`$crate` is improperly resolved if it passes through another macro

[jonas-schievink]

Current behavior:

//- /main.rs crate:main deps:lib
use lib::path;

macro_rules! vec {
  ( $e:expr ) => { $e };
}

fn f() {
    path![];
  //^^^^^^^ could not resolve macro `$crate::inner`
}

//- /lib.rs crate:lib
#[macro_export]
macro_rules! path {
    () => { vec![$crate::inner!()] };
}
#[macro_export]
macro_rules! inner {
    () => { 0 };
}

Expected behavior: The macro should be resolved correctly.

This causes incorrect diagnostics in crates/hir_ty/src/diagnostics/expr.rs, and probably all places that invoke the path! macro.

[jonas-schievink]

I think $crate needs to be resolved relative to the crate that contains the $crate tokens. I don't think we can do that in the current architecture, since we don't track hygiene of individual identifiers at the moment.

[lnicola]

Fixed in #7133.