"cannot move a value of type FnOnce" when moving a boxed function

It is unlikely FnBox becomes stable, but for the time being you can wrap the F: FnOnce(...) -> ... in an Option<F>, bind it in a mutable closure and unwrap and call it inside (so it panics if it gets called more than once); the resulting closure can be boxed as Box<FnMut(...) -> ...>, which you might want to wrap somehow to make sure it only gets used ("called") once.

See (my) boxfnonce crate.

Here's the FnOnce trait's definition (simplified a little):

pub trait FnOnce<Args> {
    type Output;

    fn call_once(self, args: Args) -> Self::Output;
}

To call a FnOnce closure, you need to be able to move the closure value itself into the invocation. Note that self has to be the actual closure type; a Box<dyn FnOnce> is a different type altogether.

Rust 1.35

Box<dyn FnOnce> is now able to be called; your original code works as-is.

Prior versions

There is a type in the standard library for working around this situation: FnBox. Unfortunately, it's unstable.

Your alternate choices are:

• Refactor the code so that instead of Box<FnOnce>, you preserve the actual closure type.
• Use Box<FnMut> instead.
• Wait for FnBox to stabilise.
• Switch to a nightly compiler.