Summary

Extend Cell to work with non-Copy types.

Motivation

It allows safe inner-mutability of non-Copy types without the overhead of RefCell's reference counting.

The key idea of Cell is to provide a primitive building block to safely support inner mutability. This must be done while maintaining Rust's aliasing requirements for mutable references. Unlike RefCell which enforces this at runtime through reference counting, Cell does this statically by disallowing any reference (mutable or immutable) to the data contained in the cell.

While the current implementation only supports Copy types, this restriction isn't actually necessary to maintain Rust's aliasing invariants. The only affected API is the get function which, by design, is only usable with Copy types.

Detailed design


# #![allow(unused_variables)]
#fn main() {
impl<T> Cell<T> {
    fn set(&self, val: T);
    fn replace(&self, val: T) -> T;
    fn into_inner(self) -> T;
}

impl<T: Copy> Cell<T> {
    fn get(&self) -> T;
}

impl<T: Default> Cell<T> {
    fn take(&self) -> T;
}
#}

The get method is kept but is only available for T: Copy.

The set method is available for all T. It will need to be implemented by calling replace and dropping the returned value. Dropping the old value in-place is unsound since the Drop impl will hold a mutable reference to the cell contents.

The into_inner and replace methods are added, which allow the value in a cell to be read even if T is not Copy. The get method can't be used since the cell must always contain a valid value.

Finally, a take method is added which is equivalent to self.replace(Default::default()).

Drawbacks

It makes the Cell type more complicated.

Cell will only be able to derive traits like Eq and Ord for types that are Copy, since there is no way to non-destructively read the contents of a non-Copy Cell.

Alternatives

The alternative is to use the MoveCell type from crates.io which provides the same functionality.

Unresolved questions

None