Improving self-referential structs

language design
71 
struct MyType {
    f1: i32,
    f2: ^i32 // A weak reference
}
let mut x = MyType { f1: 42, f2: ^f1 };
let f = x.f2;
let y = ^f.clone(); //^f promotes f to either &i32 or &mut i32
//Explicitly promote f to &mut i32, only type check when x is mut
let mut v:&mut i32 = ^f;
//f is still valid as it is still "week", and this is a "copy", not a "move"
let g = f;
//error: immutable borrow whilst mutable borrow in scope
let u: &i32 = ^f;
//error: send a weak reference to a function alone, without the full object
std::mem::drop(f);
//OK, we send both the object and the weak reference
(|v,_| std::mem::drop(v))(x,f);

Reasons behind:

  1. When initialize self-referential structs, we don’t want the usual borrow rules apply - this gives us maximal flexibility to create the struct.
  2. When using those self references however, we need to apply the borrow rules to ensure safety.
  3. So this design is to separate these requirements - by introducing a new reference type - the “weak” reference
  4. A weak reference can be promoted to usual references and enforce borrow rules at the promotion site
  5. A week reference can only be passed to another function as arguments with its owner.