Permissive Borrows

cc @RalfJung, who asked me to write up my thoughts on a new aliasing model I've spent a lot of time recently working on a new aliasing model, which I call Permissive Borrows. This post is meant to introduce the model. It's written vaguely in the style of an RFC, as I consider it to be a good style for conveying new ideas, but isn't a proper RFC or pre-RFC because it isn't suggesting any particular course of action (in particular, it does not discuss alternatives or prior art, things that shou...