False never used function warning

[leonardo-m]

I'm not sure if this is expected. In this code I've used a const test function, and I call it inside a const assert:

macro_rules! const_assert {
    ($x:expr $(,)?) => {
        const _: () = assert!($x);
    };
}

fn main() {
    const fn test(n: u32) -> bool {
        n > 0
    }
    const N: u32 = 100;
    const_assert!(test(N));
    dbg!(N);
}

This code works correctly, but rusts gives a "warning: function test is never used".

[est31]

Smaller reproduction:

    const fn test() {}
    const _: () = test();

It works if you replace the _ with FOO and print the FOO somewhere else. I guess what's missing is the special treatment of _.

[ehuss]

I think this is nearly a duplicate of #96952. See also #101532.

I think it's very possible that when using const _, the compiler assumes the value is never "used", and thus evaluating the const expression has no effect. However, that is no longer true since #89508 where panic support was added. (Just an idle speculation.)

[est31]

Oh yeah it's a duplicate. That's why it was so familiar to me. I guess this issue should be closed as dupe.