Rust-analyzer: When completing enum patterns, complete unqualifed enum names as well

[matklad]

Given the following code:

enum Animal {
    Walrus,
    MantisShrimp
}

fn feed(animal: &mut Animal) {
    match animal {
        Wa /*cursor*/
    }
}

we should suggest Animial::Walrus completion. The relevant completion handler is here. ctx.sema.type_of_pat(..) method can be used to get the type of the pattern.

[matklad]

See also https://internals.rust-lang.org/t/bring-enum-variants-in-scope-for-patterns/12104

[flodiebold]

We should do the same in expressions as well, actually! And to extend these type-based completions even further, we could e.g. suggest X::new() if the expected type is X (or any other static method on X returning X).

[djc]

In this context, would also be cool if the name resolution issue for importing the variants actually got fixed (see #3988).