From a8b3046eca631ea7eb053fabdfe2fce97c5279f2 Mon Sep 17 00:00:00 2001
From: austaras <austaras@outlook.com>
Date: Thu, 4 Sep 2025 18:24:01 +0800
Subject: [PATCH] fix: check uncertain subst when unsized coerce

---
 crates/hir-ty/src/infer/coerce.rs   | 3 ++-
 crates/hir-ty/src/infer/unify.rs    | 8 +++++---
 crates/hir-ty/src/tests/coercion.rs | 4 ++--
 3 files changed, 9 insertions(+), 6 deletions(-)

diff --git a/crates/hir-ty/src/infer/coerce.rs b/crates/hir-ty/src/infer/coerce.rs
index df516662bfd3..824caca64155 100644
--- a/crates/hir-ty/src/infer/coerce.rs
+++ b/crates/hir-ty/src/infer/coerce.rs
@@ -9,6 +9,7 @@ use std::iter;
 
 use chalk_ir::{BoundVar, Mutability, TyKind, TyVariableKind, cast::Cast};
 use hir_def::{hir::ExprId, lang_item::LangItem};
+use rustc_next_trait_solver::solve::HasChanged;
 use rustc_type_ir::solve::Certainty;
 use stdx::always;
 use triomphe::Arc;
@@ -716,7 +717,7 @@ impl<'db> InferenceTable<'db> {
         let goal: Goal = coerce_unsized_tref.cast(Interner);
 
         self.commit_if_ok(|table| match table.solve_obligation(goal) {
-            Ok(Certainty::Yes) => Ok(()),
+            Ok((_, Certainty::Yes) | (HasChanged::Yes, Certainty::Maybe(_))) => Ok(()),
             _ => Err(TypeError),
         })?;
 
diff --git a/crates/hir-ty/src/infer/unify.rs b/crates/hir-ty/src/infer/unify.rs
index 19b83d3c212d..50191e89d30b 100644
--- a/crates/hir-ty/src/infer/unify.rs
+++ b/crates/hir-ty/src/infer/unify.rs
@@ -728,11 +728,13 @@ impl<'a> InferenceTable<'a> {
     }
 
     #[tracing::instrument(level = "debug", skip(self))]
-    pub(crate) fn solve_obligation(&mut self, goal: Goal) -> Result<Certainty, NoSolution> {
+    pub(crate) fn solve_obligation(
+        &mut self,
+        goal: Goal,
+    ) -> Result<(HasChanged, Certainty), NoSolution> {
         let goal = InEnvironment::new(&self.trait_env.env, goal);
         let goal = goal.to_nextsolver(self.interner);
-        let result = next_trait_solve_in_ctxt(&self.infer_ctxt, goal);
-        result.map(|m| m.1)
+        next_trait_solve_in_ctxt(&self.infer_ctxt, goal)
     }
 
     pub(crate) fn register_obligation(&mut self, predicate: Predicate<'a>) {
diff --git a/crates/hir-ty/src/tests/coercion.rs b/crates/hir-ty/src/tests/coercion.rs
index 7ec5231a7341..bf4098cfbd6b 100644
--- a/crates/hir-ty/src/tests/coercion.rs
+++ b/crates/hir-ty/src/tests/coercion.rs
@@ -505,8 +505,8 @@ fn test() {
                                 //^ S<i8, i16>
     let obj: &dyn Bar<_, i8, i16> = &S;
                                    //^ S<i8, i16>
-    //let obj: &dyn Foo<i8, _> = &S;
-                              // S<{unknown}, {unknown}>
+    let obj: &dyn Foo<i8, usize> = &S;
+                                  //^ S<i8, {unknown}>
 }"#,
     );
 }