the trait `core::convert::From<usize>` is not implemented for the type `u64`

[kornelski]

There are various From implementations for integers, but this one is curiously absent.

I thought about making a pull request, but I can't find where the other integer From traits are defined.

[petrochenkov]

I can't find where the other integer From traits are defined.

https://github.com/rust-lang/rust/blob/master/src/libcore/num/mod.rs#L1984

The discusion about usize is in #28921 and #29220 (make sure to "Show outdated diff")
TL;DR: We need more motivating examples and maybe some mini-RFC describing the portability aspect to dispel the fears of the libs team.

[kornelski]

Thanks @petrochenkov. Both issues are about conversion to usize, which is the opposite direction than specified in this issue. I needed conversion from usize to u64 in order to use a FFI.

Anyway, I've noticed that there is a trait for usize to size_t, so I can tweak my FFI and use that instead.