Allow : after pat_param

[coolreader18]

As on the tin. Perhaps this requires a more formal decision process, but in my view this is something of a bug. Originally, pat_param was just the bikeshodden name of pat2015, c.f. #83386 (comment):

• pat_no_or would be used for closure arguments; can we have something that talks about that (e.g., pat_arg, pat_param -- pattern for a closure argument)
• pat_param is preferred because it is a parameter, not the argument passed to a call

But if it truly is a matcher for "a pattern you can use for a parameter", there shouldn't be a problem allowing it to be followed by a colon, given that that's kind of the definitional constraint for that grammar. And if, in the distant future, the language decides to separate the name and type of a parameter with something other than a colon, well, that'll need an edition change anyway.

In my opinion, this would resolve #87724: I'd imagine that most-if-not-all use cases for a pat followed by a colon are for matching function signatures or let statements, which don't let you use | patterns anyway. But, we get to have our cake and eat it too by still reserving : for future extensions to match expression (pat) patterns. If it ever is desired to make pat match the whole $pat : $ty syntax, as mentioned in #87724 (comment), that's something that can be done over an edition boundary and/or just for pat matchers.

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[nnethercote]

This definitely would need approval from the lang team.

cc @rust-lang/lang @rust-lang/lang-advisors
cc @petrochenkov

[traviscross]

Makes sense to me. Thanks @coolreader18, @nnethercote.

@rfcbot fcp merge lang

[rust-rfcbot]

@traviscross has proposed to merge this. The next step is review by the rest of the tagged team members:

• @joshtriplett
• @nikomatsakis
• @scottmcm
• @tmandry
• @traviscross

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

cc @rust-lang/lang-advisors: FCP proposed for lang, please feel free to register concerns.
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[tmandry]

It's surprising we don't already allow this. Thanks for the PR.

@rfcbot reviewed