[MemberMacro] Add missingConformancesTo argument to expansion operation

Stage in an entrypoint for member macros that allows them to learn
about which conformances that they've asked about are "missing",
meaning that they are not present on the type (ignoring those that
would be generated by an extension macro). This information is
equivalent to the information provided to extension macros, although
the member macro itself cannot create the conformance.

Author: DougGregor

Sources/SwiftSyntaxMacroExpansion/MacroExpansion.swift

@@ -236,6 +236,7 @@ public func expandAttachedMacroWithoutCollapsing<Context: MacroExpansionContext>
       let members = try attachedMacro.expansion(
         of: attributeNode,
         providingMembersOf: declGroup,
+        missingConformancesTo: conformanceList?.map(\.type) ?? [],
         in: context
       )
 

Sources/SwiftSyntaxMacros/MacroProtocols/MemberMacro.swift

@@ -28,4 +28,39 @@ public protocol MemberMacro: AttachedMacro {
     providingMembersOf declaration: some DeclGroupSyntax,
     in context: some MacroExpansionContext
   ) throws -> [DeclSyntax]
+
+  /// Expand an attached declaration macro to produce a set of members.
+  ///
+  /// - 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
+  ///     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),
+  ///     but can provide supporting declarations, such as a required
+  ///     initializer or stored property, that cannot be written in an
+  ///     extension.
+  ///   - context: The context in which to perform the macro expansion.
+  ///
+  /// - Returns: the set of member declarations introduced by this macro, which
+  /// are nested inside the `attachedTo` declaration.
+  static func expansion(
+    of node: AttributeSyntax,
+    providingMembersOf declaration: some DeclGroupSyntax,
+    missingConformancesTo protocols: [TypeSyntax],
+    in context: some MacroExpansionContext
+  ) throws -> [DeclSyntax]
+}
+
+public extension MemberMacro {
+  /// Default implementation that ignores the unhandled conformances.
+  static func expansion(
+    of node: AttributeSyntax,
+    providingMembersOf declaration: some DeclGroupSyntax,
+    missingConformancesTo protocols: [TypeSyntax],
+    in context: some MacroExpansionContext
+  ) throws -> [DeclSyntax] {
+    return try expansion(of: node, providingMembersOf: declaration, in: context)
+  }
 }