Add -enable-experimental-feature X for experimental features.

Experimental features can only be enabled in non-production (+Asserts)
builds. They can be detected with `hasFeature` in the same manner as
"future" features.

The `-enable-experimental-feature X` flag will also look for future
features by that name, so that when an experimental feature becomes an
accepted future feature, it will still be enabled in the same manner.

Switch variadic generics over to this approach, eliminating the
specific LangOption for it.

Author: DougGregor

include/swift/AST/DiagnosticsFrontend.def

@@ -36,6 +36,10 @@ ERROR(error_unsupported_target_os, none,
 ERROR(error_unsupported_target_arch, none,
       "unsupported target architecture: '%0'", (StringRef))
 
+ERROR(error_experimental_feature_not_available, none,
+      "experimental feature '%0' cannot be enabled in a production compiler",
+      (StringRef))
+
 ERROR(error_unknown_library_level, none,
       "unknown library level '%0', "
       "expected one of 'api', 'spi' or 'other'", (StringRef))

include/swift/Basic/Feature.h

@@ -18,18 +18,18 @@
 namespace swift {
 
 class LangOptions;
-  
+
 /// Enumeration describing all of the named features.
 enum class Feature {
 #define LANGUAGE_FEATURE(FeatureName, SENumber, Description, Option) \
-  FeatureName,
-  #include "swift/Basic/Features.def"
+FeatureName,
+#include "swift/Basic/Features.def"
 };
 
 constexpr unsigned numFeatures() {
   enum Features {
 #define LANGUAGE_FEATURE(FeatureName, SENumber, Description, Option) \
-    FeatureName,
+FeatureName,
 #include "swift/Basic/Features.def"
     NumFeatures
   };
@@ -51,6 +51,10 @@ inline bool featureImpliesFeature(Feature feature, Feature implied) {
   return (unsigned) feature < (unsigned) implied;
 }
 
+/// Get the feature corresponding to this "experimental" feature, if there is
+/// one.
+llvm::Optional<Feature> getExperimentalFeature(llvm::StringRef name);
+
 }
 
 #endif // SWIFT_BASIC_FEATURES_H

include/swift/Basic/Features.def

@@ -48,6 +48,12 @@
   LANGUAGE_FEATURE(FeatureName, SENumber, Description, Option)
 #endif
 
+#ifndef EXPERIMENTAL_FEATURE
+#  define EXPERIMENTAL_FEATURE(FeatureName)  \
+     LANGUAGE_FEATURE(FeatureName, 0, #FeatureName, \
+                      langOpts.hasFeature(#FeatureName))
+#endif
+
 LANGUAGE_FEATURE(StaticAssert, 0, "#assert", langOpts.EnableExperimentalStaticAssert)
 LANGUAGE_FEATURE(AsyncAwait, 296, "async/await", true)
 LANGUAGE_FEATURE(EffectfulProp, 310, "Effectful properties", true)
@@ -78,5 +84,8 @@ SUPPRESSIBLE_LANGUAGE_FEATURE(PrimaryAssociatedTypes2, 346, "Primary associated
 SUPPRESSIBLE_LANGUAGE_FEATURE(UnavailableFromAsync, 0, "@_unavailableFromAsync", true)
 SUPPRESSIBLE_LANGUAGE_FEATURE(NoAsyncAvailability, 340, "@available(*, noasync)", true)
 
+EXPERIMENTAL_FEATURE(VariadicGenerics)
+
+#undef EXPERIMENTAL_FEATURE
 #undef SUPPRESSIBLE_LANGUAGE_FEATURE
 #undef LANGUAGE_FEATURE

include/swift/Basic/LangOptions.h

@@ -18,13 +18,15 @@
 #ifndef SWIFT_BASIC_LANGOPTIONS_H
 #define SWIFT_BASIC_LANGOPTIONS_H
 
+#include "swift/Basic/Feature.h"
 #include "swift/Basic/FunctionBodySkipping.h"
 #include "swift/Basic/LLVM.h"
 #include "swift/Basic/Version.h"
 #include "swift/Config.h"
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/Hashing.h"
 #include "llvm/ADT/SmallString.h"
+#include "llvm/ADT/SmallSet.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/ADT/Triple.h"
@@ -341,9 +343,6 @@ namespace swift {
     /// Enable experimental 'move only' features.
     bool EnableExperimentalMoveOnly = false;
 
-    /// Enable variadic generics.
-    bool EnableExperimentalVariadicGenerics = false;
-
     /// Enable experimental associated type inference using type witness
     /// systems.
     bool EnableExperimentalAssociatedTypeInference = false;
@@ -369,6 +368,9 @@ namespace swift {
     /// behavior. This is a staging flag, and will be removed in the future.
     bool EnableNewOperatorLookup = false;
 
+    /// The set of features that have been enabled.
+    llvm::SmallSet<Feature, 2> Features;
+
     /// Use Clang function types for computing canonical types.
     /// If this option is false, the clang function types will still be computed
     /// but will not be used for checking type equality.
@@ -616,6 +618,13 @@ namespace swift {
       return EffectiveLanguageVersion.isVersionAtLeast(major, minor);
     }
 
+    /// Determine whether the given feature is enabled.
+    bool hasFeature(Feature feature) const;
+
+    /// Determine whether the given feature is enabled, looking up the feature
+    /// by name.
+    bool hasFeature(llvm::StringRef featureName) const;
+
     /// Returns true if the given platform condition argument represents
     /// a supported target operating system.
     ///

include/swift/Option/Options.td

@@ -691,6 +691,11 @@ def strict_concurrency : Joined<["-"], "strict-concurrency=">,
            "concurrency model, or 'complete' ('Sendable' and other checking is "
            "enabled for all code in the module)">;
 
+def enable_experimental_feature :
+  Separate<["-"], "enable-experimental-feature">,
+  Flags<[FrontendOption]>,
+  HelpText<"Enable an experimental feature">;
+
 def Rpass_EQ : Joined<["-"], "Rpass=">,
   Flags<[FrontendOption]>,
   HelpText<"Report performed transformations by optimization passes whose "

lib/AST/ASTPrinter.cpp

@@ -2999,6 +2999,10 @@ static bool usesFeatureNoAsyncAvailability(Decl *decl) {
    return decl->getAttrs().getNoAsync(decl->getASTContext()) != nullptr;
 }
 
+static bool usesFeatureVariadicGenerics(Decl *decl) {
+  return false;
+}
+
 static void
 suppressingFeatureNoAsyncAvailability(PrintOptions &options,
                                       llvm::function_ref<void()> action) {

lib/Basic/LangOptions.cpp

@@ -224,6 +224,20 @@ bool LangOptions::isCustomConditionalCompilationFlagSet(StringRef Name) const {
       != CustomConditionalCompilationFlags.end();
 }
 
+bool LangOptions::hasFeature(Feature feature) const {
+  if (Features.contains(feature))
+    return true;
+
+  return false;
+}
+
+bool LangOptions::hasFeature(llvm::StringRef featureName) const {
+  if (auto feature = getExperimentalFeature(featureName))
+    return hasFeature(*feature);
+
+  return false;
+}
+
 std::pair<bool, bool> LangOptions::setTarget(llvm::Triple triple) {
   clearAllPlatformConditionValues();
 
@@ -409,6 +423,15 @@ bool swift::isSuppressibleFeature(Feature feature) {
   llvm_unreachable("covered switch");
 }
 
+llvm::Optional<Feature> swift::getExperimentalFeature(llvm::StringRef name) {
+  return llvm::StringSwitch<Optional<Feature>>(name)
+#define LANGUAGE_FEATURE(FeatureName, SENumber, Description, Option)
+#define EXPERIMENTAL_FEATURE(FeatureName) \
+                   .Case(#FeatureName, Feature::FeatureName)
+#include "swift/Basic/Features.def"
+                   .Default(None);
+}
+
 DiagnosticBehavior LangOptions::getAccessNoteFailureLimit() const {
   switch (AccessNoteBehavior) {
   case AccessNoteDiagnosticBehavior::Ignore:

lib/Driver/ToolChains.cpp

@@ -236,6 +236,7 @@ void ToolChain::addCommonFrontendArgs(const OutputInfo &OI,
                        options::OPT_disable_actor_data_race_checks);
   inputArgs.AddLastArg(arguments, options::OPT_warn_concurrency);
   inputArgs.AddLastArg(arguments, options::OPT_strict_concurrency);
+  inputArgs.AddAllArgs(arguments, options::OPT_enable_experimental_feature);
   inputArgs.AddLastArg(arguments, options::OPT_warn_implicit_overrides);
   inputArgs.AddLastArg(arguments, options::OPT_typo_correction_limit);
   inputArgs.AddLastArg(arguments, options::OPT_enable_app_extension);

lib/Frontend/CompilerInvocation.cpp

@@ -461,9 +461,6 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
                  OPT_disable_experimental_opened_existential_types,
                  true);
 
-  Opts.EnableExperimentalVariadicGenerics |=
-    Args.hasArg(OPT_enable_experimental_variadic_generics);
-
   Opts.EnableExperimentalAssociatedTypeInference |=
       Args.hasArg(OPT_enable_experimental_associated_type_inference);
 
@@ -649,6 +646,25 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
     Opts.addCustomConditionalCompilationFlag(A->getValue());
   }
 
+  for (const Arg *A : Args.filtered(OPT_enable_experimental_feature)) {
+    // If this is a known experimental feature, allow it in +Asserts
+    // (non-release) builds for testing purposes.
+    if (auto feature = getExperimentalFeature(A->getValue())) {
+#ifdef NDEBUG
+      Diags.diagnose(SourceLoc(),
+                     diag::error_experimental_feature_not_available,
+                     A->getValue());
+#endif
+
+      Opts.Features.insert(*feature);
+    }
+  }
+
+  // Map historical flags over to experimental features. We do this for all
+  // compilers because that's how existing experimental feature flags work.
+  if (Args.hasArg(OPT_enable_experimental_variadic_generics))
+    Opts.Features.insert(Feature::VariadicGenerics);
+
   Opts.EnableAppExtensionRestrictions |= Args.hasArg(OPT_enable_app_extension);
 
   Opts.EnableSwift3ObjCInference =

lib/Parse/ParseDecl.cpp

@@ -2908,7 +2908,7 @@ bool Parser::parseNewDeclAttribute(DeclAttributes &Attributes, SourceLoc AtLoc,
     break;
   }
   case DAK_TypeSequence: {
-    if (Context.LangOpts.EnableExperimentalVariadicGenerics) {
+    if (Context.LangOpts.hasFeature(Feature::VariadicGenerics)) {
       auto range = SourceRange(Loc, Tok.getRange().getStart());
       Attributes.add(TypeSequenceAttr::create(Context, AtLoc, range));
     } else {