foreign function invalid error message

[GuillaumeGomez]

This code works:

extern "C" {
    fn foo(x: u8, ...);
}

While this one doesn't:

extern "C" fn foo(x: u8, ...); // error: only foreign functions are allowed to be variadic

I don't think this is the expected behavior.

[huonw]

What behaviour do you expect? Depending on exactly what you're hoping happens, I don't think this is a bug, since extern "C" fn foo(..) is defining a Rust function that uses the C ABI (i.e. is callable from C), not importing a C function called foo. That is, it only differs to fn foo(..) in low-level code-generation details, it still has all the normal Rust features/restrictions. This includes not working with variadic arguments.

Also, writing extern "C" fn foo(..); without a body isn't valid, e.g. if the ... is removed to allow compilation to progress, you instead get:

extern "C" fn foo(x: u8); // error: expected one of `->`, `where`, or `{`, found `;`

I suppose the error message could possibly be clearer and/or have a help or note.

[GuillaumeGomez]

Since the original message came from a lack of knowledge, I changed the title to better reflect the "real" issue here.