Add workaround flag for lazy import-as-member

In rdar://123649082, a project failed to build because of the lazy import-as-member loading changes in #71320. That project was configured in a way that broke modularization and the correct solution is to fix it, but out of an abundance of caution, add a `-disable-named-lazy-import-as-member-loading` frontend flag in case a project needs to temporarily restore the old behavior.

As a bonus, this lets us write a test to verify that lazy import-as-member loading has positive performance impact.

include/swift/Basic/LangOptions.h

@@ -377,6 +377,11 @@ namespace swift {
     /// Enable experimental eager Clang module diagnostics.
     bool EnableExperimentalEagerClangModuleDiagnostics = false;
 
+    /// Force ClangImporter's import-as-member extensions to load thier members
+    /// immediately, bypassing their SwiftLookupTables. This emulates an
+    /// implementation quirk of previous compilers.
+    bool DisableNamedLazyImportAsMemberLoading = false;
+
     /// Enable inference of Sendable conformances for public types.
     bool EnableInferPublicSendable = false;
 

include/swift/Option/FrontendOptions.td

@@ -372,7 +372,13 @@ def debug_constraints_on_line_EQ : Joined<["-"], "debug-constraints-on-line=">,
   Alias<debug_constraints_on_line>;
 
 def disable_named_lazy_member_loading : Flag<["-"], "disable-named-lazy-member-loading">,
-  HelpText<"Disable per-name lazy member loading">;
+  HelpText<"Disable per-name lazy member loading (obsolete)">;
+
+def disable_named_lazy_import_as_member_loading :
+  Flag<["-"], "disable-named-lazy-import-as-member-loading">,
+  HelpText<"Import all of a type's import-as-member globals together, as Swift "
+           "5.10 and earlier did; temporary workaround for modules that are "
+           "sensitive to this change">;
 
 def dump_requirement_machine : Flag<["-"], "dump-requirement-machine">,
   HelpText<"Enables dumping rewrite systems from the generics implementation">;

lib/AST/NameLookup.cpp

@@ -1446,10 +1446,25 @@ void MemberLookupTable::addMembers(DeclRange members) {
   }
 }
 
+static bool shouldLoadMembersImmediately(ExtensionDecl *ext) {
+  assert(ext->hasLazyMembers());
+  if (ext->wasDeserialized() || ext->hasClangNode())
+    return false;
+
+  // This extension is lazy but is not deserialized or backed by a clang node,
+  // so it's a ClangImporter extension containing import-as-member globals.
+  // Historically, Swift forced these extensions to load their members
+  // immediately, bypassing the module's SwiftLookupTable. Using the
+  // SwiftLookupTable *ought* to work the same, but in practice it sometimes
+  // gives different results when a header is not properly modularized. Provide
+  // a flag to temporarily re-enable the old behavior.
+  return ext->getASTContext().LangOpts.DisableNamedLazyImportAsMemberLoading;
+}
+
 void MemberLookupTable::addExtension(ExtensionDecl *ext) {
   // If we can lazy-load this extension, only take the members we've loaded
   // so far.
-  if (ext->hasLazyMembers()) {
+  if (ext->hasLazyMembers() && !shouldLoadMembersImmediately(ext)) {
     addMembers(ext->getCurrentMembersWithoutLoading());
     clearLazilyCompleteCache();
     clearLazilyCompleteForMacroExpansionCache();

lib/Frontend/CompilerInvocation.cpp

@@ -623,6 +623,9 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
       !Args.hasArg(OPT_disable_experimental_clang_importer_diagnostics) &&
       Args.hasArg(OPT_enable_experimental_eager_clang_module_diagnostics);
 
+  Opts.DisableNamedLazyImportAsMemberLoading |=
+      Args.hasArg(OPT_disable_named_lazy_import_as_member_loading);
+
   Opts.DisableImplicitConcurrencyModuleImport |=
     Args.hasArg(OPT_disable_implicit_concurrency_module_import);
 

test/IDE/dump_swift_lookup_tables.swift

@@ -3,7 +3,6 @@
 
 // RUN: %target-typecheck-verify-swift -dump-clang-lookup-tables -import-objc-header %S/Inputs/swift_name.h -I %S/Inputs/custom-modules > %t.frontend.log 2>&1
 // RUN: %FileCheck %s < %t.frontend.log
-// RUN: false
 
 // REQUIRES: objc_interop
 import ImportAsMember

test/NameLookup/Inputs/NamedLazyMembers/NamedLazyMembers.h

@@ -138,3 +138,6 @@
 
 @interface PrivateDoer(Category) <PrivateMethods>
 @end
+
+typedef NSString * const SimpleDoerMode NS_TYPED_ENUM NS_SWIFT_NAME(SimpleDoer.Mode);
+typedef NSString * const SimpleDoerKind NS_TYPED_ENUM NS_SWIFT_NAME(SimpleDoer.Kind);

test/NameLookup/named_lazy_member_loading_import_as_member.swift

@@ -0,0 +1,16 @@
+// REQUIRES: objc_interop
+// REQUIRES: OS=macosx
+
+// RUN: %empty-directory(%t/stats-lazy)
+// RUN: %empty-directory(%t/stats-eager)
+
+// RUN: %target-swift-frontend -typecheck -I %S/Inputs/NamedLazyMembers %s -stats-output-dir %t/stats-lazy
+// RUN: %target-swift-frontend -typecheck -I %S/Inputs/NamedLazyMembers %s -stats-output-dir %t/stats-eager -disable-named-lazy-import-as-member-loading
+
+// stats-lazy should only have imported SimpleDoer.Mode; stats-eager should also
+// have imported SimpleDoer.Kind
+// RUN: %{python} %utils/process-stats-dir.py --evaluate-delta 'NumTotalClangImportedEntities == 1' %t/stats-lazy %t/stats-eager
+
+import NamedLazyMembers
+
+func fn(_: SimpleDoer.Mode) {}