error: unsupported &mut [T] element type: opaque C++ type is not supported yet
 --> $DIR/slice_unsupported.rs:6:17
  |
6 |         fn f(_: &mut [Opaque]);
  |                 ^^^^^^^^^^^^^

error: needs a cxx::ExternType impl in order to be used as a slice element in &mut [Opaque]
 --> $DIR/slice_unsupported.rs:4:9
  |
4 |         type Opaque;
  |         ^^^^^^^^^^^