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+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 better understand how these work in Rust, read the [Closures][closures]
+chapter of the Book.
+
+[closures]: https://doc.rust-lang.org/book/ch13-01-closures.html