[AST] SE-0487: Promote NonexhaustiveAttribute to a language feature

The proposal [has been accepted](https://forums.swift.org/t/accepted-se-0487-nonexhaustive-enums/81508)
which makes the feature experimental no longer.

(cherry picked from commit a7ecd309d4)

include/swift/Basic/Features.def

@@ -268,6 +268,7 @@ LANGUAGE_FEATURE(BuiltinSelect, 0, "Builtin.select")
 LANGUAGE_FEATURE(AlwaysInheritActorContext, 472, "@_inheritActorContext(always)")
 LANGUAGE_FEATURE(NonescapableAccessorOnTrivial, 0, "Support UnsafeMutablePointer.mutableSpan")
 LANGUAGE_FEATURE(InlineArrayTypeSugar, 483, "Type sugar for InlineArray")
+SUPPRESSIBLE_LANGUAGE_FEATURE(NonexhaustiveAttribute, 487, "Nonexhaustive Enums")
 
 // Swift 6
 UPCOMING_FEATURE(ConciseMagicFile, 274, 6)
@@ -520,9 +521,6 @@ EXPERIMENTAL_FEATURE(AllowRuntimeSymbolDeclarations, true)
 /// Optimize copies of ObjectiveC blocks.
 EXPERIMENTAL_FEATURE(CopyBlockOptimization, true)
 
-/// Allow use of `@nonexhaustive` on public enums
-SUPPRESSIBLE_EXPERIMENTAL_FEATURE(NonexhaustiveAttribute, false)
-
 /// Allow use of `using` declaration that control default isolation
 /// in a file scope.
 EXPERIMENTAL_FEATURE(DefaultIsolationPerFile, false)

test/IDE/complete_decl_attribute_feature_requirement.swift

@@ -6,15 +6,14 @@
 // REQUIRES: asserts
 
 // RUN: %batch-code-completion -filecheck-additional-suffix _DISABLED
-// RUN: %batch-code-completion -filecheck-additional-suffix _ENABLED \
-// RUN:        -enable-experimental-feature NonexhaustiveAttribute
+// RUN: %batch-code-completion -filecheck-additional-suffix _ENABLED
+
+// NOTE: There are currently no experimental features that need code completion
+// testing, but this test file is being left in place for when it's needed
+// again. At that time, please remove the ABIAttribute tests.
+// REQUIRES: new_use_case
 
 // NOTE: Please do not include the ", N items" after "Begin completions". The
 // item count creates needless merge conflicts given that an "End completions"
 // line exists for each test.
 
-@#^KEYWORD4^# enum E {}
-// KEYWORD4:              Begin completions
-// KEYWORD4_ENABLED-DAG:  Keyword/None:              nonexhaustive[#{{.*}} Attribute#]; name=nonexhaustive
-// KEYWORD4_DISABLED-NOT: Keyword/None:              nonexhaustive[#{{.*}} Attribute#]; name=nonexhaustive
-// KEYWORD4:              End completions

test/IDE/complete_nonexhaustive.swift

@@ -1,6 +1,4 @@
-// RUN: %batch-code-completion -enable-experimental-feature NonexhaustiveAttribute
-
-// REQUIRES: swift_feature_NonexhaustiveAttribute
+// RUN: %batch-code-completion
 
 // NONEXHAUSTIVE-DAG: Keyword/None:                       warn; name=warn
 

test/ModuleInterface/nonexhaustive_attr.swift

@@ -1,11 +1,8 @@
 // RUN: %empty-directory(%t)
-// RUN: %target-swift-emit-module-interface(%t/Library.swiftinterface) %s -enable-experimental-feature NonexhaustiveAttribute -module-name Library
-// RUN: %target-swift-typecheck-module-from-interface(%t/Library.swiftinterface) -enable-experimental-feature NonexhaustiveAttribute -module-name Library
+// RUN: %target-swift-emit-module-interface(%t/Library.swiftinterface) %s -module-name Library
 // RUN: %target-swift-typecheck-module-from-interface(%t/Library.swiftinterface) -module-name Library
 // RUN: %FileCheck %s < %t/Library.swiftinterface
 
-// REQUIRES: swift_feature_NonexhaustiveAttribute
-
 // CHECK: #if compiler(>=5.3) && $NonexhaustiveAttribute
 // CHECK-NEXT: @nonexhaustive public enum E {
 // CHECK-NEXT: }

test/ModuleInterface/nonexhaustive_enums.swift

@@ -5,10 +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:   -enable-experimental-feature NonexhaustiveAttribute
+// RUN:   -emit-module-path %t/Lib.swiftmodule
 
-// Check that the errors are produced when using enums from module with `NonexhaustiveEnums` feature enabled.
 // RUN: %target-swift-frontend -typecheck %t/src/TestChecking.swift \
 // RUN:   -swift-version 5 -module-name Client -I %t \
 // RUN:   -verify
@@ -19,9 +17,7 @@
 // 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:   -enable-experimental-feature NonexhaustiveAttribute
-
+// RUN:   -emit-module-path %t/Lib.swiftmodule
 
 // Different module but the same package
 // RUN: %target-swift-frontend -typecheck %t/src/TestSamePackage.swift \
@@ -34,8 +30,6 @@
 // RUN:   -swift-version 6 -module-name Client -I %t \
 // RUN:   -verify
 
-// REQUIRES: swift_feature_NonexhaustiveAttribute
-
 //--- Lib.swift
 
 @nonexhaustive

test/attr/attr_nonexhaustive.swift

@@ -1,6 +1,4 @@
-// RUN: %target-typecheck-verify-swift -enable-experimental-feature NonexhaustiveAttribute
-
-// REQUIRES: swift_feature_NonexhaustiveAttribute
+// RUN: %target-typecheck-verify-swift
 
 @nonexhaustive
 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 -enable-experimental-feature NonexhaustiveAttribute
+// RUN: %target-typecheck-verify-swift -parse-as-library -verify-additional-prefix enabled- -enable-experimental-feature CompileTimeValues
 
 // REQUIRES: asserts
 
@@ -15,12 +15,3 @@ public let x = 1  // expected-disabled-error@-1 {{'const' attribute is only vali
   #error("doesn't have @const")  // expected-disabled-error {{doesn't have @const}}
 #endif
 
-@nonexhaustive
-public enum E {}  // expected-disabled-error@-1 {{'nonexhaustive' attribute is only valid when experimental feature NonexhaustiveAttribute is enabled}}
-
-#if hasAttribute(nonexhaustive)
-  #error("does have @nonexhaustive")  // expected-enabled-error {{does have @nonexhaustive}}
-#else
-  #error("doesn't have @nonexhaustive")  // expected-disabled-error {{doesn't have @nonexhaustive}}
-#endif
-