error[E0198]: negative impls cannot be unsafe
  --> $DIR/E0198.rs:5:13
   |
LL | unsafe impl !Send for Foo { }
   | ------      -^^^^
   | |           |
   | |           negative because of this
   | unsafe because of this

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0198`.