostic)]
-#[diag(lint_tail_expr_drop_order)]
-struct TailExprDropOrderLint {
-    #[label]
-    pub spans: Vec<Span>,
-}

compiler/rustc_lint_defs/src/builtin.rs

@@ -101,6 +101,7 @@ declare_lint_pass! {
         SINGLE_USE_LIFETIMES,
         SOFT_UNSTABLE,
         STABLE_FEATURES,
+        TAIL_EXPR_DROP_ORDER,
         TEST_UNSTABLE_LINT,
         TEXT_DIRECTION_CODEPOINT_IN_COMMENT,
         TRIVIAL_CASTS,
@@ -4994,6 +4995,83 @@ declare_lint! {
     "detects pointer to integer transmutes in const functions and associated constants",
 }
 
+declare_lint! {
+    /// The `tail_expr_drop_order` lint looks for those values generated at the tail expression location,
+    /// that runs a custom `Drop` destructor.
+    /// Some of them may be dropped earlier in Edition 2024 that they used to in Edition 2021 and prior.
+    /// This lint detects those cases and provides you information on those values and their custom destructor implementations.
+    /// Your discretion on this information is required.
+    ///
+    /// ### Example
+    /// ```rust,edition2021
+    /// #![warn(tail_expr_drop_order)]
+    /// struct Droppy(i32);
+    /// impl Droppy {
+    ///     fn get(&self) -> i32 {
+    ///         self.0
+    ///     }
+    /// }
+    /// impl Drop for Droppy {
+    ///     fn drop(&mut self) {
+    ///         // This is a custom destructor and it induces side-effects that is observable
+    ///         // especially when the drop order at a tail expression changes.
+    ///         println!("loud drop {}", self.0);
+    ///     }
+    /// }
+    /// fn edition_2021() -> i32 {
+    ///     let another_droppy = Droppy(0);
+    ///     Droppy(1).get()
+    /// }
+    /// fn main() {
+    ///     edition_2021();
+    /// }
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// In tail expression of blocks or function bodies,
+    /// values of type with significant `Drop` implementation has an ill-specified drop order
+    /// before Edition 2024 so that they are dropped only after dropping local variables.
+    /// Edition 2024 introduces a new rule with drop orders for them,
+    /// so that they are dropped first before dropping local variables.
+    ///
+    /// A significant `Drop::drop` destructor here refers to an explicit, arbitrary
+    /// implementation of the `Drop` trait on the type, with exceptions including `Vec`,
+    /// `Box`, `Rc`, `BTreeMap` and `HashMap` that are marked by the compiler otherwise
+    /// so long that the generic types have no significant destructor recursively.
+    /// In other words, a type has a significant drop destructor when it has a `Drop` implementation
+    /// or its destructor invokes a significant destructor on a type.
+    /// Since we cannot completely reason about the change by just inspecting the existence of
+    /// a significant destructor, this lint remains only a suggestion and is set to `allow` by default.
+    ///
+    /// This lint only points out the issue with `Droppy`, which will be dropped before `another_droppy`
+    /// does in Edition 2024.
+    /// No fix will be proposed by this lint.
+    /// However, the most probable fix is to hoist `Droppy` into its own local variable binding.
+    /// ```rust
+    /// struct Droppy(i32);
+    /// impl Droppy {
+    ///     fn get(&self) -> i32 {
+    ///         self.0
+    ///     }
+    /// }
+    /// fn edition_2024() -> i32 {
+    ///     let value = Droppy(0);
+    ///     let another_droppy = Droppy(1);
+    ///     value.get()
+    /// }
+    /// ```
+    pub TAIL_EXPR_DROP_ORDER,
+    Allow,
+    "Detect and warn on significant change in drop order in tail expression location",
+    @future_incompatible = FutureIncompatibleInfo {
+        reason: FutureIncompatibilityReason::EditionSemanticsChange(Edition::Edition2024),
+        reference: "issue #123739 <https://github.com/rust-lang/rust/issues/123739>",
+    };
+}
+
 declare_lint! {
     /// The `rust_2024_guarded_string_incompatible_syntax` lint detects `#` tokens
     /// that will be parsed as part of a guarded string literal in Rust 2024.

compiler/rustc_middle/src/middle/region.rs

@@ -236,6 +236,11 @@ pub struct ScopeTree {
     /// during type check based on a traversal of the AST.
     pub rvalue_candidates: HirIdMap<RvalueCandidateType>,
 
+    /// Backwards incompatible scoping that will be introduced in future editions.
+    /// This information is used later for linting to identify locals and
+    /// temporary values that will receive backwards-incompatible drop orders.
+    pub backwards_incompatible_scope: UnordMap<hir::ItemLocalId, Scope>,
+
     /// If there are any `yield` nested within a scope, this map
     /// stores the `Span` of the last one and its index in the
     /// postorder of the Visitor traversal on the HIR.

compiler/rustc_middle/src/mir/pretty.rs

@@ -834,6 +834,11 @@ impl Debug for Statement<'_> {
             Intrinsic(box ref intrinsic) => write!(fmt, "{intrinsic}"),
             ConstEvalCounter => write!(fmt, "ConstEvalCounter"),
             Nop => write!(fmt, "nop"),
+            BackwardIncompatibleDropHint { ref place, reason: _ } => {
+                // For now, we don't record the reason because there is only one use case,
+                // which is to report breaking change in drop order by Edition 2024
+                write!(fmt, "backward incompatible drop({place:?})")
+            }
         }
     }
 }

compiler/rustc_middle/src/mir/syntax.rs

@@ -432,6 +432,18 @@ pub enum StatementKind<'tcx> {
 
     /// No-op. Useful for deleting instructions without affecting statement indices.
     Nop,
+
+    /// Marker statement indicating where `place` would be dropped.
+    /// This is semantically equivalent to `Nop`, so codegen and MIRI should interpret this
+    /// statement as such.
+    /// The only use case of this statement is for linting in MIR to detect temporary lifetime
+    /// changes.
+    BackwardIncompatibleDropHint {
+        /// Place to drop
+        place: Box<Place<'tcx>>,
+        /// Reason for backward incompatibility
+        reason: BackwardIncompatibleDropReason,
+    },
 }
 
 impl StatementKind<'_> {
@@ -452,6 +464,7 @@ impl StatementKind<'_> {
             StatementKind::Intrinsic(..) => "Intrinsic",
             StatementKind::ConstEvalCounter => "ConstEvalCounter",
             StatementKind::Nop => "Nop",
+            StatementKind::BackwardIncompatibleDropHint { .. } => "BackwardIncompatibleDropHint",
         }
     }
 }
@@ -897,6 +910,21 @@ pub enum TerminatorKind<'tcx> {
     },
 }
 
+#[derive(
+    Clone,
+    Debug,
+    TyEncodable,
+    TyDecodable,
+    Hash,
+    HashStable,
+    PartialEq,
+    TypeFoldable,
+    TypeVisitable
+)]
+pub enum BackwardIncompatibleDropReason {
+    Edition2024,
+}
+
 impl TerminatorKind<'_> {
     /// Returns a simple string representation of a `TerminatorKind` variant, independent of any
     /// values it might hold (e.g. `TerminatorKind::Call` always returns `"Call"`).

compiler/rustc_middle/src/mir/visit.rs

@@ -452,6 +452,7 @@ macro_rules! make_mir_visitor {
                     }
                     StatementKind::ConstEvalCounter => {}
                     StatementKind::Nop => {}
+                    StatementKind::BackwardIncompatibleDropHint { .. } => {}
                 }
             }
 

compiler/rustc_middle/src/query/keys.rs

@@ -360,6 +360,14 @@ impl<'tcx> Key for (ty::ParamEnv<'tcx>, ty::TraitRef<'tcx>) {
     }
 }
 
+impl<'tcx> Key for ty::ParamEnvAnd<'tcx, Ty<'tcx>> {
+    type Cache<V> = DefaultCache<Self, V>;
+
+    fn default_span(&self, _tcx: TyCtxt<'_>) -> Span {
+        DUMMY_SP
+    }
+}
+
 impl<'tcx> Key for ty::TraitRef<'tcx> {
     type Cache<V> = DefaultCache<Self, V>;
 

compiler/rustc_middle/src/query/mod.rs

@@ -1448,6 +1448,28 @@ rustc_queries! {
         cache_on_disk_if { false }
     }
 
+    /// Returns a list of types which (a) have a potentially significant destructor
+    /// and (b) may be dropped as a result of dropping a value of some type `ty`
+    /// (in the given environment).
+    ///
+    /// The idea of "significant" drop is somewhat informal and is used only for
+    /// diagnostics and edition migrations. The idea is that a significant drop may have
+    /// some visible side-effect on execution; freeing memory is NOT considered a side-effect.
+    /// The rules are as follows:
+    /// * Type with no explicit drop impl do not have significant drop.
+    /// * Types with a drop impl are assumed to have significant drop unless they have a `#[rustc_insignificant_dtor]` annotation.
+    ///
+    /// Note that insignificant drop is a "shallow" property. A type like `Vec<LockGuard>` does not
+    /// have significant drop but the type `LockGuard` does, and so if `ty  = Vec<LockGuard>`
+    /// then the return value would be `&[LockGuard]`.
+    /// *IMPORTANT*: *DO NOT* run this query before promoted MIR body is constructed,
+    /// because this query partially depends on that query.
+    /// Otherwise, there is a risk of query cycles.
+    query list_significant_drop_tys(ty: ty::ParamEnvAnd<'tcx, Ty<'tcx>>) -> &'tcx ty::List<Ty<'tcx>> {
+        desc { |tcx| "computing when `{}` has a significant destructor", ty.value }
+        cache_on_disk_if { false }
+    }
+
     /// Computes the layout of a type. Note that this implicitly
     /// executes in "reveal all" mode, and will normalize the input type.
     query layout_of(

compiler/rustc_middle/src/thir.rs

@@ -247,13 +247,24 @@ pub struct Expr<'tcx> {
     pub ty: Ty<'tcx>,
 
     /// The lifetime of this expression if it should be spilled into a
-    /// temporary; should be `None` only if in a constant context
+    /// temporary
-    pub temp_lifetime: Option<region::Scope>,
+    pub temp_lifetime: TempLifetime,
 
     /// span of the expression in the source
     pub span: Span,
 }
 
+/// Temporary lifetime information for THIR expressions
+#[derive(Clone, Copy, Debug, HashStable)]
+pub struct TempLifetime {
+    /// Lifetime for temporaries as expected.
+    /// This should be `None` in a constant context.
+    pub temp_lifetime: Option<region::Scope>,
+    /// If `Some(lt)`, indicates that the lifetime of this temporary will change to `lt` in a future edition.
+    /// If `None`, then no changes are expected, or lints are disabled.
+    pub backwards_incompatible: Option<region::Scope>,
+}
+
 #[derive(Clone, Debug, HashStable)]
 pub enum ExprKind<'tcx> {
     /// `Scope`s are used to explicitly mark destruction scopes,
@@ -1087,7 +1098,7 @@ mod size_asserts {
     use super::*;
     // tidy-alphabetical-start
     static_assert_size!(Block, 48);
-    static_assert_size!(Expr<'_>, 64);
+    static_assert_size!(Expr<'_>, 72);
     static_assert_size!(ExprKind<'_>, 40);
     static_assert_size!(Pat<'_>, 64);
     static_assert_size!(PatKind<'_>, 48);

compiler/rustc_middle/src/ty/rvalue_scopes.rs

@@ -18,15 +18,17 @@ impl RvalueScopes {
     }
 
     /// Returns the scope when the temp created by `expr_id` will be cleaned up.
+    /// It also emits a lint on potential backwards incompatible change to the temporary scope
+    /// which is *for now* always shortening.
     pub fn temporary_scope(
         &self,
         region_scope_tree: &ScopeTree,
         expr_id: hir::ItemLocalId,
-    ) -> Option<Scope> {
+    ) -> (Option<Scope>, Option<Scope>) {
         // Check for a designated rvalue scope.
         if let Some(&s) = self.map.get(&expr_id) {
             debug!("temporary_scope({expr_id:?}) = {s:?} [custom]");
-            return s;
+            return (s, None);
         }
 
         // Otherwise, locate the innermost terminating scope
@@ -34,27 +36,40 @@ impl RvalueScopes {
         // have an enclosing scope, hence no scope will be
         // returned.
         let mut id = Scope { id: expr_id, data: ScopeData::Node };
+        let mut backwards_incompatible = None;
 
         while let Some(&(p, _)) = region_scope_tree.parent_map.get(&id) {
             match p.data {
                 ScopeData::Destruction => {
                     debug!("temporary_scope({expr_id:?}) = {id:?} [enclosing]");
-                    return Some(id);
+                    return (Some(id), backwards_incompatible);
                 }
                 ScopeData::IfThenRescope => {
                     debug!("temporary_scope({expr_id:?}) = {p:?} [enclosing]");
-                    return Some(p);
+                    return (Some(p), backwards_incompatible);
                 }
                 ScopeData::Node
                 | ScopeData::CallSite
                 | ScopeData::Arguments
                 | ScopeData::IfThen
-                | ScopeData::Remainder(_) => id = p,
+                | ScopeData::Remainder(_) => {
+                    // If we haven't already passed through a backwards-incompatible node,
+                    // then check if we are passing through one now and record it if so.
+                    // This is for now only working for cases where a temporary lifetime is
+                    // *shortened*.
+                    if backwards_incompatible.is_none() {
+                        backwards_incompatible = region_scope_tree
+                            .backwards_incompatible_scope
+                            .get(&p.item_local_id())
+                            .copied();
+                    }
+                    id = p
+                }
             }
         }
 
         debug!("temporary_scope({expr_id:?}) = None");
-        None
+        (None, backwards_incompatible)
     }
 
     /// Make an association between a sub-expression and an extended lifetime

compiler/rustc_mir_build/src/build/expr/as_operand.rs

@@ -1,6 +1,5 @@
 //! See docs in build/expr/mod.rs
 
-use rustc_middle::middle::region;
 use rustc_middle::mir::*;
 use rustc_middle::thir::*;
 use tracing::{debug, instrument};
@@ -9,6 +8,12 @@ use crate::build::expr::category::Category;
 use crate::build::{BlockAnd, BlockAndExtension, Builder, NeedsTemporary};
 
 impl<'a, 'tcx> Builder<'a, 'tcx> {
+    /// Construct a temporary lifetime restricted to just the local scope
+    pub(crate) fn local_temp_lifetime(&self) -> TempLifetime {
+        let local_scope = self.local_scope();
+        TempLifetime { temp_lifetime: Some(local_scope), backwards_incompatible: None }
+    }
+
     /// Returns an operand suitable for use until the end of the current
     /// scope expression.
     ///
@@ -21,8 +26,13 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         block: BasicBlock,
         expr_id: ExprId,
     ) -> BlockAnd<Operand<'tcx>> {
-        let local_scope = self.local_scope();
+        self.as_operand(
-        self.as_operand(block, Some(local_scope), expr_id, LocalInfo::Boring, NeedsTemporary::Maybe)
+            block,
+            self.local_temp_lifetime(),
+            expr_id,
+            LocalInfo::Boring,
+            NeedsTemporary::Maybe,
+        )
     }
 
     /// Returns an operand suitable for use until the end of the current scope expression and
@@ -80,8 +90,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         block: BasicBlock,
         expr: ExprId,
     ) -> BlockAnd<Operand<'tcx>> {
-        let local_scope = self.local_scope();
+        self.as_call_operand(block, self.local_temp_lifetime(), expr)
-        self.as_call_operand(block, Some(local_scope), expr)
     }
 
     /// Compile `expr` into a value that can be used as an operand.
@@ -102,7 +111,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
     pub(crate) fn as_operand(
         &mut self,
         mut block: BasicBlock,
-        scope: Option<region::Scope>,
+        scope: TempLifetime,
         expr_id: ExprId,
         local_info: LocalInfo<'tcx>,
         needs_temporary: NeedsTemporary,
@@ -146,7 +155,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
     pub(crate) fn as_call_operand(
         &mut self,
         mut block: BasicBlock,
-        scope: Option<region::Scope>,
+        scope: TempLifetime,
         expr_id: ExprId,
     ) -> BlockAnd<Operand<'tcx>> {
         let this = self;

compiler/rustc_mir_build/src/build/expr/as_place.rs

@@ -6,7 +6,6 @@ use std::iter;
 use rustc_abi::{FIRST_VARIANT, FieldIdx, VariantIdx};
 use rustc_hir::def_id::LocalDefId;
 use rustc_middle::hir::place::{Projection as HirProjection, ProjectionKind as HirProjectionKind};
-use rustc_middle::middle::region;
 use rustc_middle::mir::AssertKind::BoundsCheck;
 use rustc_middle::mir::*;
 use rustc_middle::thir::*;
@@ -598,7 +597,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         index: ExprId,
         mutability: Mutability,
         fake_borrow_temps: Option<&mut Vec<Local>>,
-        temp_lifetime: Option<region::Scope>,
+        temp_lifetime: TempLifetime,
         expr_span: Span,
         source_info: SourceInfo,
     ) -> BlockAnd<PlaceBuilder<'tcx>> {

compiler/rustc_mir_build/src/build/expr/as_rvalue.rs

@@ -33,14 +33,18 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         expr_id: ExprId,
     ) -> BlockAnd<Rvalue<'tcx>> {
         let local_scope = self.local_scope();
-        self.as_rvalue(block, Some(local_scope), expr_id)
+        self.as_rvalue(
+            block,
+            TempLifetime { temp_lifetime: Some(local_scope), backwards_incompatible: None },
+            expr_id,
+        )
     }
 
     /// Compile `expr`, yielding an rvalue.
     pub(crate) fn as_rvalue(
         &mut self,
         mut block: BasicBlock,
-        scope: Option<region::Scope>,
+        scope: TempLifetime,
         expr_id: ExprId,
     ) -> BlockAnd<Rvalue<'tcx>> {
         let this = self;
@@ -171,7 +175,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                     source_info,
                     kind: StatementKind::StorageLive(result),
                 });
-                if let Some(scope) = scope {
+                if let Some(scope) = scope.temp_lifetime {
                     // schedule a shallow free of that memory, lest we unwind:
                     this.schedule_drop_storage_and_value(expr_span, scope, result);
                 }
@@ -445,7 +449,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                                         block = this.limit_capture_mutability(
                                             upvar_expr.span,
                                             upvar_expr.ty,
-                                            scope,
+                                            scope.temp_lifetime,
                                             block,
                                             arg,
                                         )
@@ -705,7 +709,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         &mut self,
         mut block: BasicBlock,
         value: ExprId,
-        scope: Option<region::Scope>,
+        scope: TempLifetime,
         outer_source_info: SourceInfo,
     ) -> BlockAnd<Rvalue<'tcx>> {
         let this = self;

compiler/rustc_mir_build/src/build/expr/as_temp.rs

@@ -2,7 +2,6 @@
 
 use rustc_data_structures::stack::ensure_sufficient_stack;
 use rustc_hir::HirId;
-use rustc_middle::middle::region;
 use rustc_middle::middle::region::{Scope, ScopeData};
 use rustc_middle::mir::*;
 use rustc_middle::thir::*;
@@ -17,7 +16,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
     pub(crate) fn as_temp(
         &mut self,
         block: BasicBlock,
-        temp_lifetime: Option<region::Scope>,
+        temp_lifetime: TempLifetime,
         expr_id: ExprId,
         mutability: Mutability,
     ) -> BlockAnd<Local> {
@@ -31,7 +30,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
     fn as_temp_inner(
         &mut self,
         mut block: BasicBlock,
-        temp_lifetime: Option<region::Scope>,
+        temp_lifetime: TempLifetime,
         expr_id: ExprId,
         mutability: Mutability,
     ) -> BlockAnd<Local> {
@@ -47,8 +46,8 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         }
 
         let expr_ty = expr.ty;
-        let deduplicate_temps =
+        let deduplicate_temps = this.fixed_temps_scope.is_some()
-            this.fixed_temps_scope.is_some() && this.fixed_temps_scope == temp_lifetime;
+            && this.fixed_temps_scope == temp_lifetime.temp_lifetime;
         let temp = if deduplicate_temps && let Some(temp_index) = this.fixed_temps.get(&expr_id) {
             *temp_index
         } else {
@@ -76,7 +75,9 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                     LocalInfo::BlockTailTemp(tail_info)
                 }
 
-                _ if let Some(Scope { data: ScopeData::IfThenRescope, id }) = temp_lifetime => {
+                _ if let Some(Scope { data: ScopeData::IfThenRescope, id }) =
+                    temp_lifetime.temp_lifetime =>
+                {
                     LocalInfo::IfThenRescopeTemp {
                         if_then: HirId { owner: this.hir_id.owner, local_id: id },
                     }
@@ -117,7 +118,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 // Anything with a shorter lifetime (e.g the `&foo()` in
                 // `bar(&foo())` or anything within a block will keep the
                 // regular drops just like runtime code.
-                if let Some(temp_lifetime) = temp_lifetime {
+                if let Some(temp_lifetime) = temp_lifetime.temp_lifetime {
                     this.schedule_drop(expr_span, temp_lifetime, temp, DropKind::Storage);
                 }
             }
@@ -125,10 +126,14 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
 
         block = this.expr_into_dest(temp_place, block, expr_id).into_block();
 
-        if let Some(temp_lifetime) = temp_lifetime {
+        if let Some(temp_lifetime) = temp_lifetime.temp_lifetime {
             this.schedule_drop(expr_span, temp_lifetime, temp, DropKind::Value);
         }
 
+        if let Some(backwards_incompatible) = temp_lifetime.backwards_incompatible {
+            this.schedule_backwards_incompatible_drop(expr_span, backwards_incompatible, temp);
+        }
+
         block.and(temp)
     }
 }

compiler/rustc_mir_build/src/build/expr/into.rs

@@ -139,7 +139,8 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 // (#66975) Source could be a const of type `!`, so has to
                 // exist in the generated MIR.
                 unpack!(
-                    block = this.as_temp(block, Some(this.local_scope()), source, Mutability::Mut)
+                    block =
+                        this.as_temp(block, this.local_temp_lifetime(), source, Mutability::Mut)
                 );
 
                 // This is an optimization. If the expression was a call then we already have an
@@ -321,7 +322,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 let is_union = adt_def.is_union();
                 let active_field_index = is_union.then(|| fields[0].name);
 
-                let scope = this.local_scope();
+                let scope = this.local_temp_lifetime();
 
                 // first process the set of fields that were provided
                 // (evaluating them in order given by user)
@@ -333,7 +334,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                             unpack!(
                                 block = this.as_operand(
                                     block,
-                                    Some(scope),
+                                    scope,
                                     f.expr,
                                     LocalInfo::AggregateTemp,
                                     NeedsTemporary::Maybe,
@@ -548,15 +549,10 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             }
 
             ExprKind::Yield { value } => {
-                let scope = this.local_scope();
+                let scope = this.local_temp_lifetime();
                 let value = unpack!(
-                    block = this.as_operand(
+                    block =
-                        block,
+                        this.as_operand(block, scope, value, LocalInfo::Boring, NeedsTemporary::No)
-                        Some(scope),
-                        value,
-                        LocalInfo::Boring,
-                        NeedsTemporary::No
-                    )
                 );
                 let resume = this.cfg.start_new_block();
                 this.cfg.terminate(block, source_info, TerminatorKind::Yield {

compiler/rustc_mir_build/src/build/expr/stmt.rs

@@ -172,8 +172,17 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                     None
                 };
 
-                let temp =
+                let temp = unpack!(
-                    unpack!(block = this.as_temp(block, statement_scope, expr_id, Mutability::Not));
+                    block = this.as_temp(
+                        block,
+                        TempLifetime {
+                            temp_lifetime: statement_scope,
+                            backwards_incompatible: None
+                        },
+                        expr_id,
+                        Mutability::Not
+                    )
+                );
 
                 if let Some(span) = adjusted_span {
                     this.local_decls[temp].source_info.span = span;

compiler/rustc_mir_build/src/build/matches/mod.rs

@@ -202,8 +202,17 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 // Increment the decision depth, in case we encounter boolean expressions
                 // further down.
                 this.mcdc_increment_depth_if_enabled();
-                let place =
+                let place = unpack!(
-                    unpack!(block = this.as_temp(block, Some(temp_scope), expr_id, mutability));
+                    block = this.as_temp(
+                        block,
+                        TempLifetime {
+                            temp_lifetime: Some(temp_scope),
+                            backwards_incompatible: None
+                        },
+                        expr_id,
+                        mutability
+                    )
+                );
                 this.mcdc_decrement_depth_if_enabled();
 
                 let operand = Operand::Move(Place::from(place));

compiler/rustc_mir_build/src/build/scope.rs

@@ -89,7 +89,7 @@ use rustc_index::{IndexSlice, IndexVec};
 use rustc_middle::middle::region;
 use rustc_middle::mir::*;
 use rustc_middle::thir::{ExprId, LintLevel};
-use rustc_middle::{bug, span_bug};
+use rustc_middle::{bug, span_bug, ty};
 use rustc_session::lint::Level;
 use rustc_span::source_map::Spanned;
 use rustc_span::{DUMMY_SP, Span};
@@ -151,6 +151,9 @@ struct DropData {
 
     /// Whether this is a value Drop or a StorageDead.
     kind: DropKind,
+
+    /// Whether this is a backwards-incompatible drop lint
+    backwards_incompatible_lint: bool,
 }
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
@@ -274,8 +277,12 @@ impl DropTree {
         // represents the block in the tree that should be jumped to once all
         // of the required drops have been performed.
         let fake_source_info = SourceInfo::outermost(DUMMY_SP);
-        let fake_data =
+        let fake_data = DropData {
-            DropData { source_info: fake_source_info, local: Local::MAX, kind: DropKind::Storage };
+            source_info: fake_source_info,
+            local: Local::MAX,
+            kind: DropKind::Storage,
+            backwards_incompatible_lint: false,
+        };
         let drops = IndexVec::from_raw(vec![DropNode { data: fake_data, next: DropIdx::MAX }]);
         Self { drops, entry_points: Vec::new(), existing_drops_map: FxHashMap::default() }
     }
@@ -763,7 +770,12 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                     let local =
                         place.as_local().unwrap_or_else(|| bug!("projection in tail call args"));
 
-                    Some(DropData { source_info, local, kind: DropKind::Value })
+                    Some(DropData {
+                        source_info,
+                        local,
+                        kind: DropKind::Value,
+                        backwards_incompatible_lint: false,
+                    })
                 }
                 Operand::Constant(_) => None,
             })
@@ -1019,9 +1031,9 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 if local.index() <= self.arg_count {
                     span_bug!(
                         span,
-                        "`schedule_drop` called with local {:?} and arg_count {}",
+                        "`schedule_drop` called with body argument {:?} \
+                        but its storage does not require a drop",
                         local,
-                        self.arg_count,
                     )
                 }
                 false
@@ -1089,6 +1101,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                     source_info: SourceInfo { span: scope_end, scope: scope.source_scope },
                     local,
                     kind: drop_kind,
+                    backwards_incompatible_lint: false,
                 });
 
                 return;
@@ -1098,6 +1111,45 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         span_bug!(span, "region scope {:?} not in scope to drop {:?}", region_scope, local);
     }
 
+    /// Schedule emission of a backwards incompatible drop lint hint.
+    /// Applicable only to temporary values for now.
+    pub(crate) fn schedule_backwards_incompatible_drop(
+        &mut self,
+        span: Span,
+        region_scope: region::Scope,
+        local: Local,
+    ) {
+        if !self.local_decls[local].ty.has_significant_drop(self.tcx, ty::TypingEnv {
+            typing_mode: ty::TypingMode::non_body_analysis(),
+            param_env: self.param_env,
+        }) {
+            return;
+        }
+        for scope in self.scopes.scopes.iter_mut().rev() {
+            // Since we are inserting linting MIR statement, we have to invalidate the caches
+            scope.invalidate_cache();
+            if scope.region_scope == region_scope {
+                let region_scope_span = region_scope.span(self.tcx, self.region_scope_tree);
+                let scope_end = self.tcx.sess.source_map().end_point(region_scope_span);
+
+                scope.drops.push(DropData {
+                    source_info: SourceInfo { span: scope_end, scope: scope.source_scope },
+                    local,
+                    kind: DropKind::Value,
+                    backwards_incompatible_lint: true,
+                });
+
+                return;
+            }
+        }
+        span_bug!(
+            span,
+            "region scope {:?} not in scope to drop {:?} for linting",
+            region_scope,
+            local
+        );
+    }
+
     /// Indicates that the "local operand" stored in `local` is
     /// *moved* at some point during execution (see `local_scope` for
     /// more information about what a "local operand" is -- in short,
@@ -1378,16 +1430,25 @@ fn build_scope_drops<'tcx>(
                     continue;
                 }
 
-                unwind_drops.add_entry_point(block, unwind_to);
+                if drop_data.backwards_incompatible_lint {
-
+                    cfg.push(block, Statement {
-                let next = cfg.start_new_block();
+                        source_info,
-                cfg.terminate(block, source_info, TerminatorKind::Drop {
+                        kind: StatementKind::BackwardIncompatibleDropHint {
-                    place: local.into(),
+                            place: Box::new(local.into()),
-                    target: next,
+                            reason: BackwardIncompatibleDropReason::Edition2024,
-                    unwind: UnwindAction::Continue,
+                        },
-                    replace: false,
+                    });
-                });
+                } else {
-                block = next;
+                    unwind_drops.add_entry_point(block, unwind_to);
+                    let next = cfg.start_new_block();
+                    cfg.terminate(block, source_info, TerminatorKind::Drop {
+                        place: local.into(),
+                        target: next,
+                        unwind: UnwindAction::Continue,
+                        replace: false,
+                    });
+                    block = next;
+                }
             }
             DropKind::Storage => {
                 if storage_dead_on_unwind {

compiler/rustc_mir_build/src/thir/cx/expr.rs

@@ -23,7 +23,6 @@ use tracing::{debug, info, instrument, trace};
 
 use crate::errors;
 use crate::thir::cx::Cx;
-use crate::thir::cx::region::Scope;
 use crate::thir::util::UserAnnotatedTyHelpers;
 
 impl<'tcx> Cx<'tcx> {
@@ -240,7 +239,7 @@ impl<'tcx> Cx<'tcx> {
     fn mirror_expr_cast(
         &mut self,
         source: &'tcx hir::Expr<'tcx>,
-        temp_lifetime: Option<Scope>,
+        temp_lifetime: TempLifetime,
         span: Span,
     ) -> ExprKind<'tcx> {
         let tcx = self.tcx;
@@ -325,7 +324,7 @@ impl<'tcx> Cx<'tcx> {
     fn make_mirror_unadjusted(&mut self, expr: &'tcx hir::Expr<'tcx>) -> Expr<'tcx> {
         let tcx = self.tcx;
         let expr_ty = self.typeck_results().expr_ty(expr);
-        let temp_lifetime =
+        let (temp_lifetime, backwards_incompatible) =
             self.rvalue_scopes.temporary_scope(self.region_scope_tree, expr.hir_id.local_id);
 
         let kind = match expr.kind {
@@ -361,7 +360,7 @@ impl<'tcx> Cx<'tcx> {
                     let arg_tys = args.iter().map(|e| self.typeck_results().expr_ty_adjusted(e));
                     let tupled_args = Expr {
                         ty: Ty::new_tup_from_iter(tcx, arg_tys),
-                        temp_lifetime,
+                        temp_lifetime: TempLifetime { temp_lifetime, backwards_incompatible },
                         span: expr.span,
                         kind: ExprKind::Tuple { fields: self.mirror_exprs(args) },
                     };
@@ -391,7 +390,10 @@ impl<'tcx> Cx<'tcx> {
                                 && let [value] = args
                             {
                                 return Expr {
-                                    temp_lifetime,
+                                    temp_lifetime: TempLifetime {
+                                        temp_lifetime,
+                                        backwards_incompatible,
+                                    },
                                     ty: expr_ty,
                                     span: expr.span,
                                     kind: ExprKind::Box { value: self.mirror_expr(value) },
@@ -811,13 +813,13 @@ impl<'tcx> Cx<'tcx> {
             },
             hir::ExprKind::Loop(body, ..) => {
                 let block_ty = self.typeck_results().node_type(body.hir_id);
-                let temp_lifetime = self
+                let (temp_lifetime, backwards_incompatible) = self
                     .rvalue_scopes
                     .temporary_scope(self.region_scope_tree, body.hir_id.local_id);
                 let block = self.mirror_block(body);
                 let body = self.thir.exprs.push(Expr {
                     ty: block_ty,
-                    temp_lifetime,
+                    temp_lifetime: TempLifetime { temp_lifetime, backwards_incompatible },
                     span: self.thir[block].span,
                     kind: ExprKind::Block { block },
                 });
@@ -838,13 +840,17 @@ impl<'tcx> Cx<'tcx> {
                     expr, cast_ty.hir_id, user_ty,
                 );
 
-                let cast = self.mirror_expr_cast(source, temp_lifetime, expr.span);
+                let cast = self.mirror_expr_cast(
+                    source,
+                    TempLifetime { temp_lifetime, backwards_incompatible },
+                    expr.span,
+                );
 
                 if let Some(user_ty) = user_ty {
                     // NOTE: Creating a new Expr and wrapping a Cast inside of it may be
                     //       inefficient, revisit this when performance becomes an issue.
                     let cast_expr = self.thir.exprs.push(Expr {
-                        temp_lifetime,
+                        temp_lifetime: TempLifetime { temp_lifetime, backwards_incompatible },
                         ty: expr_ty,
                         span: expr.span,
                         kind: cast,
@@ -887,7 +893,12 @@ impl<'tcx> Cx<'tcx> {
             hir::ExprKind::Err(_) => unreachable!("cannot lower a `hir::ExprKind::Err` to THIR"),
         };
 
-        Expr { temp_lifetime, ty: expr_ty, span: expr.span, kind }
+        Expr {
+            temp_lifetime: TempLifetime { temp_lifetime, backwards_incompatible },
+            ty: expr_ty,
+            span: expr.span,
+            kind,
+        }
     }
 
     fn user_args_applied_to_res(
@@ -931,7 +942,7 @@ impl<'tcx> Cx<'tcx> {
         span: Span,
         overloaded_callee: Option<Ty<'tcx>>,
     ) -> Expr<'tcx> {
-        let temp_lifetime =
+        let (temp_lifetime, backwards_incompatible) =
             self.rvalue_scopes.temporary_scope(self.region_scope_tree, expr.hir_id.local_id);
         let (ty, user_ty) = match overloaded_callee {
             Some(fn_def) => (fn_def, None),
@@ -952,7 +963,12 @@ impl<'tcx> Cx<'tcx> {
                 )
             }
         };
-        Expr { temp_lifetime, ty, span, kind: ExprKind::ZstLiteral { user_ty } }
+        Expr {
+            temp_lifetime: TempLifetime { temp_lifetime, backwards_incompatible },
+            ty,
+            span,
+            kind: ExprKind::ZstLiteral { user_ty },
+        }
     }
 
     fn convert_arm(&mut self, arm: &'tcx hir::Arm<'tcx>) -> ArmId {
@@ -1025,7 +1041,7 @@ impl<'tcx> Cx<'tcx> {
             Res::Def(DefKind::Static { .. }, id) => {
                 // this is &raw for extern static or static mut, and & for other statics
                 let ty = self.tcx.static_ptr_ty(id, self.typing_env());
-                let temp_lifetime = self
+                let (temp_lifetime, backwards_incompatible) = self
                     .rvalue_scopes
                     .temporary_scope(self.region_scope_tree, expr.hir_id.local_id);
                 let kind = if self.tcx.is_thread_local_static(id) {
@@ -1035,7 +1051,12 @@ impl<'tcx> Cx<'tcx> {
                     ExprKind::StaticRef { alloc_id, ty, def_id: id }
                 };
                 ExprKind::Deref {
-                    arg: self.thir.exprs.push(Expr { ty, temp_lifetime, span: expr.span, kind }),
+                    arg: self.thir.exprs.push(Expr {
+                        ty,
+                        temp_lifetime: TempLifetime { temp_lifetime, backwards_incompatible },
+                        span: expr.span,
+                        kind,
+                    }),
                 }
             }
 
@@ -1106,13 +1127,13 @@ impl<'tcx> Cx<'tcx> {
 
         // construct the complete expression `foo()` for the overloaded call,
         // which will yield the &T type
-        let temp_lifetime =
+        let (temp_lifetime, backwards_incompatible) =
             self.rvalue_scopes.temporary_scope(self.region_scope_tree, expr.hir_id.local_id);
         let fun = self.method_callee(expr, span, overloaded_callee);
         let fun = self.thir.exprs.push(fun);
         let fun_ty = self.thir[fun].ty;
         let ref_expr = self.thir.exprs.push(Expr {
-            temp_lifetime,
+            temp_lifetime: TempLifetime { temp_lifetime, backwards_incompatible },
             ty: ref_ty,
             span,
             kind: ExprKind::Call { ty: fun_ty, fun, args, from_hir_call: false, fn_span: span },
@@ -1127,7 +1148,7 @@ impl<'tcx> Cx<'tcx> {
         closure_expr: &'tcx hir::Expr<'tcx>,
         place: HirPlace<'tcx>,
     ) -> Expr<'tcx> {
-        let temp_lifetime = self
+        let (temp_lifetime, backwards_incompatible) = self
             .rvalue_scopes
             .temporary_scope(self.region_scope_tree, closure_expr.hir_id.local_id);
         let var_ty = place.base_ty;
@@ -1143,7 +1164,7 @@ impl<'tcx> Cx<'tcx> {
         };
 
         let mut captured_place_expr = Expr {
-            temp_lifetime,
+            temp_lifetime: TempLifetime { temp_lifetime, backwards_incompatible },
             ty: var_ty,
             span: closure_expr.span,
             kind: self.convert_var(var_hir_id),
@@ -1168,8 +1189,12 @@ impl<'tcx> Cx<'tcx> {
                 }
             };
 
-            captured_place_expr =
+            captured_place_expr = Expr {
-                Expr { temp_lifetime, ty: proj.ty, span: closure_expr.span, kind };
+                temp_lifetime: TempLifetime { temp_lifetime, backwards_incompatible },
+                ty: proj.ty,
+                span: closure_expr.span,
+                kind,
+            };
         }
 
         captured_place_expr
@@ -1184,7 +1209,7 @@ impl<'tcx> Cx<'tcx> {
         let upvar_capture = captured_place.info.capture_kind;
         let captured_place_expr =
             self.convert_captured_hir_place(closure_expr, captured_place.place.clone());
-        let temp_lifetime = self
+        let (temp_lifetime, backwards_incompatible) = self
             .rvalue_scopes
             .temporary_scope(self.region_scope_tree, closure_expr.hir_id.local_id);
 
@@ -1201,7 +1226,7 @@ impl<'tcx> Cx<'tcx> {
                     }
                 };
                 Expr {
-                    temp_lifetime,
+                    temp_lifetime: TempLifetime { temp_lifetime, backwards_incompatible },
                     ty: upvar_ty,
                     span: closure_expr.span,
                     kind: ExprKind::Borrow {

compiler/rustc_mir_dataflow/src/impls/liveness.rs

@@ -253,6 +253,7 @@ impl<'a, 'tcx> Analysis<'tcx> for MaybeTransitiveLiveLocals<'a> {
             | StatementKind::Coverage(..)
             | StatementKind::Intrinsic(..)
             | StatementKind::ConstEvalCounter
+            | StatementKind::BackwardIncompatibleDropHint { .. }
             | StatementKind::Nop => None,
         };
         if let Some(destination) = destination {

compiler/rustc_mir_dataflow/src/impls/storage_liveness.rs

@@ -157,6 +157,7 @@ impl<'tcx> Analysis<'tcx> for MaybeRequiresStorage<'_, 'tcx> {
             | StatementKind::Nop
             | StatementKind::Retag(..)
             | StatementKind::Intrinsic(..)
+            | StatementKind::BackwardIncompatibleDropHint { .. }
             | StatementKind::StorageLive(..) => {}
         }
     }

compiler/rustc_mir_dataflow/src/move_paths/builder.rs

@@ -385,6 +385,7 @@ impl<'a, 'tcx, F: Fn(Ty<'tcx>) -> bool> MoveDataBuilder<'a, 'tcx, F> {
             | StatementKind::Coverage(..)
             | StatementKind::Intrinsic(..)
             | StatementKind::ConstEvalCounter
+            | StatementKind::BackwardIncompatibleDropHint { .. }
             | StatementKind::Nop => {}
         }
     }

compiler/rustc_mir_transform/messages.ftl

@@ -25,6 +25,31 @@ mir_transform_must_not_suspend = {$pre}`{$def_path}`{$post} held across a suspen
     .help = consider using a block (`{"{ ... }"}`) to shrink the value's scope, ending before the suspend point
 mir_transform_operation_will_panic = this operation will panic at runtime
 
+mir_transform_tail_expr_drop_order = relative drop order changing in Rust 2024
+    .temporaries = in Rust 2024, this temporary value will be dropped first
+    .observers = in Rust 2024, this local variable or temporary value will be dropped second
+    .note_dtors =
+        dropping the temporary value runs this custom `Drop` impl, which we could not prove to be side-effect free
+    .note_observer_dtors =
+        dropping the local runs this custom `Drop` impl, which we could not prove to be side-effect free
+    .drop_location =
+        now the temporary value is dropped here, before the local variables in the block or statement
+    .note_epilogue = most of the time, changing drop order is harmless; inspect the `impl Drop`s for side effects like releasing locks or sending messages
+    .label_local_epilogue = {$is_dropped_first_edition_2024 ->
+        [true] up until Edition 2021 `{$name}` is dropped last but will be dropped earlier in Edition 2024
+        *[false] `{$name}` will be dropped later as of Edition 2024
+    }
+
+mir_transform_tail_expr_dtor = {$dtor_kind ->
+    [dyn] `{$name}` may invoke a custom destructor because it contains a trait object
+    *[concrete] `{$name}` invokes this custom destructor
+    }
+
+mir_transform_tail_expr_local = {$is_generated_name ->
+        [true] this value will be stored in a temporary; let us call it `{$name}`
+        *[false] `{$name}` calls a custom destructor
+    }
+
 mir_transform_unaligned_packed_ref = reference to packed field is unaligned
     .note = packed structs are only aligned by one byte, and many modern architectures penalize unaligned field accesses
     .note_ub = creating a misaligned reference is undefined behavior (even if that reference is never dereferenced)

compiler/rustc_mir_transform/src/coroutine.rs

@@ -1772,6 +1772,7 @@ impl<'tcx> Visitor<'tcx> for EnsureCoroutineFieldAssignmentsNeverAlias<'_> {
             | StatementKind::Coverage(..)
             | StatementKind::Intrinsic(..)
             | StatementKind::ConstEvalCounter
+            | StatementKind::BackwardIncompatibleDropHint { .. }
             | StatementKind::Nop => {}
         }
     }

compiler/rustc_mir_transform/src/coverage/spans/from_mir.rs

@@ -97,6 +97,7 @@ fn filtered_statement_span(statement: &Statement<'_>) -> Option<Span> {
         StatementKind::StorageLive(_)
         | StatementKind::StorageDead(_)
         | StatementKind::ConstEvalCounter
+        | StatementKind::BackwardIncompatibleDropHint { .. }
         | StatementKind::Nop => None,
 
         // FIXME(#78546): MIR InstrumentCoverage - Can the source_info.span for `FakeRead`

compiler/rustc_mir_transform/src/dataflow_const_prop.rs

@@ -186,7 +186,8 @@ impl<'a, 'tcx> ConstAnalysis<'a, 'tcx> {
             | StatementKind::FakeRead(..)
             | StatementKind::PlaceMention(..)
             | StatementKind::Coverage(..)
-            | StatementKind::AscribeUserType(..) => (),
+            | StatementKind::BackwardIncompatibleDropHint { .. }
+            | StatementKind::AscribeUserType(..) => {}
         }
     }
 

compiler/rustc_mir_transform/src/dead_store_elimination.rs

@@ -99,7 +99,8 @@ fn eliminate<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
                 | StatementKind::Intrinsic(_)
                 | StatementKind::ConstEvalCounter
                 | StatementKind::PlaceMention(_)
-                | StatementKind::Nop => (),
+                | StatementKind::BackwardIncompatibleDropHint { .. }
+                | StatementKind::Nop => {}
 
                 StatementKind::FakeRead(_) | StatementKind::AscribeUserType(_, _) => {
                     bug!("{:?} not found in this MIR phase!", statement.kind)

compiler/rustc_mir_transform/src/dest_prop.rs

@@ -581,7 +581,7 @@ impl WriteInfo {
                     | Rvalue::RawPtr(_, _)
                     | Rvalue::Len(_)
                     | Rvalue::Discriminant(_)
-                    | Rvalue::CopyForDeref(_) => (),
+                    | Rvalue::CopyForDeref(_) => {}
                 }
             }
             // Retags are technically also reads, but reporting them as a write suffices
@@ -596,7 +596,8 @@ impl WriteInfo {
             | StatementKind::Coverage(_)
             | StatementKind::StorageLive(_)
             | StatementKind::StorageDead(_)
-            | StatementKind::PlaceMention(_) => (),
+            | StatementKind::BackwardIncompatibleDropHint { .. }
+            | StatementKind::PlaceMention(_) => {}
             StatementKind::FakeRead(_) | StatementKind::AscribeUserType(_, _) => {
                 bug!("{:?} not found in this MIR phase", statement)
             }

compiler/rustc_mir_transform/src/jump_threading.rs

@@ -352,6 +352,7 @@ impl<'a, 'tcx> TOFinder<'a, 'tcx> {
             | StatementKind::FakeRead(..)
             | StatementKind::ConstEvalCounter
             | StatementKind::PlaceMention(..)
+            | StatementKind::BackwardIncompatibleDropHint { .. }
             | StatementKind::Nop => None,
         }
     }

compiler/rustc_mir_transform/src/lib.rs

@@ -50,6 +50,7 @@ mod deduce_param_attrs;
 mod errors;
 mod ffi_unwind_calls;
 mod lint;
+mod lint_tail_expr_drop_order;
 mod shim;
 mod ssa;
 
@@ -490,6 +491,7 @@ fn mir_drops_elaborated_and_const_checked(tcx: TyCtxt<'_>, def: LocalDefId) -> &
     }
 
     let (body, _) = tcx.mir_promoted(def);
+    lint_tail_expr_drop_order::run_lint(tcx, def, &body.borrow());
     let mut body = body.steal();
 
     if let Some(error_reported) = tainted_by_errors {

compiler/rustc_mir_transform/src/lint_tail_expr_drop_order.rs

@@ -0,0 +1,701 @@
+use std::cell::RefCell;
+use std::collections::hash_map;
+use std::rc::Rc;
+
+use rustc_data_structures::fx::{FxHashMap, FxHashSet};
+use rustc_data_structures::unord::{UnordMap, UnordSet};
+use rustc_errors::Subdiagnostic;
+use rustc_hir::CRATE_HIR_ID;
+use rustc_hir::def_id::{DefId, LocalDefId};
+use rustc_index::bit_set::ChunkedBitSet;
+use rustc_index::{IndexSlice, IndexVec};
+use rustc_macros::{LintDiagnostic, Subdiagnostic};
+use rustc_middle::bug;
+use rustc_middle::mir::{
+    self, BasicBlock, Body, ClearCrossCrate, Local, Location, Place, StatementKind, TerminatorKind,
+    dump_mir,
+};
+use rustc_middle::ty::{self, Ty, TyCtxt};
+use rustc_mir_dataflow::impls::MaybeInitializedPlaces;
+use rustc_mir_dataflow::move_paths::{LookupResult, MoveData, MovePathIndex};
+use rustc_mir_dataflow::{Analysis, MaybeReachable, ResultsCursor};
+use rustc_session::lint::builtin::TAIL_EXPR_DROP_ORDER;
+use rustc_session::lint::{self};
+use rustc_span::{DUMMY_SP, Span, Symbol};
+use rustc_type_ir::data_structures::IndexMap;
+use smallvec::{SmallVec, smallvec};
+use tracing::{debug, instrument};
+
+fn place_has_common_prefix<'tcx>(left: &Place<'tcx>, right: &Place<'tcx>) -> bool {
+    left.local == right.local
+        && left.projection.iter().zip(right.projection).all(|(left, right)| left == right)
+}
+
+/// Cache entry of `drop` at a `BasicBlock`
+#[derive(Debug, Clone, Copy)]
+enum MovePathIndexAtBlock {
+    /// We know nothing yet
+    Unknown,
+    /// We know that the `drop` here has no effect
+    None,
+    /// We know that the `drop` here will invoke a destructor
+    Some(MovePathIndex),
+}
+
+struct DropsReachable<'a, 'mir, 'tcx> {
+    body: &'a Body<'tcx>,
+    place: &'a Place<'tcx>,
+    drop_span: &'a mut Option<Span>,
+    move_data: &'a MoveData<'tcx>,
+    maybe_init: &'a mut ResultsCursor<'mir, 'tcx, MaybeInitializedPlaces<'mir, 'tcx>>,
+    block_drop_value_info: &'a mut IndexSlice<BasicBlock, MovePathIndexAtBlock>,
+    collected_drops: &'a mut ChunkedBitSet<MovePathIndex>,
+    visited: FxHashMap<BasicBlock, Rc<RefCell<ChunkedBitSet<MovePathIndex>>>>,
+}
+
+impl<'a, 'mir, 'tcx> DropsReachable<'a, 'mir, 'tcx> {
+    fn visit(&mut self, block: BasicBlock) {
+        let move_set_size = self.move_data.move_paths.len();
+        let make_new_path_set = || Rc::new(RefCell::new(ChunkedBitSet::new_empty(move_set_size)));
+
+        let data = &self.body.basic_blocks[block];
+        let Some(terminator) = &data.terminator else { return };
+        // Given that we observe these dropped locals here at `block` so far,
+        // we will try to update the successor blocks.
+        // An occupied entry at `block` in `self.visited` signals that we have visited `block` before.
+        let dropped_local_here =
+            Rc::clone(self.visited.entry(block).or_insert_with(make_new_path_set));
+        // We could have invoked reverse lookup for a `MovePathIndex` every time, but unfortunately it is expensive.
+        // Let's cache them in `self.block_drop_value_info`.
+        match self.block_drop_value_info[block] {
+            MovePathIndexAtBlock::Some(dropped) => {
+                dropped_local_here.borrow_mut().insert(dropped);
+            }
+            MovePathIndexAtBlock::Unknown => {
+                if let TerminatorKind::Drop { place, .. } = &terminator.kind
+                    && let LookupResult::Exact(idx) | LookupResult::Parent(Some(idx)) =
+                        self.move_data.rev_lookup.find(place.as_ref())
+                {
+                    // Since we are working with MIRs at a very early stage,
+                    // observing a `drop` terminator is not indicative enough that
+                    // the drop will definitely happen.
+                    // That is decided in the drop elaboration pass instead.
+                    // Therefore, we need to consult with the maybe-initialization information.
+                    self.maybe_init.seek_before_primary_effect(Location {
+                        block,
+                        statement_index: data.statements.len(),
+                    });
+
+                    // Check if the drop of `place` under inspection is really in effect.
+                    // This is true only when `place` may have been initialized along a control flow path from a BID to the drop program point today.
+                    // In other words, this is where the drop of `place` will happen in the future instead.
+                    if let MaybeReachable::Reachable(maybe_init) = self.maybe_init.get()
+                        && maybe_init.contains(idx)
+                    {
+                        // We also cache the drop information, so that we do not need to check on data-flow cursor again
+                        self.block_drop_value_info[block] = MovePathIndexAtBlock::Some(idx);
+                        dropped_local_here.borrow_mut().insert(idx);
+                    } else {
+                        self.block_drop_value_info[block] = MovePathIndexAtBlock::None;
+                    }
+                }
+            }
+            MovePathIndexAtBlock::None => {}
+        }
+
+        for succ in terminator.successors() {
+            let target = &self.body.basic_blocks[succ];
+            if target.is_cleanup {
+                continue;
+            }
+
+            // As long as we are passing through a new block, or new dropped places to propagate,
+            // we will proceed with `succ`
+            let dropped_local_there = match self.visited.entry(succ) {
+                hash_map::Entry::Occupied(occupied_entry) => {
+                    if succ == block
+                        || !occupied_entry.get().borrow_mut().union(&*dropped_local_here.borrow())
+                    {
+                        // `succ` has been visited but no new drops observed so far,
+                        // so we can bail on `succ` until new drop information arrives
+                        continue;
+                    }
+                    Rc::clone(occupied_entry.get())
+                }
+                hash_map::Entry::Vacant(vacant_entry) => Rc::clone(
+                    vacant_entry.insert(Rc::new(RefCell::new(dropped_local_here.borrow().clone()))),
+                ),
+            };
+            if let Some(terminator) = &target.terminator
+                && let TerminatorKind::Drop {
+                    place: dropped_place,
+                    target: _,
+                    unwind: _,
+                    replace: _,
+                } = &terminator.kind
+                && place_has_common_prefix(dropped_place, self.place)
+            {
+                // We have now reached the current drop of the `place`.
+                // Let's check the observed dropped places in.
+                self.collected_drops.union(&*dropped_local_there.borrow());
+                if self.drop_span.is_none() {
+                    // FIXME(@dingxiangfei2009): it turns out that `self.body.source_scopes` are still a bit wonky.
+                    // There is a high chance that this span still points to a block rather than a statement semicolon.
+                    *self.drop_span = Some(terminator.source_info.span);
+                }
+                // Now we have discovered a simple control flow path from a future drop point
+                // to the current drop point.
+                // We will not continue from there.
+            } else {
+                self.visit(succ)
+            }
+        }
+    }
+}
+
+/// An additional filter to exclude well-known types from the ecosystem
+/// because their drops are trivial.
+/// This returns additional types to check if the drops are delegated to those.
+/// A typical example is `hashbrown::HashMap<K, V>`, whose drop is delegated to `K` and `V`.
+fn true_significant_drop_ty<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    ty: Ty<'tcx>,
+) -> Option<SmallVec<[Ty<'tcx>; 2]>> {
+    if let ty::Adt(def, args) = ty.kind() {
+        let mut did = def.did();
+        let mut name_rev = vec![];
+        loop {
+            let key = tcx.def_key(did);
+
+            match key.disambiguated_data.data {
+                rustc_hir::definitions::DefPathData::CrateRoot => {
+                    name_rev.push(tcx.crate_name(did.krate))
+                }
+                rustc_hir::definitions::DefPathData::TypeNs(symbol) => name_rev.push(symbol),
+                _ => return None,
+            }
+            if let Some(parent) = key.parent {
+                did = DefId { krate: did.krate, index: parent };
+            } else {
+                break;
+            }
+        }
+        let name_str: Vec<_> = name_rev.iter().rev().map(|x| x.as_str()).collect();
+        debug!(?name_str);
+        match name_str[..] {
+            // These are the types from Rust core ecosystem
+            ["sym" | "proc_macro2", ..]
+            | ["core" | "std", "task", "LocalWaker" | "Waker"]
+            | ["core" | "std", "task", "wake", "LocalWaker" | "Waker"] => Some(smallvec![]),
+            // These are important types from Rust ecosystem
+            ["tracing", "instrument", "Instrumented"] | ["bytes", "Bytes"] => Some(smallvec![]),
+            ["hashbrown", "raw", "RawTable" | "RawIntoIter"] => {
+                if let [ty, ..] = &***args
+                    && let Some(ty) = ty.as_type()
+                {
+                    Some(smallvec![ty])
+                } else {
+                    None
+                }
+            }
+            ["hashbrown", "raw", "RawDrain"] => {
+                if let [_, ty, ..] = &***args
+                    && let Some(ty) = ty.as_type()
+                {
+                    Some(smallvec![ty])
+                } else {
+                    None
+                }
+            }
+            _ => None,
+        }
+    } else {
+        None
+    }
+}
+
+/// Returns the list of types with a "potentially sigificant" that may be dropped
+/// by dropping a value of type `ty`.
+#[instrument(level = "debug", skip(tcx, param_env))]
+fn extract_component_raw<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    param_env: ty::ParamEnv<'tcx>,
+    ty: Ty<'tcx>,
+    ty_seen: &mut UnordSet<Ty<'tcx>>,
+) -> SmallVec<[Ty<'tcx>; 4]> {
+    // Droppiness does not depend on regions, so let us erase them.
+    let ty = tcx
+        .try_normalize_erasing_regions(
+            ty::TypingEnv { param_env, typing_mode: ty::TypingMode::PostAnalysis },
+            ty,
+        )
+        .unwrap_or(ty);
+
+    let tys = tcx.list_significant_drop_tys(param_env.and(ty));
+    debug!(?ty, "components");
+    let mut out_tys = smallvec![];
+    for ty in tys {
+        if let Some(tys) = true_significant_drop_ty(tcx, ty) {
+            // Some types can be further opened up because the drop is simply delegated
+            for ty in tys {
+                if ty_seen.insert(ty) {
+                    out_tys.extend(extract_component_raw(tcx, param_env, ty, ty_seen));
+                }
+            }
+        } else {
+            if ty_seen.insert(ty) {
+                out_tys.push(ty);
+            }
+        }
+    }
+    out_tys
+}
+
+#[instrument(level = "debug", skip(tcx, param_env))]
+fn extract_component_with_significant_dtor<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    param_env: ty::ParamEnv<'tcx>,
+    ty: Ty<'tcx>,
+) -> SmallVec<[Ty<'tcx>; 4]> {
+    let mut tys = extract_component_raw(tcx, param_env, ty, &mut Default::default());
+    let mut deduplicate = FxHashSet::default();
+    tys.retain(|oty| deduplicate.insert(*oty));
+    tys.into_iter().collect()
+}
+
+/// Extract the span of the custom destructor of a type
+/// especially the span of the `impl Drop` header or its entire block
+/// when we are working with current local crate.
+#[instrument(level = "debug", skip(tcx))]
+fn ty_dtor_span<'tcx>(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>) -> Option<Span> {
+    match ty.kind() {
+        ty::Bool
+        | ty::Char
+        | ty::Int(_)
+        | ty::Uint(_)
+        | ty::Float(_)
+        | ty::Error(_)
+        | ty::Str
+        | ty::Never
+        | ty::RawPtr(_, _)
+        | ty::Ref(_, _, _)
+        | ty::FnPtr(_, _)
+        | ty::Tuple(_)
+        | ty::Dynamic(_, _, _)
+        | ty::Alias(_, _)
+        | ty::Bound(_, _)
+        | ty::Pat(_, _)
+        | ty::Placeholder(_)
+        | ty::Infer(_)
+        | ty::Slice(_)
+        | ty::Array(_, _) => None,
+        ty::Adt(adt_def, _) => {
+            let did = adt_def.did();
+            let try_local_did_span = |did: DefId| {
+                if let Some(local) = did.as_local() {
+                    tcx.source_span(local)
+                } else {
+                    tcx.def_span(did)
+                }
+            };
+            let dtor = if let Some(dtor) = tcx.adt_destructor(did) {
+                dtor.did
+            } else if let Some(dtor) = tcx.adt_async_destructor(did) {
+                dtor.future
+            } else {
+                return Some(try_local_did_span(did));
+            };
+            let def_key = tcx.def_key(dtor);
+            let Some(parent_index) = def_key.parent else { return Some(try_local_did_span(dtor)) };
+            let parent_did = DefId { index: parent_index, krate: dtor.krate };
+            Some(try_local_did_span(parent_did))
+        }
+        ty::Coroutine(did, _)
+        | ty::CoroutineWitness(did, _)
+        | ty::CoroutineClosure(did, _)
+        | ty::Closure(did, _)
+        | ty::FnDef(did, _)
+        | ty::Foreign(did) => Some(tcx.def_span(did)),
+        ty::Param(_) => None,
+    }
+}
+
+/// Check if a moved place at `idx` is a part of a BID.
+/// The use of this check is that we will consider drops on these
+/// as a drop of the overall BID and, thus, we can exclude it from the diagnosis.
+fn place_descendent_of_bids<'tcx>(
+    mut idx: MovePathIndex,
+    move_data: &MoveData<'tcx>,
+    bids: &UnordSet<&Place<'tcx>>,
+) -> bool {
+    loop {
+        let path = &move_data.move_paths[idx];
+        if bids.contains(&path.place) {
+            return true;
+        }
+        if let Some(parent) = path.parent {
+            idx = parent;
+        } else {
+            return false;
+        }
+    }
+}
+
+/// The core of the lint `tail-expr-drop-order`
+pub(crate) fn run_lint<'tcx>(tcx: TyCtxt<'tcx>, def_id: LocalDefId, body: &Body<'tcx>) {
+    if matches!(tcx.def_kind(def_id), rustc_hir::def::DefKind::SyntheticCoroutineBody) {
+        // A synthetic coroutine has no HIR body and it is enough to just analyse the original body
+        return;
+    }
+    if body.span.edition().at_least_rust_2024()
+        || tcx.lints_that_dont_need_to_run(()).contains(&lint::LintId::of(TAIL_EXPR_DROP_ORDER))
+    {
+        return;
+    }
+    // ## About BIDs in blocks ##
+    // Track the set of blocks that contain a backwards-incompatible drop (BID)
+    // and, for each block, the vector of locations.
+    //
+    // We group them per-block because they tend to scheduled in the same drop ladder block.
+    let mut bid_per_block = IndexMap::default();
+    let mut bid_places = UnordSet::new();
+    let param_env = tcx.param_env(def_id).with_reveal_all_normalized(tcx);
+    let mut ty_dropped_components = UnordMap::default();
+    for (block, data) in body.basic_blocks.iter_enumerated() {
+        for (statement_index, stmt) in data.statements.iter().enumerate() {
+            if let StatementKind::BackwardIncompatibleDropHint { place, reason: _ } = &stmt.kind {
+                let ty = place.ty(body, tcx).ty;
+                if ty_dropped_components
+                    .entry(ty)
+                    .or_insert_with(|| extract_component_with_significant_dtor(tcx, param_env, ty))
+                    .is_empty()
+                {
+                    continue;
+                }
+                bid_per_block
+                    .entry(block)
+                    .or_insert(vec![])
+                    .push((Location { block, statement_index }, &**place));
+                bid_places.insert(&**place);
+            }
+        }
+    }
+    if bid_per_block.is_empty() {
+        return;
+    }
+
+    dump_mir(tcx, false, "lint_tail_expr_drop_order", &0 as _, body, |_, _| Ok(()));
+    let locals_with_user_names = collect_user_names(body);
+    let is_closure_like = tcx.is_closure_like(def_id.to_def_id());
+
+    // Compute the "maybe initialized" information for this body.
+    // When we encounter a DROP of some place P we only care
+    // about the drop if `P` may be initialized.
+    let move_data = MoveData::gather_moves(body, tcx, |_| true);
+    let maybe_init = MaybeInitializedPlaces::new(tcx, body, &move_data);
+    let mut maybe_init = maybe_init.iterate_to_fixpoint(tcx, body, None).into_results_cursor(body);
+    let mut block_drop_value_info =
+        IndexVec::from_elem_n(MovePathIndexAtBlock::Unknown, body.basic_blocks.len());
+    for (&block, candidates) in &bid_per_block {
+        // We will collect drops on locals on paths between BID points to their actual drop locations
+        // into `all_locals_dropped`.
+        let mut all_locals_dropped = ChunkedBitSet::new_empty(move_data.move_paths.len());
+        let mut drop_span = None;
+        for &(_, place) in candidates.iter() {
+            let mut collected_drops = ChunkedBitSet::new_empty(move_data.move_paths.len());
+            // ## On detecting change in relative drop order ##
+            // Iterate through each BID-containing block `block`.
+            // If the place `P` targeted by the BID is "maybe initialized",
+            // then search forward to find the actual `DROP(P)` point.
+            // Everything dropped between the BID and the actual drop point
+            // is something whose relative drop order will change.
+            DropsReachable {
+                body,
+                place,
+                drop_span: &mut drop_span,
+                move_data: &move_data,
+                maybe_init: &mut maybe_init,
+                block_drop_value_info: &mut block_drop_value_info,
+                collected_drops: &mut collected_drops,
+                visited: Default::default(),
+            }
+            .visit(block);
+            // Compute the set `all_locals_dropped` of local variables that are dropped
+            // after the BID point but before the current drop point.
+            //
+            // These are the variables whose drop impls will be reordered with respect
+            // to `place`.
+            all_locals_dropped.union(&collected_drops);
+        }
+
+        // We shall now exclude some local bindings for the following cases.
+        {
+            let mut to_exclude = ChunkedBitSet::new_empty(all_locals_dropped.domain_size());
+            // We will now do subtraction from the candidate dropped locals, because of the following reasons.
+            for path_idx in all_locals_dropped.iter() {
+                let move_path = &move_data.move_paths[path_idx];
+                let dropped_local = move_path.place.local;
+                // a) A return value _0 will eventually be used
+                // Example:
+                // fn f() -> Droppy {
+                //     let _x = Droppy;
+                //     Droppy
+                // }
+                // _0 holds the literal `Droppy` and rightfully `_x` has to be dropped first
+                if dropped_local == Local::ZERO {
+                    debug!(?dropped_local, "skip return value");
+                    to_exclude.insert(path_idx);
+                    continue;
+                }
+                // b) If we are analysing a closure, the captures are still dropped last.
+                // This is part of the closure capture lifetime contract.
+                // They are similar to the return value _0 with respect to lifetime rules.
+                if is_closure_like && matches!(dropped_local, ty::CAPTURE_STRUCT_LOCAL) {
+                    debug!(?dropped_local, "skip closure captures");
+                    to_exclude.insert(path_idx);
+                    continue;
+                }
+                // c) Sometimes we collect places that are projections into the BID locals,
+                // so they are considered dropped now.
+                // Example:
+                // struct NotVeryDroppy(Droppy);
+                // impl Drop for Droppy {..}
+                // fn f() -> NotVeryDroppy {
+                //    let x = NotVeryDroppy(droppy());
+                //    {
+                //        let y: Droppy = x.0;
+                //        NotVeryDroppy(y)
+                //    }
+                // }
+                // `y` takes `x.0`, which invalidates `x` as a complete `NotVeryDroppy`
+                // so there is no point in linting against `x` any more.
+                if place_descendent_of_bids(path_idx, &move_data, &bid_places) {
+                    debug!(?dropped_local, "skip descendent of bids");
+                    to_exclude.insert(path_idx);
+                    continue;
+                }
+                let observer_ty = move_path.place.ty(body, tcx).ty;
+                // d) The collected local has no custom destructor that passes our ecosystem filter.
+                if ty_dropped_components
+                    .entry(observer_ty)
+                    .or_insert_with(|| {
+                        extract_component_with_significant_dtor(tcx, param_env, observer_ty)
+                    })
+                    .is_empty()
+                {
+                    debug!(?dropped_local, "skip non-droppy types");
+                    to_exclude.insert(path_idx);
+                    continue;
+                }
+            }
+            // Suppose that all BIDs point into the same local,
+            // we can remove the this local from the observed drops,
+            // so that we can focus our diagnosis more on the others.
+            if candidates.iter().all(|&(_, place)| candidates[0].1.local == place.local) {
+                for path_idx in all_locals_dropped.iter() {
+                    if move_data.move_paths[path_idx].place.local == candidates[0].1.local {
+                        to_exclude.insert(path_idx);
+                    }
+                }
+            }
+            all_locals_dropped.subtract(&to_exclude);
+        }
+        if all_locals_dropped.is_empty() {
+            // No drop effect is observable, so let us move on.
+            continue;
+        }
+
+        // ## The final work to assemble the diagnosis ##
+        // First collect or generate fresh names for local variable bindings and temporary values.
+        let local_names = assign_observables_names(
+            all_locals_dropped
+                .iter()
+                .map(|path_idx| move_data.move_paths[path_idx].place.local)
+                .chain(candidates.iter().map(|(_, place)| place.local)),
+            &locals_with_user_names,
+        );
+
+        let mut lint_root = None;
+        let mut local_labels = vec![];
+        // We now collect the types with custom destructors.
+        for &(_, place) in candidates {
+            let linted_local_decl = &body.local_decls[place.local];
+            let Some(&(ref name, is_generated_name)) = local_names.get(&place.local) else {
+                bug!("a name should have been assigned")
+            };
+            let name = name.as_str();
+
+            if lint_root.is_none()
+                && let ClearCrossCrate::Set(data) =
+                    &body.source_scopes[linted_local_decl.source_info.scope].local_data
+            {
+                lint_root = Some(data.lint_root);
+            }
+
+            // Collect spans of the custom destructors.
+            let mut seen_dyn = false;
+            let destructors = ty_dropped_components
+                .get(&linted_local_decl.ty)
+                .unwrap()
+                .iter()
+                .filter_map(|&ty| {
+                    if let Some(span) = ty_dtor_span(tcx, ty) {
+                        Some(DestructorLabel { span, name, dtor_kind: "concrete" })
+                    } else if matches!(ty.kind(), ty::Dynamic(..)) {
+                        if seen_dyn {
+                            None
+                        } else {
+                            seen_dyn = true;
+                            Some(DestructorLabel { span: DUMMY_SP, name, dtor_kind: "dyn" })
+                        }
+                    } else {
+                        None
+                    }
+                })
+                .collect();
+            local_labels.push(LocalLabel {
+                span: linted_local_decl.source_info.span,
+                destructors,
+                name,
+                is_generated_name,
+                is_dropped_first_edition_2024: true,
+            });
+        }
+
+        // Similarly, custom destructors of the observed drops.
+        for path_idx in all_locals_dropped.iter() {
+            let place = &move_data.move_paths[path_idx].place;
+            // We are not using the type of the local because the drop may be partial.
+            let observer_ty = place.ty(body, tcx).ty;
+
+            let observer_local_decl = &body.local_decls[place.local];
+            let Some(&(ref name, is_generated_name)) = local_names.get(&place.local) else {
+                bug!("a name should have been assigned")
+            };
+            let name = name.as_str();
+
+            let mut seen_dyn = false;
+            let destructors = extract_component_with_significant_dtor(tcx, param_env, observer_ty)
+                .into_iter()
+                .filter_map(|ty| {
+                    if let Some(span) = ty_dtor_span(tcx, ty) {
+                        Some(DestructorLabel { span, name, dtor_kind: "concrete" })
+                    } else if matches!(ty.kind(), ty::Dynamic(..)) {
+                        if seen_dyn {
+                            None
+                        } else {
+                            seen_dyn = true;
+                            Some(DestructorLabel { span: DUMMY_SP, name, dtor_kind: "dyn" })
+                        }
+                    } else {
+                        None
+                    }
+                })
+                .collect();
+            local_labels.push(LocalLabel {
+                span: observer_local_decl.source_info.span,
+                destructors,
+                name,
+                is_generated_name,
+                is_dropped_first_edition_2024: false,
+            });
+        }
+
+        let span = local_labels[0].span;
+        tcx.emit_node_span_lint(
+            lint::builtin::TAIL_EXPR_DROP_ORDER,
+            lint_root.unwrap_or(CRATE_HIR_ID),
+            span,
+            TailExprDropOrderLint { local_labels, drop_span, _epilogue: () },
+        );
+    }
+}
+
+/// Extract binding names if available for diagnosis
+fn collect_user_names(body: &Body<'_>) -> IndexMap<Local, Symbol> {
+    let mut names = IndexMap::default();
+    for var_debug_info in &body.var_debug_info {
+        if let mir::VarDebugInfoContents::Place(place) = &var_debug_info.value
+            && let Some(local) = place.local_or_deref_local()
+        {
+            names.entry(local).or_insert(var_debug_info.name);
+        }
+    }
+    names
+}
+
+/// Assign names for anonymous or temporary values for diagnosis
+fn assign_observables_names(
+    locals: impl IntoIterator<Item = Local>,
+    user_names: &IndexMap<Local, Symbol>,
+) -> IndexMap<Local, (String, bool)> {
+    let mut names = IndexMap::default();
+    let mut assigned_names = FxHashSet::default();
+    let mut idx = 0u64;
+    let mut fresh_name = || {
+        idx += 1;
+        (format!("#{idx}"), true)
+    };
+    for local in locals {
+        let name = if let Some(name) = user_names.get(&local) {
+            let name = name.as_str();
+            if assigned_names.contains(name) { fresh_name() } else { (name.to_owned(), false) }
+        } else {
+            fresh_name()
+        };
+        assigned_names.insert(name.0.clone());
+        names.insert(local, name);
+    }
+    names
+}
+
+#[derive(LintDiagnostic)]
+#[diag(mir_transform_tail_expr_drop_order)]
+struct TailExprDropOrderLint<'a> {
+    #[subdiagnostic]
+    local_labels: Vec<LocalLabel<'a>>,
+    #[label(mir_transform_drop_location)]
+    drop_span: Option<Span>,
+    #[note(mir_transform_note_epilogue)]
+    _epilogue: (),
+}
+
+struct LocalLabel<'a> {
+    span: Span,
+    name: &'a str,
+    is_generated_name: bool,
+    is_dropped_first_edition_2024: bool,
+    destructors: Vec<DestructorLabel<'a>>,
+}
+
+/// A custom `Subdiagnostic` implementation so that the notes are delivered in a specific order
+impl Subdiagnostic for LocalLabel<'_> {
+    fn add_to_diag_with<
+        G: rustc_errors::EmissionGuarantee,
+        F: rustc_errors::SubdiagMessageOp<G>,
+    >(
+        self,
+        diag: &mut rustc_errors::Diag<'_, G>,
+        f: &F,
+    ) {
+        diag.arg("name", self.name);
+        diag.arg("is_generated_name", self.is_generated_name);
+        diag.arg("is_dropped_first_edition_2024", self.is_dropped_first_edition_2024);
+        let msg = f(diag, crate::fluent_generated::mir_transform_tail_expr_local.into());
+        diag.span_label(self.span, msg);
+        for dtor in self.destructors {
+            dtor.add_to_diag_with(diag, f);
+        }
+        let msg = f(diag, crate::fluent_generated::mir_transform_label_local_epilogue.into());
+        diag.span_label(self.span, msg);
+    }
+}
+
+#[derive(Subdiagnostic)]
+#[note(mir_transform_tail_expr_dtor)]
+struct DestructorLabel<'a> {
+    #[primary_span]
+    span: Span,
+    dtor_kind: &'static str,
+    name: &'a str,
+}

compiler/rustc_mir_transform/src/remove_noop_landing_pads.rs

@@ -92,6 +92,7 @@ impl RemoveNoopLandingPads {
                 | StatementKind::AscribeUserType(..)
                 | StatementKind::Coverage(..)
                 | StatementKind::ConstEvalCounter
+                | StatementKind::BackwardIncompatibleDropHint { .. }
                 | StatementKind::Nop => {
                     // These are all noops in a landing pad
                 }

compiler/rustc_mir_transform/src/remove_zsts.rs

@@ -125,6 +125,7 @@ impl<'tcx> MutVisitor<'tcx> for Replacer<'_, 'tcx> {
             StatementKind::Coverage(_)
             | StatementKind::Intrinsic(_)
             | StatementKind::Nop
+            | StatementKind::BackwardIncompatibleDropHint { .. }
             | StatementKind::ConstEvalCounter => None,
         };
         if let Some(place_for_ty) = place_for_ty

compiler/rustc_mir_transform/src/simplify.rs

@@ -523,7 +523,8 @@ impl<'tcx> Visitor<'tcx> for UsedLocals {
             }
 
             StatementKind::SetDiscriminant { ref place, variant_index: _ }
-            | StatementKind::Deinit(ref place) => {
+            | StatementKind::Deinit(ref place)
+            | StatementKind::BackwardIncompatibleDropHint { ref place, reason: _ } => {
                 self.visit_lhs(place, location);
             }
         }
@@ -560,6 +561,7 @@ fn remove_unused_definitions_helper(used_locals: &mut UsedLocals, body: &mut Bod
                     StatementKind::Assign(box (place, _)) => used_locals.is_used(place.local),
 
                     StatementKind::SetDiscriminant { ref place, .. }
+                    | StatementKind::BackwardIncompatibleDropHint { ref place, reason: _ }
                     | StatementKind::Deinit(ref place) => used_locals.is_used(place.local),
                     StatementKind::Nop => false,
                     _ => true,
@@ -587,6 +589,20 @@ impl<'tcx> MutVisitor<'tcx> for LocalUpdater<'tcx> {
         self.tcx
     }
 
+    fn visit_statement(&mut self, statement: &mut Statement<'tcx>, location: Location) {
+        if let StatementKind::BackwardIncompatibleDropHint { place, reason: _ } =
+            &mut statement.kind
+        {
+            self.visit_local(
+                &mut place.local,
+                PlaceContext::MutatingUse(MutatingUseContext::Store),
+                location,
+            );
+        } else {
+            self.super_statement(statement, location);
+        }
+    }
+
     fn visit_local(&mut self, l: &mut Local, _: PlaceContext, _: Location) {
         *l = self.map[*l].unwrap();
     }

compiler/rustc_mir_transform/src/validate.rs

@@ -343,6 +343,7 @@ impl<'a, 'tcx> Visitor<'tcx> for CfgChecker<'a, 'tcx> {
             | StatementKind::Intrinsic(_)
             | StatementKind::ConstEvalCounter
             | StatementKind::PlaceMention(..)
+            | StatementKind::BackwardIncompatibleDropHint { .. }
             | StatementKind::Nop => {}
         }
 
@@ -1493,6 +1494,7 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
             | StatementKind::Coverage(_)
             | StatementKind::ConstEvalCounter
             | StatementKind::PlaceMention(..)
+            | StatementKind::BackwardIncompatibleDropHint { .. }
             | StatementKind::Nop => {}
         }
 

compiler/rustc_smir/src/rustc_smir/convert/mir.rs

@@ -151,6 +151,10 @@ impl<'tcx> Stable<'tcx> for mir::StatementKind<'tcx> {
             mir::StatementKind::ConstEvalCounter => {
                 stable_mir::mir::StatementKind::ConstEvalCounter
             }
+            // BackwardIncompatibleDropHint has no semantics, so it is translated to Nop.
+            mir::StatementKind::BackwardIncompatibleDropHint { .. } => {
+                stable_mir::mir::StatementKind::Nop
+            }
             mir::StatementKind::Nop => stable_mir::mir::StatementKind::Nop,
         }
     }

compiler/rustc_ty_utils/src/needs_drop.rs

@@ -8,7 +8,7 @@ use rustc_middle::ty::util::{AlwaysRequiresDrop, needs_drop_components};
 use rustc_middle::ty::{self, EarlyBinder, GenericArgsRef, Ty, TyCtxt};
 use rustc_session::Limit;
 use rustc_span::sym;
-use tracing::debug;
+use tracing::{debug, instrument};
 
 use crate::errors::NeedsDropOverflow;
 
@@ -23,7 +23,7 @@ fn needs_drop_raw<'tcx>(
     // needs drop.
     let adt_has_dtor =
         |adt_def: ty::AdtDef<'tcx>| adt_def.destructor(tcx).map(|_| DtorType::Significant);
-    let res = drop_tys_helper(tcx, query.value, query.typing_env, adt_has_dtor, false)
+    let res = drop_tys_helper(tcx, query.value, query.typing_env, adt_has_dtor, false, false)
         .filter(filter_array_elements(tcx, query.typing_env))
         .next()
         .is_some();
@@ -41,7 +41,7 @@ fn needs_async_drop_raw<'tcx>(
     // it needs async drop.
     let adt_has_async_dtor =
         |adt_def: ty::AdtDef<'tcx>| adt_def.async_destructor(tcx).map(|_| DtorType::Significant);
-    let res = drop_tys_helper(tcx, query.value, query.typing_env, adt_has_async_dtor, false)
+    let res = drop_tys_helper(tcx, query.value, query.typing_env, adt_has_async_dtor, false, false)
         .filter(filter_array_elements(tcx, query.typing_env))
         .next()
         .is_some();
@@ -77,6 +77,7 @@ fn has_significant_drop_raw<'tcx>(
         query.typing_env,
         adt_consider_insignificant_dtor(tcx),
         true,
+        false,
     )
     .filter(filter_array_elements(tcx, query.typing_env))
     .next()
@@ -88,8 +89,8 @@ fn has_significant_drop_raw<'tcx>(
 struct NeedsDropTypes<'tcx, F> {
     tcx: TyCtxt<'tcx>,
     typing_env: ty::TypingEnv<'tcx>,
-    // Whether to reveal coroutine witnesses, this is set
+    /// Whether to reveal coroutine witnesses, this is set
-    // to `false` unless we compute `needs_drop` for a coroutine witness.
+    /// to `false` unless we compute `needs_drop` for a coroutine witness.
     reveal_coroutine_witnesses: bool,
     query_ty: Ty<'tcx>,
     seen_tys: FxHashSet<Ty<'tcx>>,
@@ -100,6 +101,9 @@ struct NeedsDropTypes<'tcx, F> {
     unchecked_tys: Vec<(Ty<'tcx>, usize)>,
     recursion_limit: Limit,
     adt_components: F,
+    /// Set this to true if an exhaustive list of types involved in
+    /// drop obligation is requested.
+    exhaustive: bool,
 }
 
 impl<'tcx, F> NeedsDropTypes<'tcx, F> {
@@ -107,6 +111,7 @@ impl<'tcx, F> NeedsDropTypes<'tcx, F> {
         tcx: TyCtxt<'tcx>,
         typing_env: ty::TypingEnv<'tcx>,
         ty: Ty<'tcx>,
+        exhaustive: bool,
         adt_components: F,
     ) -> Self {
         let mut seen_tys = FxHashSet::default();
@@ -114,14 +119,22 @@ impl<'tcx, F> NeedsDropTypes<'tcx, F> {
         Self {
             tcx,
             typing_env,
-            reveal_coroutine_witnesses: false,
+            reveal_coroutine_witnesses: exhaustive,
             seen_tys,
             query_ty: ty,
             unchecked_tys: vec![(ty, 0)],
             recursion_limit: tcx.recursion_limit(),
             adt_components,
+            exhaustive,
         }
     }
+
+    /// Called when `ty` is found to always require drop.
+    /// If the exhaustive flag is true, then `Ok(ty)` is returned like any other type.
+    /// Otherwise, `Err(AlwaysRequireDrop)` is returned, which will cause iteration to abort.
+    fn always_drop_component(&self, ty: Ty<'tcx>) -> NeedsDropResult<Ty<'tcx>> {
+        if self.exhaustive { Ok(ty) } else { Err(AlwaysRequiresDrop) }
+    }
 }
 
 impl<'tcx, F, I> Iterator for NeedsDropTypes<'tcx, F>
@@ -131,19 +144,22 @@ where
 {
     type Item = NeedsDropResult<Ty<'tcx>>;
 
+    #[instrument(level = "debug", skip(self), ret)]
     fn next(&mut self) -> Option<NeedsDropResult<Ty<'tcx>>> {
         let tcx = self.tcx;
 
         while let Some((ty, level)) = self.unchecked_tys.pop() {
+            debug!(?ty, "needs_drop_components: inspect");
             if !self.recursion_limit.value_within_limit(level) {
                 // Not having a `Span` isn't great. But there's hopefully some other
                 // recursion limit error as well.
+                debug!("needs_drop_components: recursion limit exceeded");
                 tcx.dcx().emit_err(NeedsDropOverflow { query_ty: self.query_ty });
-                return Some(Err(AlwaysRequiresDrop));
+                return Some(self.always_drop_component(ty));
             }
 
             let components = match needs_drop_components(tcx, ty) {
-                Err(e) => return Some(Err(e)),
+                Err(AlwaysRequiresDrop) => return Some(self.always_drop_component(ty)),
                 Ok(components) => components,
             };
             debug!("needs_drop_components({:?}) = {:?}", ty, components);
@@ -171,7 +187,7 @@ where
                         if self.reveal_coroutine_witnesses {
                             queue_type(self, args.as_coroutine().witness());
                         } else {
-                            return Some(Err(AlwaysRequiresDrop));
+                            return Some(self.always_drop_component(ty));
                         }
                     }
                     ty::CoroutineWitness(def_id, args) => {
@@ -186,7 +202,7 @@ where
                         }
                     }
 
-                    _ if component.is_copy_modulo_regions(tcx, self.typing_env) => (),
+                    _ if component.is_copy_modulo_regions(tcx, self.typing_env) => {}
 
                     ty::Closure(_, args) => {
                         for upvar in args.as_closure().upvar_tys() {
@@ -205,7 +221,9 @@ where
                     // impl then check whether the field types need `Drop`.
                     ty::Adt(adt_def, args) => {
                         let tys = match (self.adt_components)(adt_def, args) {
-                            Err(e) => return Some(Err(e)),
+                            Err(AlwaysRequiresDrop) => {
+                                return Some(self.always_drop_component(ty));
+                            }
                             Ok(tys) => tys,
                         };
                         for required_ty in tys {
@@ -230,7 +248,8 @@ where
                     }
 
                     ty::Foreign(_) | ty::Dynamic(..) => {
-                        return Some(Err(AlwaysRequiresDrop));
+                        debug!("needs_drop_components: foreign or dynamic");
+                        return Some(self.always_drop_component(ty));
                     }
 
                     ty::Bool
@@ -280,6 +299,7 @@ fn drop_tys_helper<'tcx>(
     typing_env: ty::TypingEnv<'tcx>,
     adt_has_dtor: impl Fn(ty::AdtDef<'tcx>) -> Option<DtorType>,
     only_significant: bool,
+    exhaustive: bool,
 ) -> impl Iterator<Item = NeedsDropResult<Ty<'tcx>>> {
     fn with_query_cache<'tcx>(
         tcx: TyCtxt<'tcx>,
@@ -343,7 +363,7 @@ fn drop_tys_helper<'tcx>(
         .map(|v| v.into_iter())
     };
 
-    NeedsDropTypes::new(tcx, typing_env, ty, adt_components)
+    NeedsDropTypes::new(tcx, typing_env, ty, exhaustive, adt_components)
 }
 
 fn adt_consider_insignificant_dtor<'tcx>(
@@ -384,6 +404,7 @@ fn adt_drop_tys<'tcx>(
         ty::TypingEnv::non_body_analysis(tcx, def_id),
         adt_has_dtor,
         false,
+        false,
     )
     .collect::<Result<Vec<_>, _>>()
     .map(|components| tcx.mk_type_list(&components))
@@ -401,11 +422,31 @@ fn adt_significant_drop_tys(
         ty::TypingEnv::non_body_analysis(tcx, def_id),
         adt_consider_insignificant_dtor(tcx),
         true,
+        false,
     )
     .collect::<Result<Vec<_>, _>>()
     .map(|components| tcx.mk_type_list(&components))
 }
 
+#[instrument(level = "debug", skip(tcx), ret)]
+fn list_significant_drop_tys<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    ty: ty::ParamEnvAnd<'tcx, Ty<'tcx>>,
+) -> &'tcx ty::List<Ty<'tcx>> {
+    tcx.mk_type_list(
+        &drop_tys_helper(
+            tcx,
+            ty.value,
+            ty::TypingEnv { typing_mode: ty::TypingMode::PostAnalysis, param_env: ty.param_env },
+            adt_consider_insignificant_dtor(tcx),
+            true,
+            true,
+        )
+        .filter_map(|res| res.ok())
+        .collect::<Vec<_>>(),
+    )
+}
+
 pub(crate) fn provide(providers: &mut Providers) {
     *providers = Providers {
         needs_drop_raw,
@@ -413,6 +454,7 @@ pub(crate) fn provide(providers: &mut Providers) {
         has_significant_drop_raw,
         adt_drop_tys,
         adt_significant_drop_tys,
+        list_significant_drop_tys,
         ..*providers
     };
 }

src/tools/clippy/clippy_utils/src/qualify_min_const_fn.rs

@@ -233,6 +233,7 @@ fn check_statement<'tcx>(
         | StatementKind::PlaceMention(..)
         | StatementKind::Coverage(..)
         | StatementKind::ConstEvalCounter
+        | StatementKind::BackwardIncompatibleDropHint { .. }
         | StatementKind::Nop => Ok(()),
     }
 }

tests/ui/drop/lint-tail-expr-drop-order-gated.rs

@@ -1,9 +1,10 @@
-// This test is to demonstrate that the lint is gated behind Edition and
+// This test ensures that `tail_expr_drop_order` does not activate in case Edition 2024 is used
-// is triggered only for Edition 2021 and before.
+// because this is a migration lint.
+// Only `cargo fix --edition 2024` shall activate this lint.
 
 //@ check-pass
-//@ edition: 2024
 //@ compile-flags: -Z unstable-options
+//@ edition: 2024
 
 #![deny(tail_expr_drop_order)]
 

tests/ui/drop/lint-tail-expr-drop-order.rs

@@ -1,13 +1,26 @@
-//@ edition: 2021
-
 // Edition 2024 lint for change in drop order at tail expression
 // This lint is to capture potential change in program semantics
 // due to implementation of RFC 3606 <https://github.com/rust-lang/rfcs/pull/3606>
+//@ edition: 2021
+//@ build-fail
 
-#![deny(tail_expr_drop_order)]
+#![deny(tail_expr_drop_order)] //~ NOTE: the lint level is defined here
+#![allow(dropping_copy_types)]
 
 struct LoudDropper;
 impl Drop for LoudDropper {
+    //~^ NOTE: `#1` invokes this custom destructor
+    //~| NOTE: `x` invokes this custom destructor
+    //~| NOTE: `#1` invokes this custom destructor
+    //~| NOTE: `x` invokes this custom destructor
+    //~| NOTE: `#1` invokes this custom destructor
+    //~| NOTE: `x` invokes this custom destructor
+    //~| NOTE: `#1` invokes this custom destructor
+    //~| NOTE: `x` invokes this custom destructor
+    //~| NOTE: `#1` invokes this custom destructor
+    //~| NOTE: `future` invokes this custom destructor
+    //~| NOTE: `_x` invokes this custom destructor
+    //~| NOTE: `#1` invokes this custom destructor
     fn drop(&mut self) {
         // This destructor should be considered significant because it is a custom destructor
         // and we will assume that the destructor can generate side effects arbitrarily so that
@@ -23,30 +36,70 @@ impl LoudDropper {
 
 fn should_lint() -> i32 {
     let x = LoudDropper;
+    //~^ NOTE: `x` calls a custom destructor
+    //~| NOTE: `x` will be dropped later as of Edition 2024
     // Should lint
     x.get() + LoudDropper.get()
-    //~^ ERROR: these values and local bindings have significant drop implementation that will have a different drop order from that of Edition 2021
+    //~^ ERROR: relative drop order changing in Rust 2024
+    //~| NOTE: this value will be stored in a temporary; let us call it `#1`
+    //~| NOTE: up until Edition 2021 `#1` is dropped last but will be dropped earlier in Edition 2024
     //~| WARN: this changes meaning in Rust 2024
+    //~| NOTE: most of the time, changing drop order is harmless; inspect the `impl Drop`s for side effects
+    //~| NOTE: for more information, see issue #123739
+}
+//~^ NOTE: now the temporary value is dropped here, before the local variables in the block or statement
+
+fn should_not_lint_closure() -> impl FnOnce() -> i32 {
+    let x = LoudDropper;
+    move || {
+        // Should not lint because ...
+        x.get() + LoudDropper.get()
+    }
+    // ^ closure captures like `x` are always dropped last by contract
 }
 
-fn should_lint_closure() -> impl FnOnce() -> i32 {
+fn should_lint_in_nested_items() {
-    let x = LoudDropper;
+    fn should_lint_me() -> i32 {
-    move || x.get() + LoudDropper.get()
+        let x = LoudDropper;
-    //~^ ERROR: these values and local bindings have significant drop implementation that will have a different drop order from that of Edition 2021
+        //~^ NOTE: `x` calls a custom destructor
-    //~| WARN: this changes meaning in Rust 2024
+        //~| NOTE: `x` will be dropped later as of Edition 2024
+        // Should lint
+        x.get() + LoudDropper.get()
+        //~^ ERROR: relative drop order changing in Rust 2024
+        //~| NOTE: this value will be stored in a temporary; let us call it `#1`
+        //~| NOTE: up until Edition 2021 `#1` is dropped last but will be dropped earlier in Edition 2024
+        //~| WARN: this changes meaning in Rust 2024
+        //~| NOTE: most of the time, changing drop order is harmless; inspect the `impl Drop`s for side effects
+        //~| NOTE: for more information, see issue #123739
+    }
+    //~^ NOTE: now the temporary value is dropped here, before the local variables in the block or statement
 }
 
+fn should_not_lint_params(x: LoudDropper) -> i32 {
+    // Should not lint because ...
+    x.get() + LoudDropper.get()
+}
+// ^ function parameters like `x` are always dropped last
+
 fn should_not_lint() -> i32 {
     let x = LoudDropper;
     // Should not lint
     x.get()
 }
 
-fn should_not_lint_in_nested_block() -> i32 {
+fn should_lint_in_nested_block() -> i32 {
     let x = LoudDropper;
-    // Should not lint because Edition 2021 drops temporaries in blocks earlier already
+    //~^ NOTE: `x` calls a custom destructor
+    //~| NOTE: `x` will be dropped later as of Edition 2024
     { LoudDropper.get() }
+    //~^ ERROR: relative drop order changing in Rust 2024
+    //~| NOTE: this value will be stored in a temporary; let us call it `#1`
+    //~| NOTE: up until Edition 2021 `#1` is dropped last but will be dropped earlier in Edition 2024
+    //~| WARN: this changes meaning in Rust 2024
+    //~| NOTE: most of the time, changing drop order is harmless; inspect the `impl Drop`s for side effects
+    //~| NOTE: for more information, see issue #123739
 }
+//~^ NOTE: now the temporary value is dropped here, before the local variables in the block or statement
 
 fn should_not_lint_in_match_arm() -> i32 {
     let x = LoudDropper;
@@ -56,14 +109,144 @@ fn should_not_lint_in_match_arm() -> i32 {
     }
 }
 
-fn should_lint_in_nested_items() {
+fn should_not_lint_when_consumed() -> (LoudDropper, i32) {
-    fn should_lint_me() -> i32 {
+    let x = LoudDropper;
+    // Should not lint because `LoudDropper` is consumed by the return value
+    (LoudDropper, x.get())
+}
+
+struct MyAdt {
+    a: LoudDropper,
+    b: LoudDropper,
+}
+
+fn should_not_lint_when_consumed_in_ctor() -> MyAdt {
+    let a = LoudDropper;
+    // Should not lint
+    MyAdt { a, b: LoudDropper }
+}
+
+fn should_not_lint_when_moved() -> i32 {
+    let x = LoudDropper;
+    drop(x);
+    // Should not lint because `x` is not live
+    LoudDropper.get()
+}
+
+fn should_lint_into_async_body() -> i32 {
+    async fn f() {
+        async fn f() {}
         let x = LoudDropper;
-        // Should lint
+        f().await;
-        x.get() + LoudDropper.get()
+        drop(x);
-        //~^ ERROR: these values and local bindings have significant drop implementation that will have a different drop order from that of Edition 2021
-        //~| WARN: this changes meaning in Rust 2024
     }
+
+    let future = f();
+    //~^ NOTE: `future` calls a custom destructor
+    //~| NOTE: `future` will be dropped later as of Edition 2024
+    LoudDropper.get()
+    //~^ ERROR: relative drop order changing in Rust 2024
+    //~| WARN: this changes meaning in Rust 2024
+    //~| NOTE: this value will be stored in a temporary; let us call it `#1`
+    //~| NOTE: up until Edition 2021 `#1` is dropped last but will be dropped earlier in Edition 2024
+    //~| NOTE: most of the time, changing drop order is harmless; inspect the `impl Drop`s for side effects
+    //~| NOTE: for more information, see issue #123739
+}
+//~^ NOTE: now the temporary value is dropped here, before the local variables in the block or statement
+
+fn should_lint_generics<T: Default>() -> &'static str {
+    fn extract<T>(_: &T) -> &'static str {
+        todo!()
+    }
+    let x = T::default();
+    //~^ NOTE: `x` calls a custom destructor
+    //~| NOTE: `x` will be dropped later as of Edition 2024
+    extract(&T::default())
+    //~^ ERROR: relative drop order changing in Rust 2024
+    //~| WARN: this changes meaning in Rust 2024
+    //~| NOTE: this value will be stored in a temporary; let us call it `#1`
+    //~| NOTE: up until Edition 2021 `#1` is dropped last but will be dropped earlier in Edition 2024
+    //~| NOTE: most of the time, changing drop order is harmless; inspect the `impl Drop`s for side effects
+    //~| NOTE: for more information, see issue #123739
+}
+//~^ NOTE: now the temporary value is dropped here, before the local variables in the block or statement
+
+fn should_lint_adt() -> i32 {
+    let x: Result<LoudDropper, ()> = Ok(LoudDropper);
+    //~^ NOTE: `x` calls a custom destructor
+    //~| NOTE: `x` will be dropped later as of Edition 2024
+    LoudDropper.get()
+    //~^ ERROR: relative drop order changing in Rust 2024
+    //~| WARN: this changes meaning in Rust 2024
+    //~| NOTE: this value will be stored in a temporary; let us call it `#1`
+    //~| NOTE: up until Edition 2021 `#1` is dropped last but will be dropped earlier in Edition 2024
+    //~| NOTE: most of the time, changing drop order is harmless; inspect the `impl Drop`s for side effects
+    //~| NOTE: for more information, see issue #123739
+}
+//~^ NOTE: now the temporary value is dropped here, before the local variables in the block or statement
+
+fn should_not_lint_insign_dtor() -> i32 {
+    let x = String::new();
+    LoudDropper.get()
+}
+
+fn should_lint_with_dtor_span() -> i32 {
+    struct LoudDropper3;
+    impl Drop for LoudDropper3 {
+        //~^ NOTE: `#1` invokes this custom destructor
+        fn drop(&mut self) {
+            println!("loud drop");
+        }
+    }
+    impl LoudDropper3 {
+        fn get(&self) -> i32 {
+            0
+        }
+    }
+    struct LoudDropper2;
+    impl Drop for LoudDropper2 {
+        //~^ NOTE: `x` invokes this custom destructor
+        fn drop(&mut self) {
+            println!("loud drop");
+        }
+    }
+    impl LoudDropper2 {
+        fn get(&self) -> i32 {
+            0
+        }
+    }
+
+    let x = LoudDropper2;
+    //~^ NOTE: `x` calls a custom destructor
+    //~| NOTE: `x` will be dropped later as of Edition 2024
+    LoudDropper3.get()
+    //~^ ERROR: relative drop order changing in Rust 2024
+    //~| NOTE: this value will be stored in a temporary; let us call it `#1`
+    //~| NOTE: up until Edition 2021 `#1` is dropped last but will be dropped earlier in Edition 2024
+    //~| WARN: this changes meaning in Rust 2024
+    //~| NOTE: most of the time, changing drop order is harmless; inspect the `impl Drop`s for side effects
+    //~| NOTE: for more information, see issue #123739
+}
+//~^ NOTE: now the temporary value is dropped here, before the local variables in the block or statement
+
+fn should_lint_with_transient_drops() {
+    drop((
+        {
+            LoudDropper.get()
+            //~^ ERROR: relative drop order changing in Rust 2024
+            //~| NOTE: this value will be stored in a temporary; let us call it `#1`
+            //~| NOTE: up until Edition 2021 `#1` is dropped last but will be dropped earlier in Edition 2024
+            //~| WARN: this changes meaning in Rust 2024
+            //~| NOTE: most of the time, changing drop order is harmless; inspect the `impl Drop`s for side effects
+            //~| NOTE: for more information, see issue #123739
+        },
+        {
+            let _x = LoudDropper;
+            //~^ NOTE: `_x` calls a custom destructor
+            //~| NOTE: `_x` will be dropped later as of Edition 2024
+        },
+    ));
+    //~^ NOTE: now the temporary value is dropped here, before the local variables in the block or statement
 }
 
 fn main() {}

tests/ui/drop/lint-tail-expr-drop-order.stderr

@@ -1,42 +1,335 @@
-error: these values and local bindings have significant drop implementation that will have a different drop order from that of Edition 2021
+error: relative drop order changing in Rust 2024
-  --> $DIR/lint-tail-expr-drop-order.rs:27:15
+  --> $DIR/lint-tail-expr-drop-order.rs:42:15
    |
 LL |     let x = LoudDropper;
-   |         - these values have significant drop implementation and will observe changes in drop order under Edition 2024
+   |         -
-LL |     // Should lint
+   |         |
+   |         `x` calls a custom destructor
+   |         `x` will be dropped later as of Edition 2024
+...
 LL |     x.get() + LoudDropper.get()
    |               ^^^^^^^^^^^
+   |               |
+   |               this value will be stored in a temporary; let us call it `#1`
+   |               up until Edition 2021 `#1` is dropped last but will be dropped earlier in Edition 2024
+...
+LL | }
+   | - now the temporary value is dropped here, before the local variables in the block or statement
    |
    = warning: this changes meaning in Rust 2024
    = note: for more information, see issue #123739 <https://github.com/rust-lang/rust/issues/123739>
+note: `#1` invokes this custom destructor
+  --> $DIR/lint-tail-expr-drop-order.rs:11:1
+   |
+LL | / impl Drop for LoudDropper {
+LL | |
+LL | |
+LL | |
+...  |
+LL | |     }
+LL | | }
+   | |_^
+note: `x` invokes this custom destructor
+  --> $DIR/lint-tail-expr-drop-order.rs:11:1
+   |
+LL | / impl Drop for LoudDropper {
+LL | |
+LL | |
+LL | |
+...  |
+LL | |     }
+LL | | }
+   | |_^
+   = note: most of the time, changing drop order is harmless; inspect the `impl Drop`s for side effects like releasing locks or sending messages
 note: the lint level is defined here
   --> $DIR/lint-tail-expr-drop-order.rs:7:9
    |
 LL | #![deny(tail_expr_drop_order)]
    |         ^^^^^^^^^^^^^^^^^^^^
 
-error: these values and local bindings have significant drop implementation that will have a different drop order from that of Edition 2021
+error: relative drop order changing in Rust 2024
-  --> $DIR/lint-tail-expr-drop-order.rs:34:23
+  --> $DIR/lint-tail-expr-drop-order.rs:67:19
-   |
-LL |     let x = LoudDropper;
-   |         - these values have significant drop implementation and will observe changes in drop order under Edition 2024
-LL |     move || x.get() + LoudDropper.get()
-   |                       ^^^^^^^^^^^
-   |
-   = warning: this changes meaning in Rust 2024
-   = note: for more information, see issue #123739 <https://github.com/rust-lang/rust/issues/123739>
-
-error: these values and local bindings have significant drop implementation that will have a different drop order from that of Edition 2021
-  --> $DIR/lint-tail-expr-drop-order.rs:63:19
    |
 LL |         let x = LoudDropper;
-   |             - these values have significant drop implementation and will observe changes in drop order under Edition 2024
+   |             -
-LL |         // Should lint
+   |             |
+   |             `x` calls a custom destructor
+   |             `x` will be dropped later as of Edition 2024
+...
 LL |         x.get() + LoudDropper.get()
    |                   ^^^^^^^^^^^
+   |                   |
+   |                   this value will be stored in a temporary; let us call it `#1`
+   |                   up until Edition 2021 `#1` is dropped last but will be dropped earlier in Edition 2024
+...
+LL |     }
+   |     - now the temporary value is dropped here, before the local variables in the block or statement
    |
    = warning: this changes meaning in Rust 2024
    = note: for more information, see issue #123739 <https://github.com/rust-lang/rust/issues/123739>
+note: `#1` invokes this custom destructor
+  --> $DIR/lint-tail-expr-drop-order.rs:11:1
+   |
+LL | / impl Drop for LoudDropper {
+LL | |
+LL | |
+LL | |
+...  |
+LL | |     }
+LL | | }
+   | |_^
+note: `x` invokes this custom destructor
+  --> $DIR/lint-tail-expr-drop-order.rs:11:1
+   |
+LL | / impl Drop for LoudDropper {
+LL | |
+LL | |
+LL | |
+...  |
+LL | |     }
+LL | | }
+   | |_^
+   = note: most of the time, changing drop order is harmless; inspect the `impl Drop`s for side effects like releasing locks or sending messages
 
-error: aborting due to 3 previous errors
+error: relative drop order changing in Rust 2024
+  --> $DIR/lint-tail-expr-drop-order.rs:94:7
+   |
+LL |     let x = LoudDropper;
+   |         -
+   |         |
+   |         `x` calls a custom destructor
+   |         `x` will be dropped later as of Edition 2024
+...
+LL |     { LoudDropper.get() }
+   |       ^^^^^^^^^^^
+   |       |
+   |       this value will be stored in a temporary; let us call it `#1`
+   |       up until Edition 2021 `#1` is dropped last but will be dropped earlier in Edition 2024
+...
+LL | }
+   | - now the temporary value is dropped here, before the local variables in the block or statement
+   |
+   = warning: this changes meaning in Rust 2024
+   = note: for more information, see issue #123739 <https://github.com/rust-lang/rust/issues/123739>
+note: `#1` invokes this custom destructor
+  --> $DIR/lint-tail-expr-drop-order.rs:11:1
+   |
+LL | / impl Drop for LoudDropper {
+LL | |
+LL | |
+LL | |
+...  |
+LL | |     }
+LL | | }
+   | |_^
+note: `x` invokes this custom destructor
+  --> $DIR/lint-tail-expr-drop-order.rs:11:1
+   |
+LL | / impl Drop for LoudDropper {
+LL | |
+LL | |
+LL | |
+...  |
+LL | |     }
+LL | | }
+   | |_^
+   = note: most of the time, changing drop order is harmless; inspect the `impl Drop`s for side effects like releasing locks or sending messages
+
+error: relative drop order changing in Rust 2024
+  --> $DIR/lint-tail-expr-drop-order.rs:147:5
+   |
+LL |     let future = f();
+   |         ------
+   |         |
+   |         `future` calls a custom destructor
+   |         `future` will be dropped later as of Edition 2024
+...
+LL |     LoudDropper.get()
+   |     ^^^^^^^^^^^
+   |     |
+   |     this value will be stored in a temporary; let us call it `#1`
+   |     up until Edition 2021 `#1` is dropped last but will be dropped earlier in Edition 2024
+...
+LL | }
+   | - now the temporary value is dropped here, before the local variables in the block or statement
+   |
+   = warning: this changes meaning in Rust 2024
+   = note: for more information, see issue #123739 <https://github.com/rust-lang/rust/issues/123739>
+note: `#1` invokes this custom destructor
+  --> $DIR/lint-tail-expr-drop-order.rs:11:1
+   |
+LL | / impl Drop for LoudDropper {
+LL | |
+LL | |
+LL | |
+...  |
+LL | |     }
+LL | | }
+   | |_^
+note: `future` invokes this custom destructor
+  --> $DIR/lint-tail-expr-drop-order.rs:11:1
+   |
+LL | / impl Drop for LoudDropper {
+LL | |
+LL | |
+LL | |
+...  |
+LL | |     }
+LL | | }
+   | |_^
+   = note: most of the time, changing drop order is harmless; inspect the `impl Drop`s for side effects like releasing locks or sending messages
+
+error: relative drop order changing in Rust 2024
+  --> $DIR/lint-tail-expr-drop-order.rs:164:14
+   |
+LL |     let x = T::default();
+   |         -
+   |         |
+   |         `x` calls a custom destructor
+   |         `x` will be dropped later as of Edition 2024
+...
+LL |     extract(&T::default())
+   |              ^^^^^^^^^^^^
+   |              |
+   |              this value will be stored in a temporary; let us call it `#1`
+   |              up until Edition 2021 `#1` is dropped last but will be dropped earlier in Edition 2024
+...
+LL | }
+   | - now the temporary value is dropped here, before the local variables in the block or statement
+   |
+   = warning: this changes meaning in Rust 2024
+   = note: for more information, see issue #123739 <https://github.com/rust-lang/rust/issues/123739>
+   = note: most of the time, changing drop order is harmless; inspect the `impl Drop`s for side effects like releasing locks or sending messages
+
+error: relative drop order changing in Rust 2024
+  --> $DIR/lint-tail-expr-drop-order.rs:178:5
+   |
+LL |     let x: Result<LoudDropper, ()> = Ok(LoudDropper);
+   |         -
+   |         |
+   |         `x` calls a custom destructor
+   |         `x` will be dropped later as of Edition 2024
+...
+LL |     LoudDropper.get()
+   |     ^^^^^^^^^^^
+   |     |
+   |     this value will be stored in a temporary; let us call it `#1`
+   |     up until Edition 2021 `#1` is dropped last but will be dropped earlier in Edition 2024
+...
+LL | }
+   | - now the temporary value is dropped here, before the local variables in the block or statement
+   |
+   = warning: this changes meaning in Rust 2024
+   = note: for more information, see issue #123739 <https://github.com/rust-lang/rust/issues/123739>
+note: `#1` invokes this custom destructor
+  --> $DIR/lint-tail-expr-drop-order.rs:11:1
+   |
+LL | / impl Drop for LoudDropper {
+LL | |
+LL | |
+LL | |
+...  |
+LL | |     }
+LL | | }
+   | |_^
+note: `x` invokes this custom destructor
+  --> $DIR/lint-tail-expr-drop-order.rs:11:1
+   |
+LL | / impl Drop for LoudDropper {
+LL | |
+LL | |
+LL | |
+...  |
+LL | |     }
+LL | | }
+   | |_^
+   = note: most of the time, changing drop order is harmless; inspect the `impl Drop`s for side effects like releasing locks or sending messages
+
+error: relative drop order changing in Rust 2024
+  --> $DIR/lint-tail-expr-drop-order.rs:222:5
+   |
+LL |     let x = LoudDropper2;
+   |         -
+   |         |
+   |         `x` calls a custom destructor
+   |         `x` will be dropped later as of Edition 2024
+...
+LL |     LoudDropper3.get()
+   |     ^^^^^^^^^^^^
+   |     |
+   |     this value will be stored in a temporary; let us call it `#1`
+   |     up until Edition 2021 `#1` is dropped last but will be dropped earlier in Edition 2024
+...
+LL | }
+   | - now the temporary value is dropped here, before the local variables in the block or statement
+   |
+   = warning: this changes meaning in Rust 2024
+   = note: for more information, see issue #123739 <https://github.com/rust-lang/rust/issues/123739>
+note: `#1` invokes this custom destructor
+  --> $DIR/lint-tail-expr-drop-order.rs:195:5
+   |
+LL | /     impl Drop for LoudDropper3 {
+LL | |
+LL | |         fn drop(&mut self) {
+LL | |             println!("loud drop");
+LL | |         }
+LL | |     }
+   | |_____^
+note: `x` invokes this custom destructor
+  --> $DIR/lint-tail-expr-drop-order.rs:207:5
+   |
+LL | /     impl Drop for LoudDropper2 {
+LL | |
+LL | |         fn drop(&mut self) {
+LL | |             println!("loud drop");
+LL | |         }
+LL | |     }
+   | |_____^
+   = note: most of the time, changing drop order is harmless; inspect the `impl Drop`s for side effects like releasing locks or sending messages
+
+error: relative drop order changing in Rust 2024
+  --> $DIR/lint-tail-expr-drop-order.rs:235:13
+   |
+LL |             LoudDropper.get()
+   |             ^^^^^^^^^^^
+   |             |
+   |             this value will be stored in a temporary; let us call it `#1`
+   |             up until Edition 2021 `#1` is dropped last but will be dropped earlier in Edition 2024
+...
+LL |             let _x = LoudDropper;
+   |                 --
+   |                 |
+   |                 `_x` calls a custom destructor
+   |                 `_x` will be dropped later as of Edition 2024
+...
+LL |     ));
+   |       - now the temporary value is dropped here, before the local variables in the block or statement
+   |
+   = warning: this changes meaning in Rust 2024
+   = note: for more information, see issue #123739 <https://github.com/rust-lang/rust/issues/123739>
+note: `#1` invokes this custom destructor
+  --> $DIR/lint-tail-expr-drop-order.rs:11:1
+   |
+LL | / impl Drop for LoudDropper {
+LL | |
+LL | |
+LL | |
+...  |
+LL | |     }
+LL | | }
+   | |_^
+note: `_x` invokes this custom destructor
+  --> $DIR/lint-tail-expr-drop-order.rs:11:1
+   |
+LL | / impl Drop for LoudDropper {
+LL | |
+LL | |
+LL | |
+...  |
+LL | |     }
+LL | | }
+   | |_^
+   = note: most of the time, changing drop order is harmless; inspect the `impl Drop`s for side effects like releasing locks or sending messages
+
+error: aborting due to 8 previous errors
 

tests/ui/thir-print/thir-flat-const-variant.stdout

@@ -11,9 +11,12 @@ Thir {
                 fields: [],
             },
             ty: (),
-            temp_lifetime: Some(
+            temp_lifetime: TempLifetime {
-                Node(3),
+                temp_lifetime: Some(
-            ),
+                    Node(3),
+                ),
+                backwards_incompatible: None,
+            },
             span: $DIR/thir-flat-const-variant.rs:12:32: 12:34 (#0),
         },
         Expr {
@@ -25,9 +28,12 @@ Thir {
                 value: e0,
             },
             ty: (),
-            temp_lifetime: Some(
+            temp_lifetime: TempLifetime {
-                Node(3),
+                temp_lifetime: Some(
-            ),
+                    Node(3),
+                ),
+                backwards_incompatible: None,
+            },
             span: $DIR/thir-flat-const-variant.rs:12:32: 12:34 (#0),
         },
         Expr {
@@ -47,9 +53,12 @@ Thir {
                 },
             ),
             ty: Foo,
-            temp_lifetime: Some(
+            temp_lifetime: TempLifetime {
-                Node(3),
+                temp_lifetime: Some(
-            ),
+                    Node(3),
+                ),
+                backwards_incompatible: None,
+            },
             span: $DIR/thir-flat-const-variant.rs:12:23: 12:35 (#0),
         },
         Expr {
@@ -61,9 +70,12 @@ Thir {
                 value: e2,
             },
             ty: Foo,
-            temp_lifetime: Some(
+            temp_lifetime: TempLifetime {
-                Node(3),
+                temp_lifetime: Some(
-            ),
+                    Node(3),
+                ),
+                backwards_incompatible: None,
+            },
             span: $DIR/thir-flat-const-variant.rs:12:23: 12:35 (#0),
         },
     ],
@@ -84,9 +96,12 @@ Thir {
                 fields: [],
             },
             ty: (),
-            temp_lifetime: Some(
+            temp_lifetime: TempLifetime {
-                Node(3),
+                temp_lifetime: Some(
-            ),
+                    Node(3),
+                ),
+                backwards_incompatible: None,
+            },
             span: $DIR/thir-flat-const-variant.rs:13:33: 13:35 (#0),
         },
         Expr {
@@ -98,9 +113,12 @@ Thir {
                 value: e0,
             },
             ty: (),
-            temp_lifetime: Some(
+            temp_lifetime: TempLifetime {
-                Node(3),
+                temp_lifetime: Some(
-            ),
+                    Node(3),
+                ),
+                backwards_incompatible: None,
+            },
             span: $DIR/thir-flat-const-variant.rs:13:33: 13:35 (#0),
         },
         Expr {
@@ -120,9 +138,12 @@ Thir {
                 },
             ),
             ty: Foo,
-            temp_lifetime: Some(
+            temp_lifetime: TempLifetime {
-                Node(3),
+                temp_lifetime: Some(
-            ),
+                    Node(3),
+                ),
+                backwards_incompatible: None,
+            },
             span: $DIR/thir-flat-const-variant.rs:13:23: 13:36 (#0),
         },
         Expr {
@@ -134,9 +155,12 @@ Thir {
                 value: e2,
             },
             ty: Foo,
-            temp_lifetime: Some(
+            temp_lifetime: TempLifetime {
-                Node(3),
+                temp_lifetime: Some(
-            ),
+                    Node(3),
+                ),
+                backwards_incompatible: None,
+            },
             span: $DIR/thir-flat-const-variant.rs:13:23: 13:36 (#0),
         },
     ],
@@ -157,9 +181,12 @@ Thir {
                 fields: [],
             },
             ty: (),
-            temp_lifetime: Some(
+            temp_lifetime: TempLifetime {
-                Node(3),
+                temp_lifetime: Some(
-            ),
+                    Node(3),
+                ),
+                backwards_incompatible: None,
+            },
             span: $DIR/thir-flat-const-variant.rs:14:33: 14:35 (#0),
         },
         Expr {
@@ -171,9 +198,12 @@ Thir {
                 value: e0,
             },
             ty: (),
-            temp_lifetime: Some(
+            temp_lifetime: TempLifetime {
-                Node(3),
+                temp_lifetime: Some(
-            ),
+                    Node(3),
+                ),
+                backwards_incompatible: None,
+            },
             span: $DIR/thir-flat-const-variant.rs:14:33: 14:35 (#0),
         },
         Expr {
@@ -193,9 +223,12 @@ Thir {
                 },
             ),
             ty: Foo,
-            temp_lifetime: Some(
+            temp_lifetime: TempLifetime {
-                Node(3),
+                temp_lifetime: Some(
-            ),
+                    Node(3),
+                ),
+                backwards_incompatible: None,
+            },
             span: $DIR/thir-flat-const-variant.rs:14:24: 14:36 (#0),
         },
         Expr {
@@ -207,9 +240,12 @@ Thir {
                 value: e2,
             },
             ty: Foo,
-            temp_lifetime: Some(
+            temp_lifetime: TempLifetime {
-                Node(3),
+                temp_lifetime: Some(
-            ),
+                    Node(3),
+                ),
+                backwards_incompatible: None,
+            },
             span: $DIR/thir-flat-const-variant.rs:14:24: 14:36 (#0),
         },
     ],
@@ -230,9 +266,12 @@ Thir {
                 fields: [],
             },
             ty: (),
-            temp_lifetime: Some(
+            temp_lifetime: TempLifetime {
-                Node(3),
+                temp_lifetime: Some(
-            ),
+                    Node(3),
+                ),
+                backwards_incompatible: None,
+            },
             span: $DIR/thir-flat-const-variant.rs:15:34: 15:36 (#0),
         },
         Expr {
@@ -244,9 +283,12 @@ Thir {
                 value: e0,
             },
             ty: (),
-            temp_lifetime: Some(
+            temp_lifetime: TempLifetime {
-                Node(3),
+                temp_lifetime: Some(
-            ),
+                    Node(3),
+                ),
+                backwards_incompatible: None,
+            },
             span: $DIR/thir-flat-const-variant.rs:15:34: 15:36 (#0),
         },
         Expr {
@@ -266,9 +308,12 @@ Thir {
                 },
             ),
             ty: Foo,
-            temp_lifetime: Some(
+            temp_lifetime: TempLifetime {
-                Node(3),
+                temp_lifetime: Some(
-            ),
+                    Node(3),
+                ),
+                backwards_incompatible: None,
+            },
             span: $DIR/thir-flat-const-variant.rs:15:24: 15:37 (#0),
         },
         Expr {
@@ -280,9 +325,12 @@ Thir {
                 value: e2,
             },
             ty: Foo,
-            temp_lifetime: Some(
+            temp_lifetime: TempLifetime {
-                Node(3),
+                temp_lifetime: Some(
-            ),
+                    Node(3),
+                ),
+                backwards_incompatible: None,
+            },
             span: $DIR/thir-flat-const-variant.rs:15:24: 15:37 (#0),
         },
     ],
@@ -312,9 +360,12 @@ Thir {
                 block: b0,
             },
             ty: (),
-            temp_lifetime: Some(
+            temp_lifetime: TempLifetime {
-                Node(2),
+                temp_lifetime: Some(
-            ),
+                    Node(2),
+                ),
+                backwards_incompatible: None,
+            },
             span: $DIR/thir-flat-const-variant.rs:18:11: 18:13 (#0),
         },
         Expr {
@@ -326,9 +377,12 @@ Thir {
                 value: e0,
             },
             ty: (),
-            temp_lifetime: Some(
+            temp_lifetime: TempLifetime {
-                Node(2),
+                temp_lifetime: Some(
-            ),
+                    Node(2),
+                ),
+                backwards_incompatible: None,
+            },
             span: $DIR/thir-flat-const-variant.rs:18:11: 18:13 (#0),
         },
     ],

tests/ui/thir-print/thir-flat.stdout

@@ -20,9 +20,12 @@ Thir {
                 block: b0,
             },
             ty: (),
-            temp_lifetime: Some(
+            temp_lifetime: TempLifetime {
-                Node(2),
+                temp_lifetime: Some(
-            ),
+                    Node(2),
+                ),
+                backwards_incompatible: None,
+            },
             span: $DIR/thir-flat.rs:4:15: 4:17 (#0),
         },
         Expr {
@@ -34,9 +37,12 @@ Thir {
                 value: e0,
             },
             ty: (),
-            temp_lifetime: Some(
+            temp_lifetime: TempLifetime {
-                Node(2),
+                temp_lifetime: Some(
-            ),
+                    Node(2),
+                ),
+                backwards_incompatible: None,
+            },
             span: $DIR/thir-flat.rs:4:15: 4:17 (#0),
         },
     ],

tests/ui/thir-print/thir-tree-match.stdout

@@ -26,7 +26,7 @@ params: [
 body:
     Expr {
         ty: bool
-        temp_lifetime: Some(Node(26))
+        temp_lifetime: TempLifetime { temp_lifetime: Some(Node(26)), backwards_incompatible: None }
         span: $DIR/thir-tree-match.rs:15:32: 21:2 (#0)
         kind: 
             Scope {
@@ -35,7 +35,7 @@ body:
                 value:
                     Expr {
                         ty: bool
-                        temp_lifetime: Some(Node(26))
+                        temp_lifetime: TempLifetime { temp_lifetime: Some(Node(26)), backwards_incompatible: None }
                         span: $DIR/thir-tree-match.rs:15:32: 21:2 (#0)
                         kind: 
                             Block {
@@ -47,7 +47,7 @@ body:
                                 expr:
                                     Expr {
                                         ty: bool
-                                        temp_lifetime: Some(Node(26))
+                                        temp_lifetime: TempLifetime { temp_lifetime: Some(Node(26)), backwards_incompatible: None }
                                         span: $DIR/thir-tree-match.rs:16:5: 20:6 (#0)
                                         kind: 
                                             Scope {
@@ -56,14 +56,14 @@ body:
                                                 value:
                                                     Expr {
                                                         ty: bool
-                                                        temp_lifetime: Some(Node(26))
+                                                        temp_lifetime: TempLifetime { temp_lifetime: Some(Node(26)), backwards_incompatible: None }
                                                         span: $DIR/thir-tree-match.rs:16:5: 20:6 (#0)
                                                         kind: 
                                                             Match {
                                                                 scrutinee:
                                                                     Expr {
                                                                         ty: Foo
-                                                                        temp_lifetime: Some(Node(26))
+                                                                        temp_lifetime: TempLifetime { temp_lifetime: Some(Node(26)), backwards_incompatible: None }
                                                                         span: $DIR/thir-tree-match.rs:16:11: 16:14 (#0)
                                                                         kind: 
                                                                             Scope {
@@ -72,7 +72,7 @@ body:
                                                                                 value:
                                                                                     Expr {
                                                                                         ty: Foo
-                                                                                        temp_lifetime: Some(Node(26))
+                                                                                        temp_lifetime: TempLifetime { temp_lifetime: Some(Node(26)), backwards_incompatible: None }
                                                                                         span: $DIR/thir-tree-match.rs:16:11: 16:14 (#0)
                                                                                         kind: 
                                                                                             VarRef {
@@ -123,7 +123,7 @@ body:
                                                                         body: 
                                                                             Expr {
                                                                                 ty: bool
-                                                                                temp_lifetime: Some(Node(13))
+                                                                                temp_lifetime: TempLifetime { temp_lifetime: Some(Node(13)), backwards_incompatible: None }
                                                                                 span: $DIR/thir-tree-match.rs:17:36: 17:40 (#0)
                                                                                 kind: 
                                                                                     Scope {
@@ -132,7 +132,7 @@ body:
                                                                                         value:
                                                                                             Expr {
                                                                                                 ty: bool
-                                                                                                temp_lifetime: Some(Node(13))
+                                                                                                temp_lifetime: TempLifetime { temp_lifetime: Some(Node(13)), backwards_incompatible: None }
                                                                                                 span: $DIR/thir-tree-match.rs:17:36: 17:40 (#0)
                                                                                                 kind: 
                                                                                                     Literal( lit: Spanned { node: Bool(true), span: $DIR/thir-tree-match.rs:17:36: 17:40 (#0) }, neg: false)
@@ -175,7 +175,7 @@ body:
                                                                         body: 
                                                                             Expr {
                                                                                 ty: bool
-                                                                                temp_lifetime: Some(Node(19))
+                                                                                temp_lifetime: TempLifetime { temp_lifetime: Some(Node(19)), backwards_incompatible: None }
                                                                                 span: $DIR/thir-tree-match.rs:18:27: 18:32 (#0)
                                                                                 kind: 
                                                                                     Scope {
@@ -184,7 +184,7 @@ body:
                                                                                         value:
                                                                                             Expr {
                                                                                                 ty: bool
-                                                                                                temp_lifetime: Some(Node(19))
+                                                                                                temp_lifetime: TempLifetime { temp_lifetime: Some(Node(19)), backwards_incompatible: None }
                                                                                                 span: $DIR/thir-tree-match.rs:18:27: 18:32 (#0)
                                                                                                 kind: 
                                                                                                     Literal( lit: Spanned { node: Bool(false), span: $DIR/thir-tree-match.rs:18:27: 18:32 (#0) }, neg: false)
@@ -219,7 +219,7 @@ body:
                                                                         body: 
                                                                             Expr {
                                                                                 ty: bool
-                                                                                temp_lifetime: Some(Node(24))
+                                                                                temp_lifetime: TempLifetime { temp_lifetime: Some(Node(24)), backwards_incompatible: None }
                                                                                 span: $DIR/thir-tree-match.rs:19:24: 19:28 (#0)
                                                                                 kind: 
                                                                                     Scope {
@@ -228,7 +228,7 @@ body:
                                                                                         value:
                                                                                             Expr {
                                                                                                 ty: bool
-                                                                                                temp_lifetime: Some(Node(24))
+                                                                                                temp_lifetime: TempLifetime { temp_lifetime: Some(Node(24)), backwards_incompatible: None }
                                                                                                 span: $DIR/thir-tree-match.rs:19:24: 19:28 (#0)
                                                                                                 kind: 
                                                                                                     Literal( lit: Spanned { node: Bool(true), span: $DIR/thir-tree-match.rs:19:24: 19:28 (#0) }, neg: false)
@@ -257,7 +257,7 @@ params: [
 body:
     Expr {
         ty: ()
-        temp_lifetime: Some(Node(2))
+        temp_lifetime: TempLifetime { temp_lifetime: Some(Node(2)), backwards_incompatible: None }
         span: $DIR/thir-tree-match.rs:23:11: 23:13 (#0)
         kind: 
             Scope {
@@ -266,7 +266,7 @@ body:
                 value:
                     Expr {
                         ty: ()
-                        temp_lifetime: Some(Node(2))
+                        temp_lifetime: TempLifetime { temp_lifetime: Some(Node(2)), backwards_incompatible: None }
                         span: $DIR/thir-tree-match.rs:23:11: 23:13 (#0)
                         kind: 
                             Block {

tests/ui/thir-print/thir-tree.stdout

@@ -4,7 +4,7 @@ params: [
 body:
     Expr {
         ty: ()
-        temp_lifetime: Some(Node(2))
+        temp_lifetime: TempLifetime { temp_lifetime: Some(Node(2)), backwards_incompatible: None }
         span: $DIR/thir-tree.rs:4:15: 4:17 (#0)
         kind: 
             Scope {
@@ -13,7 +13,7 @@ body:
                 value:
                     Expr {
                         ty: ()
-                        temp_lifetime: Some(Node(2))
+                        temp_lifetime: TempLifetime { temp_lifetime: Some(Node(2)), backwards_incompatible: None }
                         span: $DIR/thir-tree.rs:4:15: 4:17 (#0)
                         kind: 
                             Block {