error[E0391]: cycle detected when computing the supertraits of `Processor`
  --> $DIR/issue-20825.rs:15:1
   |
LL | pub trait Processor: Subscriber<Input = Self::Input> {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: ...which again requires computing the supertraits of `Processor`, completing the cycle

error: aborting due to previous error

For more information about this error, try `rustc --explain E0391`.