[Macros] Implement the skeleton of invoking peer macro expansions.

Author: hborla

include/swift/AST/TypeCheckRequests.h

@@ -3884,6 +3884,23 @@ class CompilerPluginLoadRequest
   bool isCached() const { return true; }
 };
 
+/// Expand peer macros attached to the given declaration.
+class ExpandPeerMacroRequest
+    : public SimpleRequest<ExpandPeerMacroRequest,
+                           bool(Decl *),
+                           RequestFlags::Cached> {
+public:
+  using SimpleRequest::SimpleRequest;
+
+private:
+  friend SimpleRequest;
+
+  bool evaluate(Evaluator &evaluator, Decl *decl) const;
+
+public:
+  bool isCached() const { return true; }
+};
+
 /// Resolve an external macro given its module and type name.
 class ExternalMacroDefinitionRequest
     : public SimpleRequest<ExternalMacroDefinitionRequest,

include/swift/AST/TypeCheckerTypeIDZone.def

@@ -437,6 +437,9 @@ SWIFT_REQUEST(TypeChecker, ExpandMemberAttributeMacros,
 SWIFT_REQUEST(TypeCHecker, ExpandSynthesizedMemberMacroRequest,
               bool(Decl *),
               Cached, NoLocationInfo)
+SWIFT_REQUEST(TypeChecker, ExpandPeerMacroRequest,
+              bool(Decl *),
+              Cached, NoLocationInfo)
 SWIFT_REQUEST(TypeChecker, SynthesizeRuntimeMetadataAttrGenerator,
               Expr *(CustomAttr *, ValueDecl *),
               Cached, NoLocationInfo)

lib/Sema/TypeCheckMacros.cpp

@@ -413,6 +413,24 @@ bool ExpandSynthesizedMemberMacroRequest::evaluate(Evaluator &evaluator,
   return synthesizedMembers;
 }
 
+bool ExpandPeerMacroRequest::evaluate(Evaluator &evaluator, Decl *decl) const {
+  SmallVector<Decl *, 2> peers;
+  decl->forEachAttachedMacro(MacroRole::Peer,
+      [&](CustomAttr *attr, MacroDecl *macro) {
+        expandPeers(attr, macro, decl, peers);
+      });
+
+  // Expand all peers that have attached peer macros.
+  for (auto *peer : peers) {
+    (void)evaluateOrDefault(
+        evaluator,
+        ExpandPeerMacroRequest{peer},
+        false);
+  }
+
+  return !peers.empty();
+}
+
 /// Determine whether the given source file is from an expansion of the given
 /// macro.
 static bool isFromExpansionOfMacro(SourceFile *sourceFile, MacroDecl *macro,
@@ -1065,6 +1083,34 @@ bool swift::expandMembers(CustomAttr *attr, MacroDecl *macro, Decl *decl) {
   return synthesizedMembers;
 }
 
+void swift::expandPeers(CustomAttr *attr, MacroDecl *macro, Decl *decl,
+                        SmallVectorImpl<Decl *> &peers) {
+  auto macroSourceFile = evaluateAttachedMacro(macro, decl, attr,
+                                               /*passParentContext*/false,
+                                               MacroRole::Peer);
+  if (!macroSourceFile)
+    return;
+
+  PrettyStackTraceDecl debugStack("applying expanded peer macro", decl);
+
+  auto *parent = decl->getDeclContext();
+  auto topLevelDecls = macroSourceFile->getTopLevelDecls();
+  for (auto peer : topLevelDecls) {
+    peer->setDeclContext(parent);
+
+    if (auto *nominal = dyn_cast<NominalTypeDecl>(parent)) {
+      nominal->addMember(peer);
+    } else if (auto *extension = dyn_cast<ExtensionDecl>(parent)) {
+      extension->addMember(peer);
+    } else {
+      // TODO: Add peers to global or local contexts.
+      continue;
+    }
+
+    peers.push_back(peer);
+  }
+}
+
 MacroDecl *
 ResolveMacroRequest::evaluate(Evaluator &evaluator,
                               UnresolvedMacroReference macroRef,

lib/Sema/TypeCheckMacros.h

@@ -64,6 +64,13 @@ bool expandAttributes(CustomAttr *attr, MacroDecl *macro, Decl *member);
 /// otherwise.
 bool expandMembers(CustomAttr *attr, MacroDecl *macro, Decl *decl);
 
+/// Expand the peer declarations for the given declaration based on
+/// the custom attribute that references the given macro.
+///
+/// Populates the \c peers vector with the expanded peer declarations.
+void expandPeers(CustomAttr *attr, MacroDecl *macro, Decl *decl,
+                 SmallVectorImpl<Decl *> &peers);
+
 } // end namespace swift
 
 #endif /* SWIFT_SEMA_TYPECHECKMACROS_H */