From<usize> missing for u64

[MavenRain]

Why doesn't u64 implement From<usize>? The transformation is infallible.

[terrarier2111]

It's probably because in the future platforms with usize > 64 bits might be added and this would lead to backwards compatability issues.

[MavenRain]

Perhaps this should be a documentation issue, given that, since it isn't clear from what is present for the usize that the type is intended to be perpetually forward-looking: https://doc.rust-lang.org/std/primitive.usize.html

[KamilaBorowska]

This was previously brought up in #30495. The issue is that in the future we may have 128-bit CPUs where usize would exceed u64, and From must not fail.

[MavenRain]

Yeah, that definitely sounds right. Thank you guys for clarifying so quickly!