feat(ide-completion): explictly show `async` keyword on `impl trait` methods OLD: <img width="676" alt="image" src="https://github.com/user-attachments/assets/f6fa626f-6b6d-4c22-af27-b0755e7a6bf8"> Now: <img width="684" alt="image" src="https://github.com/user-attachments/assets/efbaac0e-c805-4dd2-859d-3e44b2886dbb"> --- This is an preparation for https://github.com/rust-lang/rust-analyzer/issues/17719. ```rust use std::future::Future; trait DesugaredAsyncTrait { fn foo(&self) -> impl Future<Output = usize> + Send; fn bar(&self) -> impl Future<Output = usize> + Send; } struct Foo; impl DesugaredAsyncTrait for Foo { fn foo(&self) -> impl Future<Output = usize> + Send { async { 1 } } // async fn bar(&self) -> usize { 1 } } fn main() { let fut = Foo.bar(); fn _assert_send<T: Send>(_: T) {} _assert_send(fut); } ``` If we don't distinguish `async` or not. It would be confusing to generate sugared version `async fn foo ....` and original form `fn foo` for `async fn in trait` that is defined in desugar form.