Implement Random for num::{Saturating,Wrapping}

[sorairolake]

rust-lang/libs-team#393 (comment) states:

We can implement Random for many more types, such as NonZero<T> and Wrapping<T>.

Tracking issue: #130703

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[sorairolake]

r? @joboet

[joboet]

The implementation looks correct, but this will need T-libs-api review as well.

r? libs-api

[sorairolake]

I think Saturating<T> and Wrapping<T> are "uniform" because all values ​​of T are valid. Unlike these, all values ​​of T ​​except zero are valid in NonZero<T>, so I think it is "almost uniform" rather than "uniform". So unless there is a way to generate an "uniform" NonZero<T>, it may be better not to add an implementation for this.

[ericlagergren]

I think Saturating<T> and Wrapping<T> are "uniform" because all values ​​of T are valid. Unlike these, all values ​​of T ​​except zero are valid in NonZero<T>, so I think it is "almost uniform" rather than "uniform". So unless there is a way to generate an "uniform" NonZero<T>, it may be better not to add an implementation for this.

RandomSource outputs uniformly random bytes, so NonZero<T> is uniformly random. It's just that its range is [1, N] instead of [0, N].

What would not be uniformly random is if (for example) NonZeroU32(42) were more likely than NonZeroU32(123). But that is not the case.