[Macros] Add accessor declaration macros, to add accessors to a stored property

Author: DougGregor

Sources/_SwiftSyntaxMacros/AccessorDeclarationMacro.swift

@@ -0,0 +1,22 @@
+//
+// Copyright (c) 2014 - 2023 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
+
+/// Describes a macro that adds accessors to a given declaration.
+public protocol AccessorDeclarationMacro: DeclarationMacro {
+  /// Expand a macro that's expressed as a custom attribute attached to
+  /// the given declaration. The result is a set of accessors for the
+  /// declaration.
+  static func expansion(
+    of node: CustomAttributeSyntax,
+    attachedTo declaration: DeclSyntax,
+    in context: inout MacroExpansionContext
+  ) throws -> [AccessorDeclSyntax]
+}

Sources/_SwiftSyntaxMacros/CMakeLists.txt

@@ -7,6 +7,7 @@
 # See http://swift.org/CONTRIBUTORS.txt for Swift project authors
 
 add_swift_host_library(_SwiftSyntaxMacros
+  AccessorDeclarationMacro.swift
   DeclarationMacro.swift
   ExpressionMacro.swift
   FreestandingDeclarationMacro.swift

Sources/_SwiftSyntaxMacros/MacroSystem.swift

@@ -106,7 +106,9 @@ class MacroApplication: SyntaxRewriter {
           return true
         }
 
-        return !(macro is PeerDeclarationMacro.Type || macro is MemberDeclarationMacro.Type)
+        return !(macro is PeerDeclarationMacro.Type ||
+                 macro is MemberDeclarationMacro.Type ||
+                 macro is AccessorDeclarationMacro.Type)
       }
 
       if newAttributes.isEmpty {
@@ -254,6 +256,52 @@ class MacroApplication: SyntaxRewriter {
   override func visit(_ node: ExtensionDeclSyntax) -> DeclSyntax {
     return visit(declGroup: node)
   }
+
+  // Properties
+  override func visit(_ node: VariableDeclSyntax) -> DeclSyntax {
+    let visitedNode = super.visit(node)
+    guard let visitedVarDecl = visitedNode.as(VariableDeclSyntax.self) else {
+      return visitedNode
+    }
+
+    guard let binding = visitedVarDecl.bindings.first,
+          visitedVarDecl.bindings.count == 1 else {
+      return DeclSyntax(node)
+    }
+
+    var accessors: [AccessorDeclSyntax] = []
+
+    let accessorMacroAttributes = getMacroAttributes(attachedTo: DeclSyntax(node), ofType: AccessorDeclarationMacro.Type.self)
+    for (accessorAttr, accessorMacro) in accessorMacroAttributes {
+      do {
+        let newAccessors = try accessorMacro.expansion(
+          of: accessorAttr,
+          attachedTo: DeclSyntax(visitedNode),
+          in: &context
+        )
+
+        accessors.append(contentsOf: newAccessors)
+      } catch {
+        // FIXME: record the error
+      }
+    }
+
+    if accessors.isEmpty {
+      return visitedNode
+    }
+
+    return DeclSyntax(
+      visitedVarDecl.withBindings(
+      visitedVarDecl.bindings.replacing(childAt: 0, with: binding.withAccessor(.accessors(
+        .init(
+          leftBrace: .leftBraceToken(leadingTrivia: .space),
+          accessors: .init(accessors),
+          rightBrace: .rightBraceToken(leadingTrivia: .newline))
+      ))))
+    )
+  }
+
+  // Subscripts
 }
 
 extension MacroApplication {

Tests/SwiftSyntaxMacrosTest/MacroSystemTests.swift

@@ -208,6 +208,44 @@ struct DefineBitwidthNumberedStructsMacro: FreestandingDeclarationMacro {
   }
 }
 
+public struct PropertyWrapper: AccessorDeclarationMacro {
+  public static func expansion(
+    of node: CustomAttributeSyntax,
+    attachedTo declaration: DeclSyntax,
+    in context: inout MacroExpansionContext
+  ) throws -> [AccessorDeclSyntax] {
+    guard let varDecl = declaration.as(VariableDeclSyntax.self),
+       let binding = varDecl.bindings.first,
+       let identifier =     binding.pattern.as(IdentifierPatternSyntax.self)?.identifier,
+       let type = binding.typeAnnotation?.type,
+       binding.accessor == nil else {
+      return []
+    }
+
+    guard let wrapperTypeNameExpr = node.argumentList?.first?.expression,
+       let stringLiteral = wrapperTypeNameExpr.as(StringLiteralExprSyntax.self),
+       stringLiteral.segments.count == 1,
+       case let .stringSegment(wrapperTypeNameSegment)? = stringLiteral.segments.first else {
+      return []
+    }
+
+    return [
+      """
+
+        get {
+          _\(identifier).wrappedValue
+        }
+      """,
+      """
+
+        set {
+          _\(identifier).wrappedValue = newValue
+        }
+      """
+    ]
+  }
+}
+
 public struct AddCompletionHandler: PeerDeclarationMacro {
   public static func expansion(
     of node: CustomAttributeSyntax,
@@ -395,6 +433,7 @@ public let testMacros: [String: Macro.Type] = [
   "stringify": StringifyMacro.self,
   "myError": ErrorMacro.self,
   "bitwidthNumberedStructs": DefineBitwidthNumberedStructsMacro.self,
+  "wrapProperty" : PropertyWrapper.self,
   "addCompletionHandler": AddCompletionHandler.self,
   "addBackingStorage": AddBackingStorage.self,
 ]
@@ -515,6 +554,26 @@ final class MacroSystemTests: XCTestCase {
     )
   }
 
+  func testPropertyWrapper() {
+    AssertMacroExpansion(
+      macros: testMacros,
+      """
+      @wrapProperty("MyWrapperType")
+      var x: Int
+      """,
+      """
+
+      var x: Int {
+        get {
+          _x.wrappedValue
+        }
+        set {
+          _x.wrappedValue = newValue
+        }
+      }
+      """)
+  }
+
   func testAddCompletionHandler() {
     AssertMacroExpansion(
       macros: testMacros,