[yaml] Extract output logic from yaml::Output

[njames93]

Summary

Refactor all logic for writing a yaml file to its own class OutputStream.
This enables outputting yaml without having to specify a mapping.
It can also be used in cases when the mapping isn't trivial.

[njames93]

Fixed failing test cases by reverting Bitset Flow change
Added infrastructure to simplify emitting complex types.

[njames93]

Tweak alias/anchor output.
remove code duplication.

llvm/include/llvm/Support/YAMLTraits.h

 #include "llvm/ADT/StringRef.h"
 #include "llvm/ADT/Twine.h"
 #include "llvm/Support/AlignOf.h"
 #include "llvm/Support/Allocator.h"
 #include "llvm/Support/Endian.h"
 #include "llvm/Support/SMLoc.h"
 #include "llvm/Support/SourceMgr.h"
 #include "llvm/Support/YAMLParser.h"
+#include "llvm/Support/YamlSerializer.h"
 #include "llvm/Support/raw_ostream.h"
 #include <cassert>
 #include <map>
 #include <memory>
 #include <new>
 #include <string>
 #include <system_error>
 #include <type_traits>

 ///          io.bitSetCase(value, "round", flagRound);
 ///        }
 ///      };
 template <typename T, typename Enable = void> struct ScalarBitSetTraits {
   // Must provide:
   // static void bitset(IO &io, T &value);
 };
 
-/// Describe which type of quotes should be used when quoting is necessary.
-/// Some non-printable characters need to be double-quoted, while some others
-/// are fine with simple-quoting, and some don't need any quoting.
-enum class QuotingType { None, Single, Double };
-
 /// This class should be specialized by type that requires custom conversion
 /// to/from a yaml scalar.  For example:
 ///
 ///    template<>
 ///    struct ScalarTraits<MyType> {
 ///      static void output(const MyType &val, void*, llvm::raw_ostream &out) {
 ///        // stream out custom formatting
 ///        out << llvm::format("%x", val);

 }
 
 inline bool isBool(StringRef S) {
   // FIXME: using parseBool is causing multiple tests to fail.
   return S.equals("true") || S.equals("True") || S.equals("TRUE") ||
          S.equals("false") || S.equals("False") || S.equals("FALSE");
 }
 
-// 5.1. Character Set
-// The allowed character range explicitly excludes the C0 control block #x0-#x1F
-// (except for TAB #x9, LF #xA, and CR #xD which are allowed), DEL #x7F, the C1
-// control block #x80-#x9F (except for NEL #x85 which is allowed), the surrogate
-// block #xD800-#xDFFF, #xFFFE, and #xFFFF.
-inline QuotingType needsQuotes(StringRef S) {
-  if (S.empty())
-    return QuotingType::Single;
-
-  QuotingType MaxQuotingNeeded = QuotingType::None;
-  if (isSpace(static_cast<unsigned char>(S.front())) ||
-      isSpace(static_cast<unsigned char>(S.back())))
-    MaxQuotingNeeded = QuotingType::Single;
-  if (isNull(S))
-    MaxQuotingNeeded = QuotingType::Single;
-  if (isBool(S))
-    MaxQuotingNeeded = QuotingType::Single;
-  if (isNumeric(S))
-    MaxQuotingNeeded = QuotingType::Single;
-
-  // 7.3.3 Plain Style
-  // Plain scalars must not begin with most indicators, as this would cause
-  // ambiguity with other YAML constructs.
-  if (std::strchr(R"(-?:\,[]{}#&*!|>'"%@`)", S[0]) != nullptr)
-    MaxQuotingNeeded = QuotingType::Single;
-
-  for (unsigned char C : S) {
-    // Alphanum is safe.
-    if (isAlnum(C))
-      continue;
-
-    switch (C) {
-    // Safe scalar characters.
-    case '_':
-    case '-':
-    case '^':
-    case '.':
-    case ',':
-    case ' ':
-    // TAB (0x9) is allowed in unquoted strings.
-    case 0x9:
-      continue;
-    // LF(0xA) and CR(0xD) may delimit values and so require at least single
-    // quotes. LLVM YAML parser cannot handle single quoted multiline so use
-    // double quoting to produce valid YAML.
-    case 0xA:
-    case 0xD:
-      return QuotingType::Double;
-    // DEL (0x7F) are excluded from the allowed character range.
-    case 0x7F:
-      return QuotingType::Double;
-    // Forward slash is allowed to be unquoted, but we quote it anyway.  We have
-    // many tests that use FileCheck against YAML output, and this output often
-    // contains paths.  If we quote backslashes but not forward slashes then
-    // paths will come out either quoted or unquoted depending on which platform
-    // the test is run on, making FileCheck comparisons difficult.
-    case '/':
-    default: {
-      // C0 control block (0x0 - 0x1F) is excluded from the allowed character
-      // range.
-      if (C <= 0x1F)
-        return QuotingType::Double;
-
-      // Always double quote UTF-8.
-      if ((C & 0x80) != 0)
-        return QuotingType::Double;
-
-      // The character is not safe, at least simple quoting needed.
-      MaxQuotingNeeded = QuotingType::Single;
-    }
-    }
-  }
-
-  return MaxQuotingNeeded;
-}
-
 template <typename T, typename Context>
 struct missingTraits
     : public std::integral_constant<bool,
                                     !has_ScalarEnumerationTraits<T>::value &&
                                         !has_ScalarBitSetTraits<T>::value &&
                                         !has_ScalarTraits<T>::value &&
                                         !has_BlockScalarTraits<T>::value &&
                                         !has_TaggedScalarTraits<T>::value &&
public:
   // These are only used by operator<<. They could be private
   // if that templated operator could be made a friend.
   void beginDocuments();
   bool preflightDocument(unsigned);
   void postflightDocument();
   void endDocuments();
 
 private:
-  void output(StringRef s);
-  void outputUpToEndOfLine(StringRef s);
-  void newLineCheck(bool EmptySequence = false);
-  void outputNewLine();
-  void paddedKey(StringRef key);
-  void flowKey(StringRef Key);
-
-  enum InState {
-    inSeqFirstElement,
-    inSeqOtherElement,
-    inFlowSeqFirstElement,
-    inFlowSeqOtherElement,
-    inMapFirstKey,
-    inMapOtherKey,
-    inFlowMapFirstKey,
-    inFlowMapOtherKey
-  };
-
-  static bool inSeqAnyElement(InState State);
-  static bool inFlowSeqAnyElement(InState State);
-  static bool inMapAnyKey(InState State);
-  static bool inFlowMapAnyKey(InState State);
-
-  raw_ostream &Out;
-  int WrapColumn;
-  SmallVector<InState, 8> StateStack;
-  int Column = 0;
-  int ColumnAtFlowStart = 0;
-  int ColumnAtMapFlowStart = 0;
-  bool NeedBitValueComma = false;
-  bool NeedFlowSequenceComma = false;
+  OutputStream Out;
   bool EnumerationMatchFound = false;
   bool WriteDefaultValues = false;
-  StringRef Padding;
-  StringRef PaddingBeforeContainer;
 };
 
 template <typename T, typename Context>
 void IO::processKeyWithDefault(const char *Key, Optional<T> &Val,
                                const Optional<T> &DefaultValue, bool Required,
                                Context &Ctx) {
   assert(DefaultValue.hasValue() == false &&
          "Optional<T> shouldn't have a value!");

llvm/include/llvm/Support/YamlSerializer.h

+//===- YAMLParser.h - Simple YAML parser ------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_SUPPORT_YAMLSERIALIZER_H
+#define LLVM_SUPPORT_YAMLSERIALIZER_H
+
+#include "llvm/ADT/BitmaskEnum.h"
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/ADT/StringRef.h"
+#include "llvm/Support/Compiler.h"
+#include "llvm/Support/PointerLikeTypeTraits.h"
+namespace llvm {
+class raw_ostream;
+
+namespace yaml {
+
+/// Describe which type of quotes should be used when quoting is necessary.
+/// Some non-printable characters need to be double-quoted, while some others
+/// are fine with simple-quoting, and some don't need any quoting.
+enum class QuotingType { None, Single, Double };
+
+// 5.1. Character Set
+// The allowed character range explicitly excludes the C0 control block #x0-#x1F
+// (except for TAB #x9, LF #xA, and CR #xD which are allowed), DEL #x7F, the C1
+// control block #x80-#x9F (except for NEL #x85 which is allowed), the surrogate
+// block #xD800-#xDFFF, #xFFFE, and #xFFFF.
+QuotingType needsQuotes(StringRef S);
+
+class OutputStream {
+public:
+  OutputStream(raw_ostream &Underlying, int WrapColumn = 70)
+      : Out(Underlying), WrapColumn(WrapColumn) {
+    // Silence any warnings about unused private variables
+    (void)IsDocEmpty;
+    (void)CanEmitValues;
+  }
+  ~OutputStream() {
+    if (InDocument)
+      endDocuments();
+  }
+  struct LLVM_NODISCARD MapBuilder {
+  private:
+    friend class OutputStream;
+    MapBuilder(OutputStream &Out) : Out(&Out) {}
+
+  public:
+    ~MapBuilder() {
+      if (Out)
+        Out->exitMap();
+    }
+    MapBuilder &emit(StringRef Key,
+                     llvm::function_ref<void(OutputStream &)> BuildValue) {
+      assert(Out && "Map already disposed");
+      Out->emitMapKey(Key);
+      BuildValue(*Out);
+      Out->advanceMap();
+      return *this;
+    }
+
+    MapBuilder &emit(StringRef Key, StringRef Value) {
+      return emit(Key, Value, needsQuotes(Value));
+    }
+
+    MapBuilder &emit(StringRef Key, StringRef Value,
+                     QuotingType MustQuoteValue) {
+      assert(Out && "Map already disposed");
+      Out->emitMapKey(Key);
+      Out->outputScalar(Value, MustQuoteValue);
+      Out->advanceMap();
+      return *this;
+    }
+
+    void close() {
+      assert(Out && "Map already disposed");
+      Out->exitMap();
+      Out = nullptr;
+    }
+
+  private:
+    OutputStream *Out;
+  };
+
+  struct LLVM_NODISCARD SequenceBuilder {
+  private:
+    friend class OutputStream;
+    SequenceBuilder(OutputStream &Out) : Out(&Out) {}
+
+  public:
+    ~SequenceBuilder() {
+      if (Out)
+        Out->exitSequence();
+    }
+    SequenceBuilder &emit(llvm::function_ref<void(OutputStream &)> EmitValue) {
+      assert(Out && "Sequence already disposed");
+      Out->preEmitSequence();
+      EmitValue(*Out);
+      Out->advanceSequence();
+      return *this;
+    }
+
+    SequenceBuilder &emit(StringRef Item) {
+      return emit(Item, needsQuotes(Item));
+    }
+
+    SequenceBuilder &emit(StringRef Item, QuotingType MustQuote) {
+      assert(Out && "Sequence already disposed");
+      Out->preEmitSequence();
+      Out->outputScalar(Item, MustQuote);
+      Out->advanceSequence();
+      return *this;
+    }
+
+    void close() {
+      assert(Out && "Sequence already disposed");
+      Out->exitSequence();
+      Out = nullptr;
+    }
+
+  private:
+    OutputStream *Out;
+  };
+
+  struct LLVM_NODISCARD BitsetBuilder {
+  private:
+    friend class OutputStream;
+    BitsetBuilder(OutputStream &Out) : Out(&Out) {}
+
+  public:
+    ~BitsetBuilder() {
+      if (Out)
+        Out->exitBitset();
+    }
+    BitsetBuilder &emit(StringRef Str) {
+      assert(Out && "Bitset already disposed");
+      Out->emitBitset(Str);
+      return *this;
+    }
+
+    void close() {
+      assert(Out && "Bitset already disposed");
+      Out->exitBitset();
+      Out = nullptr;
+    }
+
+  private:
+    OutputStream *Out;
+  };
+
+  MapBuilder buildMap(bool Flow) {
+    enterMap(Flow);
+    return MapBuilder(*this);
+  }
+
+  SequenceBuilder buildSequence(bool Flow) {
+    enterSequence(Flow);
+    return SequenceBuilder(*this);
+  }
+
+  BitsetBuilder buildBitset() {
+    enterBitset();
+    return BitsetBuilder(*this);
+  }
+
+  void newDocument(bool First);
+  void endDocuments();
+
+  void enterMap(bool IsFlow);
+  void emitMapKey(StringRef Key);
+  void advanceMap();
+  void exitMap();
+
+  void enterSequence(bool IsFlow);
+  void preEmitSequence();
+  void advanceSequence();
+  void exitSequence();
+
+  void enterBitset();
+  void emitBitset(StringRef Str);
+  void exitBitset();
+
+  void outputScalar(StringRef Str);
+  void outputScalar(StringRef Str, QuotingType Quotes);
+  void outputBlockScalar(StringRef Str);
+  void outputTag(StringRef Tag);
+  void outputScalarTag(StringRef Tag);
+  bool canElideEmptySequence() const;
+
+private:
+  enum InState : unsigned char {
+    inSeqFirstElement,
+    inFlowSeqFirstElement,
+    inSeqOtherElement,
+    inFlowSeqOtherElement,
+
+    inMapFirstKey,
+    inFlowMapFirstKey,
+    inMapOtherKey,
+    inFlowMapOtherKey,
+
+    inBitsetFirst,
+    inBitsetOther
+
+  };
+
+  class State {
+    unsigned Type : llvm::detail::ConstantLog2<inBitsetOther>::value + 1;
+    unsigned Column : 31 - llvm::detail::ConstantLog2<inBitsetOther>::value;
+
+  public:
+    State(InState State) : Type(State) {
+      // As we flow after we create the state, we have to set column afterwards.
+    }
+    InState getState() const { return static_cast<InState>(Type); }
+    void setState(InState NewState) { Type = NewState; }
+    unsigned getColumn() const { return Column; }
+    void setColumn(unsigned NewColumn) { Column = NewColumn; }
+  };
+
+  static constexpr bool inSeqAnyElement(InState State) {
+    return State == inSeqFirstElement || State == inSeqOtherElement;
+  }
+
+  static constexpr bool inFlowSeqAnyElement(InState State) {
+    return State == inFlowSeqFirstElement || State == inFlowSeqOtherElement;
+  }
+
+  static constexpr bool inMapAnyKey(InState State) {
+    return State == inMapFirstKey || State == inMapOtherKey;
+  }
+
+  static constexpr bool inFlowMapAnyKey(InState State) {
+    return State == inFlowMapFirstKey || State == inFlowMapOtherKey;
+  }
+
+  static constexpr bool inAnySequence(InState State) {
+    return State <= inFlowSeqOtherElement;
+  }
+
+  static constexpr bool inAnyMap(InState State) {
+    return State >= inMapFirstKey && State <= inFlowMapOtherKey;
+  }
+
+  static constexpr bool inBitset(InState State) {
+    return State >= inBitsetFirst;
+  }
+
+  void output(StringRef S);
+  void outputUpToEndOfLine(StringRef S);
+  void newLineCheck(bool EmptySequence = false);
+  void outputNewLine();
+  void wrapFlow();
+
+  raw_ostream &Out;
+  SmallVector<State, 8> StateStack;
+  SmallVector<StringRef, 3> ContainerPaddingStack;
+  int WrapColumn;
+  int Column = 0;
+  StringRef Padding;
+  bool InDocument = false;
+  bool IsDocEmpty = true;
+  bool CanEmitValues = true;
+};
+} // namespace yaml
+} // namespace llvm
+
+#endif // LLVM_SUPPORT_YAMLSERIALIZER_H

llvm/lib/Support/CMakeLists.txt

add_llvm_component_library(LLVMSupport
   TypeSize.cpp
   Unicode.cpp
   UnicodeCaseFold.cpp
   VersionTuple.cpp
   VirtualFileSystem.cpp
   WithColor.cpp
   X86TargetParser.cpp
   YAMLParser.cpp
+  YAMLSerializer.cpp
   YAMLTraits.cpp
   raw_os_ostream.cpp
   raw_ostream.cpp
   regcomp.c
   regerror.c
   regexec.c
   regfree.c
   regstrlcpy.c

llvm/lib/Support/YAMLSerializer.cpp

+//===- YAMLSerializer.cpp - Simple YAML Serializer ------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+//  This file implements a YAML serializer.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Support/YamlSerializer.h"
+#include "llvm/Support/LineIterator.h"
+#include "llvm/Support/YAMLParser.h"
+#include "llvm/Support/YAMLTraits.h"
+#include "llvm/Support/raw_ostream.h"
+
+#if NDEBUG
+#define DEBUGONLY(X)                                                           \
+  do {                                                                         \
+  } while (false)
+#else
+#define DEBUGONLY(X)                                                           \
+  do {                                                                         \
+    X;                                                                         \
+  } while (false)
+#endif
+
+using namespace llvm;
+using namespace yaml;
+
+QuotingType llvm::yaml::needsQuotes(StringRef S) {
+  if (S.empty())
+    return QuotingType::Single;
+
+  QuotingType MaxQuotingNeeded = QuotingType::None;
+  if (isSpace(static_cast<unsigned char>(S.front())) ||
+      isSpace(static_cast<unsigned char>(S.back())))
+    MaxQuotingNeeded = QuotingType::Single;
+  if (isNull(S))
+    MaxQuotingNeeded = QuotingType::Single;
+  if (isBool(S))
+    MaxQuotingNeeded = QuotingType::Single;
+  if (isNumeric(S))
+    MaxQuotingNeeded = QuotingType::Single;
+
+  // 7.3.3 Plain Style
+  // Plain scalars must not begin with most indicators, as this would cause
+  // ambiguity with other YAML constructs.
+  if (std::strchr(R"(-?:\,[]{}#&*!|>'"%@`)", S[0]) != nullptr)
+    MaxQuotingNeeded = QuotingType::Single;
+
+  for (unsigned char C : S) {
+    // Alphanum is safe.
+    if (isAlnum(C))
+      continue;
+
+    switch (C) {
+    // Safe scalar characters.
+    case '_':
+    case '-':
+    case '^':
+    case '.':
+    case ',':
+    case ' ':
+    // TAB (0x9) is allowed in unquoted strings.
+    case 0x9:
+      continue;
+    // LF(0xA) and CR(0xD) may delimit values and so require at least single
+    // quotes. LLVM YAML parser cannot handle single quoted multiline so use
+    // double quoting to produce valid YAML.
+    case 0xA:
+    case 0xD:
+      return QuotingType::Double;
+    // DEL (0x7F) are excluded from the allowed character range.
+    case 0x7F:
+      return QuotingType::Double;
+    // Forward slash is allowed to be unquoted, but we quote it anyway.  We have
+    // many tests that use FileCheck against YAML output, and this output often
+    // contains paths.  If we quote backslashes but not forward slashes then
+    // paths will come out either quoted or unquoted depending on which platform
+    // the test is run on, making FileCheck comparisons difficult.
+    case '/':
+    default: {
+      // C0 control block (0x0 - 0x1F) is excluded from the allowed character
+      // range.
+      if (C <= 0x1F)
+        return QuotingType::Double;
+
+      // Always double quote UTF-8.
+      if ((C & 0x80) != 0)
+        return QuotingType::Double;
+
+      // The character is not safe, at least simple quoting needed.
+      MaxQuotingNeeded = QuotingType::Single;
+    }
+    }
+  }
+
+  return MaxQuotingNeeded;
+}
+
+bool llvm::yaml::OutputStream::canElideEmptySequence() const {
+  // Normally, with an optional key/value where the value is an empty sequence,
+  // the whole key/value can be not written.  But, that produces wrong yaml
+  // if the key/value is the only thing in the map and the map is used in
+  // a sequence.  This detects if the this sequence is the first key/value
+  // in map that itself is embedded in a sequence.
+  if (StateStack.size() < 2)
+    return true;
+  if (StateStack.back().getState() != inMapFirstKey)
+    return true;
+  return !inSeqAnyElement(StateStack[StateStack.size() - 2].getState());
+}
+
+void OutputStream::output(StringRef s) {
+  Column += s.size();
+  Out << s;
+  DEBUGONLY(IsDocEmpty = false);
+}
+
+void OutputStream::outputUpToEndOfLine(StringRef s) {
+  output(s);
+  if (StateStack.empty() ||
+      (!inFlowSeqAnyElement(StateStack.back().getState()) &&
+       !inFlowMapAnyKey(StateStack.back().getState()) &&
+       !inBitset(StateStack.back().getState())))
+    Padding = "\n";
+}
+
+void OutputStream::outputNewLine() {
+  Out << "\n";
+  Column = 0;
+}
+
+// if seq at top, indent as if map, then add "- "
+// if seq in middle, use "- " if firstKey, else use "  "
+//
+void OutputStream::newLineCheck(bool EmptySequence) {
+  if (Padding != "\n") {
+    output(Padding);
+    Padding = {};
+    return;
+  }
+  outputNewLine();
+  Padding = {};
+
+  if (StateStack.size() == 0 || EmptySequence)
+    return;
+
+  unsigned Indent = StateStack.size() - 1;
+  bool OutputDash = false;
+
+  if (inSeqAnyElement(StateStack.back().getState())) {
+    OutputDash = true;
+  } else if ((StateStack.size() > 1) &&
+             ((StateStack.back().getState() == inMapFirstKey) ||
+              inFlowSeqAnyElement(StateStack.back().getState()) ||
+              (StateStack.back().getState() == inFlowMapFirstKey)) &&
+             inSeqAnyElement(StateStack[StateStack.size() - 2].getState())) {
+    --Indent;
+    OutputDash = true;
+  }
+
+  for (unsigned i = 0; i < Indent; ++i) {
+    output("  ");
+  }
+  if (OutputDash) {
+    output("- ");
+  }
+}
+
+void OutputStream::wrapFlow() {
+  if (WrapColumn && Column > WrapColumn) {
+    output("\n");
+    for (unsigned i = 0; i < StateStack.back().getColumn(); ++i)
+      output(" ");
+    Column = StateStack.back().getColumn();
+    output("  ");
+  }
+}
+
+void OutputStream::outputScalar(StringRef Str) {
+  outputScalar(Str, needsQuotes(Str));
+}
+
+void OutputStream::outputScalar(StringRef S, QuotingType MustQuote) {
+  assert(CanEmitValues);
+  DEBUGONLY(CanEmitValues = false);
+  newLineCheck();
+  if (S.empty()) {
+    // Print '' for the empty string because leaving the field empty is not
+    // allowed.
+    outputUpToEndOfLine("''");
+    return;
+  }
+  if (MustQuote == QuotingType::None) {
+    // Only quote if we must.
+    outputUpToEndOfLine(S);
+    return;
+  }
+
+  const char *const Quote = MustQuote == QuotingType::Single ? "'" : "\"";
+  output(Quote); // Starting quote.
+
+  // When using double-quoted strings (and only in that case), non-printable
+  // characters may be present, and will be escaped using a variety of
+  // unicode-scalar and special short-form escapes. This is handled in
+  // yaml::escape.
+  if (MustQuote == QuotingType::Double) {
+    output(yaml::escape(S, /* EscapePrintable= */ false));
+    outputUpToEndOfLine(Quote);
+    return;
+  }
+
+  unsigned i = 0;
+  unsigned j = 0;
+  unsigned End = S.size();
+  const char *Base = S.data();
+
+  // When using single-quoted strings, any single quote ' must be doubled to be
+  // escaped.
+  while (j < End) {
+    if (S[j] == '\'') {                   // Escape quotes.
+      output(StringRef(&Base[i], j - i)); // "flush".
+      output(StringLiteral("''"));        // Print it as ''
+      i = j + 1;
+    }
+    ++j;
+  }
+  output(StringRef(&Base[i], j - i));
+  outputUpToEndOfLine(Quote); // Ending quote.
+}
+
+void OutputStream::outputBlockScalar(StringRef S) {
+  assert(CanEmitValues);
+  if (!StateStack.empty())
+    newLineCheck();
+  output(" |");
+  outputNewLine();
+
+  unsigned Indent = StateStack.empty() ? 1 : StateStack.size();
+
+  auto Buffer = MemoryBuffer::getMemBuffer(S, "", false);
+  for (line_iterator Lines(*Buffer, false); !Lines.is_at_end(); ++Lines) {
+    for (unsigned I = 0; I < Indent; ++I) {
+      output("  ");
+    }
+    output(*Lines);
+    outputNewLine();
+  }
+  DEBUGONLY(CanEmitValues = false);
+}
+
+void OutputStream::outputScalarTag(StringRef Tag) {
+  if (Tag.empty())
+    return;
+  newLineCheck();
+  output(Tag);
+  output(" ");
+}
+
+void OutputStream::outputTag(StringRef Tag) {
+  // If this tag is being written inside a sequence we should write the start
+  // of the sequence before writing the tag, otherwise the tag won't be
+  // attached to the element in the sequence, but rather the sequence itself.
+  bool OuterSequenceElement =
+      StateStack.size() > 1
+          ? inAnySequence(StateStack[StateStack.size() - 2].getState())
+          : false;
+  if (OuterSequenceElement && StateStack.back().getState() == inMapFirstKey) {
+    newLineCheck();
+  } else {
+    output(" ");
+  }
+  output(Tag);
+  if (OuterSequenceElement) {
+    // If we're writing the tag during the first element of a map, the tag
+    // takes the place of the first element in the sequence.
+    if (StateStack.back().getState() == inMapFirstKey)
+      StateStack.back().setState(inMapOtherKey);
+    // Tags inside maps in sequences should act as keys in the map from a
+    // formatting perspective, so we always want a newline in a sequence.
+    Padding = "\n";
+  }
+}
+
+void llvm::yaml::OutputStream::newDocument(bool First) {
+  assert(StateStack.empty() &&
+         "Cannot create new document while state is non-empty");
+  assert(IsDocEmpty == CanEmitValues);
+  if (First)
+    outputUpToEndOfLine("---");
+  else
+    outputUpToEndOfLine("\n---");
+  InDocument = true;
+  DEBUGONLY(IsDocEmpty = false);
+  DEBUGONLY(CanEmitValues = true);
+}
+
+void llvm::yaml::OutputStream::endDocuments() {
+  assert(IsDocEmpty == CanEmitValues);
+  outputUpToEndOfLine("\n...\n");
+  InDocument = false;
+}
+
+void llvm::yaml::OutputStream::enterMap(bool IsFlow) {
+  assert(CanEmitValues);
+  if (IsFlow) {
+    StateStack.push_back(inFlowMapFirstKey);
+    newLineCheck();
+    StateStack.back().setColumn(Column);
+    output("{ ");
+  } else {
+    StateStack.push_back(inMapFirstKey);
+    ContainerPaddingStack.push_back(Padding);
+    Padding = "\n";
+  }
+  DEBUGONLY(CanEmitValues = false);
+}
+
+void llvm::yaml::OutputStream::emitMapKey(StringRef Key) {
+  assert(!CanEmitValues);
+  if (inMapAnyKey(StateStack.back().getState())) {
+    newLineCheck();
+    output(Key);
+    output(":");
+    const char *Spaces = "                ";
+    if (Key.size() < strlen(Spaces))
+      Padding = &Spaces[Key.size()];
+    else
+      Padding = " ";
+  } else {
+    assert(inFlowMapAnyKey(StateStack.back().getState()));
+    if (StateStack.back().getState() == inFlowMapOtherKey)
+      output(", ");
+    wrapFlow();
+    output(Key);
+    output(": ");
+  }
+  DEBUGONLY(CanEmitValues = true);
+}
+
+void llvm::yaml::OutputStream::advanceMap() {
+  assert(inAnyMap(StateStack.back().getState()));
+  assert(!CanEmitValues && "Waiting for value to be emitted");
+  StateStack.back().setState(
+      static_cast<InState>(StateStack.back().getState() | 2U));
+  DEBUGONLY(CanEmitValues = false);
+}
+
+void llvm::yaml::OutputStream::exitMap() {
+  assert(!CanEmitValues && "Waiting for value to be emitted");
+  assert(inAnyMap(StateStack.back().getState()));
+  if (inFlowMapAnyKey(StateStack.back().getState())) {
+    StateStack.pop_back();
+    outputUpToEndOfLine(" }");
+  } else {
+    assert(inMapAnyKey(StateStack.back().getState()));
+    // If we did not map anything, we should explicitly emit an empty map
+    if (StateStack.back().getState() == inMapFirstKey) {
+      Padding = ContainerPaddingStack.pop_back_val();
+      newLineCheck();
+      output("{}");
+      Padding = "\n";
+    }
+    StateStack.pop_back();
+  }
+}
+
+void llvm::yaml::OutputStream::enterSequence(bool IsFlow) {
+  assert(CanEmitValues);
+  if (IsFlow) {
+    StateStack.push_back(inFlowSeqFirstElement);
+    newLineCheck();
+    StateStack.back().setColumn(Column);
+    output("[ ");
+  } else {
+    StateStack.emplace_back(inSeqFirstElement);
+    ContainerPaddingStack.push_back(Padding);
+    Padding = "\n";
+  }
+  // Prevent emitting values until preEmitSequence is called.
+  DEBUGONLY(CanEmitValues = false);
+}
+
+void llvm::yaml::OutputStream::preEmitSequence() {
+  assert(inAnySequence(StateStack.back().getState()));
+  assert(!CanEmitValues);
+  if (inFlowSeqAnyElement(StateStack.back().getState())) {
+    if (StateStack.back().getState() == inFlowSeqOtherElement)
+      output(", ");
+    wrapFlow();
+  }
+  DEBUGONLY(CanEmitValues = true);
+}
+
+void llvm::yaml::OutputStream::advanceSequence() {
+  assert(inAnySequence(StateStack.back().getState()));
+  assert(!CanEmitValues && "Waiting for sequence value to be emitted");
+  StateStack.back().setState(
+      static_cast<InState>(StateStack.back().getState() | 2U));
+}
+
+void llvm::yaml::OutputStream::exitSequence() {
+  assert(!CanEmitValues && "Waiting for sequence value to be emitted");
+  if (inFlowSeqAnyElement(StateStack.back().getState())) {
+    StateStack.pop_back();
+    outputUpToEndOfLine(" ]");
+  } else {
+    assert(inSeqAnyElement(StateStack.back().getState()));
+    // If we did not emit anything, we should explicitly emit an empty
+    // sequence
+    if (StateStack.back().getState() == inSeqFirstElement) {
+      Padding = ContainerPaddingStack.pop_back_val();
+      newLineCheck(/*EmptySequence=*/true);
+      output("[]");
+      Padding = "\n";
+    }
+    StateStack.pop_back();
+  }
+  DEBUGONLY(CanEmitValues = false);
+}
+
+void llvm::yaml::OutputStream::enterBitset() {
+  assert(CanEmitValues);
+  newLineCheck();
+  StateStack.emplace_back(inBitsetFirst).setColumn(Column);
+  output("[ ");
+  // Not technically correct, but all bitset output should go through emitBitset
+  // we can ignore that logic in there.
+  DEBUGONLY(CanEmitValues = false);
+}
+
+void llvm::yaml::OutputStream::emitBitset(StringRef Str) {
+  assert(!CanEmitValues);
+  if (StateStack.back().getState() == inBitsetOther)
+    output(", ");
+  else {
+    assert(StateStack.back().getState() == inBitsetFirst);
+    StateStack.back().setState(inBitsetOther);
+  }
+  wrapFlow();
+  output(Str);
+}
+
+void llvm::yaml::OutputStream::exitBitset() {
+  assert(!CanEmitValues);
+  assert(inBitset(StateStack.back().getState()));
+  StateStack.pop_back();
+  outputUpToEndOfLine(" ]");
+}

llvm/lib/Support/YAMLTraits.cpp

bool Input::canElideEmptySequence() {
   return false;
 }
 
 //===----------------------------------------------------------------------===//
 //  Output
 //===----------------------------------------------------------------------===//
 
 Output::Output(raw_ostream &yout, void *context, int WrapColumn)
-    : IO(context), Out(yout), WrapColumn(WrapColumn) {}
+    : IO(context), Out(yout, WrapColumn) {}
 
 Output::~Output() = default;
 
 bool Output::outputting() const {
   return true;
 }
 
-void Output::beginMapping() {
-  StateStack.push_back(inMapFirstKey);
-  PaddingBeforeContainer = Padding;
-  Padding = "\n";
-}
+void Output::beginMapping() { Out.enterMap(false); }
 
 bool Output::mapTag(StringRef Tag, bool Use) {
-  if (Use) {
-    // If this tag is being written inside a sequence we should write the start
-    // of the sequence before writing the tag, otherwise the tag won't be
-    // attached to the element in the sequence, but rather the sequence itself.
-    bool SequenceElement = false;
-    if (StateStack.size() > 1) {
-      auto &E = StateStack[StateStack.size() - 2];
-      SequenceElement = inSeqAnyElement(E) || inFlowSeqAnyElement(E);
-    }
-    if (SequenceElement && StateStack.back() == inMapFirstKey) {
-      newLineCheck();
-    } else {
-      output(" ");
-    }
-    output(Tag);
-    if (SequenceElement) {
-      // If we're writing the tag during the first element of a map, the tag
-      // takes the place of the first element in the sequence.
-      if (StateStack.back() == inMapFirstKey) {
-        StateStack.pop_back();
-        StateStack.push_back(inMapOtherKey);
-      }
-      // Tags inside maps in sequences should act as keys in the map from a
-      // formatting perspective, so we always want a newline in a sequence.
-      Padding = "\n";
-    }
-  }
+  if (Use)
+    Out.outputTag(Tag);
   return Use;
 }
 
-void Output::endMapping() {
-  // If we did not map anything, we should explicitly emit an empty map
-  if (StateStack.back() == inMapFirstKey) {
-    Padding = PaddingBeforeContainer;
-    newLineCheck();
-    output("{}");
-    Padding = "\n";
-  }
-  StateStack.pop_back();
-}
+void Output::endMapping() { Out.exitMap(); }
 
 std::vector<StringRef> Output::keys() {
   report_fatal_error("invalid call");
 }
 
 bool Output::preflightKey(const char *Key, bool Required, bool SameAsDefault,
                           bool &UseDefault, void *&SaveInfo) {
   UseDefault = false;
   SaveInfo = nullptr;
   if (Required || !SameAsDefault || WriteDefaultValues) {
-    auto State = StateStack.back();
-    if (State == inFlowMapFirstKey || State == inFlowMapOtherKey) {
-      flowKey(Key);
-    } else {
-      newLineCheck();
-      paddedKey(Key);
-    }
+    Out.emitMapKey(Key);
     return true;
   }
   return false;
 }
 
-void Output::postflightKey(void *) {
-  if (StateStack.back() == inMapFirstKey) {
-    StateStack.pop_back();
-    StateStack.push_back(inMapOtherKey);
-  } else if (StateStack.back() == inFlowMapFirstKey) {
-    StateStack.pop_back();
-    StateStack.push_back(inFlowMapOtherKey);
-  }
-}
+void Output::postflightKey(void *) { Out.advanceMap(); }
 
-void Output::beginFlowMapping() {
-  StateStack.push_back(inFlowMapFirstKey);
-  newLineCheck();
-  ColumnAtMapFlowStart = Column;
-  output("{ ");
-}
+void Output::beginFlowMapping() { Out.enterMap(true); }
 
-void Output::endFlowMapping() {
-  StateStack.pop_back();
-  outputUpToEndOfLine(" }");
-}
+void Output::endFlowMapping() { Out.exitMap(); }
 
-void Output::beginDocuments() {
-  outputUpToEndOfLine("---");
-}
+void Output::beginDocuments() {}
 
 bool Output::preflightDocument(unsigned index) {
-  if (index > 0)
-    outputUpToEndOfLine("\n---");
+  Out.newDocument(index == 0);
   return true;
 }
 
 void Output::postflightDocument() {
 }
 
-void Output::endDocuments() {
-  output("\n...\n");
-}
+void Output::endDocuments() { Out.endDocuments(); }
 
 unsigned Output::beginSequence() {
-  StateStack.push_back(inSeqFirstElement);
-  PaddingBeforeContainer = Padding;
-  Padding = "\n";
+  Out.enterSequence(false);
   return 0;
 }
 
-void Output::endSequence() {
-  // If we did not emit anything, we should explicitly emit an empty sequence
-  if (StateStack.back() == inSeqFirstElement) {
-    Padding = PaddingBeforeContainer;
-    newLineCheck(/*EmptySequence=*/true);
-    output("[]");
-    Padding = "\n";
-  }
-  StateStack.pop_back();
-}
+void Output::endSequence() { Out.exitSequence(); }
 
 bool Output::preflightElement(unsigned, void *&SaveInfo) {
+  Out.preEmitSequence();
   SaveInfo = nullptr;
   return true;
 }
 
-void Output::postflightElement(void *) {
-  if (StateStack.back() == inSeqFirstElement) {
-    StateStack.pop_back();
-    StateStack.push_back(inSeqOtherElement);
-  } else if (StateStack.back() == inFlowSeqFirstElement) {
-    StateStack.pop_back();
-    StateStack.push_back(inFlowSeqOtherElement);
-  }
-}
+void Output::postflightElement(void *) { Out.advanceSequence(); }
 
 unsigned Output::beginFlowSequence() {
-  StateStack.push_back(inFlowSeqFirstElement);
-  newLineCheck();
-  ColumnAtFlowStart = Column;
-  output("[ ");
-  NeedFlowSequenceComma = false;
+  Out.enterSequence(true);
   return 0;
 }
 
-void Output::endFlowSequence() {
-  StateStack.pop_back();
-  outputUpToEndOfLine(" ]");
-}
+void Output::endFlowSequence() { Out.exitSequence(); }
 
 bool Output::preflightFlowElement(unsigned, void *&SaveInfo) {
-  if (NeedFlowSequenceComma)
-    output(", ");
-  if (WrapColumn && Column > WrapColumn) {
-    output("\n");
-    for (int i = 0; i < ColumnAtFlowStart; ++i)
-      output(" ");
-    Column = ColumnAtFlowStart;
-    output("  ");
-  }
+  Out.preEmitSequence();
   SaveInfo = nullptr;
   return true;
 }
 
-void Output::postflightFlowElement(void *) {
-  NeedFlowSequenceComma = true;
-}
+void Output::postflightFlowElement(void *) { Out.advanceSequence(); }
 
 void Output::beginEnumScalar() {
   EnumerationMatchFound = false;
 }
 
 bool Output::matchEnumScalar(const char *Str, bool Match) {
   if (Match && !EnumerationMatchFound) {
-    newLineCheck();
-    outputUpToEndOfLine(Str);
+    Out.outputScalar(Str, QuotingType::None);
     EnumerationMatchFound = true;
   }
   return false;
 }
 
 bool Output::matchEnumFallback() {
   if (EnumerationMatchFound)
     return false;
   EnumerationMatchFound = true;
   return true;
 }
 
 void Output::endEnumScalar() {
   if (!EnumerationMatchFound)
     llvm_unreachable("bad runtime enum value");
 }
 
 bool Output::beginBitSetScalar(bool &DoClear) {
-  newLineCheck();
-  output("[ ");
-  NeedBitValueComma = false;
+  Out.enterBitset();
   DoClear = false;
   return true;
 }
 
 bool Output::bitSetMatch(const char *Str, bool Matches) {
   if (Matches) {
-    if (NeedBitValueComma)
-      output(", ");
-    output(Str);
-    NeedBitValueComma = true;
+    Out.emitBitset(Str);
   }
   return false;
 }
 
-void Output::endBitSetScalar() {
-  outputUpToEndOfLine(" ]");
-}
+void Output::endBitSetScalar() { Out.exitBitset(); }
 
 void Output::scalarString(StringRef &S, QuotingType MustQuote) {
-  newLineCheck();
-  if (S.empty()) {
-    // Print '' for the empty string because leaving the field empty is not
-    // allowed.
-    outputUpToEndOfLine("''");
-    return;
-  }
-  if (MustQuote == QuotingType::None) {
-    // Only quote if we must.
-    outputUpToEndOfLine(S);
-    return;
-  }
-
-  const char *const Quote = MustQuote == QuotingType::Single ? "'" : "\"";
-  output(Quote); // Starting quote.
+  Out.outputScalar(S, MustQuote);
 
-  // When using double-quoted strings (and only in that case), non-printable characters may be
-  // present, and will be escaped using a variety of unicode-scalar and special short-form
-  // escapes. This is handled in yaml::escape.
-  if (MustQuote == QuotingType::Double) {
-    output(yaml::escape(S, /* EscapePrintable= */ false));
-    outputUpToEndOfLine(Quote);
-    return;
-  }
+void Output::blockScalarString(StringRef &S) { Out.outputBlockScalar(S); }
 
-  unsigned i = 0;
-  unsigned j = 0;
-  unsigned End = S.size();
-  const char *Base = S.data();
-
-  // When using single-quoted strings, any single quote ' must be doubled to be escaped.
-  while (j < End) {
-    if (S[j] == '\'') {                    // Escape quotes.
-      output(StringRef(&Base[i], j - i));  // "flush".
-      output(StringLiteral("''"));         // Print it as ''
-      i = j + 1;
-    }
-    ++j;
-  }
-  output(StringRef(&Base[i], j - i));
-  outputUpToEndOfLine(Quote); // Ending quote.
-}
-
-void Output::blockScalarString(StringRef &S) {
-  if (!StateStack.empty())
-    newLineCheck();
-  output(" |");
-  outputNewLine();
-
-  unsigned Indent = StateStack.empty() ? 1 : StateStack.size();
-
-  auto Buffer = MemoryBuffer::getMemBuffer(S, "", false);
-  for (line_iterator Lines(*Buffer, false); !Lines.is_at_end(); ++Lines) {
-    for (unsigned I = 0; I < Indent; ++I) {
-      output("  ");
-    }
-    output(*Lines);
-    outputNewLine();
-  }
-}
-
-void Output::scalarTag(std::string &Tag) {
-  if (Tag.empty())
-    return;
-  newLineCheck();
-  output(Tag);
-  output(" ");
-}
+void Output::scalarTag(std::string &Tag) { Out.outputScalarTag(Tag); }
 
 void Output::setError(const Twine &message) {
 }
 
-bool Output::canElideEmptySequence() {
-  // Normally, with an optional key/value where the value is an empty sequence,
-  // the whole key/value can be not written.  But, that produces wrong yaml
-  // if the key/value is the only thing in the map and the map is used in
-  // a sequence.  This detects if the this sequence is the first key/value
-  // in map that itself is embedded in a sequence.
-  if (StateStack.size() < 2)
-    return true;
-  if (StateStack.back() != inMapFirstKey)
-    return true;
-  return !inSeqAnyElement(StateStack[StateStack.size() - 2]);
-}
-
-void Output::output(StringRef s) {
-  Column += s.size();
-  Out << s;
-}
-
-void Output::outputUpToEndOfLine(StringRef s) {
-  output(s);
-  if (StateStack.empty() || (!inFlowSeqAnyElement(StateStack.back()) &&
-                             !inFlowMapAnyKey(StateStack.back())))
-    Padding = "\n";
-}
-
-void Output::outputNewLine() {
-  Out << "\n";
-  Column = 0;
-}
-
-// if seq at top, indent as if map, then add "- "
-// if seq in middle, use "- " if firstKey, else use "  "
-//
-
-void Output::newLineCheck(bool EmptySequence) {
-  if (Padding != "\n") {
-    output(Padding);
-    Padding = {};
-    return;
-  }
-  outputNewLine();
-  Padding = {};
-
-  if (StateStack.size() == 0 || EmptySequence)
-    return;
-
-  unsigned Indent = StateStack.size() - 1;
-  bool OutputDash = false;
-
-  if (StateStack.back() == inSeqFirstElement ||
-      StateStack.back() == inSeqOtherElement) {
-    OutputDash = true;
-  } else if ((StateStack.size() > 1) &&
-             ((StateStack.back() == inMapFirstKey) ||
-              inFlowSeqAnyElement(StateStack.back()) ||
-              (StateStack.back() == inFlowMapFirstKey)) &&
-             inSeqAnyElement(StateStack[StateStack.size() - 2])) {
-    --Indent;
-    OutputDash = true;
-  }
-
-  for (unsigned i = 0; i < Indent; ++i) {
-    output("  ");
-  }
-  if (OutputDash) {
-    output("- ");
-  }
-}
-
-void Output::paddedKey(StringRef key) {
-  output(key);
-  output(":");
-  const char *spaces = "                ";
-  if (key.size() < strlen(spaces))
-    Padding = &spaces[key.size()];
-  else
-    Padding = " ";
-}
-
-void Output::flowKey(StringRef Key) {
-  if (StateStack.back() == inFlowMapOtherKey)
-    output(", ");
-  if (WrapColumn && Column > WrapColumn) {
-    output("\n");
-    for (int I = 0; I < ColumnAtMapFlowStart; ++I)
-      output(" ");
-    Column = ColumnAtMapFlowStart;
-    output("  ");
-  }
-  output(Key);
-  output(": ");
-}
+bool Output::canElideEmptySequence() { return Out.canElideEmptySequence(); }
 
 NodeKind Output::getNodeKind() { report_fatal_error("invalid call"); }
 
-bool Output::inSeqAnyElement(InState State) {
-  return State == inSeqFirstElement || State == inSeqOtherElement;
-}
-
-bool Output::inFlowSeqAnyElement(InState State) {
-  return State == inFlowSeqFirstElement || State == inFlowSeqOtherElement;
-}
-
-bool Output::inMapAnyKey(InState State) {
-  return State == inMapFirstKey || State == inMapOtherKey;
-}
-
-bool Output::inFlowMapAnyKey(InState State) {
-  return State == inFlowMapFirstKey || State == inFlowMapOtherKey;
-}
-
 //===----------------------------------------------------------------------===//
 //  traits for built-in types
 //===----------------------------------------------------------------------===//
 
 void ScalarTraits<bool>::output(const bool &Val, void *, raw_ostream &Out) {
   Out << (Val ? "true" : "false");
 }
 

llvm/unittests/Support/CMakeLists.txt

add_llvm_unittest(SupportTests
   TrailingObjectsTest.cpp
   TrigramIndexTest.cpp
   UnicodeTest.cpp
   VersionTupleTest.cpp
   VirtualFileSystemTest.cpp
   WithColorTest.cpp
   YAMLIOTest.cpp
   YAMLParserTest.cpp
+  YAMLSerializerTest.cpp
   buffer_ostream_test.cpp
   formatted_raw_ostream_test.cpp
   raw_fd_stream_test.cpp
   raw_ostream_test.cpp
   raw_pwrite_stream_test.cpp
   raw_sha1_ostream_test.cpp
   xxhashTest.cpp
   )

llvm/unittests/Support/YAMLSerializerTest.cpp

+//===- unittest/Support/YAMLSerializerTest --------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Support/YamlSerializer.h"
+#include "llvm/Support/SourceMgr.h"
+#include "llvm/Support/YAMLParser.h"
+#include "llvm/Support/raw_ostream.h"
+#include "gtest/gtest.h"
+#include <string>
+
+namespace llvm {
+using yaml::OutputStream;
+
+static void eatDiagnostic(const SMDiagnostic &, void *) {}
+
+static bool isParseable(StringRef Input) {
+  SourceMgr Mgr;
+  Mgr.setDiagHandler(eatDiagnostic);
+  yaml::Stream InStream(Input, Mgr);
+  InStream.skip();
+  return !InStream.failed();
+}
+
+TEST(YAMLSerializer, Builders) {
+  std::string Output;
+  llvm::raw_string_ostream Stream(Output);
+  {
+    yaml::OutputStream Yaml(Stream);
+    Yaml.newDocument(/*First=*/true);
+    auto BuildMap = Yaml.buildMap(/*Flow=*/false);
+    BuildMap.emit("ScalarTest", "Value");
+    BuildMap.emit("FlowMap", [](OutputStream &O) {
+      O.buildMap(/*Flow=*/true)
+          .emit("X", "5", yaml::QuotingType::None)
+          .emit("Y", "4", yaml::QuotingType::Single)
+          .emit("Z", "3", yaml::QuotingType::Double);
+    });
+    BuildMap.emit("MapTest", [](OutputStream &O) {
+      O.buildMap(/*Flow=*/false)
+          .emit("FlowFirst",
+                [](OutputStream &O) {
+                  O.buildSequence(/*Flow=*/true)
+                      .emit("Item1")
+                      .emit("Item2")
+                      .emit("Item3");
+                })
+          .emit("NonFlow",
+                [](OutputStream &O) {
+                  O.buildSequence(/*Flow=*/false)
+                      .emit("ItemA")
+                      .emit("ItemB")
+                      .emit("ItemC");
+                })
+          .emit("Bitset", [](OutputStream &O) {
+            O.buildBitset().emit("Bits1").emit("Bits2").emit("Bits3");
+          });
+    });
+  }
+  Stream.flush();
+  EXPECT_EQ(Output, R"(---
+ScalarTest:      Value
+FlowMap:         { X: 5, Y: '4', Z: "3" }
+MapTest:
+  FlowFirst:       [ Item1, Item2, Item3 ]
+  NonFlow:
+    - ItemA
+    - ItemB
+    - ItemC
+  Bitset:          [ Bits1, Bits2, Bits3 ]
+...
+)");
+  EXPECT_TRUE(isParseable(Output));
+}
+
+TEST(YAMLSerializer, WriteVector) {
+  std::vector<int> Values = {1, 2, 3, 4, 5};
+  std::string Output;
+  llvm::raw_string_ostream Stream(Output);
+  {
+    yaml::OutputStream Yaml(Stream);
+    auto Seq = Yaml.buildSequence(true);
+    for (auto Item : Values)
+      Seq.emit(std::to_string(Item), yaml::QuotingType::None);
+  }
+  Stream.flush();
+  EXPECT_EQ(Output, R"([ 1, 2, 3, 4, 5 ])");
+  EXPECT_TRUE(isParseable(Output));
+}
+} // namespace llvm