add back noalias to &mut T pointer parameters

[thestinger]

This was removed because these could alias with &const T or @mut T
and those are now gone from the language. There are still aliasing
issues within local scopes, but this is correct for function parameters.

This also removes the no-op noalias marker on proc (not a pointer) and
leaves out the mention of #6750 because real type-based alias analysis
is not within the scope of best effort usage of the noalias attribute.

Test case:

pub fn foo(x: &mut &mut u32) {
    **x = 5;
    **x = 5;
}

Before:

define void @_ZN3foo20h0ce94c9671b0150bdaa4v0.0E(i32** nocapture readonly) unnamed_addr #0 {
entry-block:
  %1 = load i32** %0, align 8
  store i32 5, i32* %1, align 4
  %2 = load i32** %0, align 8
  store i32 5, i32* %2, align 4
  ret void
}

After:

define void @_ZN3foo20h0ce94c9671b0150bdaa4v0.0E(i32** noalias nocapture readonly) unnamed_addr #0 {
entry-block:
  %1 = load i32** %0, align 8
  store i32 5, i32* %1, align 4
  ret void
}

Closes #12436

[alexcrichton]

Can't &mut T still alias *U and *mut U? Does a check need to be done of the other argument types to make sure none of them are pointers as well?

[thestinger]

It can't alias in safe code or valid unsafe code. The noalias attribute guarantees that the parameter can not be used to observe writes through other pointers not based (created from) on it and it can't cause writes observable through other pointers. It's about memory dependencies rather than pointer equality.

• http://llvm.org/docs/LangRef.html#parameter-attributes
• http://llvm.org/docs/AliasAnalysis.html#must-may-and-no-alias-responses

[thestinger]

cc @nikomatsakis

[nikomatsakis]

I am not sure if I agree (nor sure if I disagree...) regarding potential aliasing of &mut T and *mut T. That is, I don't think we've strictly defined this.