[Macros] Pass a list of protocols to conform to when expanding an extension

macro.

Author: hborla

Sources/SwiftCompilerPluginMessageHandling/CompilerPluginMessageHandler.swift

@@ -122,15 +122,16 @@ extension CompilerPluginMessageHandler {
         expandingSyntax: expandingSyntax
       )
 
-    case .expandAttachedMacro(let macro, let macroRole, let discriminator, let attributeSyntax, let declSyntax, let parentDeclSyntax, let extendedTypeSyntax):
+    case .expandAttachedMacro(let macro, let macroRole, let discriminator, let attributeSyntax, let declSyntax, let parentDeclSyntax, let extendedTypeSyntax, let conformanceListSyntax):
       try expandAttachedMacro(
         macro: macro,
         macroRole: macroRole,
         discriminator: discriminator,
         attributeSyntax: attributeSyntax,
         declSyntax: declSyntax,
         parentDeclSyntax: parentDeclSyntax,
-        extendedTypeSyntax: extendedTypeSyntax
+        extendedTypeSyntax: extendedTypeSyntax,
+        conformanceListSyntax: conformanceListSyntax
       )
 
     case .loadPluginLibrary(let libraryPath, let moduleName):

Sources/SwiftCompilerPluginMessageHandling/Macros.swift

@@ -87,7 +87,8 @@ extension CompilerPluginMessageHandler {
     attributeSyntax: PluginMessage.Syntax,
     declSyntax: PluginMessage.Syntax,
     parentDeclSyntax: PluginMessage.Syntax?,
-    extendedTypeSyntax: PluginMessage.Syntax?
+    extendedTypeSyntax: PluginMessage.Syntax?,
+    conformanceListSyntax: PluginMessage.Syntax?
   ) throws {
     let sourceManager = SourceManager()
     let context = PluginMacroExpansionContext(
@@ -104,6 +105,10 @@ extension CompilerPluginMessageHandler {
     let extendedType = extendedTypeSyntax.map {
       sourceManager.add($0).cast(TypeSyntax.self)
     }
+    let conformanceList = conformanceListSyntax.map {
+      let placeholderStruct = sourceManager.add($0).cast(StructDeclSyntax.self)
+      return placeholderStruct.inheritanceClause!.inheritedTypeCollection
+    }
 
     // TODO: Make this a 'String?' and remove non-'hasExpandMacroResult' branches.
     let expandedSources: [String]?
@@ -120,6 +125,7 @@ extension CompilerPluginMessageHandler {
         declarationNode: declarationNode,
         parentDeclNode: parentDeclNode,
         extendedType: extendedType,
+        conformanceList: conformanceList,
         in: context
       )
       if let expansions, hostCapability.hasExpandMacroResult {

Sources/SwiftCompilerPluginMessageHandling/PluginMessages.swift

@@ -34,7 +34,8 @@
     attributeSyntax: PluginMessage.Syntax,
     declSyntax: PluginMessage.Syntax,
     parentDeclSyntax: PluginMessage.Syntax?,
-    extendedTypeSyntax: PluginMessage.Syntax?
+    extendedTypeSyntax: PluginMessage.Syntax?,
+    conformanceListSyntax: PluginMessage.Syntax?
   )
 
   /// Optionally implemented message to load a dynamic link library.

Sources/SwiftSyntaxMacroExpansion/MacroExpansion.swift

@@ -185,6 +185,7 @@ public func expandAttachedMacroWithoutCollapsing<Context: MacroExpansionContext>
   declarationNode: DeclSyntax,
   parentDeclNode: DeclSyntax?,
   extendedType: TypeSyntax?,
+  conformanceList: InheritedTypeListSyntax?,
   in context: Context
 ) -> [String]? {
   do {
@@ -276,6 +277,8 @@ public func expandAttachedMacroWithoutCollapsing<Context: MacroExpansionContext>
         throw MacroExpansionError.noExtendedTypeSyntax
       }
 
+      let protocols = conformanceList?.map(\.typeName) ?? []
+
       // Local function to expand an extension macro once we've opened up
       // the existential.
       func expandExtensionMacro(
@@ -285,6 +288,7 @@ public func expandAttachedMacroWithoutCollapsing<Context: MacroExpansionContext>
           of: attributeNode,
           attachedTo: node,
           providingExtensionsOf: extendedType,
+          conformingTo: protocols,
           in: context
         )
       }
@@ -328,6 +332,7 @@ public func expandAttachedMacro<Context: MacroExpansionContext>(
   declarationNode: DeclSyntax,
   parentDeclNode: DeclSyntax?,
   extendedType: TypeSyntax?,
+  conformanceList: InheritedTypeListSyntax?,
   in context: Context
 ) -> String? {
   let expandedSources = expandAttachedMacroWithoutCollapsing(
@@ -337,6 +342,7 @@ public func expandAttachedMacro<Context: MacroExpansionContext>(
     declarationNode: declarationNode,
     parentDeclNode: parentDeclNode,
     extendedType: extendedType,
+    conformanceList: conformanceList,
     in: context
   )
   return expandedSources.map {

Sources/SwiftSyntaxMacros/MacroProtocols/ConformanceMacro.swift

@@ -37,6 +37,7 @@ extension ConformanceMacro {
     of node: AttributeSyntax,
     attachedTo declaration: some DeclGroupSyntax,
     providingExtensionsOf type: some TypeSyntaxProtocol,
+    conformingTo protocols: [TypeSyntax],
     in context: some MacroExpansionContext
   ) throws -> [ExtensionDeclSyntax] {
 

Sources/SwiftSyntaxMacros/MacroProtocols/ExtensionMacro.swift

@@ -21,6 +21,9 @@ public protocol ExtensionMacro: AttachedMacro {
   ///   - node: The custom attribute describing the attached macro.
   ///   - declaration: The declaration the macro attribute is attached to.
   ///   - type: The type to provide extensions of.
+  ///   - protocols: The list of protocols to add conformances to. These will
+  ///     always be protocols that `type` does not already state a conformance
+  ///     to.
   ///   - context: The context in which to perform the macro expansion.
   ///
   /// - Returns: the set of extension declarations introduced by the macro,
@@ -30,6 +33,7 @@ public protocol ExtensionMacro: AttachedMacro {
     of node: AttributeSyntax,
     attachedTo declaration: some DeclGroupSyntax,
     providingExtensionsOf type: some TypeSyntaxProtocol,
+    conformingTo protocols: [TypeSyntax],
     in context: some MacroExpansionContext
   ) throws -> [ExtensionDeclSyntax]
 }

Sources/SwiftSyntaxMacros/MacroSystem.swift

@@ -422,10 +422,14 @@ extension MacroApplication {
     let extensionMacroAttrs = getMacroAttributes(attachedTo: decl.as(DeclSyntax.self)!, ofType: ExtensionMacro.Type.self)
     for (attribute, extensionMacro) in extensionMacroAttrs {
       do {
+        // FIXME: We need a way for unit tests of extension macros to
+        // specify protocols already stated in source (e.g. as arguments
+        // to `assertMacroExpansion`).
         let newExtensions = try extensionMacro.expansion(
           of: attribute,
           attachedTo: decl,
           providingExtensionsOf: extendedType,
+          conformingTo: [],
           in: context
         )
 

Tests/SwiftSyntaxMacrosTest/MacroSystemTests.swift

@@ -687,8 +687,13 @@ public struct SendableExtensionMacro: ExtensionMacro {
     of node: AttributeSyntax,
     attachedTo: some DeclGroupSyntax,
     providingExtensionsOf type: some TypeSyntaxProtocol,
+    conformingTo protocols: [TypeSyntax],
     in context: some MacroExpansionContext
   ) throws -> [ExtensionDeclSyntax] {
+    if protocols.isEmpty {
+      return []
+    }
+
     let sendableExtension: DeclSyntax =
       """
       extension \(type.trimmed): Sendable {}