Improve RTN support (resolution + analysis; jump to definition, etc)

rust-analyzer version: rust-analyzer version: 0.3.2330-standalone (02862f5d52 2025-03-03)

rustc version: rustc 1.87.0-nightly (f9e0239a7 2025-03-04)

editor or extension: VSCode

relevant settings: N/A

repository link (if public, optional): N/A

code snippet to reproduce:

#![feature(return_type_notation)]

trait Foo {
    async fn method();
}

async fn foo<T: Foo>() where T::method(..): Send {}

Hiya. r-a still lacks RTN support in resolution and presumably elsewhere in the type system, so I'm opening a tracking issue for that :) I'd be happy to mentor any technical work here to improve RTN (especially to help understand how RTN works in rustc), since I'm looking to stabilize RTN soon.

Start with the Rust reproducer using return type notation and compare rust-analyzer's current resolution behavior with the reported gap. Trace the resolution and type-system entry points involved in RTN, then identify the scope needed for analysis and jump-to-definition support. Done means RTN is supported in the relevant IDE features and covered by tests.