[Frontend] Add BuiltinModule experimental feature

include/swift/Basic/Features.def

@@ -183,6 +183,9 @@ EXPERIMENTAL_FEATURE(GenerateBindingsForThrowingFunctionsInCXX, false)
 /// Enable reference bindings.
 EXPERIMENTAL_FEATURE(ReferenceBindings, false)
 
+/// Enable the explicit 'import Builtin' and allow Builtin usage.
+EXPERIMENTAL_FEATURE(BuiltinModule, true)
+
 #undef EXPERIMENTAL_FEATURE
 #undef UPCOMING_FEATURE
 #undef SUPPRESSIBLE_LANGUAGE_FEATURE

include/swift/Basic/LangOptions.h

@@ -536,9 +536,6 @@ namespace swift {
     /// The model of concurrency to be used.
     ConcurrencyModel ActiveConcurrencyModel = ConcurrencyModel::Standard;
 
-    /// Allows the explicit 'import Builtin' within Swift modules.
-    bool EnableBuiltinModule = false;
-
     bool isConcurrencyModelTaskToThread() const {
       return ActiveConcurrencyModel == ConcurrencyModel::TaskToThread;
     }

lib/AST/ASTPrinter.cpp

@@ -3279,6 +3279,10 @@ static bool usesFeatureReferenceBindings(Decl *decl) {
   return vd && vd->getIntroducer() == VarDecl::Introducer::InOut;
 }
 
+static bool usesFeatureBuiltinModule(Decl *decl) {
+  return false;
+}
+
 /// Suppress the printing of a particular feature.
 static void suppressingFeature(PrintOptions &options, Feature feature,
                                llvm::function_ref<void()> action) {

lib/Frontend/CompilerInvocation.cpp

@@ -735,6 +735,9 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
   if (Args.hasArg(OPT_enable_experimental_opaque_type_erasure))
     Opts.Features.insert(Feature::OpaqueTypeErasure);
 
+  if (Args.hasArg(OPT_enable_builtin_module))
+    Opts.Features.insert(Feature::BuiltinModule);
+
   Opts.EnableAppExtensionRestrictions |= Args.hasArg(OPT_enable_app_extension);
 
   Opts.EnableSwift3ObjCInference =
@@ -1153,8 +1156,6 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
             .Default(ConcurrencyModel::Standard);
   }
 
-  Opts.EnableBuiltinModule = Args.hasArg(OPT_enable_builtin_module);
-
   return HadError || UnsupportedOS || UnsupportedArch;
 }
 

lib/Frontend/ModuleInterfaceSupport.cpp

@@ -295,10 +295,12 @@ static void printImports(raw_ostream &out,
       continue;
     }
 
-    // Unless '-enable-builtin-module' was passed, do not print 'import Builtin'
-    // in the interface. '-parse-stdlib' still implicitly imports it however...
+    // Unless '-enable-builtin-module' /
+    // '-enable-experimental-feature BuiltinModule' was passed, do not print
+    // 'import Builtin' in the interface. '-parse-stdlib' still implicitly
+    // imports it however...
     if (importedModule->isBuiltinModule() &&
-        !M->getASTContext().LangOpts.EnableBuiltinModule) {
+        !M->getASTContext().LangOpts.hasFeature(Feature::BuiltinModule)) {
       continue;
     }
 

lib/Sema/ImportResolution.cpp

@@ -365,11 +365,13 @@ ImportResolver::getModule(ImportPath::Module modulePath) {
 
   // The Builtin module cannot be explicitly imported unless:
   // 1. We're in a .sil file
-  // 2. '-enable-builtin-module' was passed.
+  // 2. '-enable-builtin-module'/'-enable-experimental-feature BuiltinModule'
+  //    was passed.
   //
   // FIXME: Eventually, it would be nice to separate '-parse-stdlib' from
   // implicitly importing Builtin, but we're not there yet.
-  if (SF.Kind == SourceFileKind::SIL || ctx.LangOpts.EnableBuiltinModule) {
+  if (SF.Kind == SourceFileKind::SIL ||
+      ctx.LangOpts.hasFeature(Feature::BuiltinModule)) {
     if (moduleID.Item == ctx.TheBuiltinModule->getName()) {
       return ctx.TheBuiltinModule;
     }

test/Frontend/enable_builtin_module.swift

@@ -1,6 +1,7 @@
 // RUN: not %target-swift-frontend -typecheck -show-diagnostics-after-fatal -D BUILTIN_IMPORT %s 2>&1 | %FileCheck -check-prefix CHECK-NO-BUILTIN %s
 // RUN: not %target-swift-frontend -typecheck -show-diagnostics-after-fatal  -enable-builtin-module %s 2>&1 | %FileCheck -check-prefix CHECK-NO-BUILTIN-IMPORT %s
 // RUN: %target-swift-frontend -typecheck -enable-builtin-module -D BUILTIN_IMPORT %s
+// RUN: %target-swift-frontend -typecheck -enable-experimental-feature BuiltinModule -D BUILTIN_IMPORT %s
 
 // CHECK-NO-BUILTIN: no such module 'Builtin'