[Macros] Introduce a ConformanceMacro protocol.

Author: hborla

Sources/SwiftSyntaxMacros/CMakeLists.txt

@@ -10,6 +10,7 @@ add_swift_host_library(SwiftSyntaxMacros
   MacroProtocols/AccessorMacro.swift
   MacroProtocols/AttachedMacro.swift
   MacroProtocols/CodeItemMacro.swift
+  MacroProtocols/ConformanceMacro.swift
   MacroProtocols/DeclarationMacro.swift
   MacroProtocols/ExpressionMacro.swift
   MacroProtocols/FreestandingMacro.swift

Sources/SwiftSyntaxMacros/MacroProtocols/ConformanceMacro.swift

@@ -0,0 +1,36 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// 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 can add conformances to the declaration it's
+/// attached to.
+public protocol ConformanceMacro: AttachedMacro {
+  /// Expand an attached conformance macro to produce a set of conformances.
+  ///
+  /// - Parameters:
+  ///   - node: The custom attribute describing the attached macro.
+  ///   - declaration: The declaration the macro attribute is attached to.
+  ///   - context: The context in which to perform the macro expansion.
+  ///
+  /// - Returns: the set of `(type, where-clause?)` pairs that each provide the
+  ///   protocol type to which the declared type conforms, along with
+  ///   an optional where clause.
+  static func expansion<
+    Declaration: DeclGroupSyntax,
+    Context: MacroExpansionContext
+  >(
+    of node: AttributeSyntax,
+    providingConformancesOf declaration: Declaration,
+    in context: Context
+  ) throws -> [(TypeSyntax, WhereClauseSyntax?)]
+}

Sources/SwiftSyntaxMacros/MacroSystem.swift

@@ -109,7 +109,7 @@ class MacroApplication<Context: MacroExpansionContext>: SyntaxRewriter {
           return true
         }
 
-        return !(macro is PeerMacro.Type || macro is MemberMacro.Type || macro is AccessorMacro.Type || macro is MemberAttributeMacro.Type)
+        return !(macro is PeerMacro.Type || macro is MemberMacro.Type || macro is AccessorMacro.Type || macro is MemberAttributeMacro.Type || macro is ConformanceMacro.Type)
       }
 
       if newAttributes.isEmpty {