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#66 in Rust patterns
465,584 downloads per month
Used in 274 crates
(51 directly)
28KB
245 lines
replace_with
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()
.
Example
Consider this motivating example:
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:
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:
# Cargo.toml
replace_with = { version = "0.1", default-features = false }
The nightly
feature can be enabled to use core::intrinsics::abort()
instead of triggering an abort via std::process::abort()
or extern "C" fn abort() { panic!() } abort()
.
License
Licensed under either of
- Apache License, Version 2.0, (LICENSE-APACHE.txt or http://www.apache.org/licenses/LICENSE-2.0)
- MIT license (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.