Implement Read for some wrapper of Rng in terms of fill_bytes or try_fill_bytes

[sdleffler]

Hey folks. I wanted to be able to take an Rng and shove it into a function that expects R: Read, i.e. treat it as an endless bytestream. There doesn't seem to be a nice convenient way out there to, say, turn the Rng into an iterator and then that into a Read, anyhow it makes sense to me to give Rng the capabilities of Read (since Read just says, "fill up this buffer with random bytes, please!") which one might note, Rng already supports. Why not implement Read for some wrapper around Rng in terms of try_fill_bytes? Perhaps struct ReadableRng<R: Rng>(R);.

[dhardy]

That shouldn't be hard to do. Is there good reason for including it within the rand lib though? You can create and implement this type externally from rand.

[sdleffler]

That's absolutely true. I would argue that in the same way, it's possible to implement ReadRng outside of rand as well, but it happens to be there nonetheless; this makes me feel like there's a reasonable argument for types involving stdlib Read living inside rand. Also, I'm left wondering about whether or not it might even be reasonable to give Read to Rng as a parent trait, perhaps as an alternative to fill_bytes or try_fill_bytes (although I strongly suspect that ship has sailed.)

[dhardy]

ReadRng is in rand because it was used by OsRng, however that now has its own version so ReadRng could be removed. I've not seen any actual proposal to do that however, and would consider a PR adding a wrapper implementing Read.

Making Rng itself extend Read was proposed by @huonw a while back IIRC; unfortunately it is problematic for no-std support because Read and io::Result are not available outside of std.

[pitdicker]

This could live in read.rs.

[pitdicker]

Now io::Read is implemented for all RNGs.