Shared ownership, and handles that may be empty

Decided

D-0062, settled by the language’s author: part of spec §18. Older text cites it as 18.62.

Rc<T>, Weak<T> and Box<T> in std, each of which may be null, like C++’s shared_ptr/weak_ptr/unique_ptr rather than Rust’s never-empty Rc: null is the default and the moved-from state, so no rule for “a type without a default” is needed, and a field of one is = null. null, not none: null is for handles — pointers included — and none for a value that is absent; the two stay apart so x = null reads as releasing something. A handle’s value is reached only through ==/!= null, or and if let — never a bare . — so an empty handle is always checked. or and if let on an Rc or Weak hold a strong Rc of their own until the if let block, or the block enclosing the or, ends, so the value cannot be freed while referred to — the cost being one plain count up and down. if let takes a Weak directly, with no upgrade(). Forced unloading is one owner plus Weaks, since a value others own cannot be deleted from under them; Rc<T?> is a shared slot, not a nullable Rc. The compiler knows the prelude trait Nullable (Target, Guard, is_null, guard, ptr) rather than the three names. A Box guards with nothing and can dangle as a ^T can (§1.1). Revised by D-0094 point 3 (r103): a Box’s value is in the box, @value, so emptying, replacing or moving the box ends a reference to its value, an error at compile time rather than a dangling one; and to the ties check every Rc<T> holds one value (point 2). Deferred: Arc, Rc<Derived> as Rc<Base>, handles to trait objects (§14.5)