Implementation of Index<Foo> prevents indexing arrays by other index types

[ghost]

use std::ops::Index;

enum Foo {
    Zero,
    One,
    Two,
}

impl Index<Foo> for [u8; 3] {
    type Output = u8;

    fn index(&self, index: Foo) -> &u8 {
        unsafe { self.get_unchecked(index as usize) }
    }
}

fn main() {
    let arr: [u8; 3] = [0, 1, 2];

    // OK:
    let _ = arr[Foo::Two];

    // Error:
    let _ = arr[2usize];
    //          ^^^^^^ expected enum `Foo`, found `usize`

    // Error:
    let _ = &arr[0usize..2];
    //           ^^^^^^^^^ expected enum `Foo`, found struct `std::ops::Range<usize>`

    // Error:
    let _ = &arr[..];
    //           ^^ expected enum `Foo`, found struct `std::ops::RangeFull`
}

[hanna-kruppe]

see also #24066

[arielb1]

@rkruppe

Different problem. Here you are looking for the <[u8] as Index<RangeFull>> impl, which is reachable only through an unsizing coercion. Because your impl matches, the unsizing is skipped.

[hanna-kruppe]

Who's "me"? This issue isn't filed by me.

But yes, it does seem to be a different cause on closer inspection, sorry for the noise.