Rollup merge of rust-lang#86666 - ptrojahn:compare_kinds, r=petrochenkov

Fix misleading "impl Trait" error

The kinds can't be compared directly, as types with references are treated as different because the lifetimes aren't bound in ty, but are in expected.
Closes rust-lang#84160

compiler/rustc_typeck/src/check/coercion.rs

@@ -1481,6 +1481,7 @@ impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
                     expected,
                     found,
                     can_suggest,
+                    fcx.tcx.hir().get_parent_item(id),
                 );
             }
             if !pointing_at_return_type {

compiler/rustc_typeck/src/check/fn_ctxt/suggestions.rs

@@ -52,9 +52,15 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         }
         let mut pointing_at_return_type = false;
         if let Some((fn_decl, can_suggest)) = self.get_fn_decl(blk_id) {
-            pointing_at_return_type =
-                self.suggest_missing_return_type(err, &fn_decl, expected, found, can_suggest);
             let fn_id = self.tcx.hir().get_return_block(blk_id).unwrap();
+            pointing_at_return_type = self.suggest_missing_return_type(
+                err,
+                &fn_decl,
+                expected,
+                found,
+                can_suggest,
+                fn_id,
+            );
             self.suggest_missing_break_or_return_expr(
                 err, expr, &fn_decl, expected, found, blk_id, fn_id,
             );
@@ -433,6 +439,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         expected: Ty<'tcx>,
         found: Ty<'tcx>,
         can_suggest: bool,
+        fn_id: hir::HirId,
     ) -> bool {
         // Only suggest changing the return type for methods that
         // haven't set a return type at all (and aren't `fn main()` or an impl).
@@ -465,7 +472,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 let ty = <dyn AstConv<'_>>::ast_ty_to_ty(self, ty);
                 debug!("suggest_missing_return_type: return type {:?}", ty);
                 debug!("suggest_missing_return_type: expected type {:?}", ty);
-                if ty.kind() == expected.kind() {
+                let bound_vars = self.tcx.late_bound_vars(fn_id);
+                let ty = self.tcx.erase_late_bound_regions(Binder::bind_with_vars(ty, bound_vars));
+                let ty = self.normalize_associated_types_in(sp, ty);
+                if self.can_coerce(expected, ty) {
                     err.span_label(sp, format!("expected `{}` because of return type", expected));
                     return true;
                 }

src/test/ui/extern/extern-types-distinct-types.stderr

@@ -6,6 +6,8 @@ LL |     type A;
 LL |     type B;
    |     ------- the expected foreign type
 ...
+LL | fn foo(r: &A) -> &B {
+   |                  -- expected `&B` because of return type
 LL |     r
    |     ^ expected extern type `B`, found extern type `A`
    |

src/test/ui/retslot-cast.stderr

@@ -1,6 +1,9 @@
 error[E0308]: mismatched types
   --> $DIR/retslot-cast.rs:13:5
    |
+LL |             -> Option<&Iterator<Item=()>> {
+   |                -------------------------- expected `Option<&dyn Iterator<Item = ()>>` because of return type
+...
 LL |     inner(x)
    |     ^^^^^^^^ expected trait `Iterator<Item = ()>`, found trait `Iterator<Item = ()> + Send`
    |

src/test/ui/typeck/issue-84160.rs

@@ -0,0 +1,9 @@
+fn mismatched_types_with_reference(x: &u32) -> &u32 {
+    if false {
+        return x;
+    }
+    return "test";
+    //~^ERROR mismatched types
+}
+
+fn main() {}

src/test/ui/typeck/issue-84160.stderr

@@ -0,0 +1,15 @@
+error[E0308]: mismatched types
+  --> $DIR/issue-84160.rs:5:12
+   |
+LL | fn mismatched_types_with_reference(x: &u32) -> &u32 {
+   |                                                ---- expected `&u32` because of return type
+...
+LL |     return "test";
+   |            ^^^^^^ expected `u32`, found `str`
+   |
+   = note: expected reference `&u32`
+              found reference `&'static str`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.