Collective bikeshed of `#[marker] trait Foo { .. }`

language design
19

auto traits are not implicitly #[marker], but can be. Positive and negative impls still conflict:

error[E0119]: conflicting implementations of trait `Foo` for type `u32`:
 --> src/lib.rs:7:1
  |
6 | impl<T:Copy> Foo for T {} 
  | ---------------------- first implementation here
7 | impl !Foo for u32 {}
  | ^^^^^^^^^^^^^^^^^ conflicting implementation for `u32`