C-variadic function definitions are rejected, blocking varargs_without_pattern lint

[lucasly-ba]

The rustc varargs_without_pattern lint warns when ... is used as a C-variadic argument in a function definition without a pattern name.

gccrs hard-errors on any c_variadic function definition, even the well-formed unsafe extern "C" form:

#![feature(c_variadic)]
pub unsafe extern "C" fn f(args: ...) {}
// gccrs: only foreign or `unsafe extern "C"` functions may be C-variadic
pub unsafe extern "C" fn g(...) {}
// same error

Since the construct cannot be compiled at all, the lint is moot until gccrs supports C-variadic function definitions.

[CohenArthur]

We do allow variadic functions but yes not in all the cases we should. The second function is also covered by the c_variadic feature.

We do cover defining an external C function so that it is usable in Rust:

extern "C" {
    fn printf(fmt: *const i8, args: ...) -> i32;
}

but yes we are missing defining a Rust function with the C ABI/calling convention like in your example