ICE on extern "Rust" fn declaration

[huonw]

extern "Rust" fn foo() {}

fn main() {}

error: internal compiler error: unimplemented Foreign functions with Rust ABI
This message reflects a bug in the Rust compiler. 

(On first blush, I'd think that fn foo() and extern "Rust" fn foo() should be identical, but there might something I'm missing?)

[meadori]

Hmmm, does this fall under the purview of #3678 ? I see the following in librustc/middle/trans/foreign.rs:

 Rust => {
                // FIXME(#3678) Implement linking to foreign fns with Rust ABI
                ccx.sess.unimpl(
                    format!("Foreign functions with Rust ABI"));
            }