How do I store a closure in a struct in Rust?

Before Rust 1.0, I could write a structure using this obsolete closure syntax:

struct Foo {
    pub foo: |usize| -> usize,
}

Now I can do something like:

struct Foo<F: FnMut(usize) -> usize> {
    pub foo: F,
}

But then what's the type of a Foo object I create?

let foo: Foo<???> = Foo { foo: |x| x + 1 };

I could also use a reference:

struct Foo<'a> {
    pub foo: &'a mut FnMut(usize) -> usize,
}

I think this is slower because

1. the pointer dereference
2. there's no specialization for the type of FnMut that actually ends up being used

Complementing the existing answer with some more code for demonstration purposes:

Unboxed closure

Use a generic type:

struct Foo<F>
where
    F: Fn(usize) -> usize,
{
    pub foo: F,
}

impl<F> Foo<F>
where
    F: Fn(usize) -> usize,
{
    fn new(foo: F) -> Self {
        Self { foo }
    }
}

fn main() {
    let foo = Foo { foo: |a| a + 1 };
    (foo.foo)(42);
    
    (Foo::new(|a| a + 1).foo)(42);
}

Boxed trait object

struct Foo {
    pub foo: Box<dyn Fn(usize) -> usize>,
}

impl Foo {
    fn new(foo: impl Fn(usize) -> usize + 'static) -> Self {
        Self { foo: Box::new(foo) }
    }
}

fn main() {
    let foo = Foo {
        foo: Box::new(|a| a + 1),
    };
    (foo.foo)(42);
    
    (Foo::new(|a| a + 1).foo)(42);
}

Trait object reference

struct Foo<'a> {
    pub foo: &'a dyn Fn(usize) -> usize,
}

impl<'a> Foo<'a> {
    fn new(foo: &'a dyn Fn(usize) -> usize) -> Self {
        Self { foo }
    }
}

fn main() {
    let foo = Foo { foo: &|a| a + 1 };
    (foo.foo)(42);
    
    (Foo::new(&|a| a + 1).foo)(42);
}

Function pointer

struct Foo {
    pub foo: fn(usize) -> usize,
}

impl Foo {
    fn new(foo: fn(usize) -> usize) -> Self {
        Self { foo }
    }
}

fn main() {
    let foo = Foo { foo: |a| a + 1 };
    (foo.foo)(42);
    
    (Foo::new(|a| a + 1).foo)(42);
}

---

> what's the type of a Foo object I create?

It's an unnameable, automatically generated type.

> I could also use a reference [...] slower because [...] the pointer deref [...] no specialization

Perhaps, but it can be much easier on the caller.

See also:

• https://stackoverflow.com/q/27994509/155423
• https://stackoverflow.com/q/25445761/155423
• https://stackoverflow.com/q/49980850/155423
• https://stackoverflow.com/q/41082409/155423
• https://stackoverflow.com/q/27874683/155423
• https://stackoverflow.com/q/52696907/155423
• https://stackoverflow.com/q/50650070/155423

For what type you'd use in your third code snippet, there isn't one; closure types are anonymous and cannot be directly named. Instead, you'd write:

let foo = Foo { foo: |x| x + 1 };

If you're writing code in a context where you need to specify that you want a Foo, you'd write:

let foo: Foo<_> = Foo { foo: |x| x + 1 };

The _ tells the type system to infer the actual generic type for you.

The general rule of thumb as to which to use, in descending order:

• Generic parameters: struct Foo<F: FnMut(usize) -> usize>. This is the most efficient, but it does mean that a specific Foo instance can only ever store one closure, since every closure has a different concrete type.
• Trait references: &'a mut dyn FnMut(usize) -> usize. There's a pointer indirection, but now you can store a reference to any closure that has a compatible call signature.
• Boxed closures: Box<dyn FnMut(usize) -> usize>. This involves allocating the closure on the heap, but you don't have to worry about lifetimes. As with a reference, you can store any closure with a compatible signature.

Before Rust 1.0

Closures that used the || syntax were references to closures stored on the stack, making them equivalent to &'a mut FnMut(usize) -> usize. Old-style procs were heap-allocated and were equivalent to Box<dyn FnOnce(usize) -> usize> (you can only call a proc once).