Implement @ObservableDefault macro

Package.resolved

@@ -0,0 +1,15 @@
+{
+  "originHash" : "1ba60ebd54db82e47e39bc8db179589187c069067eb0a8cd6ec19d2301c5abc4",
+  "pins" : [
+    {
+      "identity" : "swift-syntax",
+      "kind" : "remoteSourceControl",
+      "location" : "https://github.com/swiftlang/swift-syntax",
+      "state" : {
+        "revision" : "cb53fa1bd3219b0b23ded7dfdd3b2baff266fd25",
+        "version" : "600.0.0"
+      }
+    }
+  ],
+  "version" : 3
+}

Package.swift

@@ -1,5 +1,6 @@
 // swift-tools-version:5.10
 import PackageDescription
+import CompilerPluginSupport
 
 let package = Package(
 	name: "Defaults",
@@ -16,18 +17,55 @@ let package = Package(
 			targets: [
 				"Defaults"
 			]
+		),
+		.library(
+			name: "DefaultsMacros",
+			targets: [
+				"DefaultsMacros"
+			]
 		)
 	],
+	dependencies: [
+		.package(url: "https://github.com/swiftlang/swift-syntax", from: "600.0.0")
+	],
 	targets: [
 		.target(
 			name: "Defaults",
 			resources: [.copy("PrivacyInfo.xcprivacy")]
 		),
+		.macro(
+			name: "DefaultsMacrosDeclarations",
+			dependencies: [
+				"Defaults",
+				.product(name: "SwiftSyntaxMacros", package: "swift-syntax"),
+				.product(name: "SwiftCompilerPlugin", package: "swift-syntax")
+			]
+		),
+		.target(
+			name: "DefaultsMacros",
+			dependencies: ["Defaults", "DefaultsMacrosDeclarations"]
+		),
 		.testTarget(
 			name: "DefaultsTests",
 			dependencies: [
 				"Defaults"
 			]
+		),
+		.testTarget(
+			name: "DefaultsMacrosDeclarationsTests",
+			dependencies: [
+				"DefaultsMacros",
+				"DefaultsMacrosDeclarations",
+				.product(name: "SwiftSyntaxMacros", package: "swift-syntax"),
+				.product(name: "SwiftSyntaxMacrosTestSupport", package: "swift-syntax")
+			]
+		),
+		.testTarget(
+			name: "DefaultsMacrosTests",
+			dependencies: [
+				"Defaults",
+				"DefaultsMacros"
+			]
 		)
 	]
 )

Sources/DefaultsMacros/ObservableDefault.swift

@@ -0,0 +1,47 @@
+import Defaults
+import Foundation
+
+/**
+Attached macro that adds support for using ``Defaults`` in ``@Observable`` classes.
+
+- Important: To prevent issues with ``@Observable``, you need to also add ``@ObservationIgnored`` to the attached property.
+
+This macro adds accessor blocks to the attached property similar to those added by `@Observable`.
+
+For example, given the following source:
+
+```swift
+@Observable
+final class CatModel {
+	@ObservableDefault(.cat)
+	@ObservationIgnored
+	var catName: String
+}
+```
+
+The macro will generate the following expansion:
+
+```swift
+@Observable
+final class CatModel {
+	@ObservationIgnored
+	var catName: String {
+		get {
+			access(keypath: \.catName)
+			return Defaults[.cat]
+		}
+		set {
+			withMutation(keyPath: \catName) {
+				Defaults[.cat] = newValue
+			}
+		}
+	}
+}
+```
+*/
+@attached(accessor, names: named(get), named(set))
+public macro ObservableDefault<Value>(_ key: Defaults.Key<Value>) =
+	#externalMacro(
+		module: "DefaultsMacrosDeclarations",
+		type: "ObservableDefaultMacro"
+	)

Sources/DefaultsMacrosDeclarations/DefaultsMacrosPlugin.swift

@@ -0,0 +1,9 @@
+import SwiftCompilerPlugin
+import SwiftSyntaxMacros
+
+@main
+struct DefaultsMacrosPlugin: CompilerPlugin {
+	let providingMacros: [Macro.Type] = [
+		ObservableDefaultMacro.self
+	]
+}

Sources/DefaultsMacrosDeclarations/ObservableDefaultMacro.swift

@@ -0,0 +1,117 @@
+import SwiftCompilerPlugin
+import SwiftDiagnostics
+import SwiftSyntax
+import SwiftSyntaxBuilder
+import SwiftSyntaxMacros
+
+public struct ObservableDefaultMacro: AccessorMacro {
+	public static func expansion(
+		of node: AttributeSyntax,
+		providingAccessorsOf declaration: some DeclSyntaxProtocol,
+		in context: some MacroExpansionContext
+	) throws -> [AccessorDeclSyntax] {
+		// Must be attached to a property declaration.
+		guard let variableDeclaration = declaration.as(VariableDeclSyntax.self) else {
+			throw ObservableDefaultMacroError.notAttachedToProperty
+		}
+
+		// Must be attached to a variable property (i.e. `var` and not `let`).
+		guard variableDeclaration.bindingSpecifier.tokenKind == .keyword(.var) else {
+			throw ObservableDefaultMacroError.notAttachedToVariable
+		}
+
+		// Must be attached to a single property.
+		guard variableDeclaration.bindings.count == 1, let binding = variableDeclaration.bindings.first else {
+			throw ObservableDefaultMacroError.notAttachedToSingleProperty
+		}
+
+		// Must not provide an initializer for the property (i.e. not assign a value).
+		guard binding.initializer == nil else {
+			throw ObservableDefaultMacroError.attachedToPropertyWithInitializer
+		}
+
+		// Must not be attached to property with existing accessor block.
+		guard binding.accessorBlock == nil else {
+			throw ObservableDefaultMacroError.attachedToPropertyWithAccessorBlock
+		}
+
+		// Must use Identifier Pattern.
+		// See https://swiftinit.org/docs/swift-syntax/swiftsyntax/identifierpatternsyntax
+		guard let pattern = binding.pattern.as(IdentifierPatternSyntax.self)?.identifier else {
+			throw ObservableDefaultMacroError.attachedToPropertyWithoutIdentifierProperty
+		}
+
+		// Must receive arguments
+		guard let arguments = node.arguments else {
+			throw ObservableDefaultMacroError.calledWithoutArguments
+		}
+
+		// Must be called with Labeled Expression.
+		// See https://swiftinit.org/docs/swift-syntax/swiftsyntax/labeledexprlistsyntax
+		guard let expressionList = arguments.as(LabeledExprListSyntax.self) else {
+			throw ObservableDefaultMacroError.calledWithoutLabeledExpression
+		}
+
+		// Must only receive one argument.
+		guard expressionList.count == 1, let expression = expressionList.first?.expression else {
+			throw ObservableDefaultMacroError.calledWithMultipleArguments
+		}
+
+		return [
+			#"""
+			get {
+				access(keyPath: \.\#(pattern))
+				return Defaults[\#(expression)]
+			}
+			"""#,
+			#"""
+			set {
+				withMutation(keyPath: \.\#(pattern)) {
+					Defaults[\#(expression)] = newValue
+				}
+			}
+			"""#
+		]
+	}
+}
+
+enum ObservableDefaultMacroError: Error {
+	case notAttachedToProperty
+	case notAttachedToVariable
+	case notAttachedToSingleProperty
+	case attachedToPropertyWithInitializer
+	case attachedToPropertyWithAccessorBlock
+	case attachedToPropertyWithoutIdentifierProperty
+	case calledWithoutArguments
+	case calledWithoutLabeledExpression
+	case calledWithMultipleArguments
+	case calledWithoutFunctionSyntax
+	case calledWithoutKeyArgument
+	case calledWithUnsupportedExpression
+}
+
+extension ObservableDefaultMacroError: CustomStringConvertible {
+	var description: String {
+		switch self {
+		case .notAttachedToProperty:
+			"@ObservableDefault must be attached to a property."
+		case .notAttachedToVariable:
+			"@ObservableDefault must be attached to a `var` property."
+		case .notAttachedToSingleProperty:
+			"@ObservableDefault can only be attached to a single property."
+		case .attachedToPropertyWithInitializer:
+			"@ObservableDefault must not be attached with a property with a value assigned. To create set default value, provide it in the `Defaults.Key` definition."
+		case .attachedToPropertyWithAccessorBlock:
+			"@ObservableDefault must not be attached to a property with accessor block."
+		case .attachedToPropertyWithoutIdentifierProperty:
+			"@ObservableDefault could not identify the attached property."
+		case .calledWithoutArguments,
+			 .calledWithoutLabeledExpression,
+			 .calledWithMultipleArguments,
+			 .calledWithoutFunctionSyntax,
+			 .calledWithoutKeyArgument,
+			 .calledWithUnsupportedExpression:
+			"@ObservableDefault must be called with (1) argument of type `Defaults.Key`"
+		}
+	}
+}