Rust already has __restrict; it is spelled &mut and is one of the most fundamental parts of the language. The key difference, of course, is that it's checked by the compiler, so is useful for correctness and not just performance. Also, for a long time it wasn't used for optimization, because the corresponding LLVM feature (noalias) was full of miscompilation bugs, because not that much attention was being paid to it, because hardly anyone actually uses restrict in C or __restrict in C++. But those days are finally over.

__builtin_assume is available on stable (though of course it's unsafe): https://doc.rust-lang.org/std/hint/fn.assert_unchecked.html

There's an open issue to stabilize the prefetch APIs: https://github.com/rust-lang/rust/issues/146941 As is usually the case when a minor standard-library feature remains unstable, the primary reason is that nobody has found the problem urgent enough to put in the required work to stabilize it. (There's an argument that this process is currently too inefficient, but that's a separate issue.) In the meantime, there are third-party libraries available that use inline assembly to offer this functionality, though this means they only support a couple of the most popular architectures.