Rust error E0525: expected a closure that implements the `Fn` trait, but this closure only implements `FnOnce`

What it meansA closure was used but didn't implement the expected trait.

What rustc prints

error[E0525]: expected a closure that implements the `Fn` trait, but this closure only implements `FnOnce`
  --> main.rs:8:19
   |
 8 |     let closure = |_| foo(x); // error: expected a closure that implements
   |                   ^^^     - closure is `FnOnce` because it moves the variable `x` out of its environment
   |                   |
   |                   this closure implements `FnOnce`, not `Fn`
...
11 |     bar(closure);
   |     --- ------- the requirement to implement `Fn` derives from here
   |     |
   |     required by a bound introduced by this call
   |
note: required by a bound in `bar`
  --> main.rs:4:11
   |
 4 | fn bar<T: Fn(u32)>(_: T) {}
   |           ^^^^^^^ required by this bound in `bar`

Real output of rustc 1.99.0 compiling the official example below (2021 edition). Your names and line numbers will differ; the first line is the part to match.

E0525Types and traits

Activein rustc 1.99.0

8related errors linked below

The compiler team's explanation

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);
}

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

From rustc --explain E0525 (1.99.0), part of the Rust project, MIT or Apache-2.0. Official page.

Up next

Often hit nextE0308: mismatched typesA related error from the same family.Read it next →

Also worth your time

Did this answer your question?

Rust error E0525: expected a closure that implements the `Fn` trait, but this closure only implements `FnOnce`

What rustc prints

The compiler team's explanation

Related errors

Popular in the shop