Description
In a whole bunch of security-related contexts it's important to write code that runs in constant time regardless of input; the obvious example is that if you compare two strings character-by-character for equality and break out of the loop when you hit the first difference, someone might learn how much of their forged authentication token (for example) was correct by measuring the time it took to fail.
Constant-time code requires careful programming, but in the general case it also requires compiler support -- at minimum, it has to be possible to disable optimizations that would convert a|b to a||b in the name of speed, or similar. Ideally, though, the compiler would verify that an annotated block would execute in constant time, and fail the compilation if it was impossible to guarantee that of the generated code.
I think the natural way to expose this in Rust is a #[constant_time] annotation that could be applied to blocks. Most of the heavy lifting, however, probably needs to be done in the LLVM core.