Tracking issue for `macros_in_extern` feature

[abonander]

This is a tracking issue for the macros_in_extern. This feature flag enables the usage of macro invocations in extern {} blocks:

macro_rules! emit_foreign_item(
    () => (fn read(fd: int, buf: *mut u8, size: usize) -> int;)
);

extern {
    emit_foreign_item!();
}

Declarative maco, proc-macro and proc-macro-attribute invocations are allowed as well. Macro defintions, however, are not.

This feature is unstable because it was recently implemented in #49350 without an RFC. It is considered a discrepancy in macro support and not a new feature. It should be completely backwards-compatible.

[abonander]

cc @petrochenkov, @nrc, @jseyfried

[abonander]

@petrochenkov I should get started on an Unstable Book entry for this feature, shouldn't I

[dtolnay]

This leak / infinite alloc on invalid syntax needs to be fixed before stabilization: #54441.