Define coherence restrictions in the manual impl section.

[nejucomo]

I'm familiar with coherence restrictions on trait impls only from IRC conversations ages ago. It came up in IRC again, and I briefly skimmed the manual sections on impls and crates to see if this restriction was spelled out.

I may have missed it. Even in that case, I feel it would be most useful in the manual section describing impls: 6.1.9 Implementations (v0.8 manual)

pcwalton has a good blog post providing background: Coherence, Modularity, and Extensibility for Typeclasses

There was some confusion on IRC as to whether coherence restrictions are per-module or per-crate.

Some questions I'd like to see answered in the manual:

• Are trait impls "always public" even when buried in non-public modules?
• Can a crate define a trait in one module, A, and an impl in another module B coherently?

[huonw]

Triage: coherence is per-crate, since the compiler sees the whole crate at once (and so can reason about that).

Answers (if someone wishes to take this on):

Are trait impls "always public" even when buried in non-public modules?

Yes, impls are scoped with the trait, so if the trait is public, so are all impls no matter where the impls are placed.

Can a crate define a trait in one module, A, and an impl in another module B coherently?

Yes, that's fine, since coherence is per-crate.

[pnkfelix]

This seems like a sub-bug of #11166 to me...

[huonw]

@pnkfelix pointed out that I'd accidentally said "coherence is per-module" originally; fixed now.

[reem]

Triage: @pnkfelix is correct that this is part of #11166, so should be closed or #11166 should be converted to a meta-bug.

[steveklabnik]

I'm just going to close this in favor of that one.