Tracking issue for `const extern fn` and `const unsafe extern fn`

[Aaron1011]

This is a tracking issue for implementing and stabilizing defining a const extern fn and const unsafe extern fn

• Add a test according to Add support for const unsafe? extern fn #64906 (comment) (added in Ensure that we error when calling "const extern fn" with wrong convention #68370)

Stabilizaton Report

Summary

This allows writing const unsafe extern "calling-convention" fn and const extern "calling-convention" fn:

Today Rust and C calling conventions are stable (as of 1.62, see #95346). All other calling conventions are still unstable.

const extern fn foo1(val: u8) -> u8 { val + 1 }
const extern "C" fn foo2(val: u8) -> u8 { val + 1}
const unsafe extern fn bar1(val: bool) -> bool { !val }
const unsafe extern "C" fn bar2(val: bool) -> bool { !val }

This can be used to const-ify an extern fn (or equivalently, to make a const fn callable from external code).

Test cases

1. Test for defining and calling in Rust code: https://github.com/rust-lang/rust/blob/master/src/test/ui/consts/const-extern-fn/const-extern-fn.rs
2. Test that calling with the wrong calling convention (from a const context) leads to an error: https://github.com/rust-lang/rust/blob/master/src/test/ui/consts/miri_unleashed/abi-mismatch.rs

[RalfJung]

Could you explain a bit more what this means and what the point is? What does the bare extern keyword even do? extern "Rust" fn and fn are the same thing, after all. (extern fn is poorly named; AFAIK it really is about specifying the ABI and has little to do with extern visibility.)

[Aaron1011]

@RalfJung: The point is to be able to define an const extern fn (i.e. const extern "C" fn), which has a C-compatible ABI and can be invoked from const contexts.

[Centril]

Implemented in #64906 (2019-10-07).