src/test/ui/syntax-trait-polarity-feature-gate.stderr

error[E0658]: negative trait bounds are not yet fully implemented; use marker types for now (see issue #13231)
  --> $DIR/syntax-trait-polarity-feature-gate.rs:17:1
   |
LL | impl !Send for TestType {}
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = help: add #![feature(optin_builtin_traits)] to the crate attributes to enable

error: aborting due to previous error

For more information about this error, try `rustc --explain E0658`.