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+Manual implementation of a `Fn*` trait.
+
+Erroneous code example:
+
+```compile_fail,E0183
+struct MyClosure {
+    foo: i32
+}
+
+impl FnOnce<()> for MyClosure {  // error
+    type Output = ();
+    extern "rust-call" fn call_once(self, args: ()) -> Self::Output {
+        println!("{}", self.foo);
+    }
+}
+```
+
+Manually implementing `Fn`, `FnMut` or `FnOnce` is unstable
+and requires `#![feature(fn_traits, unboxed_closures)]`.
+
+```
+#![feature(fn_traits, unboxed_closures)]
+
+struct MyClosure {
+    foo: i32
+}
+
+impl FnOnce<()> for MyClosure {  // ok!
+    type Output = ();
+    extern "rust-call" fn call_once(self, args: ()) -> Self::Output {
+        println!("{}", self.foo);
+    }
+}
+```
+
+The arguments must be a tuple representing the argument list.
+For more info, see the [tracking issue][iss29625]:
+
+[iss29625]: https://github.com/rust-lang/rust/issues/29625