Rollup merge of #119766 - oli-obk:split_tait_and_atpit, r=compiler-errors

Split tait and impl trait in assoc items logic

And simplify the assoc item logic where applicable.

This separation shows that it is easier to reason about impl trait in assoc items compared with TAITs. See https://rust-lang.zulipchat.com/#narrow/stream/315482-t-compiler.2Fetc.2Fopaque-types/topic/impl.20trait.20in.20associated.20type for some discussion.

The current plan is to try to stabilize impl trait in associated items before TAIT, as they do not have any issues with their defining scopes (see https://github.com/rust-lang/rust/issues/107645 for why this is not a trivial or uncontroversial topic).

compiler/rustc_middle/src/query/mod.rs

@@ -343,6 +343,15 @@ rustc_queries! {
         }
     }
 
+    query impl_trait_in_assoc_types_defined_by(
+        key: LocalDefId
+    ) -> &'tcx ty::List<LocalDefId> {
+        desc {
+            |tcx| "computing the opaque types defined by `{}`",
+            tcx.def_path_str(key.to_def_id())
+        }
+    }
+
     /// Returns the list of bounds that can be used for
     /// `SelectionCandidate::ProjectionCandidate(_)` and
     /// `ProjectionTyCandidate::TraitDef`.