[scanner] setup the clang importer injected redirecting overlay vfs for correct clang dependency scanning

This makes its possible to use -explicit-module-build to correctly find 'crt' swift module on windows

Author: hyp

include/swift/ClangImporter/ClangImporter.h

@@ -753,6 +753,14 @@ ClangInvocationFileMapping getClangInvocationFileMapping(
     llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> vfs = nullptr,
     bool suppressDiagnostic = false);
 
+/// Construct the clang overlay VFS that's needed for the clang instance
+/// used by the clang importer to find injected platform-specific modulemaps
+/// that are created by `getClangInvocationFileMapping`.
+llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem>
+createClangInvocationFileMappingVFS(
+    const ClangInvocationFileMapping &fileMapping, ASTContext &ctx,
+    llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> baseVFS);
+
 } // end namespace swift
 
 #endif

lib/ClangImporter/ClangImporter.cpp

@@ -1318,39 +1318,8 @@ ClangImporter::create(ASTContext &ctx,
   // Avoid creating indirect file system when using include tree.
   if (!ctx.ClangImporterOpts.HasClangIncludeTreeRoot) {
     // Wrap Swift's FS to allow Clang to override the working directory
-    VFS = llvm::vfs::RedirectingFileSystem::create(
-        fileMapping.redirectedFiles, true, *ctx.SourceMgr.getFileSystem());
-    if (importerOpts.DumpClangDiagnostics) {
-      llvm::errs() << "clang importer redirected file mappings:\n";
-      for (const auto &mapping : fileMapping.redirectedFiles) {
-        llvm::errs() << "   mapping real file '" << mapping.second
-                     << "' to virtual file '" << mapping.first << "'\n";
-      }
-      llvm::errs() << "\n";
-    }
-
-    if (!fileMapping.overridenFiles.empty()) {
-      llvm::IntrusiveRefCntPtr<llvm::vfs::InMemoryFileSystem> overridenVFS =
-          new llvm::vfs::InMemoryFileSystem();
-      for (const auto &file : fileMapping.overridenFiles) {
-        if (importerOpts.DumpClangDiagnostics) {
-          llvm::errs() << "clang importer overriding file '" << file.first
-                       << "' with the following contents:\n";
-          llvm::errs() << file.second << "\n";
-        }
-        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);
-    }
+    VFS = createClangInvocationFileMappingVFS(fileMapping, ctx,
+                                              ctx.SourceMgr.getFileSystem());
   }
 
   // Create a new Clang compiler invocation.

lib/ClangImporter/ClangIncludePaths.cpp

@@ -634,5 +634,46 @@ ClangInvocationFileMapping swift::getClangInvocationFileMapping(
 
   GetWindowsFileMappings(result, ctx, vfs,
                          result.requiresBuiltinHeadersInSystemModules);
+
   return result;
 }
+
+llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem>
+swift::createClangInvocationFileMappingVFS(
+    const ClangInvocationFileMapping &fileMapping, ASTContext &ctx,
+    llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> baseVFS) {
+  llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> redirectedVFS =
+      llvm::vfs::RedirectingFileSystem::create(fileMapping.redirectedFiles,
+                                               true, *baseVFS);
+  if (ctx.ClangImporterOpts.DumpClangDiagnostics) {
+    llvm::errs() << "clang importer redirected file mappings:\n";
+    for (const auto &mapping : fileMapping.redirectedFiles) {
+      llvm::errs() << "   mapping real file '" << mapping.second
+                   << "' to virtual file '" << mapping.first << "'\n";
+    }
+    llvm::errs() << "\n";
+  }
+
+  if (fileMapping.overridenFiles.empty())
+    return redirectedVFS;
+  llvm::IntrusiveRefCntPtr<llvm::vfs::InMemoryFileSystem> overridenVFS =
+      new llvm::vfs::InMemoryFileSystem();
+  for (const auto &file : fileMapping.overridenFiles) {
+    if (ctx.ClangImporterOpts.DumpClangDiagnostics) {
+      llvm::errs() << "clang importer overriding file '" << file.first
+                   << "' with the following contents:\n";
+      llvm::errs() << file.second << "\n";
+    }
+    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(redirectedVFS);
+  overlayVFS->pushOverlay(overridenVFS);
+  return overlayVFS;
+}

lib/DependencyScan/ModuleDependencyScanner.cpp

@@ -169,13 +169,54 @@ static bool isSwiftDependencyKind(ModuleDependencyKind Kind) {
          Kind == ModuleDependencyKind::SwiftPlaceholder;
 }
 
+// Wrap the clang file mapping overlay filesystems in a proxy filesystem,
+// to avoid enumerating the overlay filesystems in a `visitChildFileSystems`,
+// as clang's dependency scanner tries to validate that the number of
+// overlay file systems used is the same as the number of `-ivfsoverlay`
+// flags passed through the clang invocation. This validation causes an
+// assertion failure if the filesystem isn't hidden.
+// We don't need to expose the clang importer overlay filesystem to clang's
+// dependency scanner, as it only needs to optimize overlay VFS that are
+// explicitly passed to it via `-ivfsoverlay`.
+class SilentClangImporterOverlayWrapperFS : public llvm::vfs::ProxyFileSystem {
+public:
+  SilentClangImporterOverlayWrapperFS(
+      llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> fs,
+      llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> baseFS)
+      : llvm::vfs::ProxyFileSystem(fs), baseFS(baseFS) {}
+
+  void visitChildFileSystems(VisitCallbackTy Callback) override {
+    // Do not forward to proxy, instead count the base and forward
+    // to base directly, bypassing the installed clang overlay filesystems.
+    if (baseFS) {
+      Callback(*baseFS);
+      baseFS->visitChildFileSystems(Callback);
+    }
+  }
+
+private:
+  llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> baseFS;
+};
+
+static llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem>
+getClangInvocationOverlayScanningVFS(
+    ASTContext &ctx, llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> baseFS) {
+  auto fileMapping = swift::getClangInvocationFileMapping(
+      ctx, baseFS, /*suppressDiagnostic=*/true);
+  if (fileMapping.redirectedFiles.empty())
+    return baseFS;
+  return createClangInvocationFileMappingVFS(fileMapping, ctx, baseFS);
+}
+
 ModuleDependencyScanningWorker::ModuleDependencyScanningWorker(
     SwiftDependencyScanningService &globalScanningService,
     const CompilerInvocation &ScanCompilerInvocation,
     const SILOptions &SILOptions, ASTContext &ScanASTContext,
     swift::DependencyTracker &DependencyTracker, DiagnosticEngine &Diagnostics)
-    : clangScanningTool(*globalScanningService.ClangScanningService,
-                        globalScanningService.getClangScanningFS()) {
+    : clangScanningTool(
+          *globalScanningService.ClangScanningService,
+          getClangInvocationOverlayScanningVFS(
+              ScanASTContext, globalScanningService.getClangScanningFS())) {
   // Create a scanner-specific Invocation and ASTContext.
   workerCompilerInvocation =
       std::make_unique<CompilerInvocation>(ScanCompilerInvocation);

test/ScanDependencies/win-crt.swift

@@ -0,0 +1,11 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend -scan-dependencies -Xcc -v %s -o - | %validate-json | %FileCheck %s
+
+// We want to explicitly import WinSDK's CRT.
+// REQUIRES: OS=windows-msvc
+
+import CRT
+
+// CHECK:        "modulePath": "{{.*}}\\ucrt-{{.*}}.pcm",
+// CHECK-NEXT:   "sourceFiles": [
+// CHECK-NEXT:      "{{.*}}\\ucrt\\module.modulemap"