Future of `block` module

[dhardy]

The block module is designed to assist implementing Rng for a block-generator. #31 (and the linked PRs) brought up a number of issues with prior designs, some of which have been solved. Some remaining issues:

• The internal buffer prevents RNGs from simply deriving serde (or other serialization) support; methods have been added to make this possible but they are often harder to use.
• Fn Generator::drop exists to provide compatibility with zeroize; while functional the design is inelegant.
• There is a significant amount of code for few downstream use-cases.

To the latter point: the block module is currently used by the following crates:

• chacha20
• rand_chacha
• rand_hc
• rand_isaac

All of these have at some point been used to provide rand::rngs::StdRng. The latter two already have very few downloads on crates.io for the latest versions, indicating few (if any) direct users. chacha20 is value-compatible with rand_chacha. It may therefore be viable to deprecate all three of the latter crates.

It may therefore make sense to move the block code into chacha20 (and any other block RNG impl which doesn't get deprecated), possibly with one more addition to rand_core::utils to cover the fill_bytes implementation.

[benjamin-lieser]

It seems there not to much value in having it in rand_core. It would also be hard to update it, because we probably don't want to make rand_core v2.
I also don't think there will ever be a lot of implementers of the Rng trait anyway, so there is less value in any helper functions.

[dhardy]

I agree with what you said, but the unanswered question is what to do with this code.

Simply merging it into crates implementing a block RNG. Might work okay.

Putting it in another crate like rand_utils could work, but we have had people complain about the number of rand crates in the past.

[benjamin-lieser]

I think merging it where it is used is perfectly fine.

I also don't think we need more rand_* crates.

[tarcieri]

I would be fine with vendoring it into chacha20, especially if the other crates go away and it's the only one using it

[newpavlov]

BlockRng is also used by ThreadRng, rand_isaac, and rand_hc. We technically could expose buffering code from chacha20, but at this point it may be better to move it into a separate crate (e.g. we could make it part of block-buffer).