alias analysis pass in rustc

[graydon]

Currently we permit forming aliases in ways that are not safe: aliasing a mutable cell within a non-pinned box, doubly aliasing mutable values, etc. We need a pass which prohibits these sorts of things (as well as, er, a principled enumeration of which types of aliasing are bad).

[graydon]

First cut of this landed in 4bd5f83

[marijnh]

A more complete implementation was added in beda82d