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

If there is a `-fmodule-map-file` argument whose file doesn’t exist and SwiftShims is not in the module cache, we fail to build it, because clang throws an error about the missing module map. This causes SourceKit to drop all semantic functionality, even if the missing module map isn’t required.

To work around this, drop all `-fmodule-map-file` arguments with missing files from the clang importer’s arguments, reporting the eror that `clang` would throw manually.

Fixes rdar://77449671

Author: ahoppen

include/swift/AST/DiagnosticsClangImporter.def

@@ -87,5 +87,7 @@ WARNING(import_multiple_mainactor_attr,none,
       "this attribute for global actor '%0' is invalid; the declaration already has attribute for global actor '%1'",
       (StringRef, StringRef))
 
+ERROR(module_map_not_found, none, "module map file '%0' not found", (StringRef))
+
 #define UNDEFINE_DIAGNOSTIC_MACROS
 #include "DefineDiagnosticMacros.h"

lib/ClangImporter/ClangImporter.cpp

@@ -993,8 +993,38 @@ 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.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