From 91c53c97a990e9d1ab87b2d932362a53236fba49 Mon Sep 17 00:00:00 2001
From: Michael Goulet <michael@errs.io>
Date: Wed, 2 Jul 2025 00:32:54 +0000
Subject: [PATCH] Consider polarity in sizedness fast path

---
 compiler/rustc_trait_selection/src/traits/util.rs |  7 ++++---
 tests/ui/traits/negative-bounds/negative-sized.rs |  8 ++++++++
 .../traits/negative-bounds/negative-sized.stderr  | 15 +++++++++++++++
 3 files changed, 27 insertions(+), 3 deletions(-)
 create mode 100644 tests/ui/traits/negative-bounds/negative-sized.rs
 create mode 100644 tests/ui/traits/negative-bounds/negative-sized.stderr

diff --git a/compiler/rustc_trait_selection/src/traits/util.rs b/compiler/rustc_trait_selection/src/traits/util.rs
index a05bae535663..141454bfe375 100644
--- a/compiler/rustc_trait_selection/src/traits/util.rs
+++ b/compiler/rustc_trait_selection/src/traits/util.rs
@@ -368,16 +368,17 @@ pub fn sizedness_fast_path<'tcx>(tcx: TyCtxt<'tcx>, predicate: ty::Predicate<'tc
     // Proving `Sized`/`MetaSized`, very often on "obviously sized" types like
     // `&T`, accounts for about 60% percentage of the predicates we have to prove. No need to
     // canonicalize and all that for such cases.
-    if let ty::PredicateKind::Clause(ty::ClauseKind::Trait(trait_ref)) =
+    if let ty::PredicateKind::Clause(ty::ClauseKind::Trait(trait_pred)) =
         predicate.kind().skip_binder()
+        && trait_pred.polarity == ty::PredicatePolarity::Positive
     {
-        let sizedness = match tcx.as_lang_item(trait_ref.def_id()) {
+        let sizedness = match tcx.as_lang_item(trait_pred.def_id()) {
             Some(LangItem::Sized) => SizedTraitKind::Sized,
             Some(LangItem::MetaSized) => SizedTraitKind::MetaSized,
             _ => return false,
         };
 
-        if trait_ref.self_ty().has_trivial_sizedness(tcx, sizedness) {
+        if trait_pred.self_ty().has_trivial_sizedness(tcx, sizedness) {
             debug!("fast path -- trivial sizedness");
             return true;
         }
diff --git a/tests/ui/traits/negative-bounds/negative-sized.rs b/tests/ui/traits/negative-bounds/negative-sized.rs
new file mode 100644
index 000000000000..18369c784275
--- /dev/null
+++ b/tests/ui/traits/negative-bounds/negative-sized.rs
@@ -0,0 +1,8 @@
+#![feature(negative_bounds)]
+
+fn foo<T: !Sized>() {}
+
+fn main() {
+    foo::<()>();
+    //~^ ERROR the trait bound `(): !Sized` is not satisfied
+}
diff --git a/tests/ui/traits/negative-bounds/negative-sized.stderr b/tests/ui/traits/negative-bounds/negative-sized.stderr
new file mode 100644
index 000000000000..143933803b81
--- /dev/null
+++ b/tests/ui/traits/negative-bounds/negative-sized.stderr
@@ -0,0 +1,15 @@
+error[E0277]: the trait bound `(): !Sized` is not satisfied
+  --> $DIR/negative-sized.rs:6:11
+   |
+LL |     foo::<()>();
+   |           ^^ the trait bound `(): !Sized` is not satisfied
+   |
+note: required by a bound in `foo`
+  --> $DIR/negative-sized.rs:3:11
+   |
+LL | fn foo<T: !Sized>() {}
+   |           ^^^^^^ required by this bound in `foo`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.