[libSyntax] Hide null AbsoluteRawSyntax behind a custom OptionalStorage implementation

This splits the previous null AbsoluteRawSyntax type into two categories
and removes the public null initializer and isNull method.

1. The default initializer of AbsoluteRawSyntax now create uninitialized
   memory. This is exactly what we need since we just need to allocate
   the memory to initialise  it using the SyntaxDataRef::getChild method
   wherever we use it.
2. Make Optional<AbsoluteRawSyntax> and Optional<SyntaxDataRef> zero-cost
   wrappers around their underlying type. These use the old null type
   to indicate a missing optional value.

Overall, I believe this makes the code both safer (we now enforce null
types properly in the type system) and potentially faster (although I
haven't been able to measure an improvement)

include/swift/Syntax/AbsoluteRawSyntax.h

@@ -28,6 +28,8 @@ class SyntaxIndexInTree {
   explicit SyntaxIndexInTree(size_t IndexInTree) : IndexInTree(IndexInTree) {}
 
 public:
+  /// Create an *uninitialized* \c SyntaxIndexInTree.
+  SyntaxIndexInTree() {}
   static SyntaxIndexInTree zero() { return SyntaxIndexInTree(0); }
 
   /// Assuming that this index points to the start of \p Raw, advance it so that
@@ -68,6 +70,9 @@ private:
   SyntaxIndexInTree IndexInTree;
 
 public:
+  /// Create an *uninitialized* \c SyntaxIdentifier.
+  SyntaxIdentifier() {}
+
   SyntaxIdentifier(RootIdType RootId, SyntaxIndexInTree IndexInTree)
       : RootId(RootId), IndexInTree(IndexInTree) {
     assert(RootId < NextUnusedRootId && "New RootIds should only be created "
@@ -124,6 +129,9 @@ private:
   IndexInParentType IndexInParent;
 
 public:
+  /// Create an *uninitialized* \c AbsoluteSyntaxPosition.
+  AbsoluteSyntaxPosition() {}
+
   AbsoluteSyntaxPosition(OffsetType Offset, IndexInParentType IndexInParent)
       : Offset(Offset), IndexInParent(IndexInParent) {}
 
@@ -156,6 +164,8 @@ class AbsoluteOffsetPosition {
   AbsoluteSyntaxPosition::OffsetType Offset;
 
 public:
+  /// Create an *uninitialized* \c AbsoluteOffsetPosition.
+  AbsoluteOffsetPosition() {}
   explicit AbsoluteOffsetPosition(AbsoluteSyntaxPosition::OffsetType Offset)
       : Offset(Offset) {}
   AbsoluteOffsetPosition(AbsoluteSyntaxPosition Position)
@@ -176,6 +186,8 @@ class AbsoluteSyntaxInfo {
   SyntaxIdentifier NodeId;
 
 public:
+  /// Create an *uninitialized* \c AbsoluteSyntaxInfo.
+  AbsoluteSyntaxInfo() {}
   AbsoluteSyntaxInfo(AbsoluteSyntaxPosition Position, SyntaxIdentifier NodeId)
       : Position(Position), NodeId(NodeId) {}
 
@@ -213,16 +225,25 @@ public:
 
 /// A \c RawSyntax node that is enrichted with information of its position
 /// within the syntax tree it lives in.
-struct AbsoluteRawSyntax {
+class AbsoluteRawSyntax {
+  /// OptionalStorage is a friend so it can access the \c nullptr initializer
+  /// and \c isNull.
+  template <typename, bool>
+  friend class llvm::optional_detail::OptionalStorage;
+
   const RawSyntax *Raw;
-  const AbsoluteSyntaxInfo Info;
+  AbsoluteSyntaxInfo Info;
+
+  /// Whether this is a null \c AbsoluteRawSyntax.
+  bool isNull() const { return Raw == nullptr; }
+
+  /// Create a null \c AbsoluteRawSyntax. This should only be used in \c
+  /// AbsoluteRawSyntax's \c OptionalStorage.
+  explicit AbsoluteRawSyntax(std::nullptr_t) : Raw(nullptr) {}
 
 public:
-  /// Create a null \c AbsoluteRawSyntax to which a real \c AbsoluteRawSyntax
-  /// can be stored later.
-  explicit AbsoluteRawSyntax()
-      : Raw(nullptr), Info(AbsoluteSyntaxPosition(0, 0),
-                           SyntaxIdentifier(0, SyntaxIndexInTree::zero())) {}
+  /// Create an *uninitialized* \c AbsoluteRawSyntax.
+  explicit AbsoluteRawSyntax() {}
 
   /// Create a new \c AbsoluteRawData backed by \p Raw and with additional \p
   /// Info. The caller of this constructor is responsible to ensure that the
@@ -235,10 +256,6 @@ public:
            "should always have a RawSyntax");
   }
 
-  /// Whether this is a null \c AbsoluteRawSyntax created through the default
-  /// constructor.
-  bool isNull() const { return Raw == nullptr; }
-
   /// Construct a \c AbsoluteRawSyntax for a \c RawSyntax node that represents
   /// the syntax tree's root.
   static AbsoluteRawSyntax forRoot(const RawSyntax *Raw) {
@@ -270,16 +287,8 @@ public:
 
   /// Get the child at \p Index if it exists. If the node does not have a child
   /// at \p Index, return \c None. Asserts that \p Index < \c NumChildren
-  Optional<AbsoluteRawSyntax>
-  getChild(AbsoluteSyntaxPosition::IndexInParentType Index) const {
-    assert(Index < getNumChildren() && "Index out of bounds");
-    auto RawChild = getRaw()->getChild(Index);
-    if (RawChild) {
-      return getPresentChild(Index);
-    } else {
-      return None;
-    }
-  }
+  inline Optional<AbsoluteRawSyntax>
+  getChild(AbsoluteSyntaxPosition::IndexInParentType Index) const;
 
   /// Get the child at \p Index, asserting that it exists. This is slightly
   /// more performant than \c getChild in these cases since the \c
@@ -305,46 +314,11 @@ public:
 
   /// Get the first non-missing token node in this tree. Return \c None if
   /// this node does not contain non-missing tokens.
-  Optional<AbsoluteRawSyntax> getFirstToken() const {
-    if (getRaw()->isToken() && !getRaw()->isMissing()) {
-      return *this;
-    }
-
-    size_t NumChildren = getNumChildren();
-    for (size_t I = 0; I < NumChildren; ++I) {
-      if (auto Child = getChild(I)) {
-        if (Child->getRaw()->isMissing()) {
-          continue;
-        }
-
-        if (auto Token = Child->getFirstToken()) {
-          return Token;
-        }
-      }
-    }
-    return None;
-  }
+  inline Optional<AbsoluteRawSyntax> getFirstToken() const;
 
   /// Get the last non-missing token node in this tree. Return \c None if
   /// this node does not contain non-missing tokens.
-  Optional<AbsoluteRawSyntax> getLastToken() const {
-    if (getRaw()->isToken() && !getRaw()->isMissing()) {
-      return *this;
-    }
-
-    for (int I = getNumChildren() - 1; I >= 0; --I) {
-      if (auto Child = getChild(I)) {
-        if (Child->getRaw()->isMissing()) {
-          continue;
-        }
-
-        if (auto Token = Child->getLastToken()) {
-          return Token;
-        }
-      }
-    }
-    return None;
-  }
+  inline Optional<AbsoluteRawSyntax> getLastToken() const;
 
   /// Construct a new \c AbsoluteRawSyntax node that has the same info as the
   /// current one, but
@@ -366,6 +340,122 @@ public:
 namespace llvm {
 raw_ostream &operator<<(raw_ostream &OS,
                         swift::syntax::AbsoluteOffsetPosition Pos);
+
+namespace optional_detail {
+
+using swift::syntax::AbsoluteRawSyntax;
+
+/// A custom \c OptionalStorage implementation for \c AbsoluteRawSyntax that
+/// makes \c Optional<AbsoluteRawSyntax> a zero-cost wrapper around \c
+/// AbsoluteRawSyntax by using a special (externally not accessible) null \c
+/// AbsoluteRawSyntax to represent a missing value.
+template <>
+class OptionalStorage<AbsoluteRawSyntax> {
+  AbsoluteRawSyntax Storage;
+
+public:
+  OptionalStorage() : Storage(nullptr) {}
+  OptionalStorage(OptionalStorage const &other) = default;
+  OptionalStorage(OptionalStorage &&other) = default;
+
+  template <class... ArgTypes>
+  explicit OptionalStorage(llvm::optional_detail::in_place_t,
+                           ArgTypes &&...Args)
+      : Storage(std::forward<ArgTypes>(Args)...) {}
+
+  void reset() { Storage = AbsoluteRawSyntax(nullptr); }
+
+  bool hasValue() const { return !Storage.isNull(); }
+
+  AbsoluteRawSyntax &getValue() LLVM_LVALUE_FUNCTION {
+    assert(hasValue());
+    return Storage;
+  }
+  AbsoluteRawSyntax const &getValue() const LLVM_LVALUE_FUNCTION {
+    assert(hasValue());
+    return Storage;
+  }
+#if LLVM_HAS_RVALUE_REFERENCE_THIS
+  AbsoluteRawSyntax &&getValue() &&noexcept {
+    assert(hasValue());
+    return std::move(Storage);
+  }
+#endif
+
+  template <class... Args>
+  void emplace(Args &&...args) {
+    Storage = AbsoluteRawSyntax(std::forward<Args>(args)...);
+  }
+
+  OptionalStorage &operator=(const AbsoluteRawSyntax &AbsoluteRaw) {
+    Storage = AbsoluteRaw;
+    return *this;
+  }
+
+  OptionalStorage &operator=(AbsoluteRawSyntax &&AbsoluteRaw) {
+    Storage = std::move(AbsoluteRaw);
+    return *this;
+  }
+
+  OptionalStorage &operator=(OptionalStorage const &other) = default;
+  OptionalStorage &operator=(OptionalStorage &&other) = default;
+};
+} // namespace optional_detail
 } // end namespace llvm
 
+namespace swift {
+namespace syntax {
+
+Optional<AbsoluteRawSyntax> AbsoluteRawSyntax::getChild(
+    AbsoluteSyntaxPosition::IndexInParentType Index) const {
+  assert(Index < getNumChildren() && "Index out of bounds");
+  if (getRaw()->getChild(Index)) {
+    return getPresentChild(Index);
+  } else {
+    return None;
+  }
+}
+
+Optional<AbsoluteRawSyntax> AbsoluteRawSyntax::getFirstToken() const {
+  if (getRaw()->isToken() && !getRaw()->isMissing()) {
+    return *this;
+  }
+
+  size_t NumChildren = getNumChildren();
+  for (size_t I = 0; I < NumChildren; ++I) {
+    if (auto Child = getChild(I)) {
+      if (Child->getRaw()->isMissing()) {
+        continue;
+      }
+
+      if (auto Token = Child->getFirstToken()) {
+        return Token;
+      }
+    }
+  }
+  return None;
+}
+
+Optional<AbsoluteRawSyntax> AbsoluteRawSyntax::getLastToken() const {
+  if (getRaw()->isToken() && !getRaw()->isMissing()) {
+    return *this;
+  }
+
+  for (int I = getNumChildren() - 1; I >= 0; --I) {
+    if (auto Child = getChild(I)) {
+      if (Child->getRaw()->isMissing()) {
+        continue;
+      }
+
+      if (auto Token = Child->getLastToken()) {
+        return Token;
+      }
+    }
+  }
+  return None;
+}
+
+} // end namespace syntax
+} // end namespace swift
+
 #endif // SWIFT_SYNTAX_ABSOLUTERAWSYNTAX_H