> you can't have a writeable reference to both A and B at the same time > but you spend a lot of time in Rust getting the ownership plumbing right to

I think you maybe meant to say something different because here's the most obvious thing:

    impl A {
        fn simultaneously_writeable(&mut self) -> (&mut str, &mut str) {
            (&mut self.name, &mut self.owned.name)
        }
    }

Now it may take you a while to figure out if you've never done Rust before, but this is trivial.

Did you perhaps mean simultaneous partial field borrows where you have two separate functions that return the name fields mutably and you want to use the references returned by those functions separately simultaneously? That's hopefully going to be solved at some point, but in practice I've only seen the problem rarely so you may be overstating the true difficulty of this problem in practice.

Also, even in a more complicated example you could use RefCell to ensure that you really are grabbing the references safely at runtime while side-stepping the compile time borrow checking rules.