crypto-common: add RngError type

[tarcieri]

Adds a common error type which can be used with both getrandom and rand_core.

This eliminates the need to have getrandom as part of the public API.

[tarcieri]

Potentially it could carry along an inner code e.g. the raw_os_error as reported by either getrandom::Error or rand_core::Error, although I don't think it's bad for it to be completely opaque

[newpavlov]

I don't think we should replace the errors in such way. They may carry crucial information for finding why an error has occurred. If you don't want to expose getrandom in public API, you could convert getrandom::Error into rand_core::Error using From/Into, but it would mean that you have to pull rand_core when only the getrandom feature is enabled.

Personally, I think the current code is fine.

[newpavlov]

BTW it may be worth to add std = ["rand_core?/std", "getrandom?/std"] to crypto-common's Cargo.toml, so the errors from those crates would have the Error trait implemented when std is enabled.

[tarcieri]

They may carry crucial information for finding why an error has occurred.

Do you want anything more than the code? That's easy enough to propagate. rand_core::Error is just a thin wrapper for getrandom::Error in that regard: https://github.com/rust-random/rand/blob/ef89cbefaf484270dc3936d5d32fc2a73314173c/rand_core/src/error.rs#L178

The downside of the current approach is there are two different error types to deal with depending on which functions you're calling.

[newpavlov]

That's easy enough to propagate. rand_core::Error is just a thin wrapper for getrandom::Error in that regard

Note the #[cfg(feature = "std")] part. User-defined RNGs (e.g. wrappers around an external HW RNG) may define their own errors, without associated error code.

The downside of the current approach is there are two different error types to deal with depending on which functions you're calling.

I don't think generate_* and generate_*_with_rng will be mixed much in practice. And even if they are, you can easily convert Result<T, getrandom::Error> into Result<T, rand_core::Error> by using .map_err(Into::into). In some cases it even can be done as part of error bubbling by ?.

[tarcieri]

Alright, I guess I'll close this