Don't structurally resolve in may_coerce

Author: compiler-errors

compiler/rustc_hir_typeck/src/coercion.rs

@@ -1091,16 +1091,12 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     ///
     /// This should only be used for diagnostics.
     pub(crate) fn may_coerce(&self, expr_ty: Ty<'tcx>, target_ty: Ty<'tcx>) -> bool {
-        // FIXME(-Znext-solver): We need to structurally resolve both types here.
-        let source = self.resolve_vars_with_obligations(expr_ty);
-        debug!("coercion::can_with_predicates({:?} -> {:?})", source, target_ty);
-
         let cause = self.cause(DUMMY_SP, ObligationCauseCode::ExprAssignable);
         // We don't ever need two-phase here since we throw out the result of the coercion.
         // We also just always set `coerce_never` to true, since this is a heuristic.
         let coerce = Coerce::new(self, cause, AllowTwoPhase::No, true);
         self.probe(|_| {
-            let Ok(ok) = coerce.coerce(source, target_ty) else {
+            let Ok(ok) = coerce.coerce(expr_ty, target_ty) else {
                 return false;
             };
             let ocx = ObligationCtxt::new(self);

tests/ui/type-alias-impl-trait/lazy_subtyping_of_opaques.stderr

@@ -1,3 +1,15 @@
+error[E0308]: mismatched types
+  --> $DIR/lazy_subtyping_of_opaques.rs:11:5
+   |
+LL | fn reify_as_tait() -> Thunk<Tait> {
+   |                       ----------- expected `Thunk<_>` because of return type
+LL |
+LL |     Thunk::new(|cont| cont)
+   |     ^^^^^^^^^^^^^^^^^^^^^^^ expected `Thunk<_>`, found `()`
+   |
+   = note: expected struct `Thunk<_>`
+           found unit type `()`
+
 error[E0277]: expected a `FnOnce()` closure, found `()`
   --> $DIR/lazy_subtyping_of_opaques.rs:11:23
    |
@@ -12,19 +24,13 @@ error[E0277]: expected a `FnOnce()` closure, found `()`
    |
 LL | fn reify_as_tait() -> Thunk<Tait> {
    |                       ^^^^^^^^^^^ expected an `FnOnce()` closure, found `()`
+LL |
+LL |     Thunk::new(|cont| cont)
+   |     ----------------------- return type was inferred to be `{type error}` here
    |
    = help: the trait `FnOnce()` is not implemented for `()`
    = note: wrap the `()` in a closure with no arguments: `|| { /* code */ }`
 
-error[E0308]: mismatched types
-  --> $DIR/lazy_subtyping_of_opaques.rs:11:5
-   |
-LL |     Thunk::new(|cont| cont)
-   |     ^^^^^^^^^^^^^^^^^^^^^^^ expected `Thunk<_>`, found `()`
-   |
-   = note: expected struct `Thunk<_>`
-           found unit type `()`
-
 error: aborting due to 3 previous errors
 
 Some errors have detailed explanations: E0277, E0308.