Type inference not working with impl Fn return value

[lnicola]

fn match_literal() -> impl Fn(&str) -> () {
    |input| {}
}

Rider can tell that input: &str, but we can't.

[flodiebold]

Yeah, we don't really implement the handling of return-position impl trait inside the function. We'd need to replace the impl Trait by a new type variable and register an obligation that it should implement the trait. It's this FIXME:
https://github.com/rust-analyzer/rust-analyzer/blob/3191a93185b34c6deebca2aad0584d2840ad6d43/crates/hir_ty/src/infer.rs#L583