The following line:

impl<T: Discovery> Discovery for Arc<T> {}

makes it so that if someone changes/incorrectly writes:

builder.add_discovery(|_| Arc::new(Foo)));

instead of

builder.add_discovery(|_| Foo);

the code suddenly does nothing because default impls for methods on Discovery will just ignore the T/Foo.

For whatever reason we did it in our code, and it was such a weird bug to debug. :D