Don't require allow_internal_unstable unless staged_api is enabled.

[ecstatic-morse]

#63770 changed qualify_min_const_fn to require allow_internal_unstable for all crates that used an unstable feature, regardless of whether staged_api was enabled or the fn that used that feature was stably const. In practice, this meant that every crate in the ecosystem that wanted to use nightly features added #![feature(const_fn)], which skips qualify_min_const_fn entirely.

After this PR, crates that do not have #![feature(staged_api)] will only need to enable the feature they are interested in. For example, #![feature(const_if_match)] will be enough to enable if and match in constants. Crates with staged_api (e.g., libstd) require #[allow_internal_unstable] to be added to a function if it uses nightly features unless that function is also marked #[rustc_const_unstable]. This prevents proliferation of #[allow_internal_unstable] into functions that are not callable in a const context on stable.

r? @oli-obk (author of #63770)
cc @Centril

[Centril]

r=me with comment ^-- added.

[Centril]

If you can follow up with changes to the rustc guide that'd be great.

[ecstatic-morse]

I'll r+ on your behalf once this passes CI. There might be some bootstrap problems now that the feature is required to be at the crate root as well as in allow_internal_unstable.

[ecstatic-morse]

@bors r=Centril

[bors]

📌 Commit fc30825 has been approved by Centril

[nbdd0121]

Note that currently there is a workaround using macros:

#![feature(allow_internal_unstable)]

#[allow_internal_unstable(const_transmute)]
macro_rules! transmute {
    ($e: expr) => {
        std::mem::transmute($e)
    };
}

#[allow_internal_unstable(wrapping_next_power_of_two)]
macro_rules! wrapping_next_power_of_two {
    ($e: expr) => {
        $e.wrapping_next_power_of_two()
    };
}


fn g(a: u64) -> u64 {
    wrapping_next_power_of_two!(a)
}

const fn h(a: u64) -> i64 {
    unsafe { transmute!(a) }
}