Role of UB / uninitialized memory

[arielb1]

@RalfJung

Rc::drop calls RcBoxPtr::dec_weak after decrementing the strong count and droping the content of the RcBox. This is somewhat interesting because RcBoxPtr::dec_weak takes &Rc<T> as an argument and is a perfectly safe function, but it is actually called on an Rc that is in a bad state -- it doesn't satisfy the usual invariants that Rc typically satisfies. However, the Rc passed to dec_weak is still a valid instance of a "syntactic Rc", so I don't think the compiler could exploit this in any way.

That's safety vs. definedness again. The language should not "guess" your types' semantics - if you don't use any "magic" attributes, the language should only care about whether your types are "syntactically" valid - e.g. you should be able to use an RcBox to store 2 random words (as long as you don't call any Rc functions in a way that would crash).