matching against an associated constant gives type error

[nikomatsakis]

Ever since #55937, the following program (playground) gives an unexpected error from NLL:

#![feature(nll)]

trait Foo {
    const Blah: &'static str;
}

struct Placeholder;

impl Foo for Placeholder {
    const Blah: &'static str = "hi";
}

fn foo(x: &str) {
    match x {
        <Placeholder as Foo>::Blah => { }
        _ => { }
    }
}

First reported by @Xanewok in this comment, because this broke the RLS.

cc @davidtwco

[nikomatsakis]

I dug into this a bit -- see the [zulip log] for full details. In short, the problem is:

• In 4be7214, we added a check that the type T of the value being matched must be a subtype of the type C of the constant
• The lexical type checker has the reverse: C <: T (link)
• Really neither is quite right, but we can match the lexical type checker by including a notion of "variance" on the pattern type annotations

Probably what we really want is to require that T: PartialEq<C>.