Temporarily take ownership of a value at a mutable location, and replace it with a new value based on the old one.
This crate provides the function replace_with(), which is like std::mem::replace() except it allows the replacement value to be mapped from the original value.
See RFC 1736 for a lot of discussion as to its merits. It was never merged, and the desired ability to temporarily move out of &mut T doesn't exist yet, so this crate is my interim solution.
It's very akin to take_mut, though uses Drop instead of std::panic::catch_unwind() to react to unwinding, which avoids the optimisation barrier of calling the extern "C" __rust_maybe_catch_panic(). As such it's up to ∞x faster. The API also attempts to make slightly more explicit the behavior on panic – replace_with() accepts two closures such that aborting in the "standard case" where the mapping closure (FnOnce(T) -> T) panics (as take_mut::take() does) is avoided. If the second closure (FnOnce() -> T) panics, however, then it does indeed abort. The "abort on first panic" behaviour is available with replace_with_or_abort().
Consider this motivating example:
```rust enum States { A(String), B(String), }
impl States { fn poll(&mut self) { // error[E0507]: cannot move out of borrowed content *self = match *self { // ^^^^^ cannot move out of borrowed content States::A(a) => States::B(a), States::B(a) => States::A(a), }; } } ```
Depending on context this can be quite tricky to work around. With this crate, however:
```rust enum States { A(String), B(String), }
impl States { fn poll(&mut self) { replacewithorabort(self, |self| match self_ { States::A(a) => States::B(a), States::B(a) => States::A(a), }); } } ```
Huzzah!
Licensed under either of
at your option.
Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.