[Frontend] Add -print-supported-features option

This is a replacement for `-emit-supported-features` that prints
all of the upcoming/experimental features supported by the compiler
with some additional meta information in JSON format to stdout.

include/swift/Basic/SupportedFeatures.h

@@ -0,0 +1,28 @@
+//===--- SupportedFeatures.h - Supported Features Output --------*- C++ -*-===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2025 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+//
+// This file provides a high-level API for supported features info
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef SWIFT_SUPPORTEDFEATURES_H
+#define SWIFT_SUPPORTEDFEATURES_H
+
+#include "swift/Basic/LLVM.h"
+
+namespace swift {
+namespace features {
+void printSupportedFeatures(llvm::raw_ostream &out);
+} // namespace features
+} // namespace swift
+
+#endif

include/swift/Frontend/FrontendOptions.h

@@ -314,6 +314,10 @@ public:
   /// exit.
   bool PrintTargetInfo = false;
 
+  /// Indicates that the frontend should print the supported features and then
+  /// exit.
+  bool PrintSupportedFeatures = false;
+
   /// See the \ref SILOptions.EmitVerboseSIL flag.
   bool EmitVerboseSIL = false;
 

include/swift/Option/Options.td

@@ -1534,6 +1534,10 @@ def print_target_info : Flag<["-"], "print-target-info">,
     Flags<[FrontendOption]>,
     HelpText<"Print target information for the given target <triple>, such as x86_64-apple-macos10.9">, MetaVarName<"<triple>">;
 
+def print_supported_features : Flag<["-"], "print-supported-features">,
+    Flags<[FrontendOption]>,
+    HelpText<"Print information about features supported by the compiler">;
+
 def target_cpu : Separate<["-"], "target-cpu">, Flags<[FrontendOption, ModuleInterfaceOption]>,
   HelpText<"Generate code for a particular CPU variant">;
 

lib/Basic/CMakeLists.txt

@@ -75,6 +75,7 @@ add_swift_host_library(swiftBasic STATIC
   StableHasher.cpp
   Statistic.cpp
   StringExtras.cpp
+  SupportedFeatures.cpp
   TargetInfo.cpp
   TaskQueue.cpp
   ThreadSafeRefCounted.cpp

lib/Basic/SupportedFeatures.cpp

@@ -0,0 +1,74 @@
+//===--- SupportedFeatures.cpp - Supported features printing --------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2025 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+#include <array>
+#include <vector>
+
+#include "swift/Basic/Feature.h"
+#include "swift/Frontend/Frontend.h"
+
+#include "llvm/Support/raw_ostream.h"
+
+using namespace swift;
+
+namespace swift {
+namespace features {
+/// Print information about what features upcoming/experimental are
+/// supported by the compiler.
+/// The information includes whether a feature is adoptable and for
+/// upcoming features - what is the first mode it's introduced.
+void printSupportedFeatures(llvm::raw_ostream &out) {
+  std::array upcoming{
+#define LANGUAGE_FEATURE(FeatureName, SENumber, Description)
+#define UPCOMING_FEATURE(FeatureName, SENumber, Version) Feature::FeatureName,
+#include "swift/Basic/Features.def"
+  };
+
+  std::vector<swift::Feature> experimental{{
+#define LANGUAGE_FEATURE(FeatureName, SENumber, Description)
+#define EXPERIMENTAL_FEATURE(FeatureName, AvailableInProd) Feature::FeatureName,
+#include "swift/Basic/Features.def"
+  }};
+
+  // Include only experimental features that are available in production.
+  llvm::erase_if(experimental, [](auto &feature) {
+    return feature.isAvailableInProduction();
+  });
+
+  out << "{\n";
+  auto printFeature = [&out](const Feature &feature) {
+    out << "      ";
+    out << "{ \"name\": \"" << feature.getName() << "\"";
+    if (feature.isAdoptable()) {
+      out << ", \"migratable\": true";
+    }
+    if (auto version = feature.getLanguageVersion()) {
+      out << ", \"enabled_in\": " << *version;
+    }
+    out << " }";
+  };
+
+  out << "  \"features\": {\n";
+  out << "    \"upcoming\": [\n";
+  llvm::interleave(upcoming, printFeature, [&out] { out << ",\n"; });
+  out << "\n    ],\n";
+
+  out << "    \"experimental\": [\n";
+  llvm::interleave(experimental, printFeature, [&out] { out << ",\n"; });
+  out << "\n    ]\n";
+
+  out << "  }\n";
+  out << "}\n";
+}
+
+} // end namespace features
+} // end namespace swift

lib/Driver/Driver.cpp

@@ -2049,6 +2049,28 @@ bool Driver::handleImmediateArgs(const ArgList &Args, const ToolChain &TC) {
     return false;
   }
 
+  if (Args.hasArg(options::OPT_print_supported_features)) {
+    SmallVector<const char *, 5> commandLine;
+    commandLine.push_back("-frontend");
+    commandLine.push_back("-print-supported-features");
+
+    std::string executable = getSwiftProgramPath();
+
+    // FIXME(https://github.com/apple/swift/issues/54554): This bypasses
+    // mechanisms like -v and -###.
+    sys::TaskQueue queue;
+    queue.addTask(executable.c_str(), commandLine);
+    queue.execute(nullptr,
+                  [](sys::ProcessId PID, int returnCode, StringRef output,
+                     StringRef errors, sys::TaskProcessInformation ProcInfo,
+                     void *unused) -> sys::TaskFinishedResponse {
+                    llvm::outs() << output;
+                    llvm::errs() << errors;
+                    return sys::TaskFinishedResponse::ContinueExecution;
+                  });
+    return false;
+  }
+
   return true;
 }
 

lib/Frontend/ArgsToFrontendOptionsConverter.cpp

@@ -206,6 +206,10 @@ bool ArgsToFrontendOptionsConverter::convert(
     Opts.PrintTargetInfo = true;
   }
 
+  if (Args.hasArg(OPT_print_supported_features)) {
+    Opts.PrintSupportedFeatures = true;
+  }
+
   if (const Arg *A = Args.getLastArg(OPT_verify_generic_signatures)) {
     Opts.VerifyGenericSignaturesInModule = A->getValue();
   }

lib/FrontendTool/FrontendTool.cpp

@@ -46,6 +46,7 @@
 #include "swift/Basic/PrettyStackTrace.h"
 #include "swift/Basic/SourceManager.h"
 #include "swift/Basic/Statistic.h"
+#include "swift/Basic/SupportedFeatures.h"
 #include "swift/Basic/TargetInfo.h"
 #include "swift/Basic/UUID.h"
 #include "swift/Basic/Version.h"
@@ -2063,6 +2064,11 @@ int swift::performFrontend(ArrayRef<const char *> Args,
     return finishDiagProcessing(0, /*verifierEnabled*/ false);
   }
 
+  if (Invocation.getFrontendOptions().PrintSupportedFeatures) {
+    swift::features::printSupportedFeatures(llvm::outs());
+    return finishDiagProcessing(0, /*verifierEnabled*/ false);
+  }
+
   if (Invocation.getFrontendOptions().RequestedAction ==
       FrontendOptions::ActionType::NoneAction) {
     Instance->getDiags().diagnose(SourceLoc(),

test/Frontend/print-supported-features.swift

@@ -0,0 +1,10 @@
+// RUN: %target-swift-frontend -print-supported-features | %FileCheck %s
+
+// CHECK: "features": {
+// CHECK-NEXT:   "upcoming": [
+// CHECK:     { "name": "{{.*}}"{{, "migratable": true}}, "enabled_in": {{[0-9]+}} }
+// CHECK:   ],
+// CHECK-NEXT:   "experimental": [
+// CHECK:     { "name": "{{.*}}"{{, "migratable": true}} }
+// CHECK:   ]
+// CHECK: }