[Features] Replace -enable-nonfrozen-enum-exhaustivity-diagnostics with an

upcoming feature.

The bespoke flag still works as a way to enable the `NonfrozenEnumExhaustivity`
upcoming feature. `NonfrozenEnumExhaustivity` is enabled by default in the
Swift 6 language mode as errors, and enabled by default in the Swift 5 language
mode as warnings.

Author: hborla

include/swift/Basic/Features.def

@@ -192,6 +192,7 @@ UPCOMING_FEATURE(InferSendableFromCaptures, 418, 6)
 UPCOMING_FEATURE(ImplicitOpenExistentials, 352, 6)
 UPCOMING_FEATURE(RegionBasedIsolation, 414, 6)
 UPCOMING_FEATURE(DynamicActorIsolation, 423, 6)
+UPCOMING_FEATURE(NonfrozenEnumExhaustivity, 192, 6)
 
 // Swift 7
 UPCOMING_FEATURE(ExistentialAny, 335, 7)

include/swift/Basic/LangOptions.h

@@ -454,10 +454,6 @@ namespace swift {
     /// Diagnose uses of NSCoding with classes that have unstable mangled names.
     bool EnableNSKeyedArchiverDiagnostics = true;
 
-    /// Diagnose switches over non-frozen enums that do not have catch-all
-    /// cases.
-    bool EnableNonFrozenEnumExhaustivityDiagnostics = false;
-
     /// Regex for the passes that should report passed and missed optimizations.
     ///
     /// These are shared_ptrs so that this class remains copyable.

lib/AST/FeatureSet.cpp

@@ -653,6 +653,8 @@ static bool usesFeatureTransferringArgsAndResults(Decl *decl) {
 
 UNINTERESTING_FEATURE(DynamicActorIsolation)
 
+UNINTERESTING_FEATURE(NonfrozenEnumExhaustivity)
+
 UNINTERESTING_FEATURE(BorrowingSwitch)
 
 UNINTERESTING_FEATURE(ClosureIsolation)

lib/Frontend/CompilerInvocation.cpp

@@ -1084,10 +1084,11 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
                    OPT_disable_nskeyedarchiver_diagnostics,
                    Opts.EnableNSKeyedArchiverDiagnostics);
 
-  Opts.EnableNonFrozenEnumExhaustivityDiagnostics =
-    Args.hasFlag(OPT_enable_nonfrozen_enum_exhaustivity_diagnostics,
-                 OPT_disable_nonfrozen_enum_exhaustivity_diagnostics,
-                 Opts.isSwiftVersionAtLeast(5));
+  if (Args.hasFlag(OPT_enable_nonfrozen_enum_exhaustivity_diagnostics,
+                   OPT_disable_nonfrozen_enum_exhaustivity_diagnostics,
+                   Opts.isSwiftVersionAtLeast(5))) {
+    Opts.enableFeature(Feature::NonfrozenEnumExhaustivity);
+  }
 
   if (Arg *A = Args.getLastArg(OPT_Rpass_EQ))
     Opts.OptimizationRemarkPassedPattern =

lib/Sema/TypeCheckSwitchStmt.cpp

@@ -1157,7 +1157,7 @@ namespace {
       case DowngradeToWarning::ForUnknownCase: {
         if (Context.LangOpts.DebuggerSupport ||
             Context.LangOpts.Playground ||
-            !Context.LangOpts.EnableNonFrozenEnumExhaustivityDiagnostics) {
+            !Context.LangOpts.hasFeature(Feature::NonfrozenEnumExhaustivity)) {
           // Don't require covering unknown cases in the debugger or in
           // playgrounds.
           return;
@@ -1259,7 +1259,7 @@ namespace {
               // will later decompose the space into cases.
               continue;
             }
-            if (!Context.LangOpts.EnableNonFrozenEnumExhaustivityDiagnostics)
+            if (!Context.LangOpts.hasFeature(Feature::NonfrozenEnumExhaustivity))
               continue;
 
             // This can occur if the switch is empty and the subject type is an

test/Sema/exhaustive_switch.swift

@@ -2,6 +2,7 @@
 // RUN: %target-swift-frontend -swift-version 5 -enable-library-evolution %S/Inputs/exhaustive_switch_testable_helper.swift -emit-module -o %t
 // RUN: %target-typecheck-verify-swift -swift-version 5 -enable-library-evolution -I %t
 // RUN: %target-typecheck-verify-swift -swift-version 4 -enable-library-evolution -enable-nonfrozen-enum-exhaustivity-diagnostics -I %t
+// RUN: %target-typecheck-verify-swift -swift-version 4 -enable-library-evolution -enable-upcoming-feature NonfrozenEnumExhaustivity -I %t
 
 import exhaustive_switch_testable_helper