[c-interop] Gate @_extern behind experimental feature flag

Author: kateinoigakukun

include/swift/AST/DiagnosticsSema.def

@@ -1872,6 +1872,8 @@ ERROR(section_empty_name,none,
       "@_section section name cannot be empty", ())
 
 // @_extern
+ERROR(attr_extern_experimental,none,
+      "@_extern requires '-enable-experimental-feature Extern'", ())
 ERROR(extern_not_at_top_level_func,none,
       "@_extern attribute can only be applied to global functions", ())
 ERROR(extern_empty_c_name,none,

include/swift/Basic/Features.def

@@ -253,6 +253,9 @@ EXPERIMENTAL_FEATURE(TypedThrows, true)
 /// Allow destructuring stored `let` bindings in structs.
 EXPERIMENTAL_FEATURE(StructLetDestructuring, true)
 
+/// Enable the `@_extern` attribute.
+EXPERIMENTAL_FEATURE(Extern, true)
+
 #undef EXPERIMENTAL_FEATURE_EXCLUDED_FROM_MODULE_INTERFACE
 #undef EXPERIMENTAL_FEATURE
 #undef UPCOMING_FEATURE

lib/AST/ASTPrinter.cpp

@@ -3586,6 +3586,10 @@ static bool usesFeatureTypedThrows(Decl *decl) {
   return false;
 }
 
+static bool usesFeatureExtern(Decl *decl) {
+  return decl->getAttrs().hasAttribute<ExternAttr>();
+}
+
 /// Suppress the printing of a particular feature.
 static void suppressingFeature(PrintOptions &options, Feature feature,
                                llvm::function_ref<void()> action) {

lib/Sema/TypeCheckAttr.cpp

@@ -2120,6 +2120,10 @@ static bool isCCompatibleFuncDecl(FuncDecl *FD) {
 }
 
 void AttributeChecker::visitExternAttr(ExternAttr *attr) {
+  if (!Ctx.LangOpts.hasFeature(Feature::Extern)) {
+    diagnoseAndRemoveAttr(attr, diag::attr_extern_experimental);
+    return;
+  }
   // Only top-level func or static func decls are currently supported.
   auto *FD = dyn_cast<FuncDecl>(D);
   if (!FD || (FD->getDeclContext()->isTypeContext() && !FD->isStatic())) {

test/IRGen/extern_c.swift

@@ -1,4 +1,4 @@
-// RUN: %target-swift-frontend -emit-ir %s | %FileCheck %s
+// RUN: %target-swift-frontend -emit-ir -enable-experimental-feature Extern %s | %FileCheck %s
 
 func test() {
   // CHECK: call void @explicit_extern_c()

test/IRGen/extern_c_abitypes.swift

@@ -1,6 +1,6 @@
 // RUN: %empty-directory(%t)
 // RUN: split-file %s %t
-// RUN: %target-swift-frontend -emit-ir %t/extern_c.swift -I%t | %FileCheck %s
+// RUN: %target-swift-frontend -emit-ir -enable-experimental-feature Extern %t/extern_c.swift -I%t | %FileCheck %s
 
 //--- c_abi_types.h
 #include <stdbool.h>

test/Interop/WebAssembly/extern-wasm-cdecls.swift

@@ -1,5 +1,5 @@
 // RUN: %empty-directory(%t)
-// RUN: %target-swift-frontend %s -emit-ir -module-name Extern | %FileCheck %s
+// RUN: %target-swift-frontend %s -emit-ir -enable-experimental-feature Extern -module-name Extern | %FileCheck %s
 
 // CHECK: declare void @import1() [[EA1:#[0-9]+]]
 @_extern(c)

test/SILGen/extern_c.swift

@@ -1,4 +1,4 @@
-// RUN: %target-swift-emit-silgen %s | %FileCheck %s
+// RUN: %target-swift-emit-silgen -enable-experimental-feature Extern %s | %FileCheck %s
 
 // CHECK-DAG: sil hidden_external @my_c_name : $@convention(c) (Int) -> Int
 @_extern(c, "my_c_name")

test/attr/attr_extern.swift

@@ -1,4 +1,4 @@
-// RUN: %target-typecheck-verify-swift -disable-availability-checking
+// RUN: %target-typecheck-verify-swift -enable-experimental-feature Extern -disable-availability-checking
 
 @_extern(wasm, module: "m1", name: "f1")
 func f1(x: Int) -> Int