Const-generic bounds should allow simple casts

[aloucks]

The as_usize wrapper function in the example below shouldn't be required.

@oli-obk
#83278 (comment)

#![feature(const_generics)]
#![feature(const_evaluatable_checked)]

use std::arch::x86_64::*;

pub const fn _MM_SHUFFLE(fp3: u32, fp2: u32, fp1: u32, fp0: u32) -> i32 {
    ((fp3 << 6) | (fp2 << 4) | (fp1 << 2) | fp0) as i32
}

pub const fn as_usize(a: i32) -> usize {
    a as usize
}

pub fn swizzle<const X: u32, const Y: u32, const Z: u32, const W: u32>(v: __m128) -> __m128
where [(); as_usize(_MM_SHUFFLE(W, Z, Y, X))]:
{
    unsafe {
        _mm_shuffle_ps(v, v, _MM_SHUFFLE(W, Z, Y, X))
    }
}

[oli-obk]

cc @rust-lang/project-const-generics

Casts are infallible and could be allowed. I think they are sufficiently common for us to allow them

[nikomatsakis]

That makes sense to me

[memoryruins]

fn as_usize is no longer needed and the where clause can be written as where [(); _MM_SHUFFLE(W, Z, Y, X) as usize]:. Presumably fixed by #86130

playground
1.55.0-nightly 2021-06-21 406d4a9cc3b9601cf98a