error[E0603]: module `b` is private
  --> $DIR/re-exported-trait.rs:11:9
   |
LL | impl a::b::Trait for S {}
   |         ^ private module
   |
note: the module `b` is defined here
  --> $DIR/re-exported-trait.rs:5:5
   |
LL |     mod b {
   |     ^^^^^
help: consider importing this trait through its public re-export instead
   |
LL | impl a::Trait for S {}
   |      ~~~~~~~~

error: aborting due to 1 previous error

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