Try to use the borrow checker of this decade to write callbacks in GUI frameworks, without polluting the code with Rc and RefCell, even though it is obvious there is only one path to the data being used.
Because the closure itself contains a reference either to the struct or to its fields. That's the second path, and it can be invalidated by the first path either a) freeing the struct or b) changing its "shape" in memory (resizing a `Vec`, changing enum variants, etc.)
You can alternatively move the struct or its fields into the closure, but then you lose the first path. I know you're not doing that because if you were you wouldn't be having lifetime problems (and you wouldn't be able to share the state across event handlers, so it's not a great solution anyway).
The quoted sentence is not true, but most practical callbacks will create multiple paths to objects via borrows. Or you can move your structs into closures (like in C++). But I think what you originally wrote (using callbacks without Rc/RefCell) is fundamentally hard. You have to guarantee somehow that the struct is still alive when you access it though the borrow in the callback, otherwise you have a memory safety problem.
