“cannot move a value of type FnOnce” when moving a

Question

I'm trying to do some higher order programming in Rust, but I'm having some difficulty dealing with closures. Here's a code snippet that illustrates one of the problems I'm having:

pub enum Foo {
    Bar(Box<FnOnce(i32)>),
}

pub fn app(i: i32, arg: Foo) {
    match arg {
        Foo::Bar(f) => f(i),
    }
}

When I compile this piece of code I get the following error message:

error[E0161]: cannot move a value of type std::ops::FnOnce(i32) + 'static: the size of std::ops::FnOnce(i32) + 'static cannot be statically determined
 --> src/main.rs:7:24
  |
7 |         Foo::Bar(f) => f(i),
  |                        ^

Since I put the function in a Box, I would have thought that that would deal with the problem of the compiler not knowing the size. How can I make the above program compile?

Answer 1

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<FnOnce> is a different type altogether.

As a result, it's more or less impossible.

Now, there is a type in the standard library for dealing with just this situation: FnBox. Unfortunately, it's only recently been added, and thus cannot be used in stable Rust yet (that's the "or less" part).

So, your 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.

Answer 2

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.