Rollup merge of #108923 - spastorino:new-rpitit-9, r=compiler-errors

Make fns from other crates with RPITIT work for -Zlower-impl-trait-in-trait-to-assoc-ty

Only the last two commits are meaningful.

r? `@compiler-errors`

tests/ui/impl-trait/in-trait/foreign.rs

@@ -1,5 +1,7 @@
 // check-pass
 // aux-build: rpitit.rs
+// [next] compile-flags: -Zlower-impl-trait-in-trait-to-assoc-ty
+// revisions: current next
 
 extern crate rpitit;