Ignore dead_code warning in test

    warning: trait `IsNotImplemented` is never used
      --> tests/marker.rs:24:27
       |
    24 |                     trait IsNotImplemented {
       |                           ^^^^^^^^^^^^^^^^
    ...
    47 | assert_impl!(Group is not Send or Sync);
       | --------------------------------------- in this macro invocation
       |
       = note: `#[warn(dead_code)]` on by default
       = note: this warning originates in the macro `assert_impl` (in Nightly builds, run with -Z macro-backtrace for more info)

tests/marker.rs

@@ -21,6 +21,7 @@ macro_rules! assert_impl {
  $(
  {
  // Implemented for types that implement $marker.
+ #[allow(dead_code)]
  trait IsNotImplemented {
  fn assert_not_implemented() {}
  }