Add support for Meta-Variable Expressions

[c410-f3r]

For more information, see rust-lang/rust#83527.

#![feature(macro_metavar_expr)]

macro_rules! mac {
    ( $($foo:ident)* ) => {
        0 + ${count(foo)}
    };
}

fn main() {
    let n = mac!(a b c);
    assert_eq!(n, 3);
}

[c410-f3r]

I guess at least dollar-dollar will be easy to implement

macro_rules! foo {
    () => {
        macro_rules! bar {
            ( $$( $$any:tt )* ) => { $$( $$any )* };
        }
    };
}

fn main() { foo!(); }

[c410-f3r]

Closing in favor of #11952

[Veykril]

Ah, I completely forgot about this issue already existing sorry