The layers don't really matter. Any side-effect or implementation detail (especially in a language with an optimizing compiler) is not a good protection against

The layers don't really matter. Any side-effect or implementation detail (especially in a language with an optimizing compiler) is not a good protection against side channel attacks. Where Rust (and most languages with a robust type system) can help prevent side channel issues is via typing - all cryptographic operations in a given library can be implemented with (for example), a TimingSafeU32 type that only implements constant-time operations (comparison, multiplication, addition, etc). This massively reduces the opportunity for mistakes. Traits make implementing this type of system much easier and more elegant. `rust-timing-shield` and `subtle` are both a good example here, as well as really everything in https://github.com/dalek-cryptography https://github.com/dalek-cryptography .