extern fn type declarations and extern fn bindings should both default to the same ABI.

[brson]

This has confused me several times:

fn main() {
    extern {
        // extern "C"
        fn foo();
    }

    // extern "Rust"
    let f: extern fn() = foo; // error
}

[brson]

Nominating back compat.

[catamorphism]

accepted backwards-compatible

[nikomatsakis]

Relates to #2202.

[huonw]

extern fn foo() {
    // do Rust things
}

seems to have the C ABI by default too.

[brson]

Is there still even an extern "Rust" type? What is the default for locals now?

[nikomatsakis]

Yes, I believe fn(T) -> U is shorthand for extern "Rust" fn(T) -> U. It occurs to me that extern fn(T) -> U could (reasonably) be shorthand for extern "C" fn(T) -> U.

[nrc]

looking at this

[nrc]

The question is, if they should both default to the same ABI - which ABI? C or Rust?

[nrc]

Niko explained via irc that his comment implies that all uses of (unadorned) extern should mean extern "C"