c_variadic: Make fn f(...) {} error like fn f(u32) {} outside of extern blocks

[beetrees]

This PR makes unnamed ... parameters (such as the one in unsafe extern "C" fn f(...) {}) a parse error to be consistent with unsafe extern "C" fn f(u32) {}: this is a source of confusion for programmers coming from C, where the ... parameter is never named and instead calling va_start is required; disallowing unnamed ... parameters also improves the overall consistency of the Rust language by matching the treatment of other unnamed parameters. Unnamed ... parameters in extern blocks (such as unsafe extern "C" { fn f(...); }) continue to compile after this PR, as they are already stable and heavily used (and don't cause the mentioned confusion as they are just being used in function declarations).

As all the syntax gating for c_variadic has been done post-expansion, this is technically a breaking change. In particular, code like this has compiled on stable since Rust 1.35.0:

#[cfg(any())] // Equivalent to the more recent #[cfg(false)]
unsafe extern "C" fn bar(_: u32, ...) {}

Since this is more or less a stability hole and is unlikely to be used in practice, I think it would be ok to break this, but this will definitely require both a crater check run and a lang FCP.

Tracking issue: #44930
cc @folkertdev @workingjubilee
r? @joshtriplett

[rustbot]

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[rustbot]

This PR changes a file inside tests/crashes. If a crash was fixed, please move into the corresponding ui subdir and add 'Fixes #' to the PR description to autoclose the issue upon merge.

[compiler-errors]

@bors2 try

for the crater

[rust-bors]

Unknown command "tryfor".