[Explicit Module Builds] Re-scan all clang modules against all targets against which they will be built. #214
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nkcsgexi
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Aug 20, 2020
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We may need to switch to the batch scanning mode soon 😄
…s against which they will be built. - Factor out placeholder dependency resolution out of the ExplicitModuleHandler into the Driver - Generate and post-process the dependency graph before passing it on to the ExplicitModuleHandler: - Run the Swift Fast Dependency Scanner. - Resolve placeholder dependencies, if any. - Re-scan and update Clang modules with a super-set of their dependencies against all compilation targets.
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Some background on this issue:
When building a Swift module at a fixed target version, the module may have dependencies which specify a different target version. It is possible that in this case, multiple swift modules, with different targets, have a dependency on the same Clang module. There is a hard requirement that the target version of the Clang module compilation match exactly the target version of the depending Swift module compilation.
The problem this PR addresses:
Dependency scanning results may vary depending on the target version specified on the dependency scanning action. If the scanning action is performed at a fixed target, and the scanned module is later compiled with a higher version target, miscomputation may occur due to dependencies present only at the higher version number and thus not detected by the dependency scanner.
Uses the newly-added Clang dependency scanning mode in the swift-frontend, added here:
swiftlang/swift#33522
Resolves rdar://67079780