Function replace_with::replace_with_or_default

pub fn replace_with_or_default<T: Default, F: FnOnce(T) -> T>(
    dest: &mut T, 
    f: F
)

Temporarily takes ownership of a value at a mutable location, and replace it with a new value based on the old one. Replaces with Default::default() on panic.

We move out of the reference temporarily, to apply a closure f, returning a new value, which is then placed at the original value's location.

An important note

On panic (or to be more precise, unwinding) of the closure f, T::default() will be called to provide a replacement value. T::default() should not panic – doing so will constitute a double panic and will most likely abort the process.

Equivalent to replace_with(dest, T::default, f).

Differs from *dest = mem::replace(dest, Default::default()) in that Default::default() will only be called on panic.

Example

enum States {
    A(String),
    B(String),
}

impl Default for States {
    fn default() -> Self {
        States::A(String::new())
    }
}

impl States {
    fn poll(&mut self) {
        replace_with_or_default(self, |self_| match self_ {
            States::A(a) => States::B(a),
            States::B(a) => States::A(a),
        });
    }
}