core: make the Rng a by-value parameter of Distribution::sample

[joboet]

Given that &mut R now implements Rng if R does (#159435), I think it makes more sense for Distribution::sample to take the Rng by-value instead of by-ref. For stateless zero-sized types like SystemRng this is more efficient and convenient, and for everything else creating a reference still works just the same.

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[tbu-]

For stateless zero-sized types like SystemRng this is more efficient and convenient, and for everything else creating a reference still works just the same.

Note that this PR can result in the exact thing it wants to prevent — more complex implementations of Distribution (that need to sample the RNG more than once), will need to create a reference. Chain that through multiple indirections (e.g. a struct containing a struct containing a struct) and you suddenly have &mut &mut &mut SystemRng. Before this PR, you still had &mut SystemRng, strictly better.

[joboet]

That's a very good point. I'll nominate for libs-api to make sure using by-ref is an explicit decision.

@rustbot label +I-libs-api-nominated

[joshtriplett]

We talked about this in today's @rust-lang/libs-api meeting.

We felt that the hazard of ending up with an &mut &mut SomeRng was unpleasant, and in practice the only case that gets a notable ergonomic improvement is SystemRng. It doesn't seem to make things substantively easier in other cases. Given that, we'd prefer not to do this.

To make the SystemRng case (and other cases) simpler, we'd prefer to consider a final fn random on Rng that takes a Distribution, so that you can call SystemRng.random(1..=6) which is nicer than (1..=6).sample(&mut SystemRng).