From aebbd424607b7797f231bc09f44226fcd3040d7e Mon Sep 17 00:00:00 2001
From: Michael Goulet <michael@errs.io>
Date: Mon, 10 Mar 2025 16:50:29 +0000
Subject: [PATCH] Only prefer Sized candidates, and only if they certainly hold
---
compiler/rustc_middle/src/traits/select.rs | 7 +++--
.../src/traits/select/candidate_assembly.rs | 6 ++--
.../src/traits/select/confirmation.rs | 4 +--
.../src/traits/select/mod.rs | 28 ++++++++-----------
.../dont-incompletely-prefer-built-in.rs | 21 ++++++++++++++
...incomplete-prefer-sized-builtin-over-wc.rs | 19 +++++++++++++
...mplete-prefer-sized-builtin-over-wc.stderr | 27 ++++++++++++++++++
7 files changed, 89 insertions(+), 23 deletions(-)
create mode 100644 tests/ui/sized/dont-incompletely-prefer-built-in.rs
create mode 100644 tests/ui/traits/lifetime-incomplete-prefer-sized-builtin-over-wc.rs
create mode 100644 tests/ui/traits/lifetime-incomplete-prefer-sized-builtin-over-wc.stderr
diff --git a/compiler/rustc_middle/src/traits/select.rs b/compiler/rustc_middle/src/traits/select.rs
index 749f3c31bf8..aa2ee756bc5 100644
--- a/compiler/rustc_middle/src/traits/select.rs
+++ b/compiler/rustc_middle/src/traits/select.rs
@@ -95,8 +95,11 @@ pub type EvaluationCache<'tcx, ENV> = Cache<(ENV, ty::PolyTraitPredicate<'tcx>),
/// parameter environment.
#[derive(PartialEq, Eq, Debug, Clone, TypeVisitable)]
pub enum SelectionCandidate<'tcx> {
- /// UwU
- SizedCandidate,
+ /// A built-in implementation for the `Sized` trait. This is preferred
+ /// over all other candidates.
+ SizedCandidate {
+ has_nested: bool,
+ },
/// A builtin implementation for some specific traits, used in cases
/// where we cannot rely an ordinary library implementations.
diff --git a/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs b/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
index 9c0efec2e6c..316198f9e01 100644
--- a/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
@@ -1062,8 +1062,10 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
candidates: &mut SelectionCandidateSet<'tcx>,
) {
match self.sized_conditions(obligation) {
- BuiltinImplConditions::Where(_) => {
- candidates.vec.push(SizedCandidate);
+ BuiltinImplConditions::Where(nested) => {
+ candidates
+ .vec
+ .push(SizedCandidate { has_nested: !nested.skip_binder().is_empty() });
}
BuiltinImplConditions::None => {}
BuiltinImplConditions::Ambiguous => {
diff --git a/compiler/rustc_trait_selection/src/traits/select/confirmation.rs b/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
index 349eab2cbe3..29e0b833665 100644
--- a/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
@@ -40,8 +40,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
candidate: SelectionCandidate<'tcx>,
) -> Result<Selection<'tcx>, SelectionError<'tcx>> {
let mut impl_src = match candidate {
- SizedCandidate => {
- let data = self.confirm_builtin_candidate(obligation, true);
+ SizedCandidate { has_nested } => {
+ let data = self.confirm_builtin_candidate(obligation, has_nested);
ImplSource::Builtin(BuiltinImplSource::Misc, data)
}
diff --git a/compiler/rustc_trait_selection/src/traits/select/mod.rs b/compiler/rustc_trait_selection/src/traits/select/mod.rs
index 674ff30786b..956417b122c 100644
--- a/compiler/rustc_trait_selection/src/traits/select/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/mod.rs
@@ -1803,25 +1803,19 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
// We prefer `Sized` candidates over everything.
let mut sized_candidates =
- candidates.iter().filter(|c| matches!(c.candidate, SizedCandidate));
- if let Some(_sized_candidate) = sized_candidates.next() {
+ candidates.iter().filter(|c| matches!(c.candidate, SizedCandidate { has_nested: _ }));
+ if let Some(sized_candidate) = sized_candidates.next() {
// There should only ever be a single sized candidate
// as they would otherwise overlap.
debug_assert_eq!(sized_candidates.next(), None);
- return Some(SizedCandidate);
- }
-
- // We prefer trivial builtin candidates, i.e. builtin impls without any nested
- // requirements, over all others. This is a fix for #53123 and prevents winnowing
- // from accidentally extending the lifetime of a variable.
- let mut trivial_builtin = candidates
- .iter()
- .filter(|c| matches!(c.candidate, BuiltinCandidate { has_nested: false }));
- if let Some(_trivial) = trivial_builtin.next() {
- // There should only ever be a single trivial builtin candidate
- // as they would otherwise overlap.
- debug_assert_eq!(trivial_builtin.next(), None);
- return Some(BuiltinCandidate { has_nested: false });
+ // Only prefer the built-in `Sized` candidate if its nested goals are certain.
+ // Otherwise, we may encounter failure later on if inference causes this candidate
+ // to not hold, but a where clause would've applied instead.
+ if sized_candidate.evaluation.must_apply_modulo_regions() {
+ return Some(sized_candidate.candidate.clone());
+ } else {
+ return None;
+ }
}
// Before we consider where-bounds, we have to deduplicate them here and also
@@ -1950,7 +1944,7 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
// Don't use impl candidates which overlap with other candidates.
// This should pretty much only ever happen with malformed impls.
if candidates.iter().all(|c| match c.candidate {
- SizedCandidate
+ SizedCandidate { has_nested: _ }
| BuiltinCandidate { has_nested: _ }
| TransmutabilityCandidate
| AutoImplCandidate
diff --git a/tests/ui/sized/dont-incompletely-prefer-built-in.rs b/tests/ui/sized/dont-incompletely-prefer-built-in.rs
new file mode 100644
index 00000000000..f5bf0c8915e
--- /dev/null
+++ b/tests/ui/sized/dont-incompletely-prefer-built-in.rs
@@ -0,0 +1,21 @@
+//@ check-pass
+//@ revisions: current next
+//@ ignore-compare-mode-next-solver (explicit revisions)
+//@[next] compile-flags: -Znext-solver
+
+struct W<T: ?Sized>(T);
+
+fn is_sized<T: Sized>(x: *const T) {}
+
+fn dummy<T: ?Sized>() -> *const T { todo!() }
+
+fn non_param_where_bound<T: ?Sized>()
+where
+ W<T>: Sized,
+{
+ let x: *const W<_> = dummy();
+ is_sized::<W<_>>(x);
+ let _: *const W<T> = x;
+}
+
+fn main() {}
diff --git a/tests/ui/traits/lifetime-incomplete-prefer-sized-builtin-over-wc.rs b/tests/ui/traits/lifetime-incomplete-prefer-sized-builtin-over-wc.rs
new file mode 100644
index 00000000000..cc3303dccd5
--- /dev/null
+++ b/tests/ui/traits/lifetime-incomplete-prefer-sized-builtin-over-wc.rs
@@ -0,0 +1,19 @@
+struct MyType<'a, T: ?Sized>(&'a (), T);
+
+fn is_sized<T>() {}
+
+fn foo<'a, T: ?Sized>()
+where
+ (MyType<'a, T>,): Sized,
+ //~^ ERROR mismatched types
+ MyType<'static, T>: Sized,
+{
+ // Preferring the builtin `Sized` impl of tuples
+ // requires proving `MyType<'a, T>: Sized` which
+ // can only be proven by using the where-clause,
+ // adding an unnecessary `'static` constraint.
+ is_sized::<(MyType<'a, T>,)>();
+ //~^ ERROR lifetime may not live long enough
+}
+
+fn main() {}
diff --git a/tests/ui/traits/lifetime-incomplete-prefer-sized-builtin-over-wc.stderr b/tests/ui/traits/lifetime-incomplete-prefer-sized-builtin-over-wc.stderr
new file mode 100644
index 00000000000..a54574f743f
--- /dev/null
+++ b/tests/ui/traits/lifetime-incomplete-prefer-sized-builtin-over-wc.stderr
@@ -0,0 +1,27 @@
+error[E0308]: mismatched types
+ --> $DIR/lifetime-incomplete-prefer-sized-builtin-over-wc.rs:7:23
+ |
+LL | (MyType<'a, T>,): Sized,
+ | ^^^^^ lifetime mismatch
+ |
+ = note: expected trait `<MyType<'a, T> as Sized>`
+ found trait `<MyType<'static, T> as Sized>`
+note: the lifetime `'a` as defined here...
+ --> $DIR/lifetime-incomplete-prefer-sized-builtin-over-wc.rs:5:8
+ |
+LL | fn foo<'a, T: ?Sized>()
+ | ^^
+ = note: ...does not necessarily outlive the static lifetime
+
+error: lifetime may not live long enough
+ --> $DIR/lifetime-incomplete-prefer-sized-builtin-over-wc.rs:15:5
+ |
+LL | fn foo<'a, T: ?Sized>()
+ | -- lifetime `'a` defined here
+...
+LL | is_sized::<(MyType<'a, T>,)>();
+ | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ requires that `'a` must outlive `'static`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0308`.