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. It's also aesthetically a little prettier (I think).
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!
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