fix: check uncertain subst when unsized coerce

Author: Austaras

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),
         })?;
 

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>) {

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<u8, usize> = &S;
+                                  //^ S<u8, {unknown}>
 }"#,
     );
 }