[5.4][ClangImporter] Load the stdlib even if there is a -fmodule-map-file argument pointing to a missing file

include/swift/AST/DiagnosticsClangImporter.def

@@ -80,5 +80,7 @@ WARNING(implicit_bridging_header_imported_from_module,none,
         "is deprecated and will be removed in a later version of Swift",
         (StringRef, Identifier))
 
+ERROR(module_map_not_found, none, "module map file '%0' not found", (StringRef))
+
 #define UNDEFINE_DIAGNOSTIC_MACROS
 #include "DefineDiagnosticMacros.h"

lib/ClangImporter/ClangImporter.cpp

@@ -987,8 +987,38 @@ ClangImporter::createClangInvocation(ClangImporter *importer,
                                                  &tempDiagClient,
                                                  /*owned*/false);
 
-  return clang::createInvocationFromCommandLine(invocationArgs, tempClangDiags,
-                                                nullptr, false, CC1Args);
+  auto CI = clang::createInvocationFromCommandLine(
+      invocationArgs, tempClangDiags, nullptr, false, CC1Args);
+
+  if (!CI) {
+    return CI;
+  }
+
+  // FIXME: clang fails to generate a module if there is a `-fmodule-map-file`
+  // argument pointing to a missing file.
+  // Such missing module files occur frequently in SourceKit. If the files are
+  // missing, SourceKit fails to build SwiftShims (which wouldn't have required
+  // the missing module file), thus fails to load the stdlib and hence looses
+  // all semantic functionality.
+  // To work around this issue, drop all `-fmodule-map-file` arguments pointing
+  // to missing files and report the error that clang would throw manually.
+  // rdar://77516546 is tracking that the clang importer should be more
+  // resilient and provide a module even if there were building it.
+  auto VFS = clang::createVFSFromCompilerInvocation(
+      *CI, *tempClangDiags,
+      importer->Impl.SwiftContext.SourceMgr.getFileSystem());
+  std::vector<std::string> FilteredModuleMapFiles;
+  for (auto ModuleMapFile : CI->getFrontendOpts().ModuleMapFiles) {
+    if (VFS->exists(ModuleMapFile)) {
+      FilteredModuleMapFiles.push_back(ModuleMapFile);
+    } else {
+      importer->Impl.SwiftContext.Diags.diagnose(
+          SourceLoc(), diag::module_map_not_found, ModuleMapFile);
+    }
+  }
+  CI->getFrontendOpts().ModuleMapFiles = FilteredModuleMapFiles;
+
+  return CI;
 }
 
 std::unique_ptr<ClangImporter>

test/SourceKit/CursorInfo/missing_module_map.swift

@@ -0,0 +1,9 @@
+// RUN: %empty-directory(%t.mcp)
+// RUN: %sourcekitd-test -req=cursor -pos=6:9 %s -- -Xcc -fmodule-map-file=/some/missing/file -module-cache-path %t.mcp %s | %FileCheck %s
+
+// We used to fail to load the stdlib if there is a `-fmodule-map-file` option pointing to a missing file
+
+let x: String = "abc"
+// CHECK: source.lang.swift.ref.struct ()
+// CHECK: String
+// CHECK: s:SS