These shouldn't be too hard to implement with two pre-allocated buffers of type T and use of the RCU pattern:

• has two states: FULL and EMPTY.
• sender can send type T to receiver
• send blocks while state is FULL
• recv blocks while state is EMPTY
• receiver can peek(). blocking or failing is fine. returns &T without setting state to EMPTY
• It needs to be fast (no spin-wait!).
• does not require T to have Clone
• Does not need SPSC (precisely two threads interact with it)
• Does not need more than 1 slot.

This may be a little tricky:

• Bonus: send() and recv() return error as soon as their counterpart is dropped.

And this depends entirely on how select is implemented, and may be a performance challenge:

• Need a way to select from a set of senders receivers (block until one becomes ready AND know which it was)