[cxx-interop] Do not emit C++ interop flag in textual interfaces

[egorzhdan]

This makes sure that the compiler does not emit -enable-experimental-cxx-interop/-cxx-interoperability-mode flags in .swiftinterface files. Those flags were breaking explicit module builds. The module can still be rebuilt from its textual interface if C++ interop was enabled in the current compilation.

rdar://140203932

[egorzhdan]

@swift-ci please test

[tshortli]

It makes sense to me that when Cxx interop is an implementation detail of a module, but not used in its interface, that the flag should be hidden to avoid affecting the mode in which downstream dependencies are compiled with respect to the client. What about when the interface does depend on Cxx interop, though?

I have not read Allan's comment before reviewing.

[egorzhdan]

What about when the interface does depend on Cxx interop, though?

If the -cxx-interoperability-flag was passed to the current compiler invocation, it should get propagated to the compiler sub-instance that rebuilds the module from its interface, and the build would succeed. If the flag wasn't passed, the user would see a compiler error when trying to build the module.

The user-facing message isn't ideal, it could explicitly mention the fix of adding the missing flag – we should improve that in the future.

[egorzhdan]

@swift-ci please test with following PR:

swiftlang/swift-driver#1737

[tshortli]

The user-facing message isn't ideal, it could explicitly mention the fix of adding the missing flag – we should improve that in the future.

Ok, yeah I'd like to see something like that in the future to make failures to build textual interfaces more explainable and debuggable. For example, we could require that owners of libraries that emit textual interfaces which externally depend on Cxx interop specify a new flag that does get printed in the interface that causes client builds to fail fast if they do not enable interop themselves. The module interface verification build task should be able to diagnose whether this new flag is needed for the library owner.

[egorzhdan]

Please test with following PR:

swiftlang/swift-driver#1737

@swift-ci please test

[egorzhdan]

Please test with following PR:

swiftlang/swift-driver#1737

@swift-ci please test

[egorzhdan]

Please test with following PR:

swiftlang/swift-driver#1737

@swift-ci please test

[egorzhdan]

Please test with following PR:

swiftlang/swift-driver#1737

@swift-ci please test