Auto merge of #146329 - lcnr:opaque-type-infer-alias-candidates, r=BoxyUwU

consider item bounds for non-yet-defined opaque types

Based on rust-lang/rust#140405.

fixes https://github.com/rust-lang/trait-system-refactor-initiative/issues/182
fixes https://github.com/rust-lang/trait-system-refactor-initiative/issues/196
fixes https://github.com/rust-lang/trait-system-refactor-initiative/issues/205

there's some jank here, see https://github.com/rust-lang/trait-system-refactor-initiative/issues/229

## Design

If the self type is an inference variable which has been sub-unified with am opaque type, we need to incompletely guide inference to avoid breakage.

In this case, we
- look at the item bounds of all sub-unified opaque types, and
- blanket impls which do not constrain the self type

Even if there are applicable candidates, we always force their certainty to be `Maybe`, so they will always have to be reproven once we've constrained the inference variable.

This is a bit iffy, see the added tests.

r? `@BoxyUwU`

1 files changed, 5 insertions, 1 deletions

diff --git a/compiler/rustc_middle/src/ty/context.rs b/compiler/rustc_middle/src/ty/context.rs
index 218ac2cfbc1..8ea767dccd3 100644
--- a/compiler/rustc_middle/src/ty/context.rs
+++ b/compiler/rustc_middle/src/ty/context.rs
@@ -651,7 +651,11 @@ impl<'tcx> Interner for TyCtxt<'tcx> {
             | ty::Bound(_, _) => bug!("unexpected self type: {self_ty}"),
         }
 
-        let trait_impls = tcx.trait_impls_of(trait_def_id);
+        #[allow(rustc::usage_of_type_ir_traits)]
+        self.for_each_blanket_impl(trait_def_id, f)
+    }
+    fn for_each_blanket_impl(self, trait_def_id: DefId, mut f: impl FnMut(DefId)) {
+        let trait_impls = self.trait_impls_of(trait_def_id);
         for &impl_def_id in trait_impls.blanket_impls() {
             f(impl_def_id);
         }