[concurrency] fixes to enable building swift with concurrency on by default

include/swift/Basic/LangOptions.h

@@ -241,6 +241,9 @@ namespace swift {
     /// Enable experimental concurrency model.
     bool EnableExperimentalConcurrency = false;
 
+    /// Disable the implicit import of the _Concurrency module.
+    bool DisableImplicitConcurrencyModuleImport = false;
+
     /// Should we check the target OSs of serialized modules to see that they're
     /// new enough?
     bool EnableTargetOSChecking = true;

include/swift/Option/FrontendOptions.td

@@ -286,6 +286,10 @@ def disable_clangimporter_source_import : Flag<["-"],
   "disable-clangimporter-source-import">,
   HelpText<"Disable ClangImporter and forward all requests straight the DWARF importer.">;
 
+def disable_implicit_concurrency_module_import : Flag<["-"],
+  "disable-implicit-concurrency-module-import">,
+  HelpText<"Disable the implicit import of the _Concurrency module.">;
+
 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/Frontend/CompilerInvocation.cpp

@@ -376,6 +376,9 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
   Opts.EnableExperimentalConcurrency |=
     Args.hasArg(OPT_enable_experimental_concurrency);
 
+  Opts.DisableImplicitConcurrencyModuleImport |=
+    Args.hasArg(OPT_disable_implicit_concurrency_module_import);
+
   Opts.EnableSubstSILFunctionTypesForFunctionValues |=
     Args.hasArg(OPT_enable_subst_sil_function_types_for_function_values);
 

lib/Frontend/Frontend.cpp

@@ -708,7 +708,8 @@ CompilerInstance::openModuleDoc(const InputFile &input) {
 }
 
 bool CompilerInvocation::shouldImportSwiftConcurrency() const {
-  return getLangOptions().EnableExperimentalConcurrency;
+  return getLangOptions().EnableExperimentalConcurrency
+      && !getLangOptions().DisableImplicitConcurrencyModuleImport;
 }
 
 /// Implicitly import the SwiftOnoneSupport module in non-optimized

stdlib/cmake/modules/AddSwiftStdlib.cmake

@@ -1657,6 +1657,12 @@ function(add_swift_target_library name)
     list(APPEND SWIFTLIB_DEPENDS clang)
   endif()
 
+  # Turn off implicit import of _Concurrency when building libraries
+  if(SWIFT_ENABLE_EXPERIMENTAL_CONCURRENCY)
+    list(APPEND SWIFTLIB_SWIFT_COMPILE_FLAGS 
+                      "-Xfrontend;-disable-implicit-concurrency-module-import")
+  endif()
+
   # If we are building this library for targets, loop through the various
   # SDKs building the variants of this library.
   list_intersect(

test/ImportResolution/disable-implicit-concurrency-module-import.swift

@@ -0,0 +1,5 @@
+// RUN: %target-typecheck-verify-swift -enable-experimental-concurrency -disable-implicit-concurrency-module-import
+// REQUIRES: concurrency
+
+
+protocol P : Actor { } // expected-error{{cannot find type 'Actor' in scope}}