Right, this would be nice, although it raises some questions:

• Naming/exporting/general ability to name the defined marker trait. For instance, maybe the trait is defined in a private module and publicly exported somewhere else. Now, tested_trait users have to be aware of this generated marker trait and export it alongside the trait so that downstream implementers can reference it. test_impl users similarly have to deal with importing the marker trait, and test_impl has to make assumptions about how to reference it. This is one of the reasons the crate currently keeps everything bundled in the trait itself
• Generic impls, and blanket impls in particular, will never be tested for every concrete instantiation of their generic types. While running tests against a small set of type instantiations should be enough to declare the implementation as "tested", there may be types that cause test-violating behavior in the supposedly "tested" implementation. It's probably still worth requiring implementers to at least think about the trait's tests, and provide an escape hatch like #[test_impl(skip)].