[interop] Inject header files into libstdcxx.modulemap dynamically

This allows the module map we use for libstdc++ to correctly include optional headers like "any" and other C++17/20 files.

Author: hyp

include/swift/AST/DiagnosticsClangImporter.def

@@ -124,6 +124,9 @@ WARNING(glibc_not_found, none,
 WARNING(libstdcxx_not_found, none,
         "libstdc++ not found for '%0'; C++ stdlib may be unavailable",
         (StringRef))
+WARNING(libstdcxx_modulemap_not_found, none,
+        "module map for libstdc++ not found for '%0'; C++ stdlib may be unavailable",
+        (StringRef))
 
 WARNING(too_many_class_template_instantiations, none,
         "template instantiation for '%0' not imported: too many instantiations",

include/swift/ClangImporter/ClangImporter.h

@@ -17,6 +17,7 @@
 #define SWIFT_CLANG_IMPORTER_H
 
 #include "swift/AST/ClangModuleLoader.h"
+#include "llvm/Support/VirtualFileSystem.h"
 
 /// The maximum number of SIMD vector elements we currently try to import.
 #define SWIFT_MAX_IMPORTED_SIMD_ELEMENTS 4
@@ -580,6 +581,18 @@ bool requiresCPlusPlus(const clang::Module *module);
 
 } // namespace importer
 
+struct ClangInvocationFileMapping {
+  SmallVector<std::pair<std::string, std::string>, 2> redirectedFiles;
+  SmallVector<std::pair<std::string, std::string>, 1> overridenFiles;
+};
+
+/// On Linux, some platform libraries (glibc, libstdc++) are not modularized.
+/// We inject modulemaps for those libraries into their include directories
+/// to allow using them from Swift.
+ClangInvocationFileMapping getClangInvocationFileMapping(
+    ASTContext &ctx,
+    llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> vfs = nullptr);
+
 } // end namespace swift
 
 #endif

lib/ClangImporter/ClangImporter.cpp

@@ -1102,8 +1102,25 @@ ClangImporter::create(ASTContext &ctx,
   auto fileMapping = getClangInvocationFileMapping(ctx);
   // Wrap Swift's FS to allow Clang to override the working directory
   llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> VFS =
-      llvm::vfs::RedirectingFileSystem::create(fileMapping, true,
-                                               *ctx.SourceMgr.getFileSystem());
+      llvm::vfs::RedirectingFileSystem::create(
+          fileMapping.redirectedFiles, true, *ctx.SourceMgr.getFileSystem());
+  if (!fileMapping.overridenFiles.empty()) {
+    llvm::IntrusiveRefCntPtr<llvm::vfs::InMemoryFileSystem> overridenVFS =
+        new llvm::vfs::InMemoryFileSystem();
+    for (const auto &file : fileMapping.overridenFiles) {
+      auto contents = ctx.Allocate<char>(file.second.size() + 1);
+      std::copy(file.second.begin(), file.second.end(), contents.begin());
+      // null terminate the buffer.
+      contents[contents.size() - 1] = '\0';
+      overridenVFS->addFile(file.first, 0,
+                            llvm::MemoryBuffer::getMemBuffer(
+                                StringRef(contents.begin(), contents.size() - 1)));
+    }
+    llvm::IntrusiveRefCntPtr<llvm::vfs::OverlayFileSystem> overlayVFS =
+        new llvm::vfs::OverlayFileSystem(VFS);
+    VFS = overlayVFS;
+    overlayVFS->pushOverlay(overridenVFS);
+  }
 
   // Create a new Clang compiler invocation.
   {