`implement missing members` doesn't apply feature flags

rust-analyzer version: 0add6e95e 2021-12-20 stable

rustc version: rustc 1.56.1 (59eed8a2a 2021-11-01)

rust-analyzer doesn't respect cfg(test) (or other feature flags) when applying the implement missing members assist. In practice this means that if I have this code:

trait Foo {
    #[cfg(test)]
    fn foo_method();
}

struct Bar;

impl Foo for Bar {}

Then applying "implement missing members" on that impl block gives:

trait Foo {
    #[cfg(test)]
    fn foo_method();
}

struct Bar;

impl Foo for Bar {
    fn foo_method() {
        todo!()
    }
}

Whereas I would like to get:

trait Foo {
    #[cfg(test)]
    fn foo_method();
}

struct Bar;

impl Foo for Bar {
    #[cfg(test)]
    fn foo_method() {
        todo!()
    }
}

I'm guessing this is a non-trivial problem, but it would be super nice if rust-analyzer could pick up on some of these cases (even if it's just cfg(test)) and handle them correctly.

Start at the implement missing members assist and trace how it copies trait members into the impl. Check how cfg(test) and feature flags are handled there; done when generated methods preserve applicable cfg attributes, with tests covering the shown case.