Swift should allow multiple `SWIFT_CONFORMS_TO_PROTOCOL` attributes on a C++ class

[egorzhdan]

Description

swift/bridging header defines a SWIFT_CONFORMS_TO_PROTOCOL macro, which expands to __attribute__((swift_attr("conforms_to:MyModule.MyProtocol"))).

This attribute lets the Swift compiler know that the annotated C++ class should be automatically marked as conforming to a Swift protocol with the given name.

Currently the Swift compiler only respects one SWIFT_CONFORMS_TO_PROTOCOL attribute on a single C++ class at a time. This is a limitation, we should support multiple SWIFT_CONFORMS_TO_PROTOCOL attributes on a single class.

Reproduction

Expected behavior

If two or more SWIFT_CONFORMS_TO_PROTOCOL attributes are provided, Swift should conform the C++ type to all of the listed Swift protocols.

Environment

Recent Swift compiler from main

Additional information

This is the part of ClangImporter that handles explicit conformances:

swift/lib/ClangImporter/ImportDecl.cpp

Lines 2840 to 2842 in 375363a

	void
	addExplicitProtocolConformances(NominalTypeDecl *decl,
	const clang::CXXRecordDecl *clangDecl) {

This method would likely need to change as part of the resolution of this issue.

[jz709u]

I am working on this issue. I have a solution already, but still testing it

[jz709u]

I have been working on validating my solution.
It's been difficult as I can't do one-off toolchain builds based on this thread:
https://forums.swift.org/t/cant-build-toolchain/70247

I have resorted to validating my solution using the build-script tool and attempting to test it via unit tests. I can't find the corresponding unit tests which test this part of the code.

I would appreciate it if someone could point me in the right direction on the quickest way to test your solutions.
Thanks again

[CrazyFanFan]

@egorzhdan @jz709u Is it okay if I take over and continue working on this issue?

@egorzhdan can you assign this to me