[SE-0407] Provide member macros with information about "missing" conformances

Provide member macros with similar information about conformances to
what extension macros receive, allowing member macros to document
which conformances they care about (e.g., Decodable) and then
receiving the list of conformances that aren't already available for
the type in question. For example, a macro such as

    @attached(member, conformances: Decodable, Encodable, names:
named(init(from:), encode(to:)))
    macro Codable() = ...

Expanded on a type that is not already Decodable/Encodable would be
provided with Decodable and Encodable (via the new
`missingConformancesTo:` argument to the macro implementation) when
the type itself does not conform to those types.

Member macros still cannot produce conformances, so this is likely to
be used in conjunction with extension macros most of the time. The
extension macro declares the conformance, and can also declare any
members that shouldn't be part of the primary type definition---such
as initializers that shouldn't suppress the memberwise initializer. On
the other hand, the member macro will need to define any members that
must be in the primary definition, such as required initializers,
members that must be overridable by subclasses, and stored properties.

Codable synthesis is an example that benefits from member macros with
conformances, because for classes it wants to introduce a required
initializer for decoding and an overridable encode operation, and
these must be members of the nominal type itself. Specifically, the
`Codable` macro above is likely to have two attached member roles:

    @attached(member, conformances: Decodable, Encodable, names:
named(init(from:), encode(to:)))
    @attached(extension, conformances: Decodable, Encodable, names:
named(init(from:), encode(to:)))
    macro Codable() = ...

where the "extension" role is responsible for defining the conformance
(always), and the "member" creates the appropriate members for classes
(`init` vs. `required init`).

Tracked by rdar://112532829.

Author: DougGregor

Diff for: CHANGELOG.md

@@ -3,6 +3,18 @@
 > **Note**\
 > This is in reverse chronological order, so newer entries are added to the top.
 
+## Swift 5.9.2
+
+* [SE-0407][]:
+
+  Member macros can specify a list of protocols via the `conformances` argument to the macro role. The macro implementation will be provided with those protocols that are listed but have not already been implemented by the type to which the member macro is attached, in the same manner as extension macros.
+
+   ```swift
+   @attached(member, conformances: Decodable, Encodable, names: named(init(from:), encode(to:)))
+@attached(extension, conformances: Decodable, Encodable, names: named(init(from:), encode(to:)))
+macro Codable() = #externalMacro(module: "MyMacros", type: "CodableMacro")
+   ```
+
 ## Swift 5.9
 
 * [SE-0382][], [SE-0389][], [SE-0394][], [SE-0397][]:
@@ -9833,6 +9845,7 @@ using the `.dynamicType` member to retrieve the type of an expression should mig
 [SE-0389]: https://github.com/apple/swift-evolution/blob/main/proposals/0389-attached-macros.md
 [SE-0394]: https://github.com/apple/swift-evolution/blob/main/proposals/0394-swiftpm-expression-macros.md
 [SE-0397]: https://github.com/apple/swift-evolution/blob/main/proposals/0397-freestanding-declaration-macros.md
+[SE-0407]: https://github.com/apple/swift-evolution/blob/main/proposals/0407-member-macro-conformances.md
 [#64927]: <https://github.com/apple/swift/issues/64927>
 [#42697]: <https://github.com/apple/swift/issues/42697>
 [#42728]: <https://github.com/apple/swift/issues/42728>

Diff for: include/swift/AST/Decl.h

@@ -8701,6 +8701,7 @@ class MacroDecl : public GenericContext, public ValueDecl {
   /// be added if this macro does not contain an extension role.
   void getIntroducedConformances(
       NominalTypeDecl *attachedTo,
+      MacroRole role,
       SmallVectorImpl<ProtocolDecl *> &conformances) const;
 
   /// Returns a DeclName that represents arbitrary names.

Diff for: include/swift/AST/TypeCheckRequests.h

@@ -3338,10 +3338,10 @@ class ResolveMacroRequest
   void noteCycleStep(DiagnosticEngine &diags) const;
 };
 
-/// Returns the resolved constraint types that an extension macro
-/// adds conformances to.
-class ResolveExtensionMacroConformances
-    : public SimpleRequest<ResolveExtensionMacroConformances,
+/// Returns the resolved constraint types that a macro references conformances
+/// to.
+class ResolveMacroConformances
+    : public SimpleRequest<ResolveMacroConformances,
                            ArrayRef<Type>(const MacroRoleAttr *, const Decl *),
                            RequestFlags::Cached> {
 public:

Diff for: include/swift/AST/TypeCheckerTypeIDZone.def

@@ -363,7 +363,7 @@ SWIFT_REQUEST(TypeChecker, ResolveImplicitMemberRequest,
 SWIFT_REQUEST(TypeChecker, ResolveMacroRequest,
               ConcreteDeclRef(UnresolvedMacroReference, const Decl *),
               Cached, NoLocationInfo)
-SWIFT_REQUEST(TypeChecker, ResolveExtensionMacroConformances,
+SWIFT_REQUEST(TypeChecker, ResolveMacroConformances,
               ArrayRef<Type>(const MacroRoleAttr *, const Decl *),
               Cached, NoLocationInfo)
 SWIFT_REQUEST(TypeChecker, ResolveTypeEraserTypeRequest,

Diff for: lib/AST/Attr.cpp

@@ -1393,7 +1393,7 @@ bool DeclAttribute::printImpl(ASTPrinter &Printer, const PrintOptions &Options,
     // Print conformances, if present.
     auto conformances = evaluateOrDefault(
         D->getASTContext().evaluator,
-        ResolveExtensionMacroConformances{Attr, D},
+        ResolveMacroConformances{Attr, D},
         {});
     if (!conformances.empty()) {
       Printer << ", conformances: ";

Diff for: lib/AST/ConformanceLookupTable.cpp

@@ -506,7 +506,8 @@ void ConformanceLookupTable::addMacroGeneratedProtocols(
       MacroRole::Extension,
       [&](CustomAttr *attr, MacroDecl *macro) {
         SmallVector<ProtocolDecl *, 2> conformances;
-        macro->getIntroducedConformances(nominal, conformances);
+        macro->getIntroducedConformances(
+            nominal, MacroRole::Extension, conformances);
 
         for (auto *protocol : conformances) {
           addProtocol(protocol, attr->getLocation(), source);

Diff for: lib/AST/Decl.cpp

@@ -10886,15 +10886,16 @@ void MacroDecl::getIntroducedNames(MacroRole role, ValueDecl *attachedTo,
 
 void MacroDecl::getIntroducedConformances(
     NominalTypeDecl *attachedTo,
+    MacroRole role,
     SmallVectorImpl<ProtocolDecl *> &conformances) const {
-  auto *attr = getMacroRoleAttr(MacroRole::Extension);
+  auto *attr = getMacroRoleAttr(role);
   if (!attr)
     return;
 
   auto &ctx = getASTContext();
   auto constraintTypes = evaluateOrDefault(
       ctx.evaluator,
-      ResolveExtensionMacroConformances{attr, this},
+      ResolveMacroConformances{attr, this},
       {});
 
   for (auto constraint : constraintTypes) {

Diff for: lib/Sema/TypeCheckAttr.cpp

@@ -6948,7 +6948,7 @@ void AttributeChecker::visitMacroRoleAttr(MacroRoleAttr *attr) {
 
   (void)evaluateOrDefault(
       Ctx.evaluator,
-      ResolveExtensionMacroConformances{attr, D},
+      ResolveMacroConformances{attr, D},
       {});
 }
 

Diff for: lib/Sema/TypeCheckMacros.cpp

@@ -1548,12 +1548,56 @@ swift::expandAttributes(CustomAttr *attr, MacroDecl *macro, Decl *member) {
   return macroSourceFile->getBufferID();
 }
 
+// Collect the protocol conformances that the macro asked about but were
+// not already present on the declaration.
+static TinyPtrVector<ProtocolDecl *> getIntroducedConformances(
+    NominalTypeDecl *nominal, MacroRole role, MacroDecl *macro,
+    SmallVectorImpl<ProtocolDecl *> *potentialConformances = nullptr) {
+  SmallVector<ProtocolDecl *, 2> potentialConformancesBuffer;
+  if (!potentialConformances)
+    potentialConformances = &potentialConformancesBuffer;
+  macro->getIntroducedConformances(nominal, role, *potentialConformances);
+
+  TinyPtrVector<ProtocolDecl *> introducedConformances;
+  for (auto protocol : *potentialConformances) {
+    SmallVector<ProtocolConformance *, 2> existingConformances;
+    nominal->lookupConformance(protocol, existingConformances);
+
+    bool hasExistingConformance = llvm::any_of(
+        existingConformances,
+        [&](ProtocolConformance *conformance) {
+          return conformance->getSourceKind() !=
+              ConformanceEntryKind::PreMacroExpansion;
+        });
+
+    if (!hasExistingConformance) {
+      introducedConformances.push_back(protocol);
+    }
+  }
+
+  return introducedConformances;
+}
+
 llvm::Optional<unsigned> swift::expandMembers(CustomAttr *attr,
                                               MacroDecl *macro, Decl *decl) {
+  auto nominal = dyn_cast<NominalTypeDecl>(decl);
+  if (!nominal) {
+    auto ext = dyn_cast<ExtensionDecl>(decl);
+    if (!ext)
+      return llvm::None;
+
+    nominal = ext->getExtendedNominal();
+    if (!nominal)
+      return llvm::None;
+  }
+  auto introducedConformances = getIntroducedConformances(
+      nominal, MacroRole::Member, macro);
+
   // Evaluate the macro.
   auto macroSourceFile =
       ::evaluateAttachedMacro(macro, decl, attr,
-                              /*passParentContext=*/false, MacroRole::Member);
+                              /*passParentContext=*/false, MacroRole::Member,
+                              introducedConformances);
   if (!macroSourceFile)
     return llvm::None;
 
@@ -1625,22 +1669,9 @@ swift::expandExtensions(CustomAttr *attr, MacroDecl *macro,
     return llvm::None;
   }
 
-  // Collect the protocol conformances that the macro can add. The
-  // macro should not add conformances that are already stated in
-  // the original source.
-
   SmallVector<ProtocolDecl *, 2> potentialConformances;
-  macro->getIntroducedConformances(nominal, potentialConformances);
-
-  SmallVector<ProtocolDecl *, 2> introducedConformances;
-  for (auto protocol : potentialConformances) {
-    SmallVector<ProtocolConformance *, 2> existingConformances;
-    nominal->lookupConformance(protocol, existingConformances);
-    if (existingConformances.empty()) {
-      introducedConformances.push_back(protocol);
-    }
-  }
-
+  auto introducedConformances = getIntroducedConformances(
+      nominal, MacroRole::Extension, macro, &potentialConformances);
   auto macroSourceFile = ::evaluateAttachedMacro(macro, nominal, attr,
                                                  /*passParentContext=*/false,
                                                  role, introducedConformances);
@@ -1830,9 +1861,9 @@ ConcreteDeclRef ResolveMacroRequest::evaluate(Evaluator &evaluator,
 }
 
 ArrayRef<Type>
-ResolveExtensionMacroConformances::evaluate(Evaluator &evaluator,
-                                            const MacroRoleAttr *attr,
-                                            const Decl *decl) const {
+ResolveMacroConformances::evaluate(Evaluator &evaluator,
+                                   const MacroRoleAttr *attr,
+                                   const Decl *decl) const {
   auto *dc = decl->getDeclContext();
   auto &ctx = dc->getASTContext();
 

Diff for: test/Macros/Inputs/syntax_macro_definitions.swift

@@ -2004,3 +2004,53 @@ public struct InitWithProjectedValueWrapperMacro: PeerMacro {
     ]
   }
 }
+
+public struct RequiredDefaultInitMacro: ExtensionMacro {
+  public static func expansion(
+    of node: AttributeSyntax,
+    attachedTo decl: some DeclGroupSyntax,
+    providingExtensionsOf type: some TypeSyntaxProtocol,
+    conformingTo protocols: [TypeSyntax],
+    in context: some MacroExpansionContext
+  ) throws -> [ExtensionDeclSyntax] {
+    if protocols.isEmpty {
+      return []
+    }
+
+    let decl: DeclSyntax =
+      """
+      extension \(type.trimmed): DefaultInit {
+      }
+
+      """
+
+    return [
+      decl.cast(ExtensionDeclSyntax.self)
+    ]
+  }
+}
+
+extension RequiredDefaultInitMacro: MemberMacro {
+  public static func expansion(
+    of node: AttributeSyntax,
+    providingMembersOf declaration: some DeclGroupSyntax,
+    in context: some MacroExpansionContext
+  ) throws -> [DeclSyntax] {
+    fatalError("old swift-syntax")
+  }
+
+  public static func expansion(
+    of node: AttributeSyntax,
+    providingMembersOf declaration: some DeclGroupSyntax,
+    conformingTo protocols: [TypeSyntax],
+    in context: some MacroExpansionContext
+  ) throws -> [DeclSyntax] {
+    let decl: DeclSyntax
+    if declaration.is(ClassDeclSyntax.self) && protocols.isEmpty {
+      decl = "required init() { }"
+    } else {
+      decl = "init() { }"
+    }
+    return [ decl ]
+  }
+}

Diff for: test/Macros/macro_expand_member_with_conformances.swift

@@ -0,0 +1,22 @@
+// REQUIRES: swift_swift_parser
+
+// RUN: %empty-directory(%t)
+// RUN: %host-build-swift -swift-version 5 -emit-library -o %t/%target-library-name(MacroDefinition) -module-name=MacroDefinition %S/Inputs/syntax_macro_definitions.swift -g -no-toolchain-stdlib-rpath
+
+// RUN: %target-typecheck-verify-swift -enable-experimental-feature ExtensionMacros -swift-version 5 -load-plugin-library %t/%target-library-name(MacroDefinition) -module-name MacroUser -DTEST_DIAGNOSTICS -swift-version 5 -I %t
+protocol DefaultInit {
+  init()
+}
+
+@attached(extension, conformances: DefaultInit)
+@attached(member, conformances: DefaultInit, names: named(init()))
+macro DefaultInit() = #externalMacro(module: "MacroDefinition", type: "RequiredDefaultInitMacro")
+
+@DefaultInit
+class C { }
+
+@DefaultInit
+class D: C { }
+
+@DefaultInit
+struct E { }