has_impl.rs

#![no_std]

macro_rules! has_impl {
	($ty:ty: $tr:path) => {
		{
			struct Test<T: ?Sized>(core::marker::PhantomData<T>);

			#[allow(dead_code)]
			impl<T: ?Sized + $tr> Test<T> {
				const HAS_IMPL: bool = true;
			}

			trait Fallback {
				const HAS_IMPL: bool = false;
			}

			impl<T: ?Sized> Fallback for T {}

			Test::<$ty>::HAS_IMPL
		}
	};
}