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C-variadic function definitions stabilization allows ... without pattern with immediate FCW #160109
Description
In #155697, we stabilized C-variadic function definitions (and C-variadic function declarations in traits).
While reviewing and revising the Reference PR for this, rust-lang/reference#2177, I noticed that we're stabilizing allowance of a behavior that we document as disallowed (with an immediate FCW against it). Prior to the stabilization, we gave a hard error for both of:
unsafe extern "C" fn f(...) {} // ERROR.
trait Tr {
unsafe extern "C" fn f(...); // ERROR.
}After the stabilization, these are both accepted:
#![allow(varargs_without_pattern)]
unsafe extern "C" fn f(...) {} // OK.#![allow(varargs_without_pattern)]
trait Tr {
unsafe extern "C" fn f(...); // OK.
}That is, we're accepting ... in parameter position without a preceding pattern (with a deny-by-default FCW).
This is a bit unusual for us. Normally we don't stabilize new behavior only to issue an immediately FCW. Presumably we want to ask for this to continue to give a hard error (semantically — this was already accepted in parsing).
cc @folkertdev @rust-lang/lang-docs @rust-lang/lang
Activity
folkertdev commentedon Jul 29, 2026
For the record I agree that we should just reject bare
...in trait method declarations and definitions (it is still allowed in foreign function declarations), probably just with an error inrustc_ast_passes.We could also decide to just deny this pre-expansion now. Relevant links
c_variadic: Add future-incompatibility warning for...arguments without a pattern outside ofexternblocks #143619varargs_without_patternlint in deps #154599Quoting from that last one
So, the impact is likely extremely low, but it is a breaking change.
Rollup merge of rust-lang#160165 - folkertdev:reject-dotdotdot-post-e…
Rollup merge of rust-lang#160165 - folkertdev:reject-dotdotdot-post-e…
Rollup merge of rust-lang#160165 - folkertdev:reject-dotdotdot-post-e…
Rollup merge of rust-lang#160165 - folkertdev:reject-dotdotdot-post-e…
Rollup merge of rust-lang#160165 - folkertdev:reject-dotdotdot-post-e…
Rollup merge of rust-lang#160165 - folkertdev:reject-dotdotdot-post-e…
Unrolled build for #160165
Rollup merge of #160165 - folkertdev:reject-dotdotdot-post-expansion,…