[Frontend] Add option to explicitly import Builtin

include/swift/Basic/LangOptions.h

@@ -532,6 +532,9 @@ 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;
     }

include/swift/Option/Options.td

@@ -1044,6 +1044,10 @@ def parse_stdlib : Flag<["-"], "parse-stdlib">,
   Flags<[FrontendOption, HelpHidden, ModuleInterfaceOption]>,
   HelpText<"Parse the input file(s) as the Swift standard library">;
 
+def enable_builtin_module : Flag<["-"], "enable-builtin-module">,
+  Flags<[FrontendOption, ModuleInterfaceOption]>,
+  HelpText<"Enables the explicit import of the Builtin module">;
+
 def modes_Group : OptionGroup<"<mode options>">, HelpText<"MODES">;
 
 class ModeOpt : Group<modes_Group>;

lib/Driver/ToolChains.cpp

@@ -307,6 +307,7 @@ void ToolChain::addCommonFrontendArgs(const OutputInfo &OI,
   inputArgs.AddLastArg(arguments, options::OPT_enable_experimental_cxx_interop);
   inputArgs.AddLastArg(arguments, options::OPT_load_plugin_library);
   inputArgs.AddLastArg(arguments, options::OPT_load_plugin_executable);
+  inputArgs.AddLastArg(arguments, options::OPT_enable_builtin_module);
 
   // Pass on any build config options
   inputArgs.AddAllArgs(arguments, options::OPT_D);

lib/Frontend/CompilerInvocation.cpp

@@ -1105,6 +1105,8 @@ 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

@@ -293,8 +293,14 @@ static void printImports(raw_ostream &out,
 
   for (auto import : allImports) {
     auto importedModule = import.importedModule;
-    if (importedModule->isOnoneSupportModule() ||
-        importedModule->isBuiltinModule()) {
+    if (importedModule->isOnoneSupportModule()) {
+      continue;
+    }
+
+    // Unless '-enable-builtin-module' was passed, do not print 'import Builtin'
+    // in the interface. '-parse-stdlib' still implicitly imports it however...
+    if (importedModule->isBuiltinModule() &&
+        !M->getASTContext().LangOpts.EnableBuiltinModule) {
       continue;
     }
 

lib/Sema/ImportResolution.cpp

@@ -354,17 +354,26 @@ void ImportResolver::addImport(const UnboundImport &I, ModuleDecl *M) {
 // MARK: Import module loading
 //===----------------------------------------------------------------------===//
 
-static ModuleDecl *
-getModuleImpl(ImportPath::Module modulePath, ModuleDecl *loadingModule,
-              bool canImportBuiltin) {
+ModuleDecl *
+ImportResolver::getModule(ImportPath::Module modulePath) {
+  auto loadingModule = SF.getParentModule();
+
   ASTContext &ctx = loadingModule->getASTContext();
 
   assert(!modulePath.empty());
   auto moduleID = modulePath[0];
 
-  // The Builtin module cannot be explicitly imported unless we're a .sil file.
-  if (canImportBuiltin && moduleID.Item == ctx.TheBuiltinModule->getName())
-    return ctx.TheBuiltinModule;
+  // The Builtin module cannot be explicitly imported unless:
+  // 1. We're in a .sil file
+  // 2. '-enable-builtin-module' 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 (moduleID.Item == ctx.TheBuiltinModule->getName()) {
+      return ctx.TheBuiltinModule;
+    }
+  }
 
   // If the imported module name is the same as the current module,
   // skip the Swift module loader and use the Clang module loader instead.
@@ -382,12 +391,6 @@ getModuleImpl(ImportPath::Module modulePath, ModuleDecl *loadingModule,
   return ctx.getModule(modulePath);
 }
 
-ModuleDecl *
-ImportResolver::getModule(ImportPath::Module modulePath) {
-  return getModuleImpl(modulePath, SF.getParentModule(),
-                       /*canImportBuiltin=*/SF.Kind == SourceFileKind::SIL);
-}
-
 NullablePtr<ModuleDecl>
 UnboundImport::getTopLevelModule(ModuleDecl *M, SourceFile &SF) {
   if (import.module.getModulePath().size() == 1)
@@ -737,11 +740,21 @@ void UnboundImport::validateResilience(NullablePtr<ModuleDecl> topLevelModule,
   if (!topLevelModule || topLevelModule.get()->isNonSwiftModule())
     return;
 
+  ASTContext &ctx = SF.getASTContext();
+
+  // If the module we're validating is the builtin one, then just return because
+  // this module is essentially a header only import and does not concern
+  // itself with resiliency. This can occur when one has passed
+  // '-enable-builtin-module' and is explicitly importing the Builtin module in
+  // their sources.
+  if (topLevelModule.get() == ctx.TheBuiltinModule) {
+    return;
+  }
+
   if (!SF.getParentModule()->isResilient() ||
       topLevelModule.get()->isResilient())
     return;
 
-  ASTContext &ctx = SF.getASTContext();
   ctx.Diags.diagnose(import.module.getModulePath().front().Loc,
                      diag::module_not_compiled_with_library_evolution,
                      topLevelModule.get()->getName(),

test/Frontend/enable_builtin_module.swift

@@ -0,0 +1,16 @@
+// 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
+
+// CHECK-NO-BUILTIN: no such module 'Builtin'
+
+#if BUILTIN_IMPORT
+import Builtin
+#endif
+
+// CHECK-NO-BUILTIN: cannot find type 'Builtin' in scope
+// CHECK-NO-BUILTIN-IMPORT: cannot find type 'Builtin' in scope
+
+func something(_: Builtin.RawPointer) {
+
+}

test/ModuleInterface/enable_builtin_module.swift

@@ -0,0 +1,17 @@
+// RUN: %target-swift-emit-module-interface(%t.swiftinterface) %s -enable-builtin-module
+// RUN: %target-swift-typecheck-module-from-interface(%t.swiftinterface)
+// RUN: %FileCheck %s < %t.swiftinterface
+
+// CHECK: -enable-builtin-module
+
+// CHECK: import Builtin
+// CHECK: import Swift
+// CHECK: import _Concurrency
+// CHECK: import _StringProcessing
+// CHECK: import _SwiftConcurrencyShims
+
+// CHECK: public func something(with x: Builtin.RawPointer)
+
+import Builtin
+
+public func something(with x: Builtin.RawPointer) {}