Use --cfg curve25519_dalek_backend to select backend

[tarcieri]

Crate features are intended to be additive, whereas only 1-of-N possible backends can be selected.

Features can also be activated by transitive dependencies, which leads to a problem of different dependences selecting conflicting backends. Using --cfg instead moves all backend selection control to the toplevel executable.

This commit switches to the following RUSTFLAGS to enable backends:

• --cfg curve25519_dalek_backend="fiat": uses fiat-crypto backend
• --cfg curve25519_dalek_backend="simd": uses nightly-only SIMD backend

[tarcieri]

This is code complete.

It still needs corresponding documentation changes.

[rozbb]

I think this is good to go. The new backend selection docs can be in a new PR, once #453 lands.

[pinkforest]

Looks great :)

[jrose-signal]

I'm sorry I didn't look at this sooner; why is bits a separate axis?

[tarcieri]

@jrose-signal because both the (default) serial backend and fiat backend both have 32-bit and 64-bit implementations, so it's something of a matrix with two dimensions

[jrose-signal]

Yes, but future backends may not come in such pairs, and the bits selection doesn't necessarily correspond to your target architecture (as we've discovered), and it's diverging further from the 3.x branch.

[pinkforest]

My initial PR was embedding it with backend and it became horrondeous to maintain not to mention merge conflicts around where we needed to put cfg() gates between as there is cross-use.

[pinkforest]

If future backend doesn't need the selection then people don't need to set those bits - this is just an override not a default.

Default is selected automatically and then there is a further lotto over that:

• Fixes curve25519_dalek_bits defaults for cross and wasm #465

I'm documenting it here:

• Document curve25519_dalek_ backend and bits selection #467

[tarcieri]

Yeah, doing anything complex with cfg is something of a nightmare.

@jrose-signal I suggest you review #465. It should hopefully eliminate the need for manual curve25519_dalek_bits configuration for your use cases

[pinkforest]

We also have an issue around that would put the backends into separate crates:

• Separating backends into their own crates? #287

I also thought why not re-export "either" via pub (e.g. both u32 and u64 would be known uX) but this would be fairly involved as every export would have to be re-exported and it would be error prone compared checking that imported one is from e.g :u64: