Comment by brianpane

With hand-written assembly, you can (with effort and care) ensure that the code runs in constant time and doesn’t leak any information through side channels (such as which memory or cache addresses it accesses). That’s important for most encryption code. It’s difficult to ensure constant time execution in pure Rust (or C, or most high level languages in general) because you can’t tell if some future compiler optimization will break your attempts at constant-time code.