error[E0200]: the trait `Foo` requires an `unsafe impl` declaration
  --> $DIR/safety-trait-impl-cc.rs:9:1
   |
LL | impl lib::Foo for Bar {
   | ^^^^^^^^^^^^^^^^^^^^^
   |
   = note: the trait `Foo` enforces invariants that the compiler can't check. Review the trait documentation and make sure this implementation upholds those invariants before adding the `unsafe` keyword
help: add `unsafe` to this trait implementation
   |
LL | unsafe impl lib::Foo for Bar {
   | ++++++

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0200`.