NewCodable macros

Package.swift

@@ -228,11 +228,25 @@ let package = Package(
             swiftSettings: availabilityMacros + featureSettings + testOnlySwiftSettings
         ),
 
+        // NewCodableMacros
+        .macro(
+            name: "NewCodableMacros",
+            dependencies: [
+                .product(name: "SwiftSyntax", package: "swift-syntax"),
+                .product(name: "SwiftSyntaxMacros", package: "swift-syntax"),
+                .product(name: "SwiftSyntaxBuilder", package: "swift-syntax"),
+                .product(name: "SwiftDiagnostics", package: "swift-syntax"),
+                .product(name: "SwiftCompilerPlugin", package: "swift-syntax"),
+            ],
+            swiftSettings: featureSettings
+        ),
+
         // NewCodable
         .target(
             name: "NewCodable",
             dependencies: [
                 .target(name: "FoundationEssentials"),
+                "NewCodableMacros",
                 .product(name: "OrderedCollections", package: "swift-collections"),
                 .product(name: "BasicContainers", package: "swift-collections"),
             ],
@@ -277,6 +291,14 @@ let package = Package(
                 .enableExperimentalFeature("Lifetimes"),
                 .enableUpcomingFeature("MemberImportVisibility"),
             ]
+        ),
+        .testTarget(
+            name: "NewCodableMacrosTests",
+            dependencies: [
+                "NewCodableMacros",
+                .product(name: "SwiftSyntaxMacrosTestSupport", package: "swift-syntax"),
+            ],
+            swiftSettings: featureSettings
         )
     ]
 )

Sources/NewCodable/JSON/JSONParserDecoder.swift

@@ -1749,6 +1749,7 @@ extension JSONParserDecoder.ArrayDecoder: CommonArrayDecoder {
 }
 
 extension JSONParserDecoder.StructDecoder: CommonStructDecoder {
+    @_lifetime(self: copy self)
     public mutating func decodeExpectedOrderField(required: Bool, matchingClosure: (UTF8Span) -> Bool, andValue valueDecoderClosure: (inout JSONParserDecoder) throws(CodingError.Decoding) -> Void) throws(CodingError.Decoding) -> Bool {
         try self.decodeExpectedOrderField(required: required, matchingClosure: matchingClosure, optimizedSafeStringKey: nil, andValue: valueDecoderClosure)
     }

Sources/NewCodable/JSON/NewJSONEncoder.swift

@@ -623,6 +623,7 @@ extension JSONDirectEncoder {
         try self.encode(arbitraryPrecisionNumber: decimal.description.utf8Span)
     }
 
+    @_lifetime(self: copy self)
     internal mutating func encodeGenericNonCopyable<T: CommonEncodable & ~Copyable>(_ value: borrowing T) throws(CodingError.Encoding) {
         try value.encode(to: &self)
     }

Sources/NewCodable/Macros.swift

@@ -0,0 +1,42 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2026 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
+//
+//===----------------------------------------------------------------------===//
+
+/// Experimental NewCodable macro API.
+///
+/// The macro spellings in this file are provisional and may evolve with the
+/// macro-based Codable design. The per-property marker macros below are
+/// especially likely to change as the feature set is refined.
+@attached(member, names: named(CodingFields))
+@attached(extension, conformances: JSONEncodable, names: named(encode))
+public macro JSONEncodable() = #externalMacro(module: "NewCodableMacros", type: "JSONEncodableMacro")
+
+/// Experimental macro that synthesizes `JSONDecodable` conformance.
+@attached(member, names: named(CodingFields))
+@attached(extension, conformances: JSONDecodable, names: named(decode))
+public macro JSONDecodable() = #externalMacro(module: "NewCodableMacros", type: "JSONDecodableMacro")
+
+/// Experimental macro that synthesizes both `JSONEncodable` and `JSONDecodable`.
+@attached(member, names: named(CodingFields))
+@attached(extension, conformances: JSONEncodable, JSONDecodable, names: named(encode), named(decode))
+public macro JSONCodable() = #externalMacro(module: "NewCodableMacros", type: "JSONCodableMacro")
+
+/// Experimental per-property marker macro for overriding the serialized key.
+@attached(peer)
+public macro CodingKey(_ name: String) = #externalMacro(module: "NewCodableMacros", type: "CodingKeyMacro")
+
+/// Experimental per-property marker macro for supplying a default decoding value.
+@attached(peer)
+public macro CodableDefault<T>(_ value: T) = #externalMacro(module: "NewCodableMacros", type: "CodableDefaultMacro")
+
+/// Experimental per-property marker macro for accepting alternate decoding keys.
+@attached(peer)
+public macro CodableAlias(_ names: String...) = #externalMacro(module: "NewCodableMacros", type: "CodableAliasMacro")

Sources/NewCodableMacros/CodableAliasMacro.swift

@@ -0,0 +1,24 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2026 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
+//
+//===----------------------------------------------------------------------===//
+
+import SwiftSyntax
+import SwiftSyntaxMacros
+
+public struct CodableAliasMacro: PeerMacro {
+    public static func expansion(
+        of node: AttributeSyntax,
+        providingPeersOf declaration: some DeclSyntaxProtocol,
+        in context: some MacroExpansionContext
+    ) throws -> [DeclSyntax] {
+        []
+    }
+}

Sources/NewCodableMacros/CodableDefaultMacro.swift

@@ -0,0 +1,24 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2026 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
+//
+//===----------------------------------------------------------------------===//
+
+import SwiftSyntax
+import SwiftSyntaxMacros
+
+public struct CodableDefaultMacro: PeerMacro {
+    public static func expansion(
+        of node: AttributeSyntax,
+        providingPeersOf declaration: some DeclSyntaxProtocol,
+        in context: some MacroExpansionContext
+    ) throws -> [DeclSyntax] {
+        []
+    }
+}

Sources/NewCodableMacros/CodingKeyMacro.swift

@@ -0,0 +1,24 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2026 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
+//
+//===----------------------------------------------------------------------===//
+
+import SwiftSyntax
+import SwiftSyntaxMacros
+
+public struct CodingKeyMacro: PeerMacro {
+    public static func expansion(
+        of node: AttributeSyntax,
+        providingPeersOf declaration: some DeclSyntaxProtocol,
+        in context: some MacroExpansionContext
+    ) throws -> [DeclSyntax] {
+        []
+    }
+}

Sources/NewCodableMacros/JSONCodableMacro.swift

@@ -0,0 +1,67 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2026 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
+//
+//===----------------------------------------------------------------------===//
+
+import SwiftSyntax
+import SwiftSyntaxMacros
+import SwiftDiagnostics
+
+public struct JSONCodableMacro { }
+
+extension JSONCodableMacro: MemberMacro {
+    public static func expansion(
+        of node: AttributeSyntax,
+        providingMembersOf declaration: some DeclGroupSyntax,
+        conformingTo protocols: [TypeSyntax],
+        in context: some MacroExpansionContext
+    ) throws -> [DeclSyntax] {
+        guard declaration.is(StructDeclSyntax.self) else {
+            context.diagnose(.init(
+                node: node,
+                message: JSONEncodableDiagnostic.notAStruct
+            ))
+            return []
+        }
+
+        let properties = extractStoredProperties(from: declaration.memberBlock, in: context)
+        guard !properties.isEmpty else {
+            return []
+        }
+
+        return [makeCodingFieldsDecl(from: properties, kind: .coding)]
+    }
+}
+
+extension JSONCodableMacro: ExtensionMacro {
+    public static func expansion(
+        of node: AttributeSyntax,
+        attachedTo declaration: some DeclGroupSyntax,
+        providingExtensionsOf type: some TypeSyntaxProtocol,
+        conformingTo protocols: [TypeSyntax],
+        in context: some MacroExpansionContext
+    ) throws -> [ExtensionDeclSyntax] {
+        let encodableExtensions = try JSONEncodableMacro.expansion(
+            of: node,
+            attachedTo: declaration,
+            providingExtensionsOf: type,
+            conformingTo: protocols,
+            in: context
+        )
+        let decodableExtensions = try JSONDecodableMacro.expansion(
+            of: node,
+            attachedTo: declaration,
+            providingExtensionsOf: type,
+            conformingTo: protocols,
+            in: context
+        )
+        return encodableExtensions + decodableExtensions
+    }
+}