1.64 treats `as` casts from non-Copy enums as not moving the LHS

[cbiffle]

This code is (rightfully) rejected by at least 1.57 - 1.63, accepted by 1.64, and rejected again by 1.65.

enum Foo {
    A = 1,
}

fn main() {
    let a = Foo::A;
    let b = a as u8;
    let c = a as u8;
}

In compiler versions other than 1.64, the second a as u8 assignment is rejected, since a is not Copy and was previously moved into the as u8 operation that created b. On 1.64 it's silently tolerated.

This does not, as far as I can tell, affect as conversions other non-Copy types, though I could only come up with one example (&mut can be as-converted without being Copy). It also appears to be specific to as conversions -- attempting to move the value twice without casting triggers the compiler warning you'd expect.

It's possible this is already covered by an existing bug report, since it appears to be fixed in 1.65, but I couldn't find one, and wanted to make sure there was a paper trail in case this surprises anyone else. We've got a repo pinned to 1.64 that contains some incorrect code being admitted by this bug. (The closest thing we could find is #91161.)

Thanks to @jgallagher for help tracking this down.

[ehuss]

I believe this was #102389.

It was accepted for a 1.64.1 stable point release, but for some reason that stable release never happened. I don't see a discussion around that decision specifically. Usually a stable point release is only made if there is sufficient motivation, so perhaps it didn't meet the threshold?

[cbiffle]

I believe you are right! Since it was previously recognized and deliberately fixed (presumably with a regression test) I'm happy to close this issue. We, at least, do not need a 1.64 point release, we'll just clamber our way up to 1.65.