mozilla/rust — pull request #13935 by thestinger (2014-05-04 16:39:33+00:00)
Title: add back `noalias` to `&mut T` pointer parameters

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