Introduce adoption mode for Swift features

.github/CODEOWNERS

@@ -269,7 +269,7 @@
 /unittests/AST/                           @hborla @slavapestov @xedin
 /unittests/AST/*Evaluator*                @CodaFi @slavapestov
 /unittests/DependencyScan/                @artemcm @cachemeifyoucan
-/unittests/FrontendTool/                  @artemcm @tshortli
+/unittests/Frontend*/                     @artemcm @tshortli
 /unittests/Parse/                         @ahoppen @bnbarham @CodaFi @DougGregor @hamishknight @rintaro
 /unittests/Reflection/                    @slavapestov
 /unittests/SIL/                           @jckarter

include/swift/AST/DiagnosticsFrontend.def

@@ -2,7 +2,7 @@
 //
 // This source file is part of the Swift.org open source project
 //
-// Copyright (c) 2014 - 2020 Apple Inc. and the Swift project authors
+// 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
@@ -50,6 +50,20 @@ GROUPED_WARNING(feature_not_experimental, StrictLanguageFeatures, DefaultIgnore,
                 "use -%select{disable|enable}1-upcoming-feature instead",
                 (StringRef, bool))
 
+GROUPED_WARNING(invalid_feature_mode, StrictLanguageFeatures, none,
+                "'%0' is not a recognized mode for feature '%1'"
+                "%select{|; did you mean '%2'?}3",
+                (StringRef, StringRef, StringRef, bool))
+
+GROUPED_WARNING(cannot_disable_feature_with_mode, StrictLanguageFeatures, none,
+                "'%0' argument '%1' cannot specify a mode",
+                (StringRef, StringRef))
+
+GROUPED_WARNING(feature_does_not_support_adoption_mode, StrictLanguageFeatures,
+                none,
+                "feature '%0' does not support adoption mode",
+                (StringRef))
+
 ERROR(error_unknown_library_level, none,
       "unknown library level '%0', "
       "expected one of 'api', 'spi', 'ipi', or 'other'", (StringRef))

include/swift/Basic/BasicBridging.h

@@ -2,7 +2,7 @@
 //
 // This source file is part of the Swift.org open source project
 //
-// Copyright (c) 2022 Apple Inc. and the Swift project authors
+// Copyright (c) 2022 - 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
@@ -214,7 +214,8 @@ void BridgedData_free(BridgedData data);
 //===----------------------------------------------------------------------===//
 
 enum ENUM_EXTENSIBILITY_ATTR(open) BridgedFeature {
-#define LANGUAGE_FEATURE(FeatureName, SENumber, Description) FeatureName,
+#define LANGUAGE_FEATURE(FeatureName, IsAdoptable, SENumber, Description)      \
+  FeatureName,
 #include "swift/Basic/Features.def"
 };
 

include/swift/Basic/Feature.h

@@ -2,7 +2,7 @@
 //
 // This source file is part of the Swift.org open source project
 //
-// Copyright (c) 2014 - 2017 Apple Inc. and the Swift project authors
+// 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
@@ -21,14 +21,16 @@ namespace swift {
 class LangOptions;
 
 /// Enumeration describing all of the named features.
-enum class Feature {
-#define LANGUAGE_FEATURE(FeatureName, SENumber, Description) FeatureName,
+enum class Feature : uint16_t {
+#define LANGUAGE_FEATURE(FeatureName, IsAdoptable, SENumber, Description)      \
+  FeatureName,
 #include "swift/Basic/Features.def"
 };
 
 constexpr unsigned numFeatures() {
   enum Features {
-#define LANGUAGE_FEATURE(FeatureName, SENumber, Description) FeatureName,
+#define LANGUAGE_FEATURE(FeatureName, IsAdoptable, SENumber, Description)      \
+  FeatureName,
 #include "swift/Basic/Features.def"
     NumFeatures
   };
@@ -61,6 +63,9 @@ std::optional<Feature> getExperimentalFeature(llvm::StringRef name);
 /// \c None if it does not have such a version.
 std::optional<unsigned> getFeatureLanguageVersion(Feature feature);
 
+/// Determine whether the given feature supports adoption mode.
+bool isFeatureAdoptable(Feature feature);
+
 /// Determine whether this feature should be included in the
 /// module interface
 bool includeInModuleInterface(Feature feature);

include/swift/Basic/Features.def

@@ -14,14 +14,28 @@
 // features.
 //
 //
-// LANGUAGE_FEATURE(FeatureName, SENumber, Description)
+// LANGUAGE_FEATURE(FeatureName, IsAdoptable, SENumber, Description)
 //
 //   The LANGUAGE_FEATURE macro describes each named feature that is
 //   introduced in Swift. It allows Swift code to check for a particular
 //   feature with "#if $FeatureName" in source code.
 //
 //     FeatureName: The name given to this feature to be used in source code,
 //       e.g., AsyncAwait.
+//     IsAdoptable: Whether the feature implements adoption mode.
+//
+//       If the feature is upcoming (source-breaking) and provides for a
+//       mechanical code migration, it should implement adoption mode.
+//
+//       Adoption mode is a feature-oriented code migration mechanism: a mode
+//       of operation that should produce compiler warnings with attached
+//       fix-its that can be applied to preserve the behavior of the code once
+//       the upcoming feature is enacted.
+//       These warnings must belong to a diagnostic group named after the
+//       feature. Adoption mode itself *and* the fix-its it produces must be
+//       source and binary compatible with how the code is compiled when the
+//       feature is disabled.
+//
 //     SENumber: The number assigned to this feature in the Swift Evolution
 //       process, or 0 if there isn't one.
 //     Description: A string literal describing the feature.
@@ -91,13 +105,14 @@
 #endif
 
 #ifndef SUPPRESSIBLE_LANGUAGE_FEATURE
-#  define SUPPRESSIBLE_LANGUAGE_FEATURE(FeatureName, SENumber, Description) \
-  LANGUAGE_FEATURE(FeatureName, SENumber, Description)
+  #define SUPPRESSIBLE_LANGUAGE_FEATURE(FeatureName, SENumber, Description)    \
+    LANGUAGE_FEATURE(FeatureName, /*IsAdoptable=*/false, SENumber,             \
+                     Description)
 #endif
 
 #ifndef OPTIONAL_LANGUAGE_FEATURE
-#  define OPTIONAL_LANGUAGE_FEATURE(FeatureName, SENumber, Description) \
-  LANGUAGE_FEATURE(FeatureName, SENumber, Description)
+  #define OPTIONAL_LANGUAGE_FEATURE(FeatureName, SENumber, Description)        \
+    LANGUAGE_FEATURE(FeatureName, /*IsAdoptable=*/false, SENumber, Description)
 #endif
 
 // A feature that's both conditionally-suppressible and experimental.
@@ -116,20 +131,35 @@
 #endif
 
 #ifndef CONDITIONALLY_SUPPRESSIBLE_LANGUAGE_FEATURE
-#  define CONDITIONALLY_SUPPRESSIBLE_LANGUAGE_FEATURE(FeatureName, SENumber, Description) \
-     LANGUAGE_FEATURE(FeatureName, SENumber, Description)
+  #define CONDITIONALLY_SUPPRESSIBLE_LANGUAGE_FEATURE(FeatureName, SENumber,   \
+                                                      Description)             \
+    LANGUAGE_FEATURE(FeatureName, /*IsAdoptable=*/false, SENumber,             \
+                     Description)
+#endif
+
+// An upcoming feature that supports adoption mode.
+#ifndef ADOPTABLE_UPCOMING_FEATURE
+  #if defined(UPCOMING_FEATURE)
+    #define ADOPTABLE_UPCOMING_FEATURE(FeatureName, SENumber, Version)         \
+      UPCOMING_FEATURE(FeatureName, SENumber, Version)
+  #else
+    #define ADOPTABLE_UPCOMING_FEATURE(FeatureName, SENumber, Version)         \
+      LANGUAGE_FEATURE(FeatureName, /*IsAdoptable=*/true, SENumber,            \
+                       #FeatureName)
+  #endif
 #endif
 
 #ifndef UPCOMING_FEATURE
-#  define UPCOMING_FEATURE(FeatureName, SENumber, Version)   \
-     LANGUAGE_FEATURE(FeatureName, SENumber, #FeatureName)
+  #define UPCOMING_FEATURE(FeatureName, SENumber, Version)                     \
+    LANGUAGE_FEATURE(FeatureName, /*IsAdoptable=*/false, SENumber,             \
+                     #FeatureName)
 #endif
 
 #ifndef EXPERIMENTAL_FEATURE
-// Warning: setting `AvailableInProd` to `true` on a feature means that the flag
-//          cannot be dropped in the future.
-#  define EXPERIMENTAL_FEATURE(FeatureName, AvailableInProd)  \
-     LANGUAGE_FEATURE(FeatureName, 0, #FeatureName)
+  // Warning: setting `AvailableInProd` to `true` on a feature means that the
+  // flag cannot be dropped in the future.
+  #define EXPERIMENTAL_FEATURE(FeatureName, AvailableInProd)                   \
+    LANGUAGE_FEATURE(FeatureName, /*IsAdoptable=*/false, 0, #FeatureName)
 #endif
 
 #ifndef EXPERIMENTAL_FEATURE_EXCLUDED_FROM_MODULE_INTERFACE
@@ -138,8 +168,9 @@
 #endif
 
 #ifndef BASELINE_LANGUAGE_FEATURE
-#  define BASELINE_LANGUAGE_FEATURE(FeatureName, SENumber, Description) \
-  LANGUAGE_FEATURE(FeatureName, SENumber, Description)
+  #define BASELINE_LANGUAGE_FEATURE(FeatureName, SENumber, Description)        \
+    LANGUAGE_FEATURE(FeatureName, /*IsAdoptable=*/false, SENumber,             \
+                     Description)
 #endif
 
 BASELINE_LANGUAGE_FEATURE(AsyncAwait, 296, "async/await")
@@ -210,10 +241,14 @@ BASELINE_LANGUAGE_FEATURE(BodyMacros, 415, "Function body macros")
 SUPPRESSIBLE_LANGUAGE_FEATURE(SendingArgsAndResults, 430, "Sending arg and results")
 BASELINE_LANGUAGE_FEATURE(BorrowingSwitch, 432, "Noncopyable type pattern matching")
 CONDITIONALLY_SUPPRESSIBLE_LANGUAGE_FEATURE(IsolatedAny, 431, "@isolated(any) function types")
-LANGUAGE_FEATURE(IsolatedAny2, 431, "@isolated(any) function types")
-LANGUAGE_FEATURE(ObjCImplementation, 436, "@objc @implementation extensions")
-LANGUAGE_FEATURE(NonescapableTypes, 446, "Nonescapable types")
-LANGUAGE_FEATURE(BuiltinEmplaceTypedThrows, 0, "Builtin.emplace typed throws")
+LANGUAGE_FEATURE(IsolatedAny2, /*IsAdoptable=*/false, 431,
+                 "@isolated(any) function types")
+LANGUAGE_FEATURE(ObjCImplementation, /*IsAdoptable=*/false, 436,
+                 "@objc @implementation extensions")
+LANGUAGE_FEATURE(NonescapableTypes, /*IsAdoptable=*/false, 446,
+                 "Nonescapable types")
+LANGUAGE_FEATURE(BuiltinEmplaceTypedThrows, /*IsAdoptable=*/false, 0,
+                 "Builtin.emplace typed throws")
 SUPPRESSIBLE_LANGUAGE_FEATURE(MemorySafetyAttributes, 458, "@unsafe attribute")
 
 // Swift 6
@@ -234,7 +269,7 @@ UPCOMING_FEATURE(NonfrozenEnumExhaustivity, 192, 6)
 UPCOMING_FEATURE(GlobalActorIsolatedTypesUsability, 0434, 6)
 
 // Swift 7
-UPCOMING_FEATURE(ExistentialAny, 335, 7)
+ADOPTABLE_UPCOMING_FEATURE(ExistentialAny, 335, 7)
 UPCOMING_FEATURE(InternalImportsByDefault, 409, 7)
 UPCOMING_FEATURE(MemberImportVisibility, 444, 7)
 
@@ -476,6 +511,7 @@ EXPERIMENTAL_FEATURE(IsolatedConformances, true)
 #undef EXPERIMENTAL_FEATURE_EXCLUDED_FROM_MODULE_INTERFACE
 #undef EXPERIMENTAL_FEATURE
 #undef UPCOMING_FEATURE
+#undef ADOPTABLE_UPCOMING_FEATURE
 #undef BASELINE_LANGUAGE_FEATURE
 #undef OPTIONAL_LANGUAGE_FEATURE
 #undef CONDITIONALLY_SUPPRESSIBLE_EXPERIMENTAL_FEATURE

include/swift/Basic/LangOptions.h

@@ -2,7 +2,7 @@
 //
 // This source file is part of the Swift.org open source project
 //
-// Copyright (c) 2014 - 2020 Apple Inc. and the Swift project authors
+// 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
@@ -20,7 +20,6 @@
 
 #include "swift/Basic/CXXStdlibKind.h"
 #include "swift/Basic/Feature.h"
-#include "swift/Basic/FixedBitSet.h"
 #include "swift/Basic/FunctionBodySkipping.h"
 #include "swift/Basic/LLVM.h"
 #include "swift/Basic/PlaygroundOption.h"
@@ -725,19 +724,6 @@ namespace swift {
       return cxxInteropCompatVersion.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;
-
-    /// Enable the given feature.
-    void enableFeature(Feature feature) { Features.insert(feature); }
-
-    /// Disable the given feature.
-    void disableFeature(Feature feature) { Features.remove(feature); }
-
     /// Sets the "_hasAtomicBitWidth" conditional.
     void setHasAtomicBitWidth(llvm::Triple triple);
 
@@ -822,10 +808,68 @@ namespace swift {
         PlatformConditionValues;
     llvm::SmallVector<std::string, 2> CustomConditionalCompilationFlags;
 
-    /// The set of features that have been enabled. Doesn't include upcoming
-    /// features, which are checked against the language version in
-    /// `hasFeature`.
-    FixedBitSet<numFeatures(), Feature> Features;
+  public:
+    // MARK: Features
+    // =========================================================================
+
+    /// A wrapper around the feature state enumeration.
+    struct FeatureState {
+      enum Kind : uint8_t { Off, EnabledForAdoption, Enabled };
+
+    private:
+      Feature feature;
+      Kind state;
+
+    public:
+      FeatureState(Feature feature, Kind state)
+          : feature(feature), state(state) {}
+
+      /// Returns whether the feature is enabled.
+      bool isEnabled() const;
+
+      /// Returns whether the feature is enabled in adoption mode. Should only
+      /// be called if the feature is known to support this mode.
+      bool isEnabledForAdoption() const;
+
+      operator Kind() const { return state; }
+    };
+
+  private:
+    class FeatureStateStorage {
+      std::vector<FeatureState::Kind> states;
+
+    public:
+      FeatureStateStorage();
+
+      /// Sets the given state for the given feature.
+      void setState(Feature feature, FeatureState::Kind state);
+
+      /// Retrieves the state of the given feature.
+      FeatureState getState(Feature feature) const;
+    };
+
+    /// The states of language features.
+    FeatureStateStorage featureStates;
+
+  public:
+    /// Retrieve the state of the given feature.
+    FeatureState getFeatureState(Feature feature) const;
+
+    /// Returns whether the given feature is enabled.
+    bool hasFeature(Feature feature) const;
+
+    /// Returns whether a feature with the given name is enabled. Returns
+    /// `false` if a feature by this name is not known.
+    bool hasFeature(llvm::StringRef featureName) const;
+
+    /// Enables the given feature (enables in adoption mode if `forAdoption` is
+    /// `true`).
+    void enableFeature(Feature feature, bool forAdoption = false);
+
+    /// Disables the given feature.
+    void disableFeature(Feature feature);
+
+    // =========================================================================
   };
 
   class TypeCheckerOptions final {

lib/AST/ASTPrinter.cpp

@@ -1,7 +1,7 @@
 //
 // This source file is part of the Swift.org open source project
 //
-// Copyright (c) 2014 - 2020 Apple Inc. and the Swift project authors
+// 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
@@ -3267,7 +3267,7 @@ suppressingFeatureExecutionAttribute(PrintOptions &options,
 static void suppressingFeature(PrintOptions &options, Feature feature,
                                llvm::function_ref<void()> action) {
   switch (feature) {
-#define LANGUAGE_FEATURE(FeatureName, SENumber, Description)                   \
+#define LANGUAGE_FEATURE(FeatureName, IsAdoptable, SENumber, Description)      \
   case Feature::FeatureName:                                                   \
     llvm_unreachable("not a suppressible feature");
 #define SUPPRESSIBLE_LANGUAGE_FEATURE(FeatureName, SENumber, Description)      \

lib/AST/FeatureSet.cpp

@@ -2,7 +2,7 @@
 //
 // This source file is part of the Swift.org open source project
 //
-// Copyright (c) 2024 Apple Inc. and the Swift project authors
+// Copyright (c) 2024 - 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
@@ -45,7 +45,7 @@ static bool usesTypeMatching(Decl *decl, llvm::function_ref<bool(Type)> fn) {
 
 #define BASELINE_LANGUAGE_FEATURE(FeatureName, SENumber, Description)          \
   static bool usesFeature##FeatureName(Decl *decl) { return false; }
-#define LANGUAGE_FEATURE(FeatureName, SENumber, Description)
+#define LANGUAGE_FEATURE(FeatureName, IsAdoptable, SENumber, Description)
 #include "swift/Basic/Features.def"
 
 #define UNINTERESTING_FEATURE(FeatureName)                                     \
@@ -542,7 +542,7 @@ void FeatureSet::collectFeaturesUsed(Decl *decl, InsertOrRemove operation) {
 
   // Go through each of the features, checking whether the
   // declaration uses that feature.
-#define LANGUAGE_FEATURE(FeatureName, SENumber, Description)                   \
+#define LANGUAGE_FEATURE(FeatureName, IsAdoptable, SENumber, Description)      \
   if (CHECK(usesFeature##FeatureName))                                         \
     collectRequiredFeature(Feature::FeatureName, operation);
 #define SUPPRESSIBLE_LANGUAGE_FEATURE(FeatureName, SENumber, Description)      \