[feat] add Unused

[muir]

Add Unused, a provider that will never be included in an injection chain.

[dncohen]

IMHO it would be even better if the function returned a type Unused. That is, explicitly in the function signature it will never be part of the chain.

I think returning Provider invites someone to write a function that conditionally returns Unused in some case, and some other provider in others. Which I expect would become hard logic to follow.

Take or leave that suggestion, I'm approving as is.

[muir]

Hmm. Both are useful. Making a type Unused is more work. I can see how it is more documenting though. The question is: what should the behavior be if someone wants to have Unused as an input?

I guess the most robust answer is that Unused is not preserved: as an input, it's magically supplied even if there is no provider and as an output, the value is not preserved.

[dncohen]

Hmm. Both are useful. Making a type Unused is more work. I can see how it is more documenting though. The question is: what should the behavior be if someone wants to have Unused as an input?

An error? I think better to fail than silently do something unexpected. Users will assume rather than understand (I say this as a user).

[dncohen]

as discussed, helper to "append to front" and "append near end"