Comment by Animats

Weak is very helpful in preventing ownership loops which prevent deallocation. Weak plus RefCell lets you do back pointers cleanly. You call ".borrow()" to get access to the data protected by a RefCell. The run-time borrow panics if someone else is using the data item. This prevents two mutable pointers to the same data, which Rust requires.

Static analysis could potentially check for those potential panics at compile time. If that was implemented, the run time check, and the potential for a panic, would go away. It's not hard to check, provided that all borrows have limited scope. You just have to determine, conservatively, that no two borrow scopes for the same thing overlap.

If you had that check, it would be possible to have something that behaves like RefCell, but is checked entirely at compile time. Then you know you're free of potential double-borrow panics.

I started a discussion on this on a Rust forum. A problem is that you have to perform that check after template expansion, and the Rust compiler is not set up to do global analysis after template expansion. This idea needs further development.

This check belongs to the same set of checks which prevent deadlocking a mutex against itself. There's been some work on Rust static deadlock analysis, but it's still a research topic.