For every location in memory, we keep track of a stack of borrows (`Uniq(_)` or `Raw`), and potentially "top off" this stack by freezing the location.
A frozen location is never written to, and no `Uniq` is pushed.
-Whenever a mutable reference is created, a matching `Uniq` is pushed onto the stack for every location "covered by" the reference -- i.e., the locations that would be accessed when the reference is used (starting at where it points to, and going on for `mem::size_of::<T>` many bytes).
+Whenever a mutable reference is created, a matching `Uniq` is pushed onto the stack for every location "covered by" the reference -- i.e., the locations that would be accessed when the reference is used (starting at where it points to, and going on for `size_of_val` many bytes).
Whenever a shared reference is created, if there is no interior mutability, we freeze the locations if they are not already frozen.
If there is interior mutability, we just push a `Raw`.
Whenever a raw pointer is created from a mutable reference, we push a `Raw`.