Support mixed Swift/clang targets with SwiftBuild #9007
Description
Description
One of the benefits we get from the SwiftBuild build system is that it already supports targets that have mixed Swift and clang language family source files. This is accomplished using a bridging header file that exposes the API to Swift, specifying those headers using the -import-bridging-header option.
I was able to quickly prototype this by removing the check in SwiftPM for mixed language targets and then manually adding in a swiftSetting unsafe flag for the bridging header.
For proper ergonomics we should add in a proper flag for this swiftSetting that would specify the path to the header. We could then use the proper build setting when generating the PIF. That appears to be SWIFT_OBJC_BRIDGING_HEADER. Digging into the swift compiler code I see that -import-bridging-header is an alias to -import-objc-header and both of those options are hidden making this setting even more necessary. Note this will require a Swift Evolution proposal and review.
Question remains what to do if the consumer of the package is using the native build system. We probably need to error out in that case. It's not worth adding mixed language support there since it wasn't really built for that.
Expected behavior
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Actual behavior
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Steps to reproduce
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Swift Package Manager version/commit hash
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Swift & OS version (output of swift --version && uname -a)
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