Random number generators should not implement Copy

[nagisa]

Currently there’s a few random number generators that implement Copy:

• StdRng
• Isaac*Rng
• ChaChaRng

All of these random number generators have a bad property of also copying their seed/internal data implicitly, rather than splitting their seed between original and copied generators. This means that Rng in question will start generating the same sequence values when generator is passed by value.

These generators should not implement Copy (implicit) and implement Clone (explicit) instead in case copying generators is intended (which is almost never a case), like XorShiftRng does. Maybe I just missed the use-case of Copyable PRNGs?

[sfackler]

I'd agree that they probably shouldn't be Copy.

[huonw]

NB. This is not a question of Copy versus Clone, since we should implement Clone for sure, and an implementation of Copy is mostly orthogonal to this.