Add an option to disable ClangImporter imports form source.

This is primarily meant to used for testing LLDB's DWARFImporterDelegate,
however, this could become the default option for LLDB once
DWARFImporterDelegate is sufficiently mature.

<rdar://problem/57880844>

Author: adrian-prantl

include/swift/ClangImporter/ClangImporterOptions.h

@@ -99,6 +99,10 @@ class ClangImporterOptions {
   /// When set, don't enforce warnings with -Werror.
   bool DebuggerSupport = false;
 
+  /// When set, ClangImporter is disabled, and all requests go to the
+  /// DWARFImporter delegate.
+  bool DisableSourceImport = false;
+
   /// Return a hash code of any components from these options that should
   /// contribute to a Swift Bridging PCH hash.
   llvm::hash_code getPCHHashComponents() const {

include/swift/Option/FrontendOptions.td

@@ -268,6 +268,10 @@ def debug_crash_after_parse : Flag<["-"], "debug-crash-after-parse">,
 def debugger_support : Flag<["-"], "debugger-support">,
   HelpText<"Process swift code as if running in the debugger">;
 
+def disable_clangimporter_source_import : Flag<["-"],
+  "disable-clangimporter-source-import">,
+  HelpText<"Disable ClangImporter and forward all requests straight the DWARF importer.">;
+
 def disable_arc_opts : Flag<["-"], "disable-arc-opts">,
   HelpText<"Don't run SIL ARC optimization passes.">;
 def disable_ossa_opts : Flag<["-"], "disable-ossa-opts">,

lib/ClangImporter/ClangImporter.cpp

@@ -1757,18 +1757,24 @@ ModuleDecl *ClangImporter::Implementation::loadModuleClang(
   return finishLoadingClangModule(clangModule, /*preferOverlay=*/false);
 }
 
-ModuleDecl *ClangImporter::loadModule(
-    SourceLoc importLoc,
-    ArrayRef<std::pair<Identifier, SourceLoc>> path) {
-  ModuleDecl *MD = Impl.loadModuleClang(importLoc, path);
+ModuleDecl *
+ClangImporter::loadModule(SourceLoc importLoc,
+                          ArrayRef<std::pair<Identifier, SourceLoc>> path) {
+  return Impl.loadModule(importLoc, path);
+}
+
+ModuleDecl *ClangImporter::Implementation::loadModule(
+    SourceLoc importLoc, ArrayRef<std::pair<Identifier, SourceLoc>> path) {
+  ModuleDecl *MD = nullptr;
+  if (!DisableSourceImport)
+    MD = loadModuleClang(importLoc, path);
   if (!MD)
-    MD = Impl.loadModuleDWARF(importLoc, path);
+    MD = loadModuleDWARF(importLoc, path);
   return MD;
 }
 
 ModuleDecl *ClangImporter::Implementation::finishLoadingClangModule(
-    const clang::Module *clangModule,
-    bool findOverlay) {
+    const clang::Module *clangModule, bool findOverlay) {
   assert(clangModule);
 
   // Bump the generation count.
@@ -1954,6 +1960,7 @@ ClangImporter::Implementation::Implementation(
       BridgingHeaderLookupTable(new SwiftLookupTable(nullptr)),
       BuffersForDiagnostics(ctx.SourceMgr),
       platformAvailability(ctx.LangOpts), nameImporter(),
+      DisableSourceImport(opts.DisableSourceImport),
       DWARFImporter(dwarfImporterDelegate) {}
 
 ClangImporter::Implementation::~Implementation() {
@@ -2613,7 +2620,8 @@ void ClangImporter::lookupTypeDecl(
   clang::LookupResult lookupResult(sema, clangName, clang::SourceLocation(),
                                    lookupKind);
   bool foundViaClang = false;
-  if (sema.LookupName(lookupResult, /*Scope=*/nullptr)) {
+  if (!Impl.DisableSourceImport &&
+      sema.LookupName(lookupResult, /*Scope=*/nullptr)) {
     for (auto clangDecl : lookupResult) {
       if (!isa<clang::TypeDecl>(clangDecl) &&
           !isa<clang::ObjCContainerDecl>(clangDecl) &&

lib/ClangImporter/ImporterImpl.h

@@ -605,6 +605,10 @@ class LLVM_LIBRARY_VISIBILITY ClangImporter::Implementation
   bool shouldIgnoreBridgeHeaderTopLevelDecl(clang::Decl *D);
 
 private:
+  /// When set, ClangImporter is disabled, and all requests go to the
+  /// DWARFImporter delegate.
+  bool DisableSourceImport;
+  
   /// The DWARF importer delegate, if installed.
   DWARFImporterDelegate *DWARFImporter = nullptr;
 public:
@@ -614,7 +618,7 @@ class LLVM_LIBRARY_VISIBILITY ClangImporter::Implementation
 private:
   /// The list of Clang modules found in the debug info.
   llvm::DenseMap<Identifier, LoadedFile *> DWARFModuleUnits;
-public:
+
   /// Load a module using the clang::CompilerInstance.
   ModuleDecl *loadModuleClang(SourceLoc importLoc,
                               ArrayRef<std::pair<Identifier, SourceLoc>> path);
@@ -624,7 +628,11 @@ class LLVM_LIBRARY_VISIBILITY ClangImporter::Implementation
   ModuleDecl *loadModuleDWARF(SourceLoc importLoc,
                               ArrayRef<std::pair<Identifier, SourceLoc>> path);
 
-  
+public:
+  /// Load a module using either method.
+  ModuleDecl *loadModule(SourceLoc importLoc,
+                         ArrayRef<std::pair<Identifier, SourceLoc>> path);
+
   void recordImplicitUnwrapForDecl(ValueDecl *decl, bool isIUO) {
     if (!isIUO)
       return;

lib/Frontend/CompilerInvocation.cpp

@@ -638,7 +638,12 @@ static bool ParseClangImporterArgs(ClangImporterOptions &Opts,
 
   if (Args.hasArg(OPT_warnings_as_errors))
     Opts.ExtraArgs.push_back("-Werror");
+
   Opts.DebuggerSupport |= Args.hasArg(OPT_debugger_support);
+
+  Opts.DisableSourceImport |=
+      Args.hasArg(OPT_disable_clangimporter_source_import);
+
   return false;
 }
 

test/ClangImporter/disable-source-import.swift

@@ -0,0 +1,18 @@
+// RUN: %empty-directory(%t.mcp)
+
+// This should fail only if -disable-clangimporter-source-import is present.
+
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) \
+// RUN:    -enable-objc-interop -typecheck -I %S/Inputs/custom-modules \
+// RUN:    -module-cache-path %t.mcp %s | %FileCheck --allow-empty %s
+
+// RUN: not %target-swift-frontend(mock-sdk: %clang-importer-sdk) \
+// RUN:    -enable-objc-interop -typecheck -o - -I %S/Inputs/custom-modules \
+// RUN:    -module-cache-path %t.mcp -disable-clangimporter-source-import %s \
+// RUN:    2>&1 | %FileCheck --check-prefix=ERROR %s
+
+import ExternIntX
+
+public let y = x // ERROR: error
+
+// CHECK-NOT: error