Mark &mut parameters as noalias

[Aatch]

This marks &mut function arguments with the noalias attribute. Since the borrow checker enforces this property, this is worth doing.

I'm not sure if the place I'm doing it in is ideal, but it generates the correct code.

Closes#6350

[brson]

Do we actually know that all &mut parameters are no-alias? This seems pretty unlikely to me.

[thestinger]

@brson: yes, LLVM's understanding of noalias is that it's the only way to mutate the value inside the scope and borrow checking provides this guarantee

You're allowed to have other references to the memory before and after the function, and the function can create more references to the value from the pointer it's passed.

Technically, & pointers are close to being noalias because LLVM regards 2 pointers as non-aliasing if neither of them is ever used to write to the destination. However, the fact that we have objects that are mutable without a mutable reference rules that out for now.

[brson]

@thestinger If all Rust code was safe and obeyed borrow semantics then this would be true, but there is plenty of unsafe code that mutates immutable pointers. Do all those unsafe blocks preserve the assumption that &mut is unaliased? I don't know, but I doubt it.

[thestinger]

@brson: I don't think we'll be able to do this for immutable pointers for a long time - but I doubt there's anything breaking the rules for &mut when it comes to function calls (we aren't marking any of the & references as noalias).

To break the rules, the function would need to have two borrowed pointer parameters with one or both being &mut, and the caller would have to pass the same for both. The result would be one of the pointers potentially not seeing a store done through the noalias pointer or vice versa.