Rollup merge of #146206 - lcnr:fix-non-defining-uses, r=BoxyUwU

identity uses are ok, even if there are no defining uses

fix rust-lang/rust#146191

I've tried moving the "is this an identity use" check to `fn clone_and_resolve_opaque_types` and this would allow the following code to compile as it now ignores `Opaque<'!a> = Opaque<'!a>` while they previously resulted in errors https://github.com/rust-lang/rust/blob/71289c378d0a406a4f537fe4001282d19362931f/tests/ui/type-alias-impl-trait/hkl_forbidden.rs#L42-L46

The closure signature gets inferred to `for<'a> fn(&'a ()) -> Inner<'a>`. The closure then has a defining use `Inner<'a_latbound> = &'a_latebound ()` while the parent function has a non-defining `Inner<'!a> = Inner<'!a>`. By eagerly discarding identity uses we don't error on the non-defining use in the parent.

r? `@BoxyUwU`

1 files changed, 10 insertions, 1 deletions

diff --git a/compiler/rustc_borrowck/src/region_infer/opaque_types/mod.rs b/compiler/rustc_borrowck/src/region_infer/opaque_types/mod.rs
index 72615cb33b3..95c5366a6c8 100644
--- a/compiler/rustc_borrowck/src/region_infer/opaque_types/mod.rs
+++ b/compiler/rustc_borrowck/src/region_infer/opaque_types/mod.rs
@@ -503,7 +503,16 @@ pub(crate) fn apply_computed_concrete_opaque_types<'tcx>(
     let mut errors = Vec::new();
     for &(key, hidden_type) in opaque_types {
         let Some(expected) = get_concrete_opaque_type(concrete_opaque_types, key.def_id) else {
-            assert!(tcx.use_typing_mode_borrowck(), "non-defining use in defining scope");
+            if !tcx.use_typing_mode_borrowck() {
+                if let ty::Alias(ty::Opaque, alias_ty) = hidden_type.ty.kind()
+                    && alias_ty.def_id == key.def_id.to_def_id()
+                    && alias_ty.args == key.args
+                {
+                    continue;
+                } else {
+                    unreachable!("non-defining use in defining scope");
+                }
+            }
             errors.push(DeferredOpaqueTypeError::NonDefiningUseInDefiningScope {
                 span: hidden_type.span,
                 opaque_type_key: key,