`$a:tt $($b:tt)*` is forbidden in macros

[nikomatsakis]

In an example like http://is.gd/TKfdKf:

macro_rules! foo {
  ( $a:tt $($b:tt)* ) => { }
}

fn main() { }

we report the following error:

<anon>:2:11: 2:19 error: `$a:tt` is followed by a sequence repetition, which is not allowed for `tt` fragments
<anon>:2   ( $a:tt $($b:tt)* ) => { }
                   ^~~~~~~~
error: aborting due to previous error
playpen: application terminated with error code 101

This is part of the macro future-proofing code but it is an unnecessary limitation. For fragments that represent at most one token tree (tt, block, ident, possibly others) there is no concern at all about being following by sequence repetition. This turns out to be a really big limitation in practice, since making macros that work over a generic sequence of token trees effectively allows for arbitrary parsing.

cc @pnkfelix

[nikomatsakis]

triage: P-medium

[nikomatsakis]

I'd be happy to mentor this, if I don't fix it first.

[nikomatsakis]

triage: P-medium

[nikomatsakis]

cc @cmr

[Stebalien]

I-ICE?