Numerous unused variable cause Rust difficulties

[coder0xff]

This issue is related to rust-lang/rust#122357. Large numbers of unused variables in fiat_p384_scalar_mul and possibly others are causing stack overflow in the Rust compiler (LLVM) on some targets. While the code in fiat_p384_scalar_mul is certainly valid Rust code, and the fault lies with the compiler, trimming dead code should alleviate this issue while a proper fix is pending. I will open a PR shortly.

[tarcieri]

Note that the code in those backends is autogenerated by fiat-crypto. An ideal fix would go into their Rust code generation backend: https://github.com/mit-plv/fiat-crypto

We do also have a tool for applying automated transformations to their output where you could also potentially apply a fix: https://github.com/RustCrypto/utils/tree/master/fiat-constify

I would definitely suggest against any hand edits to these files, as a big part of their value is formal verification.

[coder0xff]

Looking at https://github.com/mit-plv/fiat-crypto/blob/master/fiat-rust/src/p384_32.rs#L167 , since it's not a const fn and uses the mutable variables it declares, I don't think that warrants change, unless of course they would be changed to const fn. That would be an improvement, but probably harder to justify. I'll take a pass at modifying updating fiat-constify to strip the unused variables.

[tarcieri]

Sounds good!

Yeah, fiat-crypto itself should probably wait for const_mut_refs to add const fn support, since it does incur a small performance penalty.