main branch no longer implicitly enables const-extern-fn

[n1000]

Rust / System details:

$ rustc -vV
rustc 1.75.0 (82e1608df 2023-12-21)
binary: rustc
commit-hash: 82e1608dfa6e0b5569232559e3d385fea5a93112
commit-date: 2023-12-21
host: x86_64-unknown-linux-gnu
release: 1.75.0
LLVM version: 17.0.6

libc version: main branch at commit e2e6fd6 or later.

Minimal test code:

use libc::WIFEXITED;

fn main() {
    const _: () = assert!(WIFEXITED(0));
}

Build output:

$ cargo build
   Compiling libc-const-extern-test v0.1.0 (/home/nathan/src/libc-const-extern-test)
error[E0015]: cannot call non-const fn `WIFEXITED` in constants
 --> src/main.rs:4:27
  |
4 |     const _: () = assert!(WIFEXITED(0));
  |                           ^^^^^^^^^^^^
  |
  = note: calls in constants are limited to constant functions, tuple structs and tuple variants

For more information about this error, try `rustc --explain E0015`.
error: could not compile `libc-const-extern-test` (bin "libc-const-extern-test") due to previous error
zsh: exit 101   cargo build

I wasn't sure if this was intended behavior or not (this error first showed up after commit e2e6fd6). If it is intended, then I guess it is just the README.md that may need to be updated to remove this text from the Features description:

const-extern-fn: ... If you use Rust >= 1.62, this feature is implicitly enabled. ...

If it is unintended, then should the const-extern-fn feature be enabled by default now that the MSRV of libc is 1.71?

In commit e2e6fd6 the definition of safe_f! in src/macros.rs was updated to be conditional on const-extern-fn directly. Previously it was conditionally defined based on libc_const_extern_fn which is automatically turned on by build.rs based on the rustc version.

Apologies in advance if this is a known breakage / work in progress on main.

[JohnTitor]

Yeah, it's WIP. I'd like to enable it unconditionally but ctest2 emits an error which is used by libc-test. That's because it uses an ancient rustc parser and we have to make it update (and enable the const fn feature) before releasing libc v0.3.
See #3248 for what is needed to release.