error[E0200]: the trait `Bar` requires an `unsafe impl` declaration
  --> $DIR/E0200.rs:5:1
   |
LL | impl Bar for Foo { }
   | ^^^^^^^^^^^^^^^^
   |
   = note: the trait `Bar` 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 Bar for Foo { }
   | ++++++

error: aborting due to 1 previous error

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