From 765476ff7110fc40875d5c9a9207b0cecbf48793 Mon Sep 17 00:00:00 2001
From: xmakro <xmakro@users.noreply.github.com>
Date: Tue, 8 Sep 2026 12:43:29 -0700
Subject: [PATCH] Check trivial MIR sizedness before interning an obligation

[skip ci]
---
 compiler/rustc_borrowck/src/type_check/mod.rs | 27 ++++++++++++-------
 .../rustc_infer/src/infer/outlives/mod.rs     | 10 +++++++
 2 files changed, 27 insertions(+), 10 deletions(-)

diff --git a/compiler/rustc_borrowck/src/type_check/mod.rs b/compiler/rustc_borrowck/src/type_check/mod.rs
index 65cced536603a..ff4636098abb1 100644
--- a/compiler/rustc_borrowck/src/type_check/mod.rs
+++ b/compiler/rustc_borrowck/src/type_check/mod.rs
@@ -690,16 +690,23 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
                 }
 
                 if !self.tcx().features().unsized_fn_params() {
-                    let trait_ref = ty::TraitRef::new(
-                        tcx,
-                        tcx.require_lang_item(LangItem::Sized, self.last_span),
-                        [place_ty],
-                    );
-                    self.prove_trait_ref(
-                        trait_ref,
-                        location.to_locations(),
-                        ConstraintCategory::SizedBound,
-                    );
+                    let sized = tcx.require_lang_item(LangItem::Sized, self.last_span);
+                    // Apply the type-op's trivial Sized path before interning its
+                    // trait reference and predicate. Preserve the full operation
+                    // when it has region state to collect or fast paths are disabled.
+                    let trivial = !self.infcx.next_trait_solver()
+                        && !self.infcx.disable_trait_solver_fast_paths()
+                        && !self.infcx.in_snapshot()
+                        && !self.infcx.has_pending_region_state()
+                        && place_ty.has_trivial_sizedness(tcx, ty::SizedTraitKind::Sized);
+                    if !trivial {
+                        let trait_ref = ty::TraitRef::new(tcx, sized, [place_ty]);
+                        self.prove_trait_ref(
+                            trait_ref,
+                            location.to_locations(),
+                            ConstraintCategory::SizedBound,
+                        );
+                    }
                 }
             }
             StatementKind::AscribeUserType((place, projection), variance) => {
diff --git a/compiler/rustc_infer/src/infer/outlives/mod.rs b/compiler/rustc_infer/src/infer/outlives/mod.rs
index 79c1a4fb7662f..5b80da4d7ba14 100644
--- a/compiler/rustc_infer/src/infer/outlives/mod.rs
+++ b/compiler/rustc_infer/src/infer/outlives/mod.rs
@@ -32,6 +32,16 @@ pub fn explicit_outlives_bounds<'tcx>(
 }
 
 impl<'tcx> InferCtxt<'tcx> {
+    pub fn has_pending_region_state(&self) -> bool {
+        let inner = self.inner.borrow();
+        !inner.region_obligations.is_empty()
+            || !inner.region_assumptions.is_empty()
+            || inner
+                .region_constraint_storage
+                .as_ref()
+                .is_none_or(|storage| !storage.data.is_empty())
+    }
+
     /// Process the region constraints and return any errors that
     /// result. After this, no more unification operations should be
     /// done -- or the compiler will panic -- but it is legal to use