[AST] Prevent use of @extensible without ExtensibleAttribute feature

(cherry picked from commit 0ad7d8b590)

include/swift/AST/DeclAttr.def

@@ -880,6 +880,7 @@ SIMPLE_DECL_ATTR(extensible, Extensible,
   OnEnum,
   ABIStableToAdd | ABIStableToRemove | APIBreakingToAdd | APIStableToRemove | ForbiddenInABIAttr,
   169)
+DECL_ATTR_FEATURE_REQUIREMENT(Extensible, ExtensibleAttribute)
 
 SIMPLE_DECL_ATTR(concurrent, Concurrent,
   OnFunc | OnConstructor | OnSubscript | OnVar,

test/IDE/complete_decl_attribute.swift

@@ -344,7 +344,6 @@ struct _S {
 // ON_MEMBER_LAST-DAG: Keyword/None:                       freestanding[#Declaration Attribute#]; name=freestanding
 // ON_MEMBER_LAST-DAG: Keyword/None:                       storageRestrictions[#Declaration Attribute#]; name=storageRestrictions
 // ON_MEMBER_LAST-DAG: Keyword/None:                       lifetime[#Declaration Attribute#]; name=lifetime
-// ON_MEMBER_LAST-DAG: Keyword/None:                       extensible[#Declaration Attribute#]; name=extensible
 // ON_MEMBER_LAST-DAG: Keyword/None:                       concurrent[#Declaration Attribute#]; name=concurrent
 // ON_MEMBER_LAST-NOT: Keyword
 // ON_MEMBER_LAST-DAG: Decl[Struct]/CurrModule:            MyStruct[#MyStruct#]; name=MyStruct
@@ -421,7 +420,6 @@ func dummy2() {}
 // KEYWORD_LAST-DAG: Keyword/None:                       attached[#Declaration Attribute#]; name=attached
 // KEYWORD_LAST-DAG: Keyword/None:                       storageRestrictions[#Declaration Attribute#]; name=storageRestrictions
 // KEYWORD_LAST-DAG: Keyword/None:                       lifetime[#Declaration Attribute#]; name=lifetime
-// KEYWORD_LAST-DAG: Keyword/None:                       extensible[#Declaration Attribute#]; name=extensible
 // KEYWORD_LAST-DAG: Keyword/None:                       concurrent[#Declaration Attribute#]; name=concurrent
 // KEYWORD_LAST-NOT: Keyword
 // KEYWORD_LAST-DAG: Decl[Struct]/CurrModule:            MyStruct[#MyStruct#]; name=MyStruct

test/IDE/complete_decl_attribute_feature_requirement.swift

@@ -12,7 +12,8 @@
 
 // RUN: %batch-code-completion -filecheck-additional-suffix _DISABLED
 // RUN: %batch-code-completion -filecheck-additional-suffix _ENABLED \
-// RUN:        -enable-experimental-feature ABIAttribute
+// RUN:        -enable-experimental-feature ABIAttribute \
+// RUN:        -enable-experimental-feature ExtensibleAttribute
 
 // NOTE: Please do not include the ", N items" after "Begin completions". The
 // item count creates needless merge conflicts given that an "End completions"
@@ -39,6 +40,8 @@
 // KEYWORD4:              Begin completions
 // KEYWORD4_ENABLED-NOT:  Keyword/None:              abi[#{{.*}} Attribute#]; name=abi
 // KEYWORD4_DISABLED-NOT: Keyword/None:              abi[#{{.*}} Attribute#]; name=abi
+// KEYWORD4_ENABLED-DAG:  Keyword/None:              extensible[#{{.*}} Attribute#]; name=extensible
+// KEYWORD4_DISABLED-NOT: Keyword/None:              extensible[#{{.*}} Attribute#]; name=extensible
 // KEYWORD4:              End completions
 
 @#^KEYWORD5^# struct S{}

test/ModuleInterface/extensible_enums.swift

@@ -5,7 +5,8 @@
 /// Build the library
 // RUN: %target-swift-frontend -emit-module %t/src/Lib.swift \
 // RUN:   -module-name Lib \
-// RUN:   -emit-module-path %t/Lib.swiftmodule
+// RUN:   -emit-module-path %t/Lib.swiftmodule \
+// RUN:   -enable-experimental-feature ExtensibleAttribute
 
 // Check that the errors are produced when using enums from module with `ExtensibleEnums` feature enabled.
 // RUN: %target-swift-frontend -typecheck %t/src/TestChecking.swift \
@@ -18,7 +19,9 @@
 // RUN: %target-swift-frontend -emit-module %t/src/Lib.swift \
 // RUN:   -module-name Lib \
 // RUN:   -package-name Test \
-// RUN:   -emit-module-path %t/Lib.swiftmodule
+// RUN:   -emit-module-path %t/Lib.swiftmodule \
+// RUN:   -enable-experimental-feature ExtensibleAttribute
+
 
 // Different module but the same package
 // RUN: %target-swift-frontend -typecheck %t/src/TestSamePackage.swift \
@@ -26,6 +29,8 @@
 // RUN:   -package-name Test \
 // RUN:   -verify
 
+// REQUIRES: swift_feature_ExtensibleAttribute
+
 //--- Lib.swift
 
 @extensible

test/attr/attr_extensible.swift

@@ -1,4 +1,6 @@
-// RUN: %target-typecheck-verify-swift
+// RUN: %target-typecheck-verify-swift -enable-experimental-feature ExtensibleAttribute
+
+// REQUIRES: swift_feature_ExtensibleAttribute
 
 @extensible
 public enum E1 { // Ok

test/attr/feature_requirement.swift

@@ -1,5 +1,5 @@
 // RUN: %target-typecheck-verify-swift -parse-as-library -disable-experimental-parser-round-trip -verify-additional-prefix disabled-
-// RUN: %target-typecheck-verify-swift -parse-as-library -verify-additional-prefix enabled- -enable-experimental-feature CompileTimeValues
+// RUN: %target-typecheck-verify-swift -parse-as-library -verify-additional-prefix enabled- -enable-experimental-feature CompileTimeValues -enable-experimental-feature ExtensibleAttribute
 
 // REQUIRES: asserts
 
@@ -14,3 +14,13 @@ public let x = 1  // expected-disabled-error@-1 {{'const' attribute is only vali
 #else
   #error("doesn't have @const")  // expected-disabled-error {{doesn't have @const}}
 #endif
+
+@extensible
+public enum E {}  // expected-disabled-error@-1 {{'extensible' attribute is only valid when experimental feature ExtensibleAttribute is enabled}}
+
+#if hasAttribute(extensible)
+  #error("does have @extensible")  // expected-enabled-error {{does have @extensible}}
+#else
+  #error("doesn't have @extensible")  // expected-disabled-error {{doesn't have @extensible}}
+#endif
+