Allow either requirement of MemberMacro to be implemented

... and try again for the name of the new method, using
"preexistingConformances" instead of "missingConformancesTo".

Author: DougGregor

Sources/SwiftSyntaxMacroExpansion/MacroExpansion.swift

@@ -236,7 +236,7 @@ public func expandAttachedMacroWithoutCollapsing<Context: MacroExpansionContext>
       let members = try attachedMacro.expansion(
         of: attributeNode,
         providingMembersOf: declGroup,
-        missingConformancesTo: conformanceList?.map(\.type) ?? [],
+        implementingRequirementsFor: conformanceList?.map(\.type) ?? [],
         in: context
       )
 

Sources/SwiftSyntaxMacros/MacroProtocols/MemberMacro.swift

@@ -23,6 +23,7 @@ public protocol MemberMacro: AttachedMacro {
   ///
   /// - Returns: the set of member declarations introduced by this macro, which
   /// are nested inside the `attachedTo` declaration.
+  @available(*, deprecated, message: "Use expansion(of:providingMembersOf:implementingRequirementsFor:in:")
   static func expansion(
     of node: AttributeSyntax,
     providingMembersOf declaration: some DeclGroupSyntax,
@@ -34,7 +35,7 @@ public protocol MemberMacro: AttachedMacro {
   /// - Parameters:
   ///   - node: The custom attribute describing the attached macro.
   ///   - declaration: The declaration the macro attribute is attached to.
-  ///   - missingConformancesTo: The set of protocols that were declared
+  ///   - implementingRequirementsFor: The set of protocols that were declared
   ///     in the set of conformances for the macro and to which the declaration
   ///     does not explicitly conform. The member macro itself cannot declare
   ///     conformances to these protocols (only an extension macro can do that),
@@ -48,17 +49,26 @@ public protocol MemberMacro: AttachedMacro {
   static func expansion(
     of node: AttributeSyntax,
     providingMembersOf declaration: some DeclGroupSyntax,
-    missingConformancesTo protocols: [TypeSyntax],
+    implementingRequirementsFor protocols: [TypeSyntax],
     in context: some MacroExpansionContext
   ) throws -> [DeclSyntax]
 }
 
 public extension MemberMacro {
+  /// Default implementation supplies no conformances.
+  static func expansion(
+    of node: AttributeSyntax,
+    providingMembersOf declaration: some DeclGroupSyntax,
+    in context: some MacroExpansionContext
+  ) throws -> [DeclSyntax] {
+    return try expansion(of: node, providingMembersOf: declaration, implementingRequirementsFor: [], in: context)
+  }
+
   /// Default implementation that ignores the unhandled conformances.
   static func expansion(
     of node: AttributeSyntax,
     providingMembersOf declaration: some DeclGroupSyntax,
-    missingConformancesTo protocols: [TypeSyntax],
+    implementingRequirementsFor protocols: [TypeSyntax],
     in context: some MacroExpansionContext
   ) throws -> [DeclSyntax] {
     return try expansion(of: node, providingMembersOf: declaration, in: context)