# replace_with
[](https://crates.io/crates/replace_with)
[](#License)
[](https://travis-ci.com/alecmocatta/replace_with)
[Docs](https://docs.rs/replace_with/0.1.2)
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()`](https://docs.rs/replace_with/0.1.2/replace_with/fn.replace_with.html), which is like [`std::mem::replace()`](https://doc.rust-lang.org/std/mem/fn.replace.html) except it allows the replacement value to be mapped from the original value.
See [RFC 1736](https://github.com/rust-lang/rfcs/pull/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`](https://github.com/Sgeo/take_mut), though uses `Drop` instead of [`std::panic::catch_unwind()`](https://doc.rust-lang.org/std/panic/fn.catch_unwind.html) 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()`](https://docs.rs/replace_with/0.1.2/replace_with/fn.replace_with.html) accepts two closures such that aborting in the "standard case" where the mapping closure (`FnOnce(T) -> T`) panics (as [`take_mut::take()`](https://docs.rs/take_mut/0.2.2/take_mut/fn.take.html) 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()`](https://docs.rs/replace_with/0.1.2/replace_with/fn.replace_with_or_abort.html).
## Example
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) {
replace_with_or_abort(self, |self_| match self_ {
States::A(a) => States::B(a),
States::B(a) => States::A(a),
});
}
}
```
Huzzah!
## License
Licensed under either of
* Apache License, Version 2.0, ([LICENSE-APACHE.txt](LICENSE-APACHE.txt) or http://www.apache.org/licenses/LICENSE-2.0)
* MIT license ([LICENSE-MIT.txt](LICENSE-MIT.txt) or http://opensource.org/licenses/MIT)
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.