Summary

Unblock the evolution of key trait hierarchies:

• Adding Receiver
  as a supertrait of Deref.
• Allow the tower::Service trait to be split into a non-Sync supertrait and a
  Sync (thread-safe) subtrait.

The design should incorporate the feedback from the
Evolving trait hierarchies
language design meeting. The design should also set the stage for future changes to allow for
the more general case of splitting items out into supertraits.

Tasks and status

• Discussion and moral support (lang )
• Discussion and moral support (compiler )
• Discussion and moral support (types )
• Author RFC (@dingxiangfei2009 with help from @cramertj)
• Implementation (@cramertj)
• Stabilization decision (libs-api )
• Stabilization decision (lang )