Macros: limitation in the expression parser for <$:path>::<ident>

[behnam]

Background: https://users.rust-lang.org/t/macros-using-path-tokens-with-format-args/15480

When passing in a path token to a macro, then trying to suffix the metavariable with ::<ident> (or more), the parser cannot recognize the whole thing as an :expr, which causes failures on calls to macros like format_args!.

Repro:

macro_rules! something {
    ( $path:path ) => (
        //println!("Say: {}", $path::say);
        format_args!("Say: {}", $path::say);
    );
}

mod foo {
    const say: &str = "Hello";
}

mod bar {
    const say: &str = "World";

    mod baz {
        const say: &str = "Universe";
    }
}

fn main() {
    something!(foo);
    something!(bar);
    something!(bar::baz);
}

It fails with three instances of this error (with RUSTFLAGS='-Z external-macro-backtrace'):

error: expected token: `,`
  --> src/main.rs:4:9
   |
1  | / macro_rules! talk {
2  | |     ( $mod:path ) => (
3  | |         // print!("Say: {}", $mod::say);
4  | |         format_args!("Say: {}", $mod::say);
   | |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
5  | |     );
6  | | }
   | |_- in this expansion of `talk!`
...
21 |       talk!(foo);
   |       ----------- in this macro invocation

A workaround is to use:

{ use $path as base; base::say }

but would be great if we could just use:

$path::say

I couldn't find an existing report. I'm guessing it falls under RFE.

[durka]

cc @jseyfried

This is known issue, but I don't know how hard it would be to make it "just work".

[petrochenkov]

We can even do $path1::$path2 in general, this is very similar to what nested groups in imports already have to deal with.
I'll assign to myself, because this is already in my queue.