Macro static_assertions::assert_obj_safe

macro_rules! assert_obj_safe {
    ($($xs:ty),+ $(,)*) => { ... };
    ($label:ident; $($xs:tt)+) => { ... };
}

Asserts at compile-time that the traits are object-safe.

This is useful for when changes are made to a trait that accidentally prevent it from being used as an object.