error[E0308]: mismatched types
  --> $DIR/option_mut_coerce_shared.rs:5:12
   |
LL |     method(a);
   |     ------ ^ types differ in mutability
   |     |
   |     arguments to this function are incorrect
   |
   = note: expected enum `Option<&()>`
              found enum `Option<&mut ()>`
note: function defined here
  --> $DIR/option_mut_coerce_shared.rs:1:4
   |
LL | fn method(a: Option<&()>) {}
   |    ^^^^^^ --------------
help: try using `.as_deref()` to convert `Option<&mut ()>` to `Option<&()>`
   |
LL |     method(a.as_deref());
   |             +++++++++++

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0308`.