error[E0720]: cannot resolve opaque type
  --> $DIR/binding-without-value.rs:5:13
   |
LL |     let _ : impl Copy;
   |         -   ^^^^^^^^^ cannot resolve opaque type
   |         |
   |         this binding might not have a concrete type
   |
help: set the binding to a value for a concrete type to be resolved
   |
LL |     let _ : impl Copy = /* value */;
   |                       ^^^^^^^^^^^^^

error: aborting due to previous error

For more information about this error, try `rustc --explain E0720`.