const extern fn's dont need `fn` specifier

[DutchGhost]

The following program compiles, and prints 1:

#![feature(const_extern_fn)]

const extern "Rust" PUT_ANYTHING_YOU_WANT_HERE bug() -> usize { 1 }

fn main() {
    dbg!(bug());
}

[DutchGhost]

As pointed out by @Nemo157 on discord , this also works on stable:

const unsafe WhereIsFerris Now() {}

[Centril]

Regression introduced in 1.40.0, specifically this line: https://github.com/rust-lang/rust/pull/64906/files#diff-af81683dfdae5890b2c6fa2d9b047f0aR163.