error[E0603]: module `b` is private
  --> $DIR/private-trait.rs:8:9
   |
LL | impl a::b::Hidden for S {}
   |         ^  ------ trait `Hidden` is not publicly re-exported
   |         |
   |         private module
   |
note: the module `b` is defined here
  --> $DIR/private-trait.rs:2:5
   |
LL |     mod b {
   |     ^^^^^

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0603`.