replace_with

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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

[dependencies.replace_with]
version = ...
default-features = false
features = []
...

The replace_with() & replace_with_or_default() functions are available on stable Rust both, with and without std.

The replace_with_or_abort() function however by default makes use of std::process::abort() which is not available with no_std.

As such replace_with will by default call core::intrinsics::abort() 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 in the [profile] section of your crate's Cargo.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 …

# Cargo.toml

[dependencies.replace_with]
features = ["panic_abort"]
...

… the "panic_abort" feature enables the replace_with_or_abort_unchecked() 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 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.