error[E0507]: cannot move out of `*cb` which is behind a mutable reference --> $DIR/suggest-as-ref-on-mut-closure.rs:7:5 | LL | cb.map(|cb| cb()); | ^^^-------------- | | | | | `*cb` moved due to this method call | help: consider calling `.as_ref()` or `.as_mut()` to borrow the type's contents | move occurs because `*cb` has type `Option<&mut dyn FnMut()>`, which does not implement the `Copy` trait | note: `Option::::map` takes ownership of the receiver `self`, which moves `*cb` --> $SRC_DIR/core/src/option.rs:LL:COL error[E0596]: cannot borrow `*cb` as mutable, as it is behind a `&` reference --> $DIR/suggest-as-ref-on-mut-closure.rs:12:26 | LL | cb.as_ref().map(|cb| cb()); | -- ^^ `cb` is a `&` reference, so the data it refers to cannot be borrowed as mutable | | | consider changing this binding's type to be: `&mut &mut dyn FnMut()` error: aborting due to 2 previous errors Some errors have detailed explanations: E0507, E0596. For more information about an error, try `rustc --explain E0507`.