Support C23's Variadics Without a Named Parameter

[veera-sivarajan]

Fixes #123773

This PR removes the static check that disallowed extern functions
with ellipsis (varargs) as the only parameter since this is now
valid in C23.

This will not break any existing code as mentioned in the proposal
document: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2975.pdf.

Also, adds a doc comment for check_decl_cvariadic_pos() and
fixes the name of the function (varadic -> variadic).

[rustbot]

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r=me, needs T-lang approval though first

Concept seems reasonable to me.

@rfcbot merge

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

• @joshtriplett
• @nikomatsakis
• @pnkfelix
• @scottmcm
• @tmandry

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!

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Seems like a reasonable extension.

@rfcbot reviewed

@rfcbot reviewed

🔔 This is now entering its final comment period, as per the review above. 🔔

Yeah, since this is extern "C" { … } if C says it exists, then might as well allow it.
@rfcbot reviewed

@rustbot labels -I-lang-nominated

We discussed this in the lang triage call today. People felt good about this, and it's now in FCP.

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

This will be merged soon.

@bors r+

📌 Commit f005b45 has been approved by compiler-errors

It is now in the queue for this repository.

⌛ Testing commit f005b45 with merge df5b3f0...

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 mingw-w64-x86_64-cmake-3.28.1-1-any downloading...
 mingw-w64-x86_64-openssl-3.2.0-1-any downloading...
 mingw-w64-x86_64-gettext-0.22.4-3-any downloading...
 mingw-w64-x86_64-curl-8.5.0-1-any downloading...
error: failed retrieving file 'mingw-w64-x86_64-openssl-3.2.0-1-any.pkg.tar.zst.sig' from mirror.umd.edu : Operation too slow. Less than 1 bytes/sec transferred the last 10 seconds
error: failed retrieving file 'mingw-w64-x86_64-curl-8.5.0-1-any.pkg.tar.zst' from mirror.msys2.org : Operation too slow. Less than 1 bytes/sec transferred the last 10 seconds
warning: too many errors from mirror.msys2.org, skipping for the remainder of this transaction
warning: failed to retrieve some files
error: failed to commit transaction (unexpected error)

@bors retry