semantics of `$($x:tt)?*` were changed in Rust 2015, should revert

[nikomatsakis]

There has been a lot of discussion about the proper semantics of $($x:tt)?*. Ideally, ? would serve as the "optional" operator there, but that is not the semantics in stable Rust, where it serves as a separator. #49719 did a crater run which found no crates relying on the current behavior; however, that PR was merged without an official decision from the lang team on whether to change the behavior. We should revert that PR and then make an official decision (for example, one proposal implemented in #51336 is to change the behavior in the edition, though in that case we probably want a migration lint etc).

@pietroalbini is going to prepare the revert PR and backport

( cc @mark-i-m, @sgrif, @petrochenkov @alexreg @durka )

[nikomatsakis]

I'm just opening this issue to help in tracking!

[alexreg]

@nikomatsakis Can we please implement @mark-i-m's PR that changes this behaviour (to make it more consistent) for the 2018 edition though?