Rollup merge of #132047 - compiler-errors:rbv-rtn-cleanup, r=cjgillot

Robustify and genericize return-type-notation resolution in `resolve_bound_vars`

#129629 implemented return-type-notation (RTN) in its path form, like `where T::method(..): Bound`. As part of lowering, we must record the late-bound vars for the where clause introduced by the method (namely, its early- and late-bound lifetime arguments, since `where T::method(..)` turns into a higher-ranked where clause over all of the lifetimes according to [RFC 3654](https://rust-lang.github.io/rfcs/3654-return-type-notation.html#converting-to-higher-ranked-trait-bounds)).

However, this logic was only looking at the where clauses of the parent item that the `T::method(..)` bound was written on, and not any parent items. This PR generalizes that logic to look at the parent item (i.e. the outer impl or trait) instead and fixes a (debug only) assertion as an effect.

This logic is also more general and likely easier to adapt to more interesting (though likely very far off) cases like non-lifetime binder `for<T: Trait> T::method(..): Send` bounds.

Tracking:

- https://github.com/rust-lang/rust/issues/109417

1 files changed, 31 insertions, 0 deletions

diff --git a/tests/ui/associated-type-bounds/all-generics-lookup.rs b/tests/ui/associated-type-bounds/all-generics-lookup.rs
new file mode 100644
index 00000000000..c5940c14f44
--- /dev/null
+++ b/tests/ui/associated-type-bounds/all-generics-lookup.rs
@@ -0,0 +1,31 @@
+//@ check-pass
+
+#![feature(return_type_notation)]
+
+trait Trait {
+    fn method(&self) -> impl Sized;
+}
+
+impl Trait for () {
+    fn method(&self) -> impl Sized {}
+}
+
+struct Struct<T>(T);
+
+// This test used to fail a debug assertion since we weren't resolving the item
+// for `T::method(..)` correctly, leading to two bound vars being given the
+// index 0. The solution is to look at both generics of `test` and its parent impl.
+
+impl<T> Struct<T>
+where
+    T: Trait,
+{
+    fn test()
+    where
+        T::method(..): Send
+    {}
+}
+
+fn main() {
+    Struct::<()>::test();
+}