[PrintAsObjC] Clarify the 'specialNames' map in DeclAndTypePrinter

And comment the other cached members a little better.

Author: jrose-apple

lib/PrintAsObjC/DeclAndTypePrinter.cpp

@@ -1443,17 +1443,15 @@ class DeclAndTypePrinter::Implementation
   }
 
   /// If \p typeDecl is one of the standard library types used to map in Clang
-  /// primitives and basic types, return the address of the info in
-  /// \c specialNames containing the Clang name and whether it can be optional.
-  ///
-  /// Returns null if the name is not one of these known types.
-  const NameAndOptional *getKnownTypeInfo(const TypeDecl *typeDecl) {
+  /// primitives and basic types, return the info in \c specialNames containing
+  /// the Clang name and whether it can be nullable in C.
+  Optional<CTypeInfo> getKnownTypeInfo(const TypeDecl *typeDecl) {
     auto &specialNames = owningPrinter.specialNames;
     if (specialNames.empty()) {
       ASTContext &ctx = getASTContext();
 #define MAP(SWIFT_NAME, CLANG_REPR, NEEDS_NULLABILITY)                       \
       specialNames[{ctx.StdlibModuleName, ctx.getIdentifier(#SWIFT_NAME)}] = \
-        { CLANG_REPR, NEEDS_NULLABILITY}
+        { CLANG_REPR, NEEDS_NULLABILITY }
 
       MAP(CBool, "bool", false);
 
@@ -1534,8 +1532,8 @@ class DeclAndTypePrinter::Implementation
     Identifier name = typeDecl->getName();
     auto iter = specialNames.find({moduleName, name});
     if (iter == specialNames.end())
-      return nullptr;
-    return &iter->second;
+      return None;
+    return iter->second;
   }
 
   /// If \p typeDecl is one of the standard library types used to map in Clang
@@ -1546,11 +1544,11 @@ class DeclAndTypePrinter::Implementation
   /// for interfacing with C and Objective-C.
   bool printIfKnownSimpleType(const TypeDecl *typeDecl,
                               Optional<OptionalTypeKind> optionalKind) {
-    auto *knownTypeInfo = getKnownTypeInfo(typeDecl);
+    Optional<CTypeInfo> knownTypeInfo = getKnownTypeInfo(typeDecl);
     if (!knownTypeInfo)
       return false;
-    os << knownTypeInfo->first;
-    if (knownTypeInfo->second)
+    os << knownTypeInfo->name;
+    if (knownTypeInfo->canBeNullable)
       printNullability(optionalKind);
     return true;
   }

lib/PrintAsObjC/DeclAndTypePrinter.h

@@ -38,12 +38,28 @@ class DeclAndTypePrinter {
   const DelayedMemberSet &delayedMembers;
   AccessLevel minRequiredAccess;
 
-  using NameAndOptional = std::pair<StringRef, bool>;
-  llvm::DenseMap<std::pair<Identifier, Identifier>, NameAndOptional>
-    specialNames;
+  struct CTypeInfo {
+    StringRef name;
+    bool canBeNullable;
+  };
 
-  // Cached for convenience.
+  /// A map from {Module, TypeName} pairs to {C name, C nullability} pairs.
+  ///
+  /// This is populated on first use with a list of known Swift types that are
+  /// translated directly by the ObjC printer instead of structurally, allowing
+  /// it to do things like map 'Int' to 'NSInteger' and 'Float' to 'float'.
+  /// In some sense it's the reverse of the ClangImporter's MappedTypes.def.
+  llvm::DenseMap<std::pair<Identifier, Identifier>, CTypeInfo> specialNames;
+
+  /// The name 'CFTypeRef'.
+  ///
+  /// Cached for convenience.
   Identifier ID_CFTypeRef;
+
+  /// The protocol type 'NSCopying', or a null type if Foundation has not been
+  /// imported.
+  ///
+  /// Cached for convenience.
   Optional<Type> NSCopyingType;
 
   Implementation getImpl();