Description
Using @mut won't lead to dynamic failures until it ends up borrowed as & or &mut. I think this has proven to be a very confusing feature, and explaining it is a regular activity in #rust. It's very hard to reason about the failure cases until you actually run into one, especially if there are a lot of shallow copies which is the use case for managed pointers. It's usually a better idea to pass it by-value for the sake of robustness.
Resorting to @mut should already be a last resort and dynamic freezes should be a very obvious opt-in feature, with appropriately named methods + docstrings.
let x = @mut 5;
do x.borrow |r| {
...
}The current behaviour would also be incredibly hard to reproduce in a library type meant to be used in the same way like RcMut. We would need a very special trait able to run code at the end of the scope without an object + destructor involved.
In the future, this will allow us to add back the sugared borrows to @mut for the subset of cases that we can prove are safe without dynamic freezes. By putting the dynamic failure strategy in a library, we leave open the possibility of better solutions as the language evolves.
I know doing this would be a painful change, but it can be done gradually with a lint check set to deny the old implicit dynamic borrows by default.