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`.