[Someone]

But Rust doesn’t have a spec (https://doc.rust-lang.org/reference/ gets closest, but explicitly states “Rust compilers, including rustc, will perform optimizations. The reference does not specify what optimizations are allowed or disallowed” and “this book is not normative”), so it doesn’t promise what kind of error you’ll get or when.

I would think a Rust compiler could hoist the check outside of the loop at least sometimes (it might not want to do it if the loop made changes that are visible from outside the loop, such as changing a volatile variable or doing I/O)

[tialaramex]

Rust doesn't have "volatile variables" that's a weird C thing which then ends up in C++ and related languages because nobody wants to touch this mess.

The purpose of "volatile" is to mark MMIO so that the memory reads and writes don't get optimised out because they actually perform I/O. Everywhere you see volatile abused to do something else (yes including Unix signal handlers) that's because C has a hammer and so now everything looks like a nail to C programmers. In a few cases this abuse is guaranteed to work (and results in a lot of heavy lifting elsewhere to achieve that) in most cases it's just luck.

Rust has generic intrinsics which perform memory read/write that won't get optimised out, which is the actual thing C needed when volatile was invented.