[cxx-interop] Add ability to specify protocol conformance on C++ side.

[zoecarver]

No description provided.

[zoecarver]

@swift-ci please test

[ktoso]

Really awesome, thank you @zoecarver :)

[hyp]

We should error out if the protocol lookup fails to catch typos .

[hyp]

we should also error out on ambiguous lookups, no?

[zoecarver]

@swift-ci please test

[hyp]

Debug output, pls rm -rf.

[zoecarver]

@swift-ci please test

[zoecarver]

@swift-ci please test

[egorzhdan]

This is a great improvement but I'm a bit worried that importing a C++ module can now trigger loading of an unrelated Swift module, which might run into cyclic dependencies.

Could you please add a test that has something like this:

1. a C++ module A that contains a decl with conforms_to:X attr
2. a Swift module B that imports A
3. a Swift module C that imports B and contains protocol X

[zoecarver]

@swift-ci please test

[zoecarver]

This is a great improvement but I'm a bit worried that importing a C++ module can now trigger loading of an unrelated Swift module, which might run into cyclic dependencies.

Could you please add a test that has something like this:

1. a C++ module A that contains a decl with conforms_to:X attr
2. a Swift module B that imports A
3. a Swift module C that imports B and contains protocol X

Yeah this is a good point. Regardless, it is not ideal that we have to load all the modules and potentially have ambiguities. What if we just required people to specify the module in the attribute string?

[ktoso]

What if we just required people to specify the module in the attribute string?

I'd be more than happy with that; requiring to be precise here sounds good tbh.

[zoecarver]

I will address the review comments post-commit. I think we should require the user to specify both the module and the protocol.