Make CryptoRngCore trait imply CryptoRng as well

[cbeck88]

Hi,

I recently had a case where I needed to make an API that takes &mut dyn (CryptoRng + RngCore). I didn't want to make my trait generic because it would greatly complicate some FFI situations.

Since rust doesn't currently allow dyn (CryptoRng + RngCore), I needed to make an extension trait that implies both of these, and, naturally, is implied for any object with both of these traits.

It turns out that rand crate already does almost the same thing with CryptoRngCore (which is actually what I named my extension trait).

However the implementation in rand crate is slightly different: CryptoRngCore implies RngCore but not CryptoRng.

I can't see right now that there's a compelling use-case for it the way it's implemented right now: any place where I would write &mut dyn CryptoRngCore my code would also work if my function takes &mut dyn RngCore, since CryptoRngCore provides no additional functionality other than a function that converts self to &mut dyn RngCore. (But I could have started with that...)

OTOH if CryptoRngCore implies both RngCore and CryptoRng then it's quite useful, because it works around the inability to make dyn (X + Y) in rust right now. Then I would just use the upstream version and drop my version.

Let me know what you think. Thank you!

[dhardy]

See also #1187. Looks good to me but I'll give others a chance to take a look.

[cbeck88]

(I guess this is also described in the alternatives section of this RFC: #1143)