A closure was used but didn't implement the expected trait.
Erroneous code example:
```compile_fail,E0525
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);
}
```
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!
}
```
To understand better how closures work in Rust, read:
https://doc.rust-lang.org/book/ch13-01-closures.html