Macro with empty expansion in expression position causes an "unexpected <eof>" error

[huonw]

macro_rules! foo ( () => () );
fn main() { foo!() }

expand.rs:1:0: 1:0 error: unexpected token: `<eof>`
expand.rs:1 macro_rules! foo ( () => () );
            ^

(Moving the foo!() outside main works.)

[DanielKeep]

Still happening as of latest nightly:

rustc 0.12.0-pre-nightly (63eaba24d 2014-09-15 21:56:00 +0000)
binary: rustc
commit-hash: 63eaba24d6a7f93061d63337ea1f7083ca892787
commit-date: 2014-09-15 21:56:00 +0000
host: i686-w64-mingw32
release: 0.12.0-pre-nightly

[charlesetc]

This is still happening

% rustc --version                                                                                                                           
rustc 1.5.0-dev (0be1bcd06 2015-10-24)

[brson]

Still happening.

[brson]

@jseyfried just pinging you to make you aware and spark your interest.

[jseyfried]

The issue here is that foo!() is an expression position, so we expect it to be exactly one expression.
If it were foo!(); or foo! { }, then it would be in a statement(s) position and would compile.

This distinction isn't ideal, but I've already cleaned it up about as much as possible given back-compat constraints (see #34660), so I'm not sure there's much we can do about it now.

cc @nrc