[C++Interop] Do not query C++ Standard Library Swift overlays when building Swift modules which were built without C++ interop #81908
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…tual modules which were not built with c++interop When the compiler is building a module without a defined formal C++ interop mode (e.g. building a textual interface which specifies it was built without C++ interop enabled), avoid looking up the C++ standard library Swift overlay for it. This is required for the case of the Darwin module, for example, which includes headers which map to C++ stdlib headers when the compiler is operating in C++ interop mode, but the C++ standard library Swift overlay module itself depends on 'Darwin', which results in a cycle. To resolve such situations, we can rely on the fact that Swift textual interfaces of modules which were not built with C++ interop must be able to build without importing the C++ standard library Swift overlay, so we avoid specifying it as a dependency for such modules. The primary source module, as well as Swift textual module dependencies which were built with C++ interop will continue getting a direct depedency of the 'CxxStdlib' Swift module. This was previously fixed in the dependency scanner for explicitly-built modules in swiftlang#81415.
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lib/Frontend/CompilerInvocation.cpp
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| @@ -737,7 +738,10 @@ void LangOptions::setCxxInteropFromArgs(ArgList &Args, | |||
| // version, and is either 4, 5, 6, or 7 (even though only 5.9 and 6.* make | |||
| // any sense). For now, we don't actually care about the version, so we'll | |||
| // just use version 6 (i.e., 'swift-6') to mean that C++ interop mode is on. | |||
| if (EnableCXXInterop) | |||
| // | |||
| // FIXME: Always declare the 'Darwin' module as formally having been built | |||
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This FIXME reads like this is something should be fixed to do, not what currently do that needs to be fixed. Also there is a separate check for Darwin module name that probably also requires a FIXME
Textual interfaces for 'Darwin' built with recent compilers specify that it is built witout C++ interop enabled. However, to ensure compatibility with versions of the 'Darwin' module built with older compilers, we hard-code this fact. This is required to break the module cycle that occurs when building the 'Darwin' module with C++ interop enabled, where the underlying 'Darwin' clang module depends on C++ standard library for which the compiler brings in the 'CxxStdlib' Swift overlay, which depends on 'Darwin'.
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Similarly to how this was fixed in #81415 in the Dependency Scanner for builds using Explicitly-Built modules, this change causes Implicitly-Built module code path to also forego querying Swift overlay
CxxStdlibwhen the compiler is building a module with no declared "formal C++ interop mode".This change also temporarily hard-codes the
Darwinmodule to always have no formal C++ interop mode, for the sake of compatibility withDarwinSwift module textual interfaces which were constructed prior to the introduction of the-formal-cxx-interoperability-mode=option.