Error code E0525

A closure was used but didn’t implement the expected trait.

Erroneous code example:

struct X;

fn foo<T>(_: T) {}
fn bar<T: Fn(u32)>(_: T) {}

fn main() {
    let x = X;
    let closure = |_| foo(x); // error: expected a closure that implements
                              //        the `Fn` trait, but this closure only
                              //        implements `FnOnce`
    bar(closure);
}

Run

In the example above, closure is an FnOnce closure whereas the bar function expected an Fn closure. In this case, it’s simple to fix the issue, you just have to implement Copy and Clone traits on struct X and it’ll be ok:

#[derive(Clone, Copy)] // We implement `Clone` and `Copy` traits.
struct X;

fn foo<T>(_: T) {}
fn bar<T: Fn(u32)>(_: T) {}

fn main() {
    let x = X;
    let closure = |_| foo(x);
    bar(closure); // ok!
}

Run

To better understand how these work in Rust, read the Closures chapter of the Book.

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