tracking issue for `?` macro repetition

[nikomatsakis]

RFC: rust-lang/rfcs#2298

Status

• RFC accepted
• Preliminary implementation started (Implement ? macro repetition #47752)
• Preliminary implementation available
• Edition-dependent behavior proposed (see Change behavior of ? as a macro separator and Kleene op in 2018 edition #51934)
• Stabilization proposed
• Stabilized

Known bugs

None.

Unresolved questions to be answered before stabilization

• Should the ? Kleene operator accept a separator? Adding a separator is completely meaningless (since we don't accept trailing separators, and ? can accept "at most one" repetition), but allowing it is consistent with + and *. Currently, we allow a separator. We could also make it an error or lint.

[nikomatsakis]

cc @mark-i-m

[mark-i-m]

@nikomatsakis Could you update the checklist? The preliminary impl is available on nightly.

[mark-i-m]

@Centril I think you need to update the tracking issue in the RFC, too 😛

[Centril]

@mark-i-m Already done 🤣

[clarfonthey]

Is there any plan to stabilise this soon?

[mark-i-m]

I think it depends on experience if this is the right way to solve the problems involved... Frankly, I haven't really written any macros since I implemented this 😛 ...

I think there is still the one problem that was pointed out in the RFC thread: this doesn't provide a clean solution for trailing commas in * macros, since you would end up accepting foo!{,} which is a bit weird.

We could conceivably stabilize this anyway if people find it really useful. Any thoughts?