[Macros] Enable global peer macros.

Global peer macro expansions are not injected into the AST. Instead, they
are visited as "auxiliary declarations" when needed, such as in the decl
checker and during SILGen. This is the same mechanism used for local property
wrappers and local lazy variables.

Author: hborla

include/swift/AST/Decl.h

@@ -867,6 +867,15 @@ class alignas(1 << DeclAlignInBits) Decl : public ASTAllocated<Decl> {
   /// attribute macro expansion.
   DeclAttributes getSemanticAttrs() const;
 
+  using AuxiliaryDeclCallback = llvm::function_ref<void(Decl *)>;
+
+  /// Iterate over the auxiliary declarations for this declaration,
+  /// invoking the given callback with each auxiliary decl.
+  ///
+  /// Auxiliary declarations can be property wrapper backing variables,
+  /// backing variables for 'lazy' vars, or peer macro expansions.
+  void visitAuxiliaryDecls(AuxiliaryDeclCallback callback) const;
+
   using MacroCallback = llvm::function_ref<void(CustomAttr *, MacroDecl *)>;
 
   /// Iterate over each attached macro with the given role, invoking the

include/swift/AST/TypeCheckerTypeIDZone.def

@@ -441,7 +441,7 @@ SWIFT_REQUEST(TypeChecker, ExpandSynthesizedMemberMacroRequest,
               ArrayRef<unsigned>(Decl *),
               Cached, NoLocationInfo)
 SWIFT_REQUEST(TypeChecker, ExpandPeerMacroRequest,
-              bool(Decl *),
+              ArrayRef<unsigned>(Decl *),
               Cached, NoLocationInfo)
 SWIFT_REQUEST(TypeChecker, SynthesizeRuntimeMetadataAttrGenerator,
               Expr *(CustomAttr *, ValueDecl *),

lib/AST/Decl.cpp

@@ -376,6 +376,27 @@ DeclAttributes Decl::getSemanticAttrs() const {
   return getAttrs();
 }
 
+void Decl::visitAuxiliaryDecls(AuxiliaryDeclCallback callback) const {
+  auto &ctx = getASTContext();
+  auto *mutableThis = const_cast<Decl *>(this);
+  auto peerBuffers =
+      evaluateOrDefault(ctx.evaluator,
+                        ExpandPeerMacroRequest{mutableThis},
+                        {});
+
+  SourceManager &sourceMgr = ctx.SourceMgr;
+  auto *moduleDecl = getModuleContext();
+  for (auto bufferID : peerBuffers) {
+    auto startLoc = sourceMgr.getLocForBufferStart(bufferID);
+    auto *sourceFile = moduleDecl->getSourceFileContainingLocation(startLoc);
+    for (auto *peer : sourceFile->getTopLevelDecls()) {
+      callback(peer);
+    }
+  }
+
+  // FIXME: fold VarDecl::visitAuxiliaryDecls into this.
+}
+
 void Decl::forEachAttachedMacro(MacroRole role,
                                 MacroCallback macroCallback) const {
   auto *dc = getDeclContext();

lib/Parse/ParseRequests.cpp

@@ -200,8 +200,12 @@ SourceFileParsingResult ParseSourceFileRequest::evaluate(Evaluator &evaluator,
     }
 
     case GeneratedSourceInfo::PeerMacroExpansion: {
-      // FIXME: this does not work for top-level or local peer expansions.
-      parser.parseExpandedMemberList(items);
+      if (parser.CurDeclContext->isTypeContext()) {
+        parser.parseExpandedMemberList(items);
+      } else {
+        parser.parseTopLevelItems(items);
+      }
+
       break;
     }
     }

lib/SILGen/SILGen.cpp

@@ -2188,6 +2188,13 @@ class SILGenModuleRAII {
 
     SourceFileScope scope(SGM, sf);
     for (auto *D : sf->getTopLevelDecls()) {
+      // Emit auxiliary decls.
+      D->visitAuxiliaryDecls([&](Decl *auxiliaryDecl) {
+        FrontendStatsTracer StatsTracer(SGM.getASTContext().Stats,
+                                        "SILgen-decl", auxiliaryDecl);
+        SGM.visit(auxiliaryDecl);
+      });
+
       FrontendStatsTracer StatsTracer(SGM.getASTContext().Stats,
                                       "SILgen-decl", D);
       SGM.visit(D);

lib/Sema/LookupVisibleDecls.cpp

@@ -616,7 +616,7 @@ static void synthesizeMemberDeclsForLookup(NominalTypeDecl *NTD,
     (void)evaluateOrDefault(
         ctx.evaluator,
         ExpandPeerMacroRequest{member},
-        false);
+        {});
   }
 
   synthesizePropertyWrapperVariables(NTD);

lib/Sema/TypeCheckDecl.cpp

@@ -2806,7 +2806,7 @@ static ArrayRef<Decl *> evaluateMembersRequest(
     (void)evaluateOrDefault(
         ctx.evaluator,
         ExpandPeerMacroRequest{member},
-        false);
+        {});
 
     if (auto *var = dyn_cast<VarDecl>(member)) {
       // The projected storage wrapper ($foo) might have

lib/Sema/TypeCheckDeclPrimary.cpp

@@ -1852,6 +1852,11 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
   }
 
   void visit(Decl *decl) {
+    // Visit auxiliary decls first.
+    decl->visitAuxiliaryDecls([&](Decl *auxiliaryDecl) {
+      this->visit(auxiliaryDecl);
+    });
+
     if (auto *Stats = getASTContext().Stats)
       ++Stats->getFrontendCounters().NumDeclsTypechecked;
 

lib/Sema/TypeCheckMacros.cpp

@@ -1194,9 +1194,6 @@ swift::expandPeers(CustomAttr *attr, MacroDecl *macro, Decl *decl) {
       nominal->addMember(peer);
     } else if (auto *extension = dyn_cast<ExtensionDecl>(parent)) {
       extension->addMember(peer);
-    } else {
-      // TODO: Add peers to global or local contexts.
-      continue;
     }
   }
 

lib/Sema/TypeCheckStorage.cpp

@@ -121,7 +121,7 @@ static void computeLoweredStoredProperties(NominalTypeDecl *decl,
     (void)evaluateOrDefault(
         ctx.evaluator,
         ExpandPeerMacroRequest{member},
-        false);
+        {});
 
     auto *var = dyn_cast<VarDecl>(member);
     if (!var || var->isStatic())