AST: Promote Extern feature to baseline.

Author: tshortli

include/swift/Basic/Features.def

@@ -185,7 +185,7 @@ SUPPRESSIBLE_LANGUAGE_FEATURE(RetroactiveAttribute, 364, "@retroactive")
 SUPPRESSIBLE_LANGUAGE_FEATURE(ExtensionMacroAttr, 0, "@attached(extension)")
 BASELINE_LANGUAGE_FEATURE(TypedThrows, 413, "Typed throws")
 CONDITIONALLY_SUPPRESSIBLE_LANGUAGE_FEATURE(OptionalIsolatedParameters, 420, "Optional isolated parameters")
-SUPPRESSIBLE_LANGUAGE_FEATURE(Extern, 0, "@_extern")
+BASELINE_LANGUAGE_FEATURE(Extern, 0, "@_extern")
 BASELINE_LANGUAGE_FEATURE(ExpressionMacroDefaultArguments, 422, "Expression macro as caller-side default argument")
 LANGUAGE_FEATURE(BuiltinStoreRaw, 0, "Builtin.storeRaw")
 BASELINE_LANGUAGE_FEATURE(BuiltinCreateTask, 0, "Builtin.createTask and Builtin.createDiscardingTask")

lib/AST/ASTPrinter.cpp

@@ -3051,14 +3051,6 @@ static void suppressingFeatureRetroactiveAttribute(
   action();
 }
 
-static void suppressingFeatureExtern(PrintOptions &options,
-                                     llvm::function_ref<void()> action) {
-  unsigned originalExcludeAttrCount = options.ExcludeAttrList.size();
-  options.ExcludeAttrList.push_back(DeclAttrKind::Extern);
-  action();
-  options.ExcludeAttrList.resize(originalExcludeAttrCount);
-}
-
 static void suppressingFeatureIsolatedAny(PrintOptions &options,
                                           llvm::function_ref<void()> action) {
   llvm::SaveAndRestore<bool> scope(options.SuppressIsolatedAny, true);

lib/AST/FeatureSet.cpp

@@ -171,10 +171,6 @@ static bool usesFeatureOptionalIsolatedParameters(Decl *decl) {
   return false;
 }
 
-static bool usesFeatureExtern(Decl *decl) {
-  return decl->getAttrs().hasAttribute<ExternAttr>();
-}
-
 static bool usesFeatureAssociatedTypeImplements(Decl *decl) {
   return isa<TypeDecl>(decl) && decl->getAttrs().hasAttribute<ImplementsAttr>();
 }

test/ModuleInterface/extern_attr.swift

@@ -4,23 +4,11 @@
 // RUN: %target-swift-typecheck-module-from-interface(%t/Library.swiftinterface) -module-name Library
 // RUN: %FileCheck %s < %t/Library.swiftinterface
 
-// CHECK:      #if compiler(>=5.3) && $Extern
-// CHECK-NEXT:   @_extern(c) public func externalCFunc()
-// CHECK-NEXT: #else
-// CHECK-NEXT:   public func externalCFunc()
-// CHECK-NEXT: #endif
+// CHECK:   @_extern(c) public func externalCFunc()
 @_extern(c) public func externalCFunc()
 
-// CHECK:      #if compiler(>=5.3) && $Extern
-// CHECK-NEXT:   @_extern(c, "renamedCFunc") public func externalRenamedCFunc()
-// CHECK-NEXT: #else
-// CHECK-NEXT:   public func externalRenamedCFunc()
-// CHECK-NEXT: #endif
+// CHECK:   @_extern(c, "renamedCFunc") public func externalRenamedCFunc()
 @_extern(c, "renamedCFunc") public func externalRenamedCFunc()
 
-// CHECK:      #if compiler(>=5.3) && $Extern
-// CHECK-NEXT:   @_extern(wasm, module: "m", name: "f") public func wasmImportedFunc()
-// CHECK-NEXT: #else
-// CHECK-NEXT:  public func wasmImportedFunc()
-// CHECK-NEXT: #endif
+// CHECK:   @_extern(wasm, module: "m", name: "f") public func wasmImportedFunc()
 @_extern(wasm, module: "m", name: "f") public func wasmImportedFunc()