The issue isn't whether the compiler knows which data is affected and which isn't, but rather that the compiler doesn't know about the semantics of the mutex.

The issue isn't whether the compiler knows which data is affected and which isn't, but rather that the compiler doesn't know about the semantics of the mutex. However, the library knows about the semantics of the mutex, and it could explicitly instruct the processor to only flush certain memory regions from the store buffer when the mutex is unlocked. But then again, that's not Rust-specific – a C++ library could do the same.