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::<T>::map` takes ownership of the receiver `self`, which moves `*cb`
  --> $SRC_DIR/core/src/option.rs:LL:COL
help: you could `clone` the value and consume it, if the `&mut dyn FnMut(): Clone` trait bound could be satisfied
   |
LL |     <Option<&mut dyn FnMut()> as Clone>::clone(&cb).map(|cb| cb());
   |     ++++++++++++++++++++++++++++++++++++++++++++  +

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`.