mentioned items: also handle vtables

compiler/rustc_middle/src/mir/mod.rs

@@ -319,7 +319,11 @@ impl<'tcx> CoroutineInfo<'tcx> {
 pub enum MentionedItem<'tcx> {
     Fn(DefId, GenericArgsRef<'tcx>),
     Drop(Ty<'tcx>),
-    // FIXME: add Vtable { source_ty: Ty<'tcx>, target_ty: Ty<'tcx> },
+    /// Unsizing casts might require vtables, so we have to record them.
+    UnsizeCast {
+        source_ty: Ty<'tcx>,
+        target_ty: Ty<'tcx>,
+    },
     // FIXME: do we have to add closures?
 }
 

compiler/rustc_mir_transform/src/mentioned_items.rs

@@ -1,6 +1,6 @@
 use rustc_middle::mir::visit::Visitor;
 use rustc_middle::mir::{self, ConstOperand, Location, MentionedItem, MirPass};
-use rustc_middle::ty::{self, TyCtxt};
+use rustc_middle::ty::{self, adjustment::PointerCoercion, TyCtxt};
 use rustc_session::Session;
 use rustc_span::source_map::Spanned;
 
@@ -54,4 +54,28 @@ impl<'tcx> Visitor<'tcx> for MentionedItemsVisitor<'_, 'tcx> {
             _ => {}
         }
     }
+
+    fn visit_rvalue(&mut self, rvalue: &mir::Rvalue<'tcx>, location: Location) {
+        self.super_rvalue(rvalue, location);
+        match *rvalue {
+            // We need to detect unsizing casts that required vtables.
+            mir::Rvalue::Cast(
+                mir::CastKind::PointerCoercion(PointerCoercion::Unsize),
+                ref operand,
+                target_ty,
+            )
+            | mir::Rvalue::Cast(mir::CastKind::DynStar, ref operand, target_ty) => {
+                let span = self.body.source_info(location).span;
+                self.mentioned_items.push(Spanned {
+                    node: MentionedItem::UnsizeCast {
+                        source_ty: operand.ty(self.body, self.tcx),
+                        target_ty,
+                    },
+                    span,
+                });
+            }
+            // Function pointer casts are already handled by `visit_constant` above.
+            _ => {}
+        }
+    }
 }

compiler/rustc_monomorphize/src/collector.rs

@@ -736,7 +736,7 @@ impl<'a, 'tcx> MirUsedCollector<'a, 'tcx> {
     where
         T: TypeFoldable<TyCtxt<'tcx>>,
     {
-        debug!("monomorphize: self.instance={:?}", self.instance);
+        trace!("monomorphize: self.instance={:?}", self.instance);
         self.instance.instantiate_mir_and_normalize_erasing_regions(
             self.tcx,
             ty::ParamEnv::reveal_all(),
@@ -1342,13 +1342,16 @@ fn create_mono_items_for_vtable_methods<'tcx>(
 ) {
     assert!(!trait_ty.has_escaping_bound_vars() && !impl_ty.has_escaping_bound_vars());
 
-    if let ty::Dynamic(trait_ty, ..) = trait_ty.kind() {
+    let ty::Dynamic(trait_ty, ..) = trait_ty.kind() else {
+        bug!("create_mono_items_for_vtable_methods: {trait_ty:?} not a trait type");
+    };
     if let Some(principal) = trait_ty.principal() {
         let poly_trait_ref = principal.with_self_ty(tcx, impl_ty);
         assert!(!poly_trait_ref.has_escaping_bound_vars());
 
         // Walk all methods of the trait, including those of its supertraits
         let entries = tcx.vtable_entries(poly_trait_ref);
+        debug!(?entries);
         let methods = entries
             .iter()
             .filter_map(|entry| match entry {
@@ -1370,7 +1373,6 @@ fn create_mono_items_for_vtable_methods<'tcx>(
 
     // Also add the destructor.
     visit_drop_use(tcx, impl_ty, false, source, output);
-    }
 }
 
 //=-----------------------------------------------------------------------------
@@ -1693,7 +1695,8 @@ fn collect_items_of_instance<'tcx>(
     }
 }
 
-/// `item` must be already monomorphized
+/// `item` must be already monomorphized.
+#[instrument(skip(tcx, span, output), level = "debug")]
 fn visit_mentioned_item<'tcx>(
     tcx: TyCtxt<'tcx>,
     item: &MentionedItem<'tcx>,
@@ -1712,6 +1715,18 @@ fn visit_mentioned_item<'tcx>(
         MentionedItem::Drop(ty) => {
             visit_drop_use(tcx, ty, /*is_direct_call*/ true, span, output);
         }
+        MentionedItem::UnsizeCast { source_ty, target_ty } => {
+            let (source_ty, target_ty) =
+                find_vtable_types_for_unsizing(tcx.at(span), source_ty, target_ty);
+            // This could also be a different Unsize instruction, like
+            // from a fixed sized array to a slice. But we are only
+            // interested in things that produce a vtable.
+            if (target_ty.is_trait() && !source_ty.is_trait())
+                || (target_ty.is_dyn_star() && !source_ty.is_dyn_star())
+            {
+                create_mono_items_for_vtable_methods(tcx, target_ty, source_ty, span, output);
+            }
+        }
     }
 }
 

tests/ui/consts/required-consts/collect-in-dead-vtable.noopt.stderr

@@ -1,21 +1,21 @@
 error[E0080]: evaluation of `Fail::<i32>::C` failed
-  --> $DIR/collect-in-dead-vtable.rs:11:19
+  --> $DIR/collect-in-dead-vtable.rs:8:19
    |
 LL |     const C: () = panic!();
-   |                   ^^^^^^^^ the evaluated program panicked at 'explicit panic', $DIR/collect-in-dead-vtable.rs:11:19
+   |                   ^^^^^^^^ the evaluated program panicked at 'explicit panic', $DIR/collect-in-dead-vtable.rs:8:19
    |
    = note: this error originates in the macro `$crate::panic::panic_2015` which comes from the expansion of the macro `panic` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 note: erroneous constant encountered
-  --> $DIR/collect-in-dead-vtable.rs:25:21
+  --> $DIR/collect-in-dead-vtable.rs:21:21
    |
 LL |             let _ = Fail::<T>::C;
    |                     ^^^^^^^^^^^^
 
 note: the above error was encountered while instantiating `fn <std::vec::Vec<i32> as MyTrait>::not_called`
-  --> $DIR/collect-in-dead-vtable.rs:34:40
+  --> $DIR/collect-in-dead-vtable.rs:30:40
    |
-LL |         let gen_vtable: &dyn MyTrait = &v; // vtable "appears" here
+LL |         let gen_vtable: &dyn MyTrait = &v; // vtable is "mentioned" here
    |                                        ^^
 
 error: aborting due to 1 previous error

tests/ui/consts/required-consts/collect-in-dead-vtable.opt.stderr

@@ -0,0 +1,23 @@
+error[E0080]: evaluation of `Fail::<i32>::C` failed
+  --> $DIR/collect-in-dead-vtable.rs:8:19
+   |
+LL |     const C: () = panic!();
+   |                   ^^^^^^^^ the evaluated program panicked at 'explicit panic', $DIR/collect-in-dead-vtable.rs:8:19
+   |
+   = note: this error originates in the macro `$crate::panic::panic_2015` which comes from the expansion of the macro `panic` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+note: erroneous constant encountered
+  --> $DIR/collect-in-dead-vtable.rs:21:21
+   |
+LL |             let _ = Fail::<T>::C;
+   |                     ^^^^^^^^^^^^
+
+note: the above error was encountered while instantiating `fn <std::vec::Vec<i32> as MyTrait>::not_called`
+  --> $DIR/collect-in-dead-vtable.rs:30:40
+   |
+LL |         let gen_vtable: &dyn MyTrait = &v; // vtable is "mentioned" here
+   |                                        ^^
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0080`.

tests/ui/consts/required-consts/collect-in-dead-vtable.rs

@@ -1,14 +1,11 @@
 //@revisions: noopt opt
-//@[noopt] build-fail
+//@ build-fail
 //@[opt] compile-flags: -O
-//FIXME: `opt` revision currently does not stop with an error due to
-//<https://github.com/rust-lang/rust/issues/107503>.
-//@[opt] build-pass
 //! This fails without optimizations, so it should also fail with optimizations.
 
 struct Fail<T>(T);
 impl<T> Fail<T> {
-    const C: () = panic!(); //[noopt]~ERROR evaluation of `Fail::<i32>::C` failed
+    const C: () = panic!(); //~ERROR evaluation of `Fail::<i32>::C` failed
 }
 
 trait MyTrait {
@@ -17,8 +14,7 @@ trait MyTrait {
 
 // This function is not actually called, but it is mentioned in a vtable in a function that is
 // called. Make sure we still find this error.
-// This relies on mono-item collection checking `required_consts` in functions that are referenced
+// This ensures that we are properly considering vtables when gathering "mentioned" items.
-// in vtables that syntactically appear in collected functions (even inside dead code).
 impl<T> MyTrait for Vec<T> {
     fn not_called(&self) {
         if false {
@@ -31,7 +27,7 @@ impl<T> MyTrait for Vec<T> {
 fn called<T>() {
     if false {
         let v: Vec<T> = Vec::new();
-        let gen_vtable: &dyn MyTrait = &v; // vtable "appears" here
+        let gen_vtable: &dyn MyTrait = &v; // vtable is "mentioned" here
     }
 }