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# replace_with
[](https://crates.io/crates/replace_with)
[](#License)
[](https://dev.azure.com/alecmocatta/replace_with/_build?definitionId=11)
[📖 Docs](https://docs.rs/replace_with) | [💬 Chat](https://constellation.zulipchat.com/#narrow/stream/213236-subprojects)
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/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/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/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!
## `no_std`
To use `replace_with` with `no_std` you have to disable the `std` feature, which is active by default, by specifying your dependency to it like this:
```toml
# Cargo.toml
[dependencies.replace_with]
version = ...
default-features = false
features = []
...
```
The [`replace_with()`](https://docs.rs/replace_with/0.1/replace_with/fn.replace_with.html) & [`replace_with_or_default()`](https://docs.rs/replace_with/0.1/replace_with/fn.replace_with_or_default.html) functions are available on stable Rust both, with and without `std`.
The [`replace_with_or_abort()`](https://docs.rs/replace_with/0.1/replace_with/fn.replace_with_or_abort.html) function however by default makes use of [`std::process::abort()`](https://doc.rust-lang.org/std/process/fn.abort.html) which is not available with `no_std`.
As such `replace_with` will by default call [`core::intrinsics::abort()`](https://doc.rust-lang.org/core/intrinsics/fn.abort.html) instead, which in turn requires nightly Rust.
Not everything is lost for stable `no_std` though, `replace_with` has one more trick up its sleeve:
### panic = "abort"
If you define [`panic = abort`](https://doc.rust-lang.org/edition-guide/rust-2018/error-handling-and-panics/aborting-on-panic.html) in the `[profile]` section of your crate's `Cargo.toml` …
```toml
# Cargo.toml
[profile.debug]
panic = "abort"
[profile.release]
panic = "abort"
```
… and add the `"panic_abort"` feature to `replace_with` in the `dependencies` section of your crate's `Cargo.toml` …
```toml
# Cargo.toml
[dependencies.replace_with]
features = ["panic_abort"]
...
```
… the `"panic_abort"` feature enables the [`replace_with_or_abort_unchecked()`](https://docs.rs/replace_with/0.1/replace_with/fn.replace_with_or_abort_unchecked.html) function becomes on stable Rust as an `unsafe` function, a simple wrapper around `ptr::write(dest, f(ptr::read(dest)));`.
**Word of caution:** It is crucial to only ever use this function having defined `panic = "abort"`, or else bad things may happen. It's *up to you* to uphold this invariant!
## 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)
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.