as cast of non-Copy enum is no longer a move

[scottmcm]

@MinusKelvin noticed this 1.63→1.64 change in #102303 (comment)

I tried this code:

pub enum Enum { A, B, C }
pub fn func(inbounds: &Enum, array: &[i16; 3]) -> i16 {
    array[*inbounds as usize]
}

I expected to see this happen: because Enum is non-Copy, an error about moving out from behind a reference, like happened in 1.63 https://rust.godbolt.org/z/vGPqbsE6v

error[E0507]: cannot move out of `*inbounds` which is behind a shared reference
 --> <source>:3:11
  |
3 |     array[*inbounds as usize]
  |           ^^^^^^^^^ move occurs because `*inbounds` has type `Enum`, which does not implement the `Copy` trait

Instead, this happened: it compiles fine in 1.64 https://rust.godbolt.org/z/svsPMxTs8 and in nightly https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=817f19e95903785423743c2a8466c189

---

This is probably cased by

Change enum->int casts to not go through MIR casts #96862

which is mentioned in https://github.com/rust-lang/rust/blob/stable/RELEASES.md#version-1640-2022-09-22

It's unclear to me whether this is intentional under the banner of "Drop behaviour", since there's no Drop trait involved.

[joshtriplett]

Prioritized as P-critical in @rust-lang/lang meeting just now. This needs to be fixed by 1.65 at a minimum.

[nikomatsakis]

@rustbot labels +T-compiler

This needs work from T-compiler! :)

[nikomatsakis]

@rustbot labels -I-lang-nominated

Meeting consensus is that the current behavior is wrong and needs to be fixed.