[[Pointer safety principle]]
[[Variables are subject to permissions similar to files on linux]]
[[Lifetime specifier]]
[[When heap data is deallocated]]
[[Variables lives on the stack]]
[[Ownership]]

This is literally talking about heap data and references (ie `&String`): the heap data must not be freed when we try to use a reference to it.

This is enforced in one of two ways.

Firstly, if a reference is created to a variable it loses its Own permission: thus we can't drop it, because the compiler expects that permission on the place where it is dropped.

Secondly, it uses the Flow permission and requires a a lifetime specifier when we return things from functions.