Adding upstream version 124.0.1.upstream/124.0.1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

67 files changed, 15560 insertions, 0 deletions

diff --git a/intl/components/src/Bidi.cpp b/intl/components/src/Bidi.cpp
new file mode 100644
index 0000000000..5e18e0075d
--- /dev/null
+++ b/intl/components/src/Bidi.cpp
@@ -0,0 +1,212 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "mozilla/intl/Bidi.h"
+#include "mozilla/Casting.h"
+#include "mozilla/intl/ICU4CGlue.h"
+
+#if !USE_RUST_UNICODE_BIDI
+#  include "unicode/ubidi.h"
+#endif
+
+namespace mozilla::intl {
+
+#if USE_RUST_UNICODE_BIDI
+using namespace ffi;
+
+Bidi::Bidi() = default;
+Bidi::~Bidi() = default;
+#else
+Bidi::Bidi() { mBidi = ubidi_open(); }
+Bidi::~Bidi() { ubidi_close(mBidi.GetMut()); }
+#endif
+
+ICUResult Bidi::SetParagraph(Span<const char16_t> aParagraph,
+                             BidiEmbeddingLevel aLevel) {
+#if USE_RUST_UNICODE_BIDI
+  const auto* text = reinterpret_cast<const uint16_t*>(aParagraph.Elements());
+  mBidi.reset(bidi_new(text, aParagraph.Length(), aLevel));
+
+  return ToICUResult(U_ZERO_ERROR);
+#else
+  // Do not allow any reordering of the runs, as this can change the
+  // performance characteristics of working with runs. In the default mode,
+  // the levels can be iterated over directly, rather than relying on computing
+  // logical runs on the fly. This can have negative performance characteristics
+  // compared to iterating over the levels.
+  //
+  // In the UBIDI_REORDER_RUNS_ONLY the levels are encoded with additional
+  // information which can be safely ignored in this Bidi implementation.
+  // Note that this check is here since setting the mode must be done before
+  // calls to setting the paragraph.
+  MOZ_ASSERT(ubidi_getReorderingMode(mBidi.GetMut()) == UBIDI_REORDER_DEFAULT);
+
+  UErrorCode status = U_ZERO_ERROR;
+  ubidi_setPara(mBidi.GetMut(), aParagraph.Elements(),
+                AssertedCast<int32_t>(aParagraph.Length()), aLevel, nullptr,
+                &status);
+
+  mLevels = nullptr;
+
+  return ToICUResult(status);
+#endif
+}
+
+Bidi::ParagraphDirection Bidi::GetParagraphDirection() const {
+#if USE_RUST_UNICODE_BIDI
+  auto dir = bidi_get_direction(mBidi.get());
+  switch (dir) {
+    case -1:
+      return Bidi::ParagraphDirection::RTL;
+    case 0:
+      return Bidi::ParagraphDirection::Mixed;
+    case 1:
+      return Bidi::ParagraphDirection::LTR;
+    default:
+      MOZ_ASSERT_UNREACHABLE("Bad direction value");
+      return Bidi::ParagraphDirection::Mixed;
+  }
+#else
+  switch (ubidi_getDirection(mBidi.GetConst())) {
+    case UBIDI_LTR:
+      return Bidi::ParagraphDirection::LTR;
+    case UBIDI_RTL:
+      return Bidi::ParagraphDirection::RTL;
+    case UBIDI_MIXED:
+      return Bidi::ParagraphDirection::Mixed;
+    case UBIDI_NEUTRAL:
+      // This is only used in `ubidi_getBaseDirection` which is unused in this
+      // API.
+      MOZ_ASSERT_UNREACHABLE("Unexpected UBiDiDirection value.");
+  };
+  return Bidi::ParagraphDirection::Mixed;
+#endif
+}
+
+/* static */
+void Bidi::ReorderVisual(const BidiEmbeddingLevel* aLevels, int32_t aLength,
+                         int32_t* aIndexMap) {
+#if USE_RUST_UNICODE_BIDI
+  bidi_reorder_visual(reinterpret_cast<const uint8_t*>(aLevels), aLength,
+                      aIndexMap);
+#else
+  ubidi_reorderVisual(reinterpret_cast<const uint8_t*>(aLevels), aLength,
+                      aIndexMap);
+#endif
+}
+
+/* static */
+Bidi::BaseDirection Bidi::GetBaseDirection(Span<const char16_t> aText) {
+#if USE_RUST_UNICODE_BIDI
+  const auto* text = reinterpret_cast<const uint16_t*>(aText.Elements());
+  switch (bidi_get_base_direction(text, aText.Length(), false)) {
+    case -1:
+      return Bidi::BaseDirection::RTL;
+    case 0:
+      return Bidi::BaseDirection::Neutral;
+    case 1:
+      return Bidi::BaseDirection::LTR;
+    default:
+      MOZ_ASSERT_UNREACHABLE("Bad base direction value");
+      return Bidi::BaseDirection::Neutral;
+  }
+#else
+  UBiDiDirection direction = ubidi_getBaseDirection(
+      aText.Elements(), AssertedCast<int32_t>(aText.Length()));
+  switch (direction) {
+    case UBIDI_LTR:
+      return Bidi::BaseDirection::LTR;
+    case UBIDI_RTL:
+      return Bidi::BaseDirection::RTL;
+    case UBIDI_NEUTRAL:
+      return Bidi::BaseDirection::Neutral;
+    case UBIDI_MIXED:
+      MOZ_ASSERT_UNREACHABLE("Unexpected UBiDiDirection value.");
+  }
+  return Bidi::BaseDirection::Neutral;
+#endif
+}
+
+#if !USE_RUST_UNICODE_BIDI
+static BidiDirection ToBidiDirection(UBiDiDirection aDirection) {
+  switch (aDirection) {
+    case UBIDI_LTR:
+      return BidiDirection::LTR;
+    case UBIDI_RTL:
+      return BidiDirection::RTL;
+    case UBIDI_MIXED:
+    case UBIDI_NEUTRAL:
+      MOZ_ASSERT_UNREACHABLE("Unexpected UBiDiDirection value.");
+  }
+  return BidiDirection::LTR;
+}
+#endif
+
+Result<int32_t, ICUError> Bidi::CountRuns() {
+#if USE_RUST_UNICODE_BIDI
+  return bidi_count_runs(mBidi.get());
+#else
+  UErrorCode status = U_ZERO_ERROR;
+  int32_t runCount = ubidi_countRuns(mBidi.GetMut(), &status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+
+  mLength = ubidi_getProcessedLength(mBidi.GetConst());
+  mLevels = mLength > 0 ? reinterpret_cast<const BidiEmbeddingLevel*>(
+                              ubidi_getLevels(mBidi.GetMut(), &status))
+                        : nullptr;
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+
+  return runCount;
+#endif
+}
+
+void Bidi::GetLogicalRun(int32_t aLogicalStart, int32_t* aLogicalLimitOut,
+                         BidiEmbeddingLevel* aLevelOut) {
+#if USE_RUST_UNICODE_BIDI
+  const int32_t length = bidi_get_length(mBidi.get());
+  MOZ_DIAGNOSTIC_ASSERT(aLogicalStart < length);
+  const auto* levels = bidi_get_levels(mBidi.get());
+#else
+  MOZ_ASSERT(mLevels, "CountRuns hasn't been run?");
+  MOZ_RELEASE_ASSERT(aLogicalStart < mLength, "Out of bound");
+  const int32_t length = mLength;
+  const auto* levels = mLevels;
+#endif
+  const uint8_t level = levels[aLogicalStart];
+  int32_t limit;
+  for (limit = aLogicalStart + 1; limit < length; limit++) {
+    if (levels[limit] != level) {
+      break;
+    }
+  }
+  *aLogicalLimitOut = limit;
+  *aLevelOut = BidiEmbeddingLevel(level);
+}
+
+BidiEmbeddingLevel Bidi::GetParagraphEmbeddingLevel() const {
+#if USE_RUST_UNICODE_BIDI
+  return BidiEmbeddingLevel(bidi_get_paragraph_level(mBidi.get()));
+#else
+  return BidiEmbeddingLevel(ubidi_getParaLevel(mBidi.GetConst()));
+#endif
+}
+
+BidiDirection Bidi::GetVisualRun(int32_t aRunIndex, int32_t* aLogicalStart,
+                                 int32_t* aLength) {
+#if USE_RUST_UNICODE_BIDI
+  auto run = bidi_get_visual_run(mBidi.get(), aRunIndex);
+  *aLogicalStart = run.start;
+  *aLength = run.length;
+  return BidiEmbeddingLevel(run.level).Direction();
+#else
+  return ToBidiDirection(
+      ubidi_getVisualRun(mBidi.GetMut(), aRunIndex, aLogicalStart, aLength));
+#endif
+}
+
+}  // namespace mozilla::intl
diff --git a/intl/components/src/Bidi.h b/intl/components/src/Bidi.h
new file mode 100644
index 0000000000..7b901e6bfd
--- /dev/null
+++ b/intl/components/src/Bidi.h
@@ -0,0 +1,183 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+#ifndef intl_components_Bidi_h_
+#define intl_components_Bidi_h_
+
+#include "mozilla/intl/BidiEmbeddingLevel.h"
+#include "mozilla/intl/ICU4CGlue.h"
+
+// Restrict use of the Rust unicode-bidi implementation to Nightly builds,
+// pending investigation of perf regressions; Beta/Release builds will
+// continue to use the ICU4C implementation for now.
+#ifdef NIGHTLY_BUILD
+#  define USE_RUST_UNICODE_BIDI 1
+#else
+#  define USE_RUST_UNICODE_BIDI 0
+#endif
+
+#if USE_RUST_UNICODE_BIDI
+#  include "mozilla/intl/unicode_bidi_ffi_generated.h"
+#else
+struct UBiDi;
+#endif
+
+namespace mozilla::intl {
+
+/**
+ * This component is a Mozilla-focused API for working with bidirectional (bidi)
+ * text. Text is commonly displayed left to right (LTR), especially for
+ * Latin-based alphabets. However, languages like Arabic and Hebrew displays
+ * text right to left (RTL). When displaying text, LTR and RTL text can be
+ * combined together in the same paragraph. This class gives tools for working
+ * with unidirectional, and mixed direction paragraphs.
+ *
+ * See the Unicode Bidirectional Algorithm document for implementation details:
+ * https://unicode.org/reports/tr9/
+ */
+class Bidi final {
+ public:
+  Bidi();
+  ~Bidi();
+
+  // Not copyable or movable
+  Bidi(const Bidi&) = delete;
+  Bidi& operator=(const Bidi&) = delete;
+
+  /**
+   * This enum indicates the text direction for the set paragraph. Some
+   * paragraphs are unidirectional, where they only have one direction, or a
+   * paragraph could use both LTR and RTL. In this case the paragraph's
+   * direction would be mixed.
+   */
+  enum class ParagraphDirection { LTR, RTL, Mixed };
+
+  /**
+   * Set the current paragraph of text to analyze for its bidi properties. This
+   * performs the Unicode bidi algorithm as specified by:
+   * https://unicode.org/reports/tr9/
+   *
+   * After setting the text, the other getter methods can be used to find out
+   * the directionality of the paragraph text.
+   */
+  ICUResult SetParagraph(Span<const char16_t> aParagraph,
+                         BidiEmbeddingLevel aLevel);
+
+  /**
+   * Get the embedding level for the paragraph that was set by SetParagraph.
+   */
+  BidiEmbeddingLevel GetParagraphEmbeddingLevel() const;
+
+  /**
+   * Get the directionality of the paragraph text that was set by SetParagraph.
+   */
+  ParagraphDirection GetParagraphDirection() const;
+
+  /**
+   * Get the number of runs. This function may invoke the actual reordering on
+   * the Bidi object, after SetParagraph may have resolved only the levels of
+   * the text. Therefore, `CountRuns` may have to allocate memory, and may fail
+   * doing so.
+   */
+  Result<int32_t, ICUError> CountRuns();
+
+  /**
+   * Get the next logical run. The logical runs are a run of text that has the
+   * same directionality and embedding level. These runs are in memory order,
+   * and not in display order.
+   *
+   * Important! `Bidi::CountRuns` must be called before calling this method.
+   *
+   * @param aLogicalStart is the offset into the paragraph text that marks the
+   *      logical start of the text.
+   * @param aLogicalLimitOut is an out param that is the length of the string
+   *      that makes up the logical run.
+   * @param aLevelOut is an out parameter that returns the embedding level for
+   *      the run
+   */
+  void GetLogicalRun(int32_t aLogicalStart, int32_t* aLogicalLimitOut,
+                     BidiEmbeddingLevel* aLevelOut);
+
+  /**
+   * This is a convenience function that does not use the ICU Bidi object.
+   * It is intended to be used for when an application has determined the
+   * embedding levels of objects (character sequences) and just needs to have
+   * them reordered (L2).
+   *
+   * @param aLevels is an array with `aLength` levels that have been
+   *      determined by the application.
+   *
+   * @param aLength is the number of levels in the array, or, semantically,
+   *      the number of objects to be reordered. It must be greater than 0.
+   *
+   * @param aIndexMap is a pointer to an array of `aLength`
+   *      indexes which will reflect the reordering of the characters.
+   *      The array does not need to be initialized.
+   *      The index map will result in
+   *        `aIndexMap[aVisualIndex]==aLogicalIndex`.
+   */
+  static void ReorderVisual(const BidiEmbeddingLevel* aLevels, int32_t aLength,
+                            int32_t* aIndexMap);
+
+  /**
+   * This enum indicates the bidi character type of the first strong character
+   * for the set paragraph.
+   * LTR: bidi character type 'L'.
+   * RTL: bidi character type 'R' or 'AL'.
+   * Neutral: The rest of bidi character types.
+   */
+  enum class BaseDirection { LTR, RTL, Neutral };
+
+  /**
+   * Get the base direction of the text.
+   */
+  static BaseDirection GetBaseDirection(Span<const char16_t> aText);
+
+  /**
+   * Get one run's logical start, length, and directionality. In an RTL run, the
+   * character at the logical start is visually on the right of the displayed
+   * run. The length is the number of characters in the run.
+   * `Bidi::CountRuns` should be called before the runs are retrieved.
+   *
+   * @param aRunIndex is the number of the run in visual order, in the
+   *      range `[0..CountRuns-1]`.
+   *
+   * @param aLogicalStart is the first logical character index in the text.
+   *      The pointer may be `nullptr` if this index is not needed.
+   *
+   * @param aLength is the number of characters (at least one) in the run.
+   *      The pointer may be `nullptr` if this is not needed.
+   *
+   * Note that in right-to-left runs, the code places modifier letters before
+   * base characters and second surrogates before first ones.
+   */
+  BidiDirection GetVisualRun(int32_t aRunIndex, int32_t* aLogicalStart,
+                             int32_t* aLength);
+
+ private:
+#if USE_RUST_UNICODE_BIDI
+  using UnicodeBidi = mozilla::intl::ffi::UnicodeBidi;
+  struct BidiFreePolicy {
+    void operator()(void* aPtr) {
+      bidi_destroy(static_cast<UnicodeBidi*>(aPtr));
+    }
+  };
+  mozilla::UniquePtr<UnicodeBidi, BidiFreePolicy> mBidi;
+#else
+  ICUPointer<UBiDi> mBidi = ICUPointer<UBiDi>(nullptr);
+
+  /**
+   * An array of levels that is the same length as the paragraph from
+   * `Bidi::SetParagraph`.
+   */
+  const BidiEmbeddingLevel* mLevels = nullptr;
+
+  /**
+   * The length of the paragraph from `Bidi::SetParagraph`.
+   */
+  int32_t mLength = 0;
+#endif
+};
+
+}  // namespace mozilla::intl
+#endif
diff --git a/intl/components/src/BidiClass.h b/intl/components/src/BidiClass.h
new file mode 100644
index 0000000000..04a861a382
--- /dev/null
+++ b/intl/components/src/BidiClass.h
@@ -0,0 +1,49 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+#ifndef intl_components_BidiClass_h_
+#define intl_components_BidiClass_h_
+
+#include <cstdint>
+
+namespace mozilla::intl {
+
+/**
+ *  Read ftp://ftp.unicode.org/Public/UNIDATA/ReadMe-Latest.txt
+ *  section BIDIRECTIONAL PROPERTIES
+ *  for the detailed definition of the following categories
+ *
+ *  The values here must match the equivalents in %bidicategorycode in
+ *  mozilla/intl/unicharutil/tools/genUnicodePropertyData.pl,
+ *  and must also match the values used by ICU's UCharDirection.
+ */
+enum class BidiClass : uint8_t {
+  LeftToRight = 0,
+  RightToLeft = 1,
+  EuropeanNumber = 2,
+  EuropeanNumberSeparator = 3,
+  EuropeanNumberTerminator = 4,
+  ArabicNumber = 5,
+  CommonNumberSeparator = 6,
+  BlockSeparator = 7,
+  SegmentSeparator = 8,
+  WhiteSpaceNeutral = 9,
+  OtherNeutral = 10,
+  LeftToRightEmbedding = 11,
+  LeftToRightOverride = 12,
+  RightToLeftArabic = 13,
+  RightToLeftEmbedding = 14,
+  RightToLeftOverride = 15,
+  PopDirectionalFormat = 16,
+  DirNonSpacingMark = 17,
+  BoundaryNeutral = 18,
+  FirstStrongIsolate = 19,
+  LeftToRightIsolate = 20,
+  RightToLeftIsolate = 21,
+  PopDirectionalIsolate = 22,
+  BidiClassCount
+};
+
+}  // namespace mozilla::intl
+
+#endif
diff --git a/intl/components/src/BidiEmbeddingLevel.cpp b/intl/components/src/BidiEmbeddingLevel.cpp
new file mode 100644
index 0000000000..d3ef5da937
--- /dev/null
+++ b/intl/components/src/BidiEmbeddingLevel.cpp
@@ -0,0 +1,53 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "mozilla/intl/BidiEmbeddingLevel.h"
+#include "mozilla/Casting.h"
+#include "mozilla/intl/ICU4CGlue.h"
+
+#include "unicode/ubidi.h"
+
+namespace mozilla::intl {
+
+bool BidiEmbeddingLevel::IsDefaultLTR() const {
+  return mValue == UBIDI_DEFAULT_LTR;
+};
+
+bool BidiEmbeddingLevel::IsDefaultRTL() const {
+  return mValue == UBIDI_DEFAULT_RTL;
+};
+
+bool BidiEmbeddingLevel::IsRTL() const {
+  // If the least significant bit is 1, then the embedding level
+  // is right-to-left.
+  // If the least significant bit is 0, then the embedding level
+  // is left-to-right.
+  return (mValue & 0x1) == 1;
+};
+
+bool BidiEmbeddingLevel::IsLTR() const { return !IsRTL(); };
+
+bool BidiEmbeddingLevel::IsSameDirection(BidiEmbeddingLevel aOther) const {
+  return (((mValue ^ aOther) & 1) == 0);
+}
+
+BidiEmbeddingLevel BidiEmbeddingLevel::LTR() { return BidiEmbeddingLevel(0); };
+
+BidiEmbeddingLevel BidiEmbeddingLevel::RTL() { return BidiEmbeddingLevel(1); };
+
+BidiEmbeddingLevel BidiEmbeddingLevel::DefaultLTR() {
+  return BidiEmbeddingLevel(UBIDI_DEFAULT_LTR);
+};
+
+BidiEmbeddingLevel BidiEmbeddingLevel::DefaultRTL() {
+  return BidiEmbeddingLevel(UBIDI_DEFAULT_RTL);
+};
+
+BidiDirection BidiEmbeddingLevel::Direction() {
+  return IsRTL() ? BidiDirection::RTL : BidiDirection::LTR;
+};
+
+uint8_t BidiEmbeddingLevel::Value() const { return mValue; }
+
+}  // namespace mozilla::intl
diff --git a/intl/components/src/BidiEmbeddingLevel.h b/intl/components/src/BidiEmbeddingLevel.h
new file mode 100644
index 0000000000..fe0045e4a5
--- /dev/null
+++ b/intl/components/src/BidiEmbeddingLevel.h
@@ -0,0 +1,113 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+#ifndef intl_components_BidiEmbeddingLevel_h_
+#define intl_components_BidiEmbeddingLevel_h_
+
+#include <cstdint>
+
+/**
+ * This file has the BidiEmbeddingLevel and BidiDirection enum broken out from
+ * the main Bidi class for faster includes. This code is used in Layout which
+ * could trigger long build times when changing core mozilla::intl files.
+ */
+namespace mozilla::intl {
+
+/**
+ * This enum unambiguously classifies text runs as either being left to right,
+ * or right to left.
+ */
+enum class BidiDirection : uint8_t {
+  // Left to right text.
+  LTR = 0,
+  // Right to left text.
+  RTL = 1,
+};
+
+/**
+ * Embedding levels are numbers that indicate how deeply the bidi text is
+ * embedded, and the direction of text on that embedding level. When switching
+ * between strongly LTR code points and strongly RTL code points the embedding
+ * level normally switches between an embedding level of 0 (LTR) and 1 (RTL).
+ * The only time the embedding level increases is if the embedding code points
+ * are used. This is the Left-to-Right Embedding (LRE) code point (U+202A), or
+ * the Right-to-Left Embedding (RLE) code point (U+202B). The minimum
+ * embedding level of text is zero, and the maximum explicit depth is 125.
+ *
+ * The most significant bit is reserved for additional meaning. It can be used
+ * to signify in certain APIs that the text should by default be LTR or RTL if
+ * no strongly directional code points are found.
+ *
+ * Bug 1736595: At the time of this writing, some places in Gecko code use a 1
+ * in the most significant bit to indicate that an embedding level has not
+ * been set. This leads to an ambiguous understanding of what the most
+ * significant bit actually means.
+ */
+class BidiEmbeddingLevel {
+ public:
+  constexpr explicit BidiEmbeddingLevel(uint8_t aValue) : mValue(aValue) {}
+  constexpr explicit BidiEmbeddingLevel(int aValue)
+      : mValue(static_cast<uint8_t>(aValue)) {}
+
+  BidiEmbeddingLevel() = default;
+
+  // Enable the copy operators, but disable move as this is only a uint8_t.
+  BidiEmbeddingLevel(const BidiEmbeddingLevel& other) = default;
+  BidiEmbeddingLevel& operator=(const BidiEmbeddingLevel& other) = default;
+
+  /**
+   * Determine the direction of the embedding level by looking at the least
+   * significant bit. If it is 0, then it is LTR. If it is 1, then it is RTL.
+   */
+  BidiDirection Direction();
+
+  /**
+   * Create a left-to-right embedding level.
+   */
+  static BidiEmbeddingLevel LTR();
+
+  /**
+   * Create an right-to-left embedding level.
+   */
+  static BidiEmbeddingLevel RTL();
+
+  /**
+   * When passed into `SetParagraph`, the direction is determined by first
+   * strongly directional character, with the default set to left-to-right if
+   * none is found.
+   *
+   * This is encoded with the highest bit set to 1.
+   */
+  static BidiEmbeddingLevel DefaultLTR();
+
+  /**
+   * When passed into `SetParagraph`, the direction is determined by first
+   * strongly directional character, with the default set to right-to-left if
+   * none is found.
+   *
+   * * This is encoded with the highest and lowest bits set to 1.
+   */
+  static BidiEmbeddingLevel DefaultRTL();
+
+  bool IsDefaultLTR() const;
+  bool IsDefaultRTL() const;
+  bool IsLTR() const;
+  bool IsRTL() const;
+  bool IsSameDirection(BidiEmbeddingLevel aOther) const;
+
+  /**
+   * Get the underlying value as a uint8_t.
+   */
+  uint8_t Value() const;
+
+  /**
+   * Implicitly convert to the underlying value.
+   */
+  operator uint8_t() const { return mValue; }
+
+ private:
+  uint8_t mValue = 0;
+};
+
+}  // namespace mozilla::intl
+#endif
diff --git a/intl/components/src/Calendar.cpp b/intl/components/src/Calendar.cpp
new file mode 100644
index 0000000000..d44dedaaae
--- /dev/null
+++ b/intl/components/src/Calendar.cpp
@@ -0,0 +1,172 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "mozilla/intl/Calendar.h"
+
+#include "unicode/ucal.h"
+#include "unicode/uloc.h"
+#include "unicode/utypes.h"
+
+namespace mozilla::intl {
+
+/* static */
+Result<UniquePtr<Calendar>, ICUError> Calendar::TryCreate(
+    const char* aLocale, Maybe<Span<const char16_t>> aTimeZoneOverride) {
+  UErrorCode status = U_ZERO_ERROR;
+  const UChar* zoneID = nullptr;
+  int32_t zoneIDLen = 0;
+  if (aTimeZoneOverride) {
+    zoneIDLen = static_cast<int32_t>(aTimeZoneOverride->Length());
+    zoneID = aTimeZoneOverride->Elements();
+  }
+
+  UCalendar* calendar =
+      ucal_open(zoneID, zoneIDLen, aLocale, UCAL_DEFAULT, &status);
+
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+
+  return MakeUnique<Calendar>(calendar);
+}
+
+Result<Span<const char>, ICUError> Calendar::GetBcp47Type() {
+  UErrorCode status = U_ZERO_ERROR;
+  const char* oldType = ucal_getType(mCalendar, &status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+  const char* bcp47Type = uloc_toUnicodeLocaleType("calendar", oldType);
+
+  if (!bcp47Type) {
+    return Err(ICUError::InternalError);
+  }
+
+  return MakeStringSpan(bcp47Type);
+}
+
+static Weekday WeekdayFromDaysOfWeek(UCalendarDaysOfWeek weekday) {
+  switch (weekday) {
+    case UCAL_MONDAY:
+      return Weekday::Monday;
+    case UCAL_TUESDAY:
+      return Weekday::Tuesday;
+    case UCAL_WEDNESDAY:
+      return Weekday::Wednesday;
+    case UCAL_THURSDAY:
+      return Weekday::Thursday;
+    case UCAL_FRIDAY:
+      return Weekday::Friday;
+    case UCAL_SATURDAY:
+      return Weekday::Saturday;
+    case UCAL_SUNDAY:
+      return Weekday::Sunday;
+  }
+  MOZ_CRASH("unexpected weekday value");
+}
+
+Result<EnumSet<Weekday>, ICUError> Calendar::GetWeekend() {
+  static_assert(static_cast<int32_t>(UCAL_SUNDAY) == 1);
+  static_assert(static_cast<int32_t>(UCAL_SATURDAY) == 7);
+
+  UErrorCode status = U_ZERO_ERROR;
+
+  EnumSet<Weekday> weekend;
+  for (int32_t i = UCAL_SUNDAY; i <= UCAL_SATURDAY; i++) {
+    auto dayOfWeek = static_cast<UCalendarDaysOfWeek>(i);
+    auto type = ucal_getDayOfWeekType(mCalendar, dayOfWeek, &status);
+    if (U_FAILURE(status)) {
+      return Err(ToICUError(status));
+    }
+
+    switch (type) {
+      case UCAL_WEEKEND_ONSET:
+        // Treat days which start as a weekday as weekdays.
+        [[fallthrough]];
+      case UCAL_WEEKDAY:
+        break;
+
+      case UCAL_WEEKEND_CEASE:
+        // Treat days which start as a weekend day as weekend days.
+        [[fallthrough]];
+      case UCAL_WEEKEND:
+        weekend += WeekdayFromDaysOfWeek(dayOfWeek);
+        break;
+    }
+  }
+
+  return weekend;
+}
+
+Weekday Calendar::GetFirstDayOfWeek() {
+  int32_t firstDayOfWeek = ucal_getAttribute(mCalendar, UCAL_FIRST_DAY_OF_WEEK);
+  MOZ_ASSERT(UCAL_SUNDAY <= firstDayOfWeek && firstDayOfWeek <= UCAL_SATURDAY);
+
+  return WeekdayFromDaysOfWeek(
+      static_cast<UCalendarDaysOfWeek>(firstDayOfWeek));
+}
+
+int32_t Calendar::GetMinimalDaysInFirstWeek() {
+  int32_t minimalDays =
+      ucal_getAttribute(mCalendar, UCAL_MINIMAL_DAYS_IN_FIRST_WEEK);
+  MOZ_ASSERT(1 <= minimalDays && minimalDays <= 7);
+
+  return minimalDays;
+}
+
+Result<Ok, ICUError> Calendar::SetTimeInMs(double aUnixEpoch) {
+  UErrorCode status = U_ZERO_ERROR;
+  ucal_setMillis(mCalendar, aUnixEpoch, &status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+  return Ok{};
+}
+
+/* static */
+Result<SpanEnumeration<char>, ICUError>
+Calendar::GetLegacyKeywordValuesForLocale(const char* aLocale) {
+  UErrorCode status = U_ZERO_ERROR;
+  UEnumeration* enumeration = ucal_getKeywordValuesForLocale(
+      "calendar", aLocale, /* commonlyUsed */ false, &status);
+
+  if (U_SUCCESS(status)) {
+    return SpanEnumeration<char>(enumeration);
+  }
+
+  return Err(ToICUError(status));
+}
+
+/* static */
+SpanResult<char> Calendar::LegacyIdentifierToBcp47(const char* aIdentifier,
+                                                   int32_t aLength) {
+  if (aIdentifier == nullptr) {
+    return Err(InternalError{});
+  }
+  // aLength is not needed here, as the ICU call uses the null terminated
+  // string.
+  return MakeStringSpan(uloc_toUnicodeLocaleType("ca", aIdentifier));
+}
+
+/* static */
+Result<Calendar::Bcp47IdentifierEnumeration, ICUError>
+Calendar::GetBcp47KeywordValuesForLocale(const char* aLocale,
+                                         CommonlyUsed aCommonlyUsed) {
+  UErrorCode status = U_ZERO_ERROR;
+  UEnumeration* enumeration = ucal_getKeywordValuesForLocale(
+      "calendar", aLocale, static_cast<bool>(aCommonlyUsed), &status);
+
+  if (U_SUCCESS(status)) {
+    return Bcp47IdentifierEnumeration(enumeration);
+  }
+
+  return Err(ToICUError(status));
+}
+
+Calendar::~Calendar() {
+  MOZ_ASSERT(mCalendar);
+  ucal_close(mCalendar);
+}
+
+}  // namespace mozilla::intl
diff --git a/intl/components/src/Calendar.h b/intl/components/src/Calendar.h
new file mode 100644
index 0000000000..32975bc376
--- /dev/null
+++ b/intl/components/src/Calendar.h
@@ -0,0 +1,133 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+#ifndef intl_components_Calendar_h_
+#define intl_components_Calendar_h_
+
+#include "mozilla/Assertions.h"
+#include "mozilla/EnumSet.h"
+#include "mozilla/intl/ICU4CGlue.h"
+#include "mozilla/intl/ICUError.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/Result.h"
+#include "mozilla/Span.h"
+#include "mozilla/UniquePtr.h"
+
+using UCalendar = void*;
+
+namespace mozilla::intl {
+
+/**
+ * Weekdays in the ISO-8601 calendar.
+ */
+enum class Weekday : uint8_t {
+  Monday = 1,
+  Tuesday,
+  Wednesday,
+  Thursday,
+  Friday,
+  Saturday,
+  Sunday,
+};
+
+/**
+ * This component is a Mozilla-focused API for working with calendar systems in
+ * internationalization code. It is used in coordination with other operations
+ * such as datetime formatting.
+ */
+class Calendar final {
+ public:
+  explicit Calendar(UCalendar* aCalendar) : mCalendar(aCalendar) {
+    MOZ_ASSERT(aCalendar);
+  };
+
+  // Do not allow copy as this class owns the ICU resource. Move is not
+  // currently implemented, but a custom move operator could be created if
+  // needed.
+  Calendar(const Calendar&) = delete;
+  Calendar& operator=(const Calendar&) = delete;
+
+  /**
+   * Create a Calendar.
+   */
+  static Result<UniquePtr<Calendar>, ICUError> TryCreate(
+      const char* aLocale,
+      Maybe<Span<const char16_t>> aTimeZoneOverride = Nothing{});
+
+  /**
+   * Get the BCP 47 keyword value string designating the calendar type. For
+   * instance "gregory", "chinese", "islamic-civil", etc.
+   */
+  Result<Span<const char>, ICUError> GetBcp47Type();
+
+  /**
+   * Return the set of weekdays which are considered as part of the weekend.
+   */
+  Result<EnumSet<Weekday>, ICUError> GetWeekend();
+
+  /**
+   * Return the weekday which is considered the first day of the week.
+   */
+  Weekday GetFirstDayOfWeek();
+
+  /**
+   * Return the minimal number of days in the first week of a year.
+   */
+  int32_t GetMinimalDaysInFirstWeek();
+
+  /**
+   * Set the time for the calendar relative to the number of milliseconds since
+   * 1 January 1970, UTC.
+   */
+  Result<Ok, ICUError> SetTimeInMs(double aUnixEpoch);
+
+  /**
+   * Return ICU legacy keywords, such as "gregorian", "islamic",
+   * "islamic-civil", "hebrew", etc.
+   */
+  static Result<SpanEnumeration<char>, ICUError>
+  GetLegacyKeywordValuesForLocale(const char* aLocale);
+
+ private:
+  /**
+   * Internal function to convert a legacy calendar identifier to the newer
+   * BCP 47 identifier.
+   */
+  static SpanResult<char> LegacyIdentifierToBcp47(const char* aIdentifier,
+                                                  int32_t aLength);
+
+ public:
+  enum class CommonlyUsed : bool {
+    /**
+     * Select all possible values, even when not commonly used by a locale.
+     */
+    No,
+
+    /**
+     * Only select the values which are commonly used by a locale.
+     */
+    Yes,
+  };
+
+  using Bcp47IdentifierEnumeration =
+      Enumeration<char, SpanResult<char>, Calendar::LegacyIdentifierToBcp47>;
+
+  /**
+   * Return BCP 47 Unicode locale extension type keywords.
+   */
+  static Result<Bcp47IdentifierEnumeration, ICUError>
+  GetBcp47KeywordValuesForLocale(const char* aLocale,
+                                 CommonlyUsed aCommonlyUsed = CommonlyUsed::No);
+
+  ~Calendar();
+
+ private:
+  friend class DateIntervalFormat;
+  UCalendar* GetUCalendar() const { return mCalendar; }
+
+  UCalendar* mCalendar = nullptr;
+};
+
+}  // namespace mozilla::intl
+
+#endif
diff --git a/intl/components/src/Collator.cpp b/intl/components/src/Collator.cpp
new file mode 100644
index 0000000000..8835b29bde
--- /dev/null
+++ b/intl/components/src/Collator.cpp
@@ -0,0 +1,295 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include <algorithm>
+#include <string.h>
+#include "mozilla/intl/Collator.h"
+
+namespace mozilla::intl {
+
+Collator::Collator(UCollator* aCollator) : mCollator(aCollator) {
+  MOZ_ASSERT(aCollator);
+}
+
+Collator::~Collator() {
+  if (mCollator.GetMut()) {
+    ucol_close(mCollator.GetMut());
+  }
+}
+
+Result<UniquePtr<Collator>, ICUError> Collator::TryCreate(const char* aLocale) {
+  UErrorCode status = U_ZERO_ERROR;
+  UCollator* collator = ucol_open(IcuLocale(aLocale), &status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+  return MakeUnique<Collator>(collator);
+};
+
+int32_t Collator::CompareStrings(Span<const char16_t> aSource,
+                                 Span<const char16_t> aTarget) const {
+  switch (ucol_strcoll(mCollator.GetConst(), aSource.data(),
+                       static_cast<int32_t>(aSource.size()), aTarget.data(),
+                       static_cast<int32_t>(aTarget.size()))) {
+    case UCOL_LESS:
+      return -1;
+    case UCOL_EQUAL:
+      return 0;
+    case UCOL_GREATER:
+      return 1;
+  }
+  MOZ_ASSERT_UNREACHABLE("ucol_strcoll returned bad UCollationResult");
+  return 0;
+}
+
+int32_t Collator::CompareSortKeys(Span<const uint8_t> aKey1,
+                                  Span<const uint8_t> aKey2) const {
+  size_t minLength = std::min(aKey1.Length(), aKey2.Length());
+  int32_t tmpResult = strncmp((const char*)aKey1.Elements(),
+                              (const char*)aKey2.Elements(), minLength);
+  if (tmpResult < 0) {
+    return -1;
+  }
+  if (tmpResult > 0) {
+    return 1;
+  }
+  if (aKey1.Length() > minLength) {
+    // First string contains second one, so comes later, hence return > 0.
+    return 1;
+  }
+  if (aKey2.Length() > minLength) {
+    // First string is a substring of second one, so comes earlier,
+    // hence return < 0.
+    return -1;
+  }
+  return 0;
+}
+
+static UColAttributeValue CaseFirstToICU(Collator::CaseFirst caseFirst) {
+  switch (caseFirst) {
+    case Collator::CaseFirst::False:
+      return UCOL_OFF;
+    case Collator::CaseFirst::Upper:
+      return UCOL_UPPER_FIRST;
+    case Collator::CaseFirst::Lower:
+      return UCOL_LOWER_FIRST;
+  }
+
+  MOZ_ASSERT_UNREACHABLE();
+  return UCOL_DEFAULT;
+}
+
+void Collator::SetStrength(Collator::Strength aStrength) {
+  UColAttributeValue strength;
+  switch (aStrength) {
+    case Collator::Strength::Default:
+      strength = UCOL_DEFAULT_STRENGTH;
+      break;
+    case Collator::Strength::Primary:
+      strength = UCOL_PRIMARY;
+      break;
+    case Collator::Strength::Secondary:
+      strength = UCOL_SECONDARY;
+      break;
+    case Collator::Strength::Tertiary:
+      strength = UCOL_TERTIARY;
+      break;
+    case Collator::Strength::Quaternary:
+      strength = UCOL_QUATERNARY;
+      break;
+    case Collator::Strength::Identical:
+      strength = UCOL_IDENTICAL;
+      break;
+  }
+
+  ucol_setStrength(mCollator.GetMut(), strength);
+}
+
+ICUResult Collator::SetCaseLevel(Collator::Feature aFeature) {
+  UErrorCode status = U_ZERO_ERROR;
+  ucol_setAttribute(mCollator.GetMut(), UCOL_CASE_LEVEL,
+                    ToUColAttributeValue(aFeature), &status);
+  return ToICUResult(status);
+}
+
+ICUResult Collator::SetAlternateHandling(
+    Collator::AlternateHandling aAlternateHandling) {
+  UErrorCode status = U_ZERO_ERROR;
+  UColAttributeValue handling;
+  switch (aAlternateHandling) {
+    case Collator::AlternateHandling::NonIgnorable:
+      handling = UCOL_NON_IGNORABLE;
+      break;
+    case Collator::AlternateHandling::Shifted:
+      handling = UCOL_SHIFTED;
+      break;
+    case Collator::AlternateHandling::Default:
+      handling = UCOL_DEFAULT;
+      break;
+  }
+
+  ucol_setAttribute(mCollator.GetMut(), UCOL_ALTERNATE_HANDLING, handling,
+                    &status);
+  return ToICUResult(status);
+}
+
+ICUResult Collator::SetNumericCollation(Collator::Feature aFeature) {
+  UErrorCode status = U_ZERO_ERROR;
+  ucol_setAttribute(mCollator.GetMut(), UCOL_NUMERIC_COLLATION,
+                    ToUColAttributeValue(aFeature), &status);
+  return ToICUResult(status);
+}
+
+ICUResult Collator::SetNormalizationMode(Collator::Feature aFeature) {
+  UErrorCode status = U_ZERO_ERROR;
+  ucol_setAttribute(mCollator.GetMut(), UCOL_NORMALIZATION_MODE,
+                    ToUColAttributeValue(aFeature), &status);
+  return ToICUResult(status);
+}
+
+ICUResult Collator::SetCaseFirst(Collator::CaseFirst aCaseFirst) {
+  UErrorCode status = U_ZERO_ERROR;
+  ucol_setAttribute(mCollator.GetMut(), UCOL_CASE_FIRST,
+                    CaseFirstToICU(aCaseFirst), &status);
+  return ToICUResult(status);
+}
+
+ICUResult Collator::SetOptions(const Options& aOptions,
+                               const Maybe<Options&> aPrevOptions) {
+  if (aPrevOptions &&
+      // Check the equality of the previous options.
+      aPrevOptions->sensitivity == aOptions.sensitivity &&
+      aPrevOptions->caseFirst == aOptions.caseFirst &&
+      aPrevOptions->ignorePunctuation == aOptions.ignorePunctuation &&
+      aPrevOptions->numeric == aOptions.numeric) {
+    return Ok();
+  }
+
+  Collator::Strength strength = Collator::Strength::Default;
+  Collator::Feature caseLevel = Collator::Feature::Off;
+  switch (aOptions.sensitivity) {
+    case Collator::Sensitivity::Base:
+      strength = Collator::Strength::Primary;
+      break;
+    case Collator::Sensitivity::Accent:
+      strength = Collator::Strength::Secondary;
+      break;
+    case Collator::Sensitivity::Case:
+      caseLevel = Collator::Feature::On;
+      strength = Collator::Strength::Primary;
+      break;
+    case Collator::Sensitivity::Variant:
+      strength = Collator::Strength::Tertiary;
+      break;
+  }
+
+  SetStrength(strength);
+
+  ICUResult result = Ok();
+
+  // According to the ICU team, UCOL_SHIFTED causes punctuation to be
+  // ignored. Looking at Unicode Technical Report 35, Unicode Locale Data
+  // Markup Language, "shifted" causes whitespace and punctuation to be
+  // ignored - that's a bit more than asked for, but there's no way to get
+  // less.
+  result = this->SetAlternateHandling(
+      aOptions.ignorePunctuation ? Collator::AlternateHandling::Shifted
+                                 : Collator::AlternateHandling::NonIgnorable);
+  if (result.isErr()) {
+    return result;
+  }
+
+  result = SetCaseLevel(caseLevel);
+  if (result.isErr()) {
+    return result;
+  }
+
+  result = SetNumericCollation(aOptions.numeric ? Collator::Feature::On
+                                                : Collator::Feature::Off);
+  if (result.isErr()) {
+    return result;
+  }
+
+  // Normalization is always on to meet the canonical equivalence requirement.
+  result = SetNormalizationMode(Collator::Feature::On);
+  if (result.isErr()) {
+    return result;
+  }
+
+  result = SetCaseFirst(aOptions.caseFirst);
+  if (result.isErr()) {
+    return result;
+  }
+  return Ok();
+}
+
+Result<Collator::CaseFirst, ICUError> Collator::GetCaseFirst() const {
+  UErrorCode status = U_ZERO_ERROR;
+  UColAttributeValue caseFirst =
+      ucol_getAttribute(mCollator.GetConst(), UCOL_CASE_FIRST, &status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+
+  if (caseFirst == UCOL_OFF) {
+    return CaseFirst::False;
+  }
+  if (caseFirst == UCOL_UPPER_FIRST) {
+    return CaseFirst::Upper;
+  }
+  MOZ_ASSERT(caseFirst == UCOL_LOWER_FIRST);
+  return CaseFirst::Lower;
+}
+
+Result<bool, ICUError> Collator::GetIgnorePunctuation() const {
+  UErrorCode status = U_ZERO_ERROR;
+  UColAttributeValue alternateHandling =
+      ucol_getAttribute(mCollator.GetConst(), UCOL_ALTERNATE_HANDLING, &status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+
+  MOZ_ASSERT(alternateHandling == UCOL_SHIFTED ||
+             alternateHandling == UCOL_NON_IGNORABLE);
+  return alternateHandling == UCOL_SHIFTED;
+}
+
+/* static */
+Result<Collator::Bcp47ExtEnumeration, ICUError>
+Collator::GetBcp47KeywordValuesForLocale(const char* aLocale,
+                                         CommonlyUsed aCommonlyUsed) {
+  UErrorCode status = U_ZERO_ERROR;
+  UEnumeration* enumeration = ucol_getKeywordValuesForLocale(
+      "collation", aLocale, static_cast<bool>(aCommonlyUsed), &status);
+
+  if (U_SUCCESS(status)) {
+    return Bcp47ExtEnumeration(enumeration);
+  }
+
+  return Err(ToICUError(status));
+}
+
+/* static */
+Result<Collator::Bcp47ExtEnumeration, ICUError>
+Collator::GetBcp47KeywordValues() {
+  UErrorCode status = U_ZERO_ERROR;
+  UEnumeration* enumeration = ucol_getKeywordValues("collation", &status);
+
+  if (U_SUCCESS(status)) {
+    return Bcp47ExtEnumeration(enumeration);
+  }
+
+  return Err(ToICUError(status));
+}
+
+/* static */
+SpanResult<char> Collator::KeywordValueToBcp47Extension(const char* aKeyword,
+                                                        int32_t aLength) {
+  if (aKeyword == nullptr) {
+    return Err(InternalError{});
+  }
+  return MakeStringSpan(uloc_toUnicodeLocaleType("co", aKeyword));
+}
+
+}  // namespace mozilla::intl
diff --git a/intl/components/src/Collator.h b/intl/components/src/Collator.h
new file mode 100644
index 0000000000..655cb7b0fd
--- /dev/null
+++ b/intl/components/src/Collator.h
@@ -0,0 +1,348 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+#ifndef intl_components_Collator_h_
+#define intl_components_Collator_h_
+
+#ifndef JS_STANDALONE
+#  include "gtest/MozGtestFriend.h"
+#endif
+
+#include "unicode/ucol.h"
+
+#include "mozilla/Compiler.h"
+#include "mozilla/intl/ICU4CGlue.h"
+#include "mozilla/intl/ICUError.h"
+#include "mozilla/Result.h"
+#include "mozilla/Span.h"
+
+namespace mozilla::intl {
+
+class Collator final {
+ public:
+  /**
+   * Construct from a raw UCollator. This is public so that the UniquePtr can
+   * access it.
+   */
+  explicit Collator(UCollator* aCollator);
+
+  // Do not allow copy as this class owns the ICU resource. Move is not
+  // currently implemented, but a custom move operator could be created if
+  // needed.
+  Collator(const Collator&) = delete;
+  Collator& operator=(const Collator&) = delete;
+
+  /**
+   * Attempt to initialize a new collator.
+   */
+  static Result<UniquePtr<Collator>, ICUError> TryCreate(const char* aLocale);
+
+  ~Collator();
+
+  /**
+   * Get a sort key with the provided UTF-16 string, and store the sort key into
+   * the provided buffer of byte array.
+   * Every sort key ends with 0x00, and the terminating 0x00 byte is counted
+   * when calculating the length of buffer. For the purpose of other byte
+   * values, check the "Special Byte Values" document from ICU.
+   *
+   * https://icu.unicode.org/design/collation/bytes
+   */
+  template <typename B>
+  ICUResult GetSortKey(Span<const char16_t> aString, B& aBuffer) const {
+    return FillBufferWithICUCall(
+        aBuffer,
+        [this, aString](uint8_t* target, int32_t length, UErrorCode* status) {
+          // ucol_getSortKey doesn't use the error code to report
+          // U_BUFFER_OVERFLOW_ERROR, instead it uses the return value to
+          // indicate the desired length to store the key. So we update the
+          // UErrorCode accordingly to let FillBufferWithICUCall resize the
+          // buffer.
+          int32_t len = ucol_getSortKey(mCollator.GetConst(), aString.data(),
+                                        static_cast<int32_t>(aString.size()),
+                                        target, length);
+          if (len == 0) {
+            // Returns 0 means there's an internal error.
+            *status = U_INTERNAL_PROGRAM_ERROR;
+          } else if (len > length) {
+            *status = U_BUFFER_OVERFLOW_ERROR;
+          } else {
+            *status = U_ZERO_ERROR;
+          }
+          return len;
+        });
+  }
+
+  int32_t CompareStrings(Span<const char16_t> aSource,
+                         Span<const char16_t> aTarget) const;
+
+  int32_t CompareSortKeys(Span<const uint8_t> aKey1,
+                          Span<const uint8_t> aKey2) const;
+
+  /**
+   * Determine how casing affects sorting. These options map to ECMA 402
+   * collator options.
+   *
+   * https://tc39.es/ecma402/#sec-initializecollator
+   */
+  enum class CaseFirst {
+    // Sort upper case first.
+    Upper,
+    // Sort lower case first.
+    Lower,
+    // Orders upper and lower case letters in accordance to their tertiary
+    // weights.
+    False,
+  };
+
+  /**
+   * Which differences in the strings should lead to differences in collation
+   * comparisons.
+   *
+   * This setting needs to be ECMA 402 compliant.
+   * https://tc39.es/ecma402/#sec-collator-comparestrings
+   */
+  enum class Sensitivity {
+    // Only strings that differ in base letters compare as unequal.
+    // Examples: a ≠ b, a = á, a = A.
+    Base,
+    // Only strings that differ in base letters or accents and other diacritic
+    // marks compare as unequal.
+    // Examples: a ≠ b, a ≠ á, a = A.
+    Accent,
+    // Only strings that differ in base letters or case compare as unequal.
+    // Examples: a ≠ b, a = á, a ≠ A.
+    Case,
+    // Strings that differ in base letters, accents and other diacritic marks,
+    // or case compare as unequal. Other differences may also be taken into
+    // consideration.
+    // Examples: a ≠ b, a ≠ á, a ≠ A.
+    Variant,
+  };
+
+  /**
+   * These options map to ECMA 402 collator options. Make sure the defaults map
+   * to the default initialized values of ECMA 402.
+   *
+   * https://tc39.es/ecma402/#sec-initializecollator
+   */
+  struct Options {
+    Sensitivity sensitivity = Sensitivity::Variant;
+    CaseFirst caseFirst = CaseFirst::False;
+    bool ignorePunctuation = false;
+    bool numeric = false;
+  };
+
+  /**
+   * Change the configuraton of the options.
+   */
+  ICUResult SetOptions(const Options& aOptions,
+                       const Maybe<Options&> aPrevOptions = Nothing());
+
+  /**
+   * Return the case first option of this collator.
+   */
+  Result<CaseFirst, ICUError> GetCaseFirst() const;
+
+  /**
+   * Return the "ignores punctuation" option of this collator.
+   */
+  Result<bool, ICUError> GetIgnorePunctuation() const;
+
+  /**
+   * Map keywords to their BCP 47 equivalents.
+   */
+  static SpanResult<char> KeywordValueToBcp47Extension(const char* aKeyword,
+                                                       int32_t aLength);
+
+  enum class CommonlyUsed : bool {
+    /**
+     * Select all possible values, even when not commonly used by a locale.
+     */
+    No,
+
+    /**
+     * Only select the values which are commonly used by a locale.
+     */
+    Yes,
+  };
+
+  using Bcp47ExtEnumeration =
+      Enumeration<char, SpanResult<char>,
+                  Collator::KeywordValueToBcp47Extension>;
+
+  /**
+   * Returns an iterator of collator locale extensions in the preferred order.
+   * These extensions can be used in BCP 47 locales. For instance this
+   * iterator could return "phonebk" and could be appled to the German locale
+   * "de" as "de-co-phonebk" for a phonebook-style collation.
+   *
+   * The collation extensions can be found here:
+   * http://cldr.unicode.org/core-spec/#Key_Type_Definitions
+   */
+  static Result<Bcp47ExtEnumeration, ICUError> GetBcp47KeywordValuesForLocale(
+      const char* aLocale, CommonlyUsed aCommonlyUsed = CommonlyUsed::No);
+
+  /**
+   * Returns an iterator over all possible collator locale extensions.
+   * These extensions can be used in BCP 47 locales. For instance this
+   * iterator could return "phonebk" and could be appled to the German locale
+   * "de" as "de-co-phonebk" for a phonebook-style collation.
+   *
+   * The collation extensions can be found here:
+   * http://cldr.unicode.org/core-spec/#Key_Type_Definitions
+   */
+  static Result<Bcp47ExtEnumeration, ICUError> GetBcp47KeywordValues();
+
+  /**
+   * Returns an iterator over all supported collator locales.
+   *
+   * The returned strings are ICU locale identifiers and NOT BCP 47 language
+   * tags.
+   *
+   * Also see <https://unicode-org.github.io/icu/userguide/locale>.
+   */
+  static auto GetAvailableLocales() {
+    return AvailableLocalesEnumeration<ucol_countAvailable,
+                                       ucol_getAvailable>();
+  }
+
+ private:
+  /**
+   * Toggle features, or use the default setting.
+   */
+  enum class Feature {
+    // Turn the feature off.
+    On,
+    // Turn the feature off.
+    Off,
+    // Use the default setting for the feature.
+    Default,
+  };
+
+  static constexpr auto ToUColAttributeValue(Feature aFeature) {
+    switch (aFeature) {
+      case Collator::Feature::On:
+        return UCOL_ON;
+      case Collator::Feature::Off:
+        return UCOL_OFF;
+      case Collator::Feature::Default:
+        return UCOL_DEFAULT;
+    }
+#if MOZ_IS_GCC
+#  if !MOZ_GCC_VERSION_AT_LEAST(9, 1, 0)
+    return UCOL_DEFAULT;
+#  else
+    MOZ_CRASH("invalid collator feature");
+#  endif
+#else
+    MOZ_CRASH("invalid collator feature");
+#endif
+  }
+
+  /**
+   * Attribute for handling variable elements.
+   */
+  enum class AlternateHandling {
+    // Treats all the codepoints with non-ignorable primary weights in the
+    // same way (default)
+    NonIgnorable,
+    // Causes codepoints with primary weights that are equal or below the
+    // variable top value to be ignored on primary level and moved to the
+    // quaternary level.
+    Shifted,
+    Default,
+  };
+
+  /**
+   * The strength attribute.
+   *
+   * The usual strength for most locales (except Japanese) is tertiary.
+   *
+   * Quaternary strength is useful when combined with shifted setting for
+   * alternate handling attribute and for JIS X 4061 collation, when it is used
+   * to distinguish between Katakana and Hiragana. Otherwise, quaternary level
+   * is affected only by the number of non-ignorable code points in the string.
+   *
+   * Identical strength is rarely useful, as it amounts to codepoints of the NFD
+   * form of the string.
+   */
+  enum class Strength {
+    // Primary collation strength.
+    Primary,
+    // Secondary collation strength.
+    Secondary,
+    // Tertiary collation strength.
+    Tertiary,
+    // Quaternary collation strength.
+    Quaternary,
+    // Identical collation strength.
+    Identical,
+    Default,
+  };
+
+  /**
+   * Configure the Collation::Strength
+   */
+  void SetStrength(Strength strength);
+
+  /**
+   * Configure Collation::AlternateHandling.
+   */
+  ICUResult SetAlternateHandling(AlternateHandling aAlternateHandling);
+
+  /**
+   * Controls whether an extra case level (positioned before the third level) is
+   * generated or not.
+   *
+   * Contents of the case level are affected by the value of CaseFirst
+   * attribute. A simple way to ignore accent differences in a string is to set
+   * the strength to Primary and enable case level.
+   */
+  ICUResult SetCaseLevel(Feature aFeature);
+
+  /**
+   * When turned on, this attribute makes substrings of digits sort according to
+   * their numeric values.
+   *
+   * This is a way to get '100' to sort AFTER '2'. Note that the longest digit
+   * substring that can be treated as a single unit is 254 digits (not counting
+   * leading zeros). If a digit substring is longer than that, the digits beyond
+   * the limit will be treated as a separate digit substring.
+   *
+   * A "digit" in this sense is a code point with General_Category=Nd, which
+   * does not include circled numbers, roman numerals, etc. Only a contiguous
+   * digit substring is considered, that is, non-negative integers without
+   * separators. There is no support for plus/minus signs, decimals, exponents,
+   * etc.
+   */
+  ICUResult SetNumericCollation(Feature aFeature);
+
+  /**
+   * Controls whether the normalization check and necessary normalizations are
+   * performed.
+   *
+   * When off (default), no normalization check is performed. The correctness of
+   * the result is guaranteed only if the input data is in so-called FCD form
+   * When set to on, an incremental check is performed to see whether the input
+   * data is in the FCD form. If the data is not in the FCD form, incremental
+   * NFD normalization is performed.
+   */
+  ICUResult SetNormalizationMode(Feature aFeature);
+
+  /**
+   * Configure Collation::CaseFirst.
+   */
+  ICUResult SetCaseFirst(CaseFirst aCaseFirst);
+
+#ifndef JS_STANDALONE
+  FRIEND_TEST(IntlCollator, SetAttributesInternal);
+#endif
+
+  ICUPointer<UCollator> mCollator = ICUPointer<UCollator>(nullptr);
+  Maybe<Sensitivity> mLastStrategy = Nothing();
+};
+
+}  // namespace mozilla::intl
+
+#endif
diff --git a/intl/components/src/Currency.cpp b/intl/components/src/Currency.cpp
new file mode 100644
index 0000000000..4db8e0919c
--- /dev/null
+++ b/intl/components/src/Currency.cpp
@@ -0,0 +1,22 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "mozilla/intl/Currency.h"
+
+#include "unicode/ucurr.h"
+#include "unicode/uenum.h"
+#include "unicode/utypes.h"
+
+namespace mozilla::intl {
+
+Result<SpanEnumeration<char>, ICUError> Currency::GetISOCurrencies() {
+  UErrorCode status = U_ZERO_ERROR;
+  UEnumeration* enumeration = ucurr_openISOCurrencies(UCURR_ALL, &status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+  return SpanEnumeration<char>(enumeration);
+}
+
+}  // namespace mozilla::intl
diff --git a/intl/components/src/Currency.h b/intl/components/src/Currency.h
new file mode 100644
index 0000000000..d0f8eb6ee8
--- /dev/null
+++ b/intl/components/src/Currency.h
@@ -0,0 +1,30 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef intl_components_Currency_h_
+#define intl_components_Currency_h_
+
+#include "mozilla/intl/ICU4CGlue.h"
+#include "mozilla/intl/ICUError.h"
+#include "mozilla/Result.h"
+
+namespace mozilla::intl {
+
+/**
+ * This component is a Mozilla-focused API for working with currencies in
+ * internationalization code.
+ */
+class Currency final {
+ public:
+  Currency() = delete;
+
+  /**
+   * Returns an enumeration of all supported ISO currency codes.
+   */
+  static Result<SpanEnumeration<char>, ICUError> GetISOCurrencies();
+};
+
+}  // namespace mozilla::intl
+
+#endif
diff --git a/intl/components/src/DateIntervalFormat.cpp b/intl/components/src/DateIntervalFormat.cpp
new file mode 100644
index 0000000000..0097668f8b
--- /dev/null
+++ b/intl/components/src/DateIntervalFormat.cpp
@@ -0,0 +1,266 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "DateTimeFormat.h"  // for DATE_TIME_FORMAT_REPLACE_SPECIAL_SPACES
+#include "DateTimeFormatUtils.h"
+#include "ScopedICUObject.h"
+
+#include "mozilla/intl/Calendar.h"
+#include "mozilla/intl/DateIntervalFormat.h"
+
+namespace mozilla::intl {
+
+/**
+ * PartitionDateTimeRangePattern ( dateTimeFormat, x, y ), steps 9-11.
+ *
+ * Examine the formatted value to see if any interval span field is present.
+ *
+ * https://tc39.es/ecma402/#sec-partitiondatetimerangepattern
+ */
+static ICUResult DateFieldsPracticallyEqual(
+    const UFormattedValue* aFormattedValue, bool* aEqual) {
+  if (!aFormattedValue) {
+    return Err(ICUError::InternalError);
+  }
+
+  MOZ_ASSERT(aEqual);
+  *aEqual = false;
+  UErrorCode status = U_ZERO_ERROR;
+  UConstrainedFieldPosition* fpos = ucfpos_open(&status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+  ScopedICUObject<UConstrainedFieldPosition, ucfpos_close> toCloseFpos(fpos);
+
+  // We're only interested in UFIELD_CATEGORY_DATE_INTERVAL_SPAN fields.
+  ucfpos_constrainCategory(fpos, UFIELD_CATEGORY_DATE_INTERVAL_SPAN, &status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+
+  bool hasSpan = ufmtval_nextPosition(aFormattedValue, fpos, &status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+
+  // When no date interval span field was found, both dates are "practically
+  // equal" per PartitionDateTimeRangePattern.
+  *aEqual = !hasSpan;
+  return Ok();
+}
+
+/* static */
+Result<UniquePtr<DateIntervalFormat>, ICUError> DateIntervalFormat::TryCreate(
+    Span<const char> aLocale, Span<const char16_t> aSkeleton,
+    Span<const char16_t> aTimeZone) {
+  UErrorCode status = U_ZERO_ERROR;
+  UDateIntervalFormat* dif =
+      udtitvfmt_open(IcuLocale(aLocale), aSkeleton.data(),
+                     AssertedCast<int32_t>(aSkeleton.size()), aTimeZone.data(),
+                     AssertedCast<int32_t>(aTimeZone.size()), &status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+
+  return UniquePtr<DateIntervalFormat>(new DateIntervalFormat(dif));
+}
+
+DateIntervalFormat::~DateIntervalFormat() {
+  MOZ_ASSERT(mDateIntervalFormat);
+  udtitvfmt_close(mDateIntervalFormat.GetMut());
+}
+
+#if DATE_TIME_FORMAT_REPLACE_SPECIAL_SPACES
+// We reach inside the UFormattedValue and modify its internal string. (It's
+// crucial that this is just an in-place replacement that doesn't alter any
+// field positions, etc., )
+static void ReplaceSpecialSpaces(const UFormattedValue* aValue) {
+  UErrorCode status = U_ZERO_ERROR;
+  int32_t len;
+  const UChar* str = ufmtval_getString(aValue, &len, &status);
+  if (U_FAILURE(status)) {
+    return;
+  }
+
+  for (const auto& c : Span(str, len)) {
+    if (IsSpecialSpace(c)) {
+      const_cast<UChar&>(c) = ' ';
+    }
+  }
+}
+#endif
+
+ICUResult DateIntervalFormat::TryFormatCalendar(
+    const Calendar& aStart, const Calendar& aEnd,
+    AutoFormattedDateInterval& aFormatted, bool* aPracticallyEqual) const {
+  MOZ_ASSERT(aFormatted.IsValid());
+
+  UErrorCode status = U_ZERO_ERROR;
+  udtitvfmt_formatCalendarToResult(mDateIntervalFormat.GetConst(),
+                                   aStart.GetUCalendar(), aEnd.GetUCalendar(),
+                                   aFormatted.GetFormatted(), &status);
+
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+
+#if DATE_TIME_FORMAT_REPLACE_SPECIAL_SPACES
+  ReplaceSpecialSpaces(aFormatted.Value());
+#endif
+
+  MOZ_TRY(DateFieldsPracticallyEqual(aFormatted.Value(), aPracticallyEqual));
+  return Ok();
+}
+
+ICUResult DateIntervalFormat::TryFormatDateTime(
+    double aStart, double aEnd, AutoFormattedDateInterval& aFormatted,
+    bool* aPracticallyEqual) const {
+  MOZ_ASSERT(aFormatted.IsValid());
+
+  UErrorCode status = U_ZERO_ERROR;
+  udtitvfmt_formatToResult(mDateIntervalFormat.GetConst(), aStart, aEnd,
+                           aFormatted.GetFormatted(), &status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+
+#if DATE_TIME_FORMAT_REPLACE_SPECIAL_SPACES
+  ReplaceSpecialSpaces(aFormatted.Value());
+#endif
+
+  MOZ_TRY(DateFieldsPracticallyEqual(aFormatted.Value(), aPracticallyEqual));
+  return Ok();
+}
+
+ICUResult DateIntervalFormat::TryFormattedToParts(
+    const AutoFormattedDateInterval& aFormatted,
+    DateTimePartVector& aParts) const {
+  MOZ_ASSERT(aFormatted.IsValid());
+  const UFormattedValue* value = aFormatted.Value();
+  if (!value) {
+    return Err(ICUError::InternalError);
+  }
+
+  size_t lastEndIndex = 0;
+  auto AppendPart = [&](DateTimePartType type, size_t endIndex,
+                        DateTimePartSource source) {
+    if (!aParts.emplaceBack(type, endIndex, source)) {
+      return false;
+    }
+
+    lastEndIndex = endIndex;
+    return true;
+  };
+
+  UErrorCode status = U_ZERO_ERROR;
+  UConstrainedFieldPosition* fpos = ucfpos_open(&status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+  ScopedICUObject<UConstrainedFieldPosition, ucfpos_close> toCloseFpos(fpos);
+
+  size_t categoryEndIndex = 0;
+  DateTimePartSource source = DateTimePartSource::Shared;
+
+  while (true) {
+    bool hasMore = ufmtval_nextPosition(value, fpos, &status);
+    if (U_FAILURE(status)) {
+      return Err(ToICUError(status));
+    }
+    if (!hasMore) {
+      break;
+    }
+
+    int32_t category = ucfpos_getCategory(fpos, &status);
+    if (U_FAILURE(status)) {
+      return Err(ToICUError(status));
+    }
+
+    int32_t field = ucfpos_getField(fpos, &status);
+    if (U_FAILURE(status)) {
+      return Err(ToICUError(status));
+    }
+
+    int32_t beginIndexInt, endIndexInt;
+    ucfpos_getIndexes(fpos, &beginIndexInt, &endIndexInt, &status);
+    if (U_FAILURE(status)) {
+      return Err(ToICUError(status));
+    }
+
+    MOZ_ASSERT(beginIndexInt <= endIndexInt,
+               "field iterator returning invalid range");
+
+    size_t beginIndex = AssertedCast<size_t>(beginIndexInt);
+    size_t endIndex = AssertedCast<size_t>(endIndexInt);
+
+    // Indices are guaranteed to be returned in order (from left to right).
+    MOZ_ASSERT(lastEndIndex <= beginIndex,
+               "field iteration didn't return fields in order start to "
+               "finish as expected");
+
+    if (category == UFIELD_CATEGORY_DATE_INTERVAL_SPAN) {
+      // Append any remaining literal parts before changing the source kind.
+      if (lastEndIndex < beginIndex) {
+        if (!AppendPart(DateTimePartType::Literal, beginIndex, source)) {
+          return Err(ICUError::InternalError);
+        }
+      }
+
+      // The special field category UFIELD_CATEGORY_DATE_INTERVAL_SPAN has only
+      // two allowed values (0 or 1), indicating the begin of the start- resp.
+      // end-date.
+      MOZ_ASSERT(field == 0 || field == 1,
+                 "span category has unexpected value");
+
+      source = field == 0 ? DateTimePartSource::StartRange
+                          : DateTimePartSource::EndRange;
+      categoryEndIndex = endIndex;
+      continue;
+    }
+
+    // Ignore categories other than UFIELD_CATEGORY_DATE.
+    if (category != UFIELD_CATEGORY_DATE) {
+      continue;
+    }
+
+    DateTimePartType type =
+        ConvertUFormatFieldToPartType(static_cast<UDateFormatField>(field));
+    if (lastEndIndex < beginIndex) {
+      if (!AppendPart(DateTimePartType::Literal, beginIndex, source)) {
+        return Err(ICUError::InternalError);
+      }
+    }
+
+    if (!AppendPart(type, endIndex, source)) {
+      return Err(ICUError::InternalError);
+    }
+
+    if (endIndex == categoryEndIndex) {
+      // Append any remaining literal parts before changing the source kind.
+      if (lastEndIndex < endIndex) {
+        if (!AppendPart(DateTimePartType::Literal, endIndex, source)) {
+          return Err(ICUError::InternalError);
+        }
+      }
+
+      source = DateTimePartSource::Shared;
+    }
+  }
+
+  // Append any final literal.
+  auto spanResult = aFormatted.ToSpan();
+  if (spanResult.isErr()) {
+    return spanResult.propagateErr();
+  }
+  size_t formattedSize = spanResult.unwrap().size();
+  if (lastEndIndex < formattedSize) {
+    if (!AppendPart(DateTimePartType::Literal, formattedSize, source)) {
+      return Err(ICUError::InternalError);
+    }
+  }
+
+  return Ok();
+}
+
+}  // namespace mozilla::intl
diff --git a/intl/components/src/DateIntervalFormat.h b/intl/components/src/DateIntervalFormat.h
new file mode 100644
index 0000000000..c4dbce807a
--- /dev/null
+++ b/intl/components/src/DateIntervalFormat.h
@@ -0,0 +1,107 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+#ifndef intl_components_DateIntervalFormat_h_
+#define intl_components_DateIntervalFormat_h_
+
+#include "mozilla/intl/Calendar.h"
+#include "mozilla/intl/DateTimePart.h"
+#include "mozilla/intl/ICU4CGlue.h"
+#include "mozilla/intl/ICUError.h"
+#include "mozilla/Result.h"
+#include "mozilla/Span.h"
+#include "mozilla/UniquePtr.h"
+
+#include "unicode/udateintervalformat.h"
+#include "unicode/utypes.h"
+
+namespace mozilla::intl {
+class Calendar;
+
+using AutoFormattedDateInterval =
+    AutoFormattedResult<UFormattedDateInterval, udtitvfmt_openResult,
+                        udtitvfmt_resultAsValue, udtitvfmt_closeResult>;
+
+/**
+ * This component is a Mozilla-focused API for the date range formatting
+ * provided by ICU. This DateIntervalFormat class helps to format the range
+ * between two date-time values.
+ *
+ * https://tc39.es/ecma402/#sec-formatdatetimerange
+ * https://tc39.es/ecma402/#sec-formatdatetimerangetoparts
+ */
+class DateIntervalFormat final {
+ public:
+  /**
+   * Create a DateIntervalFormat object from locale, skeleton and time zone.
+   * The format of skeleton can be found in [1].
+   *
+   * Note: Skeleton will be removed in the future.
+   *
+   * [1]: https://unicode.org/reports/tr35/tr35-dates.html#Date_Format_Patterns
+   */
+  static Result<UniquePtr<DateIntervalFormat>, ICUError> TryCreate(
+      Span<const char> aLocale, Span<const char16_t> aSkeleton,
+      Span<const char16_t> aTimeZone);
+
+  ~DateIntervalFormat();
+
+  /**
+   * Format a date-time range between two Calendar objects.
+   *
+   * DateIntervalFormat cannot be changed to use a proleptic Gregorian
+   * calendar, so use this method if the start date is before the Gregorian
+   * calendar is introduced(October 15, 1582), otherwise use TryFormatDateTime
+   * instead.
+   *
+   * The result will be stored in aFormatted, caller can use
+   * AutoFormattedDateInterval::ToSpan() to get the formatted string, or pass
+   * the aFormatted to TryFormattedToParts to get the parts vector.
+   *
+   * aPracticallyEqual will be set to true if the date times of the two
+   * calendars are equal.
+   */
+  ICUResult TryFormatCalendar(const Calendar& aStart, const Calendar& aEnd,
+                              AutoFormattedDateInterval& aFormatted,
+                              bool* aPracticallyEqual) const;
+
+  /**
+   * Format a date-time range between two Unix epoch times in milliseconds.
+   *
+   * The result will be stored in aFormatted, caller can use
+   * AutoFormattedDateInterval::ToSpan() to get the formatted string, or pass
+   * the aFormatted to TryFormattedToParts to get the parts vector.
+   *
+   * aPracticallyEqual will be set to true if the date times of the two
+   * Unix epoch times are equal.
+   */
+  ICUResult TryFormatDateTime(double aStart, double aEnd,
+                              AutoFormattedDateInterval& aFormatted,
+                              bool* aPracticallyEqual) const;
+
+  /**
+   *  Convert the formatted DateIntervalFormat into several parts.
+   *
+   *  The caller get the formatted result from either TryFormatCalendar, or
+   *  TryFormatDateTime methods, and instantiate the DateTimePartVector. This
+   *  method will generate the parts and insert them into the vector.
+   *
+   *  See:
+   *  https://tc39.es/ecma402/#sec-formatdatetimerangetoparts
+   */
+  ICUResult TryFormattedToParts(const AutoFormattedDateInterval& aFormatted,
+                                DateTimePartVector& aParts) const;
+
+ private:
+  DateIntervalFormat() = delete;
+  explicit DateIntervalFormat(UDateIntervalFormat* aDif)
+      : mDateIntervalFormat(aDif) {}
+  DateIntervalFormat(const DateIntervalFormat&) = delete;
+  DateIntervalFormat& operator=(const DateIntervalFormat&) = delete;
+
+  ICUPointer<UDateIntervalFormat> mDateIntervalFormat =
+      ICUPointer<UDateIntervalFormat>(nullptr);
+};
+}  // namespace mozilla::intl
+
+#endif
diff --git a/intl/components/src/DateTimeFormat.cpp b/intl/components/src/DateTimeFormat.cpp
new file mode 100644
index 0000000000..2c09bb2adf
--- /dev/null
+++ b/intl/components/src/DateTimeFormat.cpp
@@ -0,0 +1,1148 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include <cstring>
+
+#include "unicode/ucal.h"
+#include "unicode/udat.h"
+#include "unicode/udatpg.h"
+#include "unicode/ures.h"
+
+#include "DateTimeFormatUtils.h"
+#include "ScopedICUObject.h"
+
+#include "mozilla/EnumSet.h"
+#include "mozilla/intl/Calendar.h"
+#include "mozilla/intl/DateTimeFormat.h"
+#include "mozilla/intl/DateTimePatternGenerator.h"
+
+namespace mozilla::intl {
+
+DateTimeFormat::~DateTimeFormat() {
+  MOZ_ASSERT(mDateFormat);
+  udat_close(mDateFormat);
+}
+
+static UDateFormatStyle ToUDateFormatStyle(
+    Maybe<DateTimeFormat::Style> aLength) {
+  if (!aLength) {
+    return UDAT_NONE;
+  }
+  switch (*aLength) {
+    case DateTimeFormat::Style::Full:
+      return UDAT_FULL;
+    case DateTimeFormat::Style::Long:
+      return UDAT_LONG;
+    case DateTimeFormat::Style::Medium:
+      return UDAT_MEDIUM;
+    case DateTimeFormat::Style::Short:
+      return UDAT_SHORT;
+  }
+  MOZ_ASSERT_UNREACHABLE();
+  // Do not use the default: branch so that the enum is exhaustively checked.
+  return UDAT_NONE;
+}
+
+/**
+ * Parse a pattern according to the format specified in
+ * <https://unicode.org/reports/tr35/tr35-dates.html#Date_Format_Patterns>.
+ */
+template <typename CharT>
+class PatternIterator {
+  CharT* iter;
+  const CharT* const end;
+
+ public:
+  explicit PatternIterator(mozilla::Span<CharT> aPattern)
+      : iter(aPattern.data()), end(aPattern.data() + aPattern.size()) {}
+
+  CharT* next() {
+    MOZ_ASSERT(iter != nullptr);
+
+    bool inQuote = false;
+    while (iter < end) {
+      CharT* cur = iter++;
+      if (*cur == '\'') {
+        inQuote = !inQuote;
+      } else if (!inQuote) {
+        return cur;
+      }
+    }
+
+    iter = nullptr;
+    return nullptr;
+  }
+};
+
+Maybe<DateTimeFormat::HourCycle> DateTimeFormat::HourCycleFromPattern(
+    Span<const char16_t> aPattern) {
+  PatternIterator<const char16_t> iter(aPattern);
+  while (const auto* ptr = iter.next()) {
+    switch (*ptr) {
+      case 'K':
+        return Some(DateTimeFormat::HourCycle::H11);
+      case 'h':
+        return Some(DateTimeFormat::HourCycle::H12);
+      case 'H':
+        return Some(DateTimeFormat::HourCycle::H23);
+      case 'k':
+        return Some(DateTimeFormat::HourCycle::H24);
+    }
+  }
+  return Nothing();
+}
+
+static bool IsHour12(DateTimeFormat::HourCycle aHourCycle) {
+  return aHourCycle == DateTimeFormat::HourCycle::H11 ||
+         aHourCycle == DateTimeFormat::HourCycle::H12;
+}
+
+static char16_t HourSymbol(DateTimeFormat::HourCycle aHourCycle) {
+  switch (aHourCycle) {
+    case DateTimeFormat::HourCycle::H11:
+      return 'K';
+    case DateTimeFormat::HourCycle::H12:
+      return 'h';
+    case DateTimeFormat::HourCycle::H23:
+      return 'H';
+    case DateTimeFormat::HourCycle::H24:
+      return 'k';
+  }
+  MOZ_CRASH("unexpected hour cycle");
+}
+
+enum class PatternField { Hour, Minute, Second, Other };
+
+template <typename CharT>
+static PatternField ToPatternField(CharT aCh) {
+  if (aCh == 'K' || aCh == 'h' || aCh == 'H' || aCh == 'k' || aCh == 'j') {
+    return PatternField::Hour;
+  }
+  if (aCh == 'm') {
+    return PatternField::Minute;
+  }
+  if (aCh == 's') {
+    return PatternField::Second;
+  }
+  return PatternField::Other;
+}
+
+/**
+ * Replaces all hour pattern characters in |patternOrSkeleton| to use the
+ * matching hour representation for |hourCycle|.
+ */
+/* static */
+void DateTimeFormat::ReplaceHourSymbol(
+    mozilla::Span<char16_t> aPatternOrSkeleton,
+    DateTimeFormat::HourCycle aHourCycle) {
+  char16_t replacement = HourSymbol(aHourCycle);
+  PatternIterator<char16_t> iter(aPatternOrSkeleton);
+  while (auto* ptr = iter.next()) {
+    auto field = ToPatternField(*ptr);
+    if (field == PatternField::Hour) {
+      *ptr = replacement;
+    }
+  }
+}
+
+/**
+ * Find a matching pattern using the requested hour-12 options.
+ *
+ * This function is needed to work around the following two issues.
+ * - https://unicode-org.atlassian.net/browse/ICU-21023
+ * - https://unicode-org.atlassian.net/browse/CLDR-13425
+ *
+ * We're currently using a relatively simple workaround, which doesn't give the
+ * most accurate results. For example:
+ *
+ * ```
+ * var dtf = new Intl.DateTimeFormat("en", {
+ *   timeZone: "UTC",
+ *   dateStyle: "long",
+ *   timeStyle: "long",
+ *   hourCycle: "h12",
+ * });
+ * print(dtf.format(new Date("2020-01-01T00:00Z")));
+ * ```
+ *
+ * Returns the pattern "MMMM d, y 'at' h:mm:ss a z", but when going through
+ * |DateTimePatternGenerator::GetSkeleton| and then
+ * |DateTimePatternGenerator::GetBestPattern| to find an equivalent pattern for
+ * "h23", we'll end up with the pattern "MMMM d, y, HH:mm:ss z", so the
+ * combinator element " 'at' " was lost in the process.
+ */
+/* static */
+ICUResult DateTimeFormat::FindPatternWithHourCycle(
+    DateTimePatternGenerator& aDateTimePatternGenerator,
+    DateTimeFormat::PatternVector& aPattern, bool aHour12,
+    DateTimeFormat::SkeletonVector& aSkeleton) {
+  MOZ_TRY(mozilla::intl::DateTimePatternGenerator::GetSkeleton(aPattern,
+                                                               aSkeleton));
+
+  // Input skeletons don't differentiate between "K" and "h" resp. "k" and "H".
+  DateTimeFormat::ReplaceHourSymbol(aSkeleton,
+                                    aHour12 ? DateTimeFormat::HourCycle::H12
+                                            : DateTimeFormat::HourCycle::H23);
+
+  MOZ_TRY(aDateTimePatternGenerator.GetBestPattern(aSkeleton, aPattern));
+
+  return Ok();
+}
+
+static auto PatternMatchOptions(mozilla::Span<const char16_t> aSkeleton) {
+  // Values for hour, minute, and second are:
+  // - absent: 0
+  // - numeric: 1
+  // - 2-digit: 2
+  int32_t hour = 0;
+  int32_t minute = 0;
+  int32_t second = 0;
+
+  PatternIterator<const char16_t> iter(aSkeleton);
+  while (const auto* ptr = iter.next()) {
+    switch (ToPatternField(*ptr)) {
+      case PatternField::Hour:
+        MOZ_ASSERT(hour < 2);
+        hour += 1;
+        break;
+      case PatternField::Minute:
+        MOZ_ASSERT(minute < 2);
+        minute += 1;
+        break;
+      case PatternField::Second:
+        MOZ_ASSERT(second < 2);
+        second += 1;
+        break;
+      case PatternField::Other:
+        break;
+    }
+  }
+
+  // Adjust the field length when the user requested '2-digit' representation.
+  //
+  // We can't just always adjust the field length, because
+  // 1. The default value for hour, minute, and second fields is 'numeric'. If
+  //    the length is always adjusted, |date.toLocaleTime()| will start to
+  //    return strings like "1:5:9 AM" instead of "1:05:09 AM".
+  // 2. ICU doesn't support to adjust the field length to 'numeric' in certain
+  //    cases. For example when the locale is "de" (German):
+  //      a. hour='numeric' and minute='2-digit' will return "1:05".
+  //      b. whereas hour='numeric' and minute='numeric' will return "01:05".
+  //
+  // Therefore we only support adjusting the field length when the user
+  // explicitly requested the '2-digit' representation.
+
+  using PatternMatchOption =
+      mozilla::intl::DateTimePatternGenerator::PatternMatchOption;
+  mozilla::EnumSet<PatternMatchOption> options;
+  if (hour == 2) {
+    options += PatternMatchOption::HourField;
+  }
+  if (minute == 2) {
+    options += PatternMatchOption::MinuteField;
+  }
+  if (second == 2) {
+    options += PatternMatchOption::SecondField;
+  }
+  return options;
+}
+
+/* static */
+Result<UniquePtr<DateTimeFormat>, ICUError> DateTimeFormat::TryCreateFromStyle(
+    Span<const char> aLocale, const StyleBag& aStyleBag,
+    DateTimePatternGenerator* aDateTimePatternGenerator,
+    Maybe<Span<const char16_t>> aTimeZoneOverride) {
+  auto dateStyle = ToUDateFormatStyle(aStyleBag.date);
+  auto timeStyle = ToUDateFormatStyle(aStyleBag.time);
+
+  if (dateStyle == UDAT_NONE && timeStyle == UDAT_NONE) {
+    dateStyle = UDAT_DEFAULT;
+    timeStyle = UDAT_DEFAULT;
+  }
+
+  // The time zone is optional.
+  int32_t tzIDLength = -1;
+  const UChar* tzID = nullptr;
+  if (aTimeZoneOverride) {
+    tzIDLength = static_cast<int32_t>(aTimeZoneOverride->size());
+    tzID = aTimeZoneOverride->Elements();
+  }
+
+  UErrorCode status = U_ZERO_ERROR;
+  UDateFormat* dateFormat =
+      udat_open(timeStyle, dateStyle, IcuLocale(aLocale), tzID, tzIDLength,
+                /* pattern */ nullptr, /* pattern length */ -1, &status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+
+  auto df = UniquePtr<DateTimeFormat>(new DateTimeFormat(dateFormat));
+
+  if (aStyleBag.time && (aStyleBag.hour12 || aStyleBag.hourCycle)) {
+    // Only adjust the style pattern for time if there is an override.
+    // Extract the pattern and adjust it for the preferred hour cycle.
+    DateTimeFormat::PatternVector pattern{};
+
+    VectorToBufferAdaptor buffer(pattern);
+    MOZ_TRY(df->GetPattern(buffer));
+
+    Maybe<DateTimeFormat::HourCycle> hcPattern = HourCycleFromPattern(pattern);
+    DateTimeFormat::SkeletonVector skeleton{};
+
+    if (hcPattern) {
+      bool wantHour12 =
+          aStyleBag.hour12 ? *aStyleBag.hour12 : IsHour12(*aStyleBag.hourCycle);
+      if (wantHour12 == IsHour12(*hcPattern)) {
+        // Return the date-time format when its hour-cycle settings match the
+        // requested options.
+        if (aStyleBag.hour12 || *hcPattern == *aStyleBag.hourCycle) {
+          return df;
+        }
+      } else {
+        MOZ_ASSERT(aDateTimePatternGenerator);
+        MOZ_TRY(DateTimeFormat::FindPatternWithHourCycle(
+            *aDateTimePatternGenerator, pattern, wantHour12, skeleton));
+      }
+      // Replace the hourCycle, if present, in the pattern string. But only do
+      // this if no hour12 option is present, because the latter takes
+      // precedence over hourCycle.
+      if (!aStyleBag.hour12) {
+        DateTimeFormat::ReplaceHourSymbol(pattern, *aStyleBag.hourCycle);
+      }
+
+      auto result = DateTimeFormat::TryCreateFromPattern(aLocale, pattern,
+                                                         aTimeZoneOverride);
+      if (result.isErr()) {
+        return Err(result.unwrapErr());
+      }
+      auto dateTimeFormat = result.unwrap();
+      MOZ_TRY(dateTimeFormat->CacheSkeleton(skeleton));
+      return dateTimeFormat;
+    }
+  }
+
+  return df;
+}
+
+DateTimeFormat::DateTimeFormat(UDateFormat* aDateFormat) {
+  MOZ_RELEASE_ASSERT(aDateFormat, "Expected aDateFormat to not be a nullptr.");
+  mDateFormat = aDateFormat;
+}
+
+// A helper to ergonomically push a string onto a string vector.
+template <typename V, size_t N>
+static ICUResult PushString(V& aVec, const char16_t (&aString)[N]) {
+  if (!aVec.append(aString, N - 1)) {
+    return Err(ICUError::OutOfMemory);
+  }
+  return Ok();
+}
+
+// A helper to ergonomically push a char onto a string vector.
+template <typename V>
+static ICUResult PushChar(V& aVec, char16_t aCh) {
+  if (!aVec.append(aCh)) {
+    return Err(ICUError::OutOfMemory);
+  }
+  return Ok();
+}
+
+/**
+ * Returns an ICU skeleton string representing the specified options.
+ * http://unicode.org/reports/tr35/tr35-dates.html#Date_Field_Symbol_Table
+ */
+ICUResult ToICUSkeleton(const DateTimeFormat::ComponentsBag& aBag,
+                        DateTimeFormat::SkeletonVector& aSkeleton) {
+  // Create an ICU skeleton representing the specified aBag. See
+  if (aBag.weekday) {
+    switch (*aBag.weekday) {
+      case DateTimeFormat::Text::Narrow:
+        MOZ_TRY(PushString(aSkeleton, u"EEEEE"));
+        break;
+      case DateTimeFormat::Text::Short:
+        MOZ_TRY(PushString(aSkeleton, u"E"));
+        break;
+      case DateTimeFormat::Text::Long:
+        MOZ_TRY(PushString(aSkeleton, u"EEEE"));
+    }
+  }
+  if (aBag.era) {
+    switch (*aBag.era) {
+      case DateTimeFormat::Text::Narrow:
+        MOZ_TRY(PushString(aSkeleton, u"GGGGG"));
+        break;
+      case DateTimeFormat::Text::Short:
+        // Use "GGG" instead of "G" to return the same results as other
+        // browsers. This is exploiting the following ICU bug
+        // <https://unicode-org.atlassian.net/browse/ICU-22138>. As soon as that
+        // bug has been fixed, we can change this back to "G".
+        //
+        // In practice the bug only affects "G", so we only apply it for "G"
+        // and not for other symbols like "B" or "z".
+        MOZ_TRY(PushString(aSkeleton, u"GGG"));
+        break;
+      case DateTimeFormat::Text::Long:
+        MOZ_TRY(PushString(aSkeleton, u"GGGG"));
+        break;
+    }
+  }
+  if (aBag.year) {
+    switch (*aBag.year) {
+      case DateTimeFormat::Numeric::TwoDigit:
+        MOZ_TRY(PushString(aSkeleton, u"yy"));
+        break;
+      case DateTimeFormat::Numeric::Numeric:
+        MOZ_TRY(PushString(aSkeleton, u"y"));
+        break;
+    }
+  }
+  if (aBag.month) {
+    switch (*aBag.month) {
+      case DateTimeFormat::Month::TwoDigit:
+        MOZ_TRY(PushString(aSkeleton, u"MM"));
+        break;
+      case DateTimeFormat::Month::Numeric:
+        MOZ_TRY(PushString(aSkeleton, u"M"));
+        break;
+      case DateTimeFormat::Month::Narrow:
+        MOZ_TRY(PushString(aSkeleton, u"MMMMM"));
+        break;
+      case DateTimeFormat::Month::Short:
+        MOZ_TRY(PushString(aSkeleton, u"MMM"));
+        break;
+      case DateTimeFormat::Month::Long:
+        MOZ_TRY(PushString(aSkeleton, u"MMMM"));
+        break;
+    }
+  }
+  if (aBag.day) {
+    switch (*aBag.day) {
+      case DateTimeFormat::Numeric::TwoDigit:
+        MOZ_TRY(PushString(aSkeleton, u"dd"));
+        break;
+      case DateTimeFormat::Numeric::Numeric:
+        MOZ_TRY(PushString(aSkeleton, u"d"));
+        break;
+    }
+  }
+
+  // If hour12 and hourCycle are both present, hour12 takes precedence.
+  char16_t hourSkeletonChar = 'j';
+  if (aBag.hour12) {
+    if (*aBag.hour12) {
+      hourSkeletonChar = 'h';
+    } else {
+      hourSkeletonChar = 'H';
+    }
+  } else if (aBag.hourCycle) {
+    switch (*aBag.hourCycle) {
+      case DateTimeFormat::HourCycle::H11:
+      case DateTimeFormat::HourCycle::H12:
+        hourSkeletonChar = 'h';
+        break;
+      case DateTimeFormat::HourCycle::H23:
+      case DateTimeFormat::HourCycle::H24:
+        hourSkeletonChar = 'H';
+        break;
+    }
+  }
+  if (aBag.hour) {
+    switch (*aBag.hour) {
+      case DateTimeFormat::Numeric::TwoDigit:
+        MOZ_TRY(PushChar(aSkeleton, hourSkeletonChar));
+        MOZ_TRY(PushChar(aSkeleton, hourSkeletonChar));
+        break;
+      case DateTimeFormat::Numeric::Numeric:
+        MOZ_TRY(PushChar(aSkeleton, hourSkeletonChar));
+        break;
+    }
+  }
+  // ICU requires that "B" is set after the "j" hour skeleton symbol.
+  // https://unicode-org.atlassian.net/browse/ICU-20731
+  if (aBag.dayPeriod) {
+    switch (*aBag.dayPeriod) {
+      case DateTimeFormat::Text::Narrow:
+        MOZ_TRY(PushString(aSkeleton, u"BBBBB"));
+        break;
+      case DateTimeFormat::Text::Short:
+        MOZ_TRY(PushString(aSkeleton, u"B"));
+        break;
+      case DateTimeFormat::Text::Long:
+        MOZ_TRY(PushString(aSkeleton, u"BBBB"));
+        break;
+    }
+  }
+  if (aBag.minute) {
+    switch (*aBag.minute) {
+      case DateTimeFormat::Numeric::TwoDigit:
+        MOZ_TRY(PushString(aSkeleton, u"mm"));
+        break;
+      case DateTimeFormat::Numeric::Numeric:
+        MOZ_TRY(PushString(aSkeleton, u"m"));
+        break;
+    }
+  }
+  if (aBag.second) {
+    switch (*aBag.second) {
+      case DateTimeFormat::Numeric::TwoDigit:
+        MOZ_TRY(PushString(aSkeleton, u"ss"));
+        break;
+      case DateTimeFormat::Numeric::Numeric:
+        MOZ_TRY(PushString(aSkeleton, u"s"));
+        break;
+    }
+  }
+  if (aBag.fractionalSecondDigits) {
+    switch (*aBag.fractionalSecondDigits) {
+      case 1:
+        MOZ_TRY(PushString(aSkeleton, u"S"));
+        break;
+      case 2:
+        MOZ_TRY(PushString(aSkeleton, u"SS"));
+        break;
+      default:
+        MOZ_TRY(PushString(aSkeleton, u"SSS"));
+        break;
+    }
+  }
+  if (aBag.timeZoneName) {
+    switch (*aBag.timeZoneName) {
+      case DateTimeFormat::TimeZoneName::Short:
+        MOZ_TRY(PushString(aSkeleton, u"z"));
+        break;
+      case DateTimeFormat::TimeZoneName::Long:
+        MOZ_TRY(PushString(aSkeleton, u"zzzz"));
+        break;
+      case DateTimeFormat::TimeZoneName::ShortOffset:
+        MOZ_TRY(PushString(aSkeleton, u"O"));
+        break;
+      case DateTimeFormat::TimeZoneName::LongOffset:
+        MOZ_TRY(PushString(aSkeleton, u"OOOO"));
+        break;
+      case DateTimeFormat::TimeZoneName::ShortGeneric:
+        MOZ_TRY(PushString(aSkeleton, u"v"));
+        break;
+      case DateTimeFormat::TimeZoneName::LongGeneric:
+        MOZ_TRY(PushString(aSkeleton, u"vvvv"));
+        break;
+    }
+  }
+  return Ok();
+}
+
+/* static */
+Result<UniquePtr<DateTimeFormat>, ICUError>
+DateTimeFormat::TryCreateFromComponents(
+    Span<const char> aLocale, const DateTimeFormat::ComponentsBag& aBag,
+    DateTimePatternGenerator* aDateTimePatternGenerator,
+    Maybe<Span<const char16_t>> aTimeZoneOverride) {
+  DateTimeFormat::SkeletonVector skeleton;
+  MOZ_TRY(ToICUSkeleton(aBag, skeleton));
+  return TryCreateFromSkeleton(aLocale, skeleton, aDateTimePatternGenerator,
+                               aBag.hourCycle, aTimeZoneOverride);
+}
+
+/* static */
+Result<UniquePtr<DateTimeFormat>, ICUError>
+DateTimeFormat::TryCreateFromPattern(
+    Span<const char> aLocale, Span<const char16_t> aPattern,
+    Maybe<Span<const char16_t>> aTimeZoneOverride) {
+  UErrorCode status = U_ZERO_ERROR;
+
+  // The time zone is optional.
+  int32_t tzIDLength = -1;
+  const UChar* tzID = nullptr;
+  if (aTimeZoneOverride) {
+    tzIDLength = static_cast<int32_t>(aTimeZoneOverride->size());
+    tzID = aTimeZoneOverride->data();
+  }
+
+  // Create the date formatter.
+  UDateFormat* dateFormat = udat_open(
+      UDAT_PATTERN, UDAT_PATTERN, IcuLocale(aLocale), tzID, tzIDLength,
+      aPattern.data(), static_cast<int32_t>(aPattern.size()), &status);
+
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+
+  // The DateTimeFormat wrapper will control the life cycle of the ICU
+  // dateFormat object.
+  return UniquePtr<DateTimeFormat>(new DateTimeFormat(dateFormat));
+}
+
+/* static */
+Result<UniquePtr<DateTimeFormat>, ICUError>
+DateTimeFormat::TryCreateFromSkeleton(
+    Span<const char> aLocale, Span<const char16_t> aSkeleton,
+    DateTimePatternGenerator* aDateTimePatternGenerator,
+    Maybe<DateTimeFormat::HourCycle> aHourCycle,
+    Maybe<Span<const char16_t>> aTimeZoneOverride) {
+  if (!aDateTimePatternGenerator) {
+    return Err(ICUError::InternalError);
+  }
+
+  // Compute the best pattern for the skeleton.
+  DateTimeFormat::PatternVector pattern;
+  auto options = PatternMatchOptions(aSkeleton);
+  MOZ_TRY(
+      aDateTimePatternGenerator->GetBestPattern(aSkeleton, pattern, options));
+
+  if (aHourCycle) {
+    DateTimeFormat::ReplaceHourSymbol(pattern, *aHourCycle);
+  }
+
+  auto result =
+      DateTimeFormat::TryCreateFromPattern(aLocale, pattern, aTimeZoneOverride);
+  if (result.isErr()) {
+    return Err(result.unwrapErr());
+  }
+  auto dateTimeFormat = result.unwrap();
+  MOZ_TRY(dateTimeFormat->CacheSkeleton(aSkeleton));
+  return dateTimeFormat;
+}
+
+ICUResult DateTimeFormat::CacheSkeleton(Span<const char16_t> aSkeleton) {
+  if (mOriginalSkeleton.append(aSkeleton.Elements(), aSkeleton.Length())) {
+    return Ok();
+  }
+  return Err(ICUError::OutOfMemory);
+}
+
+void DateTimeFormat::SetStartTimeIfGregorian(double aTime) {
+  UErrorCode status = U_ZERO_ERROR;
+  UCalendar* cal = const_cast<UCalendar*>(udat_getCalendar(mDateFormat));
+  ucal_setGregorianChange(cal, aTime, &status);
+  // An error here means the calendar is not Gregorian, and can be ignored.
+}
+
+/* static */
+Result<UniquePtr<Calendar>, ICUError> DateTimeFormat::CloneCalendar(
+    double aUnixEpoch) const {
+  UErrorCode status = U_ZERO_ERROR;
+  UCalendar* calendarRaw = ucal_clone(udat_getCalendar(mDateFormat), &status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+  auto calendar = MakeUnique<Calendar>(calendarRaw);
+
+  MOZ_TRY(calendar->SetTimeInMs(aUnixEpoch));
+
+  return calendar;
+}
+
+/**
+ * ICU locale identifier consisting of a language and a region subtag.
+ */
+class LanguageRegionLocaleId {
+  // unicode_language_subtag = alpha{2,3} | alpha{5,8} ;
+  static constexpr size_t LanguageLength = 8;
+
+  // unicode_region_subtag = (alpha{2} | digit{3}) ;
+  static constexpr size_t RegionLength = 3;
+
+  // Add +1 to account for the separator.
+  static constexpr size_t LRLength = LanguageLength + RegionLength + 1;
+
+  // Add +1 to zero terminate the string.
+  char mLocale[LRLength + 1] = {};
+
+  // Pointer to the start of the region subtag within |locale_|.
+  char* mRegion = nullptr;
+
+ public:
+  LanguageRegionLocaleId(Span<const char> aLanguage,
+                         Maybe<Span<const char>> aRegion);
+
+  const char* languageRegion() const { return mLocale; }
+  const char* region() const { return mRegion; }
+};
+
+LanguageRegionLocaleId::LanguageRegionLocaleId(
+    Span<const char> aLanguage, Maybe<Span<const char>> aRegion) {
+  MOZ_RELEASE_ASSERT(aLanguage.Length() <= LanguageLength);
+  MOZ_RELEASE_ASSERT(!aRegion || aRegion->Length() <= RegionLength);
+
+  size_t languageLength = aLanguage.Length();
+
+  std::memcpy(mLocale, aLanguage.Elements(), languageLength);
+
+  // ICU locale identifiers are separated by underscores.
+  mLocale[languageLength] = '_';
+
+  mRegion = mLocale + languageLength + 1;
+  if (aRegion) {
+    std::memcpy(mRegion, aRegion->Elements(), aRegion->Length());
+  } else {
+    // Use "001" (UN M.49 code for the World) as the fallback to match ICU.
+    std::strcpy(mRegion, "001");
+  }
+}
+
+/* static */
+Result<DateTimeFormat::HourCyclesVector, ICUError>
+DateTimeFormat::GetAllowedHourCycles(Span<const char> aLanguage,
+                                     Maybe<Span<const char>> aRegion) {
+  // ICU doesn't expose a public API to retrieve the hour cyles for a locale, so
+  // we have to reconstruct |DateTimePatternGenerator::getAllowedHourFormats()|
+  // using the public UResourceBundle API.
+  //
+  // The time data format is specified in UTS 35 at [1] and the data itself is
+  // located at [2].
+  //
+  // [1] https://unicode.org/reports/tr35/tr35-dates.html#Time_Data
+  // [2]
+  // https://github.com/unicode-org/cldr/blob/master/common/supplemental/supplementalData.xml
+
+  HourCyclesVector result;
+
+  // Reserve space for the maximum number of hour cycles. This call always
+  // succeeds because it matches the inline capacity. We can now infallibly
+  // append all hour cycles to the vector.
+  MOZ_ALWAYS_TRUE(result.reserve(HourCyclesVector::InlineLength));
+
+  LanguageRegionLocaleId localeId(aLanguage, aRegion);
+
+  // First open the "supplementalData" resource bundle.
+  UErrorCode status = U_ZERO_ERROR;
+  UResourceBundle* res = ures_openDirect(nullptr, "supplementalData", &status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+  ScopedICUObject<UResourceBundle, ures_close> closeRes(res);
+  MOZ_ASSERT(ures_getType(res) == URES_TABLE);
+
+  // Locate "timeDate" within the "supplementalData" resource bundle.
+  UResourceBundle* timeData = ures_getByKey(res, "timeData", nullptr, &status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+  ScopedICUObject<UResourceBundle, ures_close> closeTimeData(timeData);
+  MOZ_ASSERT(ures_getType(timeData) == URES_TABLE);
+
+  // Try to find a matching resource within "timeData". The two possible keys
+  // into the "timeData" resource bundle are `language_region` and `region`.
+  // Prefer `language_region` and otherwise fallback to `region`.
+  UResourceBundle* hclocale =
+      ures_getByKey(timeData, localeId.languageRegion(), nullptr, &status);
+  if (status == U_MISSING_RESOURCE_ERROR) {
+    status = U_ZERO_ERROR;
+    hclocale = ures_getByKey(timeData, localeId.region(), nullptr, &status);
+  }
+  if (status == U_MISSING_RESOURCE_ERROR) {
+    // Default to "h23" if no resource was found at all. This matches ICU.
+    result.infallibleAppend(HourCycle::H23);
+    return result;
+  }
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+  ScopedICUObject<UResourceBundle, ures_close> closeHcLocale(hclocale);
+  MOZ_ASSERT(ures_getType(hclocale) == URES_TABLE);
+
+  EnumSet<HourCycle> added{};
+
+  auto addToResult = [&](const UChar* str, int32_t len) {
+    // An hour cycle strings is one of "K", "h", "H", or "k"; optionally
+    // followed by the suffix "b" or "B". We ignore the suffix because day
+    // periods can't be expressed in the "hc" Unicode extension.
+    MOZ_ASSERT(len == 1 || len == 2);
+
+    // Default to "h23" for unsupported hour cycle strings.
+    HourCycle hc = HourCycle::H23;
+    switch (str[0]) {
+      case 'K':
+        hc = HourCycle::H11;
+        break;
+      case 'h':
+        hc = HourCycle::H12;
+        break;
+      case 'H':
+        hc = HourCycle::H23;
+        break;
+      case 'k':
+        hc = HourCycle::H24;
+        break;
+    }
+
+    // Add each unique hour cycle to the result array.
+    if (!added.contains(hc)) {
+      added += hc;
+
+      result.infallibleAppend(hc);
+    }
+  };
+
+  // Determine the preferred hour cycle for the locale.
+  int32_t len = 0;
+  const UChar* hc = ures_getStringByKey(hclocale, "preferred", &len, &status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+  addToResult(hc, len);
+
+  // Find any additionally allowed hour cycles of the locale.
+  UResourceBundle* allowed =
+      ures_getByKey(hclocale, "allowed", nullptr, &status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+  ScopedICUObject<UResourceBundle, ures_close> closeAllowed(allowed);
+  MOZ_ASSERT(ures_getType(allowed) == URES_ARRAY ||
+             ures_getType(allowed) == URES_STRING);
+
+  while (ures_hasNext(allowed)) {
+    int32_t len = 0;
+    const UChar* hc = ures_getNextString(allowed, &len, nullptr, &status);
+    if (U_FAILURE(status)) {
+      return Err(ToICUError(status));
+    }
+    addToResult(hc, len);
+  }
+
+  return result;
+}
+
+Result<DateTimeFormat::ComponentsBag, ICUError>
+DateTimeFormat::ResolveComponents() {
+  // Maps an ICU pattern string to a corresponding set of date-time components
+  // and their values, and adds properties for these components to the result
+  // object, which will be returned by the resolvedOptions method. For the
+  // interpretation of ICU pattern characters, see
+  // http://unicode.org/reports/tr35/tr35-dates.html#Date_Field_Symbol_Table
+
+  DateTimeFormat::PatternVector pattern{};
+  VectorToBufferAdaptor buffer(pattern);
+  MOZ_TRY(GetPattern(buffer));
+
+  DateTimeFormat::ComponentsBag bag{};
+
+  using Text = DateTimeFormat::Text;
+  using HourCycle = DateTimeFormat::HourCycle;
+  using Numeric = DateTimeFormat::Numeric;
+  using Month = DateTimeFormat::Month;
+
+  auto text = Text::Long;
+  auto numeric = Numeric::Numeric;
+  auto month = Month::Long;
+  uint8_t fractionalSecondDigits = 0;
+
+  for (size_t i = 0, len = pattern.length(); i < len;) {
+    char16_t c = pattern[i++];
+    if (c == u'\'') {
+      // Skip past string literals.
+      while (i < len && pattern[i] != u'\'') {
+        i++;
+      }
+      i++;
+      continue;
+    }
+
+    // Count how many times the character is repeated.
+    size_t count = 1;
+    while (i < len && pattern[i] == c) {
+      i++;
+      count++;
+    }
+
+    // Determine the enum case of the field.
+    switch (c) {
+      // "text" cases
+      case u'G':
+      case u'E':
+      case u'c':
+      case u'B':
+      case u'z':
+      case u'O':
+      case u'v':
+      case u'V':
+        if (count <= 3) {
+          text = Text::Short;
+        } else if (count == 4) {
+          text = Text::Long;
+        } else {
+          text = Text::Narrow;
+        }
+        break;
+      // "number" cases
+      case u'y':
+      case u'd':
+      case u'h':
+      case u'H':
+      case u'm':
+      case u's':
+      case u'k':
+      case u'K':
+        if (count == 2) {
+          numeric = Numeric::TwoDigit;
+        } else {
+          numeric = Numeric::Numeric;
+        }
+        break;
+      // "numeric" cases
+      case u'r':
+      case u'U':
+        // Both are mapped to numeric years.
+        numeric = Numeric::Numeric;
+        break;
+      // "text & number" cases
+      case u'M':
+      case u'L':
+        if (count == 1) {
+          month = Month::Numeric;
+        } else if (count == 2) {
+          month = Month::TwoDigit;
+        } else if (count == 3) {
+          month = Month::Short;
+        } else if (count == 4) {
+          month = Month::Long;
+        } else {
+          month = Month::Narrow;
+        }
+        break;
+      case u'S':
+        fractionalSecondDigits = count;
+        break;
+      default: {
+        // skip other pattern characters and literal text
+      }
+    }
+
+    // Map ICU pattern characters back to the corresponding date-time
+    // components of DateTimeFormat. See
+    // http://unicode.org/reports/tr35/tr35-dates.html#Date_Field_Symbol_Table
+    switch (c) {
+      case u'E':
+      case u'c':
+        bag.weekday = Some(text);
+        break;
+      case u'G':
+        bag.era = Some(text);
+        break;
+      case u'y':
+      case u'r':
+      case u'U':
+        bag.year = Some(numeric);
+        break;
+      case u'M':
+      case u'L':
+        bag.month = Some(month);
+        break;
+      case u'd':
+        bag.day = Some(numeric);
+        break;
+      case u'B':
+        bag.dayPeriod = Some(text);
+        break;
+      case u'K':
+        bag.hourCycle = Some(HourCycle::H11);
+        bag.hour = Some(numeric);
+        bag.hour12 = Some(true);
+        break;
+      case u'h':
+        bag.hourCycle = Some(HourCycle::H12);
+        bag.hour = Some(numeric);
+        bag.hour12 = Some(true);
+        break;
+      case u'H':
+        bag.hourCycle = Some(HourCycle::H23);
+        bag.hour = Some(numeric);
+        bag.hour12 = Some(false);
+        break;
+      case u'k':
+        bag.hourCycle = Some(HourCycle::H24);
+        bag.hour = Some(numeric);
+        bag.hour12 = Some(false);
+        break;
+      case u'm':
+        bag.minute = Some(numeric);
+        break;
+      case u's':
+        bag.second = Some(numeric);
+        break;
+      case u'S':
+        bag.fractionalSecondDigits = Some(fractionalSecondDigits);
+        break;
+      case u'z':
+        switch (text) {
+          case Text::Long:
+            bag.timeZoneName = Some(TimeZoneName::Long);
+            break;
+          case Text::Short:
+          case Text::Narrow:
+            bag.timeZoneName = Some(TimeZoneName::Short);
+            break;
+        }
+        break;
+      case u'O':
+        switch (text) {
+          case Text::Long:
+            bag.timeZoneName = Some(TimeZoneName::LongOffset);
+            break;
+          case Text::Short:
+          case Text::Narrow:
+            bag.timeZoneName = Some(TimeZoneName::ShortOffset);
+            break;
+        }
+        break;
+      case u'v':
+      case u'V':
+        switch (text) {
+          case Text::Long:
+            bag.timeZoneName = Some(TimeZoneName::LongGeneric);
+            break;
+          case Text::Short:
+          case Text::Narrow:
+            bag.timeZoneName = Some(TimeZoneName::ShortGeneric);
+            break;
+        }
+        break;
+    }
+  }
+  return bag;
+}
+
+const char* DateTimeFormat::ToString(
+    DateTimeFormat::TimeZoneName aTimeZoneName) {
+  switch (aTimeZoneName) {
+    case TimeZoneName::Long:
+      return "long";
+    case TimeZoneName::Short:
+      return "short";
+    case TimeZoneName::ShortOffset:
+      return "shortOffset";
+    case TimeZoneName::LongOffset:
+      return "longOffset";
+    case TimeZoneName::ShortGeneric:
+      return "shortGeneric";
+    case TimeZoneName::LongGeneric:
+      return "longGeneric";
+  }
+  MOZ_CRASH("Unexpected DateTimeFormat::TimeZoneName");
+}
+
+const char* DateTimeFormat::ToString(DateTimeFormat::Month aMonth) {
+  switch (aMonth) {
+    case Month::Numeric:
+      return "numeric";
+    case Month::TwoDigit:
+      return "2-digit";
+    case Month::Long:
+      return "long";
+    case Month::Short:
+      return "short";
+    case Month::Narrow:
+      return "narrow";
+  }
+  MOZ_CRASH("Unexpected DateTimeFormat::Month");
+}
+
+const char* DateTimeFormat::ToString(DateTimeFormat::Text aText) {
+  switch (aText) {
+    case Text::Long:
+      return "long";
+    case Text::Short:
+      return "short";
+    case Text::Narrow:
+      return "narrow";
+  }
+  MOZ_CRASH("Unexpected DateTimeFormat::Text");
+}
+
+const char* DateTimeFormat::ToString(DateTimeFormat::Numeric aNumeric) {
+  switch (aNumeric) {
+    case Numeric::Numeric:
+      return "numeric";
+    case Numeric::TwoDigit:
+      return "2-digit";
+  }
+  MOZ_CRASH("Unexpected DateTimeFormat::Numeric");
+}
+
+const char* DateTimeFormat::ToString(DateTimeFormat::Style aStyle) {
+  switch (aStyle) {
+    case Style::Full:
+      return "full";
+    case Style::Long:
+      return "long";
+    case Style::Medium:
+      return "medium";
+    case Style::Short:
+      return "short";
+  }
+  MOZ_CRASH("Unexpected DateTimeFormat::Style");
+}
+
+const char* DateTimeFormat::ToString(DateTimeFormat::HourCycle aHourCycle) {
+  switch (aHourCycle) {
+    case HourCycle::H11:
+      return "h11";
+    case HourCycle::H12:
+      return "h12";
+    case HourCycle::H23:
+      return "h23";
+    case HourCycle::H24:
+      return "h24";
+  }
+  MOZ_CRASH("Unexpected DateTimeFormat::HourCycle");
+}
+
+ICUResult DateTimeFormat::TryFormatToParts(
+    UFieldPositionIterator* aFieldPositionIterator, size_t aSpanSize,
+    DateTimePartVector& aParts) const {
+  ScopedICUObject<UFieldPositionIterator, ufieldpositer_close> toClose(
+      aFieldPositionIterator);
+
+  size_t lastEndIndex = 0;
+  auto AppendPart = [&](DateTimePartType type, size_t endIndex) {
+    // For the part defined in FormatDateTimeToParts, it doesn't have ||Source||
+    // property, we store Shared for simplicity,
+    if (!aParts.emplaceBack(type, endIndex, DateTimePartSource::Shared)) {
+      return false;
+    }
+
+    lastEndIndex = endIndex;
+    return true;
+  };
+
+  int32_t fieldInt, beginIndexInt, endIndexInt;
+  while ((fieldInt = ufieldpositer_next(aFieldPositionIterator, &beginIndexInt,
+                                        &endIndexInt)) >= 0) {
+    MOZ_ASSERT(beginIndexInt <= endIndexInt,
+               "field iterator returning invalid range");
+
+    size_t beginIndex = AssertedCast<size_t>(beginIndexInt);
+    size_t endIndex = AssertedCast<size_t>(endIndexInt);
+
+    // Technically this isn't guaranteed.  But it appears true in pratice,
+    // and http://bugs.icu-project.org/trac/ticket/12024 is expected to
+    // correct the documentation lapse.
+    MOZ_ASSERT(lastEndIndex <= beginIndex,
+               "field iteration didn't return fields in order start to "
+               "finish as expected");
+
+    DateTimePartType type =
+        ConvertUFormatFieldToPartType(static_cast<UDateFormatField>(fieldInt));
+    if (lastEndIndex < beginIndex) {
+      if (!AppendPart(DateTimePartType::Literal, beginIndex)) {
+        return Err(ICUError::InternalError);
+      }
+    }
+
+    if (!AppendPart(type, endIndex)) {
+      return Err(ICUError::InternalError);
+    }
+  }
+
+  // Append any final literal.
+  if (lastEndIndex < aSpanSize) {
+    if (!AppendPart(DateTimePartType::Literal, aSpanSize)) {
+      return Err(ICUError::InternalError);
+    }
+  }
+
+  return Ok();
+}
+
+}  // namespace mozilla::intl
diff --git a/intl/components/src/DateTimeFormat.h b/intl/components/src/DateTimeFormat.h
new file mode 100644
index 0000000000..4853d9e3b2
--- /dev/null
+++ b/intl/components/src/DateTimeFormat.h
@@ -0,0 +1,593 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+#ifndef intl_components_DateTimeFormat_h_
+#define intl_components_DateTimeFormat_h_
+#include <functional>
+#include "unicode/udat.h"
+
+#include "mozilla/Assertions.h"
+#include "mozilla/intl/ICU4CGlue.h"
+#include "mozilla/intl/ICUError.h"
+
+#include "mozilla/intl/DateTimePart.h"
+#include "mozilla/intl/DateTimePatternGenerator.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/Span.h"
+#include "mozilla/Try.h"
+#include "mozilla/UniquePtr.h"
+#include "mozilla/Utf8.h"
+#include "mozilla/Variant.h"
+#include "mozilla/Vector.h"
+
+/*
+ * To work around webcompat problems caused by Narrow No-Break Space in
+ * formatted date/time output, where existing code on the web naively
+ * assumes there will be a normal Space, we replace any occurrences of
+ * U+202F in the formatted results with U+0020.
+ *
+ * The intention is to undo this hack once other major browsers are also
+ * ready to ship with the updated (ICU72) i18n data that uses NNBSP.
+ *
+ * See https://bugzilla.mozilla.org/show_bug.cgi?id=1806042 for details,
+ * and see DateIntervalFormat.cpp for the other piece of this hack.
+ */
+#define DATE_TIME_FORMAT_REPLACE_SPECIAL_SPACES 1
+
+namespace mozilla::intl {
+
+#if DATE_TIME_FORMAT_REPLACE_SPECIAL_SPACES
+static inline bool IsSpecialSpace(char16_t c) {
+  // NARROW NO-BREAK SPACE and THIN SPACE
+  return c == 0x202F || c == 0x2009;
+}
+#endif
+
+class Calendar;
+
+/**
+ * Intro to mozilla::intl::DateTimeFormat
+ * ======================================
+ *
+ * This component is a Mozilla-focused API for the date formatting provided by
+ * ICU. The methods internally call out to ICU4C. This is responsible for and
+ * owns any resources opened through ICU, through RAII.
+ *
+ * The construction of a DateTimeFormat contains the majority of the cost
+ * of the DateTimeFormat operation. DateTimeFormat::TryFormat should be
+ * relatively inexpensive after the initial construction.
+ *
+ * This class supports creating from Styles (a fixed set of options) and from a
+ * components bag (a list of components and their lengths).
+ *
+ * This API serves to back the ECMA-402 Intl.DateTimeFormat API.
+ * https://tc39.es/ecma402/#datetimeformat-objects
+ *
+ *
+ * ECMA-402 Intl.DateTimeFormat API and implementation details with ICU
+ * skeletons and patterns.
+ * ====================================================================
+ *
+ * Different locales have different ways to display dates using the same
+ * basic components. For example, en-US might use "Sept. 24, 2012" while
+ * fr-FR might use "24 Sept. 2012". The intent of Intl.DateTimeFormat is to
+ * permit production of a format for the locale that best matches the
+ * set of date-time components and their desired representation as specified
+ * by the API client.
+ *
+ * ICU4C supports specification of date and time formats in three ways:
+ *
+ * 1) A style is just one of the identifiers FULL, LONG, MEDIUM, or SHORT.
+ *    The date-time components included in each style and their representation
+ *    are defined by ICU using CLDR locale data (CLDR is the Unicode
+ *    Consortium's Common Locale Data Repository).
+ *
+ * 2) A skeleton is a string specifying which date-time components to include,
+ *    and which representations to use for them. For example, "yyyyMMMMdd"
+ *    specifies a year with at least four digits, a full month name, and a
+ *    two-digit day. It does not specify in which order the components appear,
+ *    how they are separated, the localized strings for textual components
+ *    (such as weekday or month), whether the month is in format or
+ *    stand-alone form¹, or the numbering system used for numeric components.
+ *    All that information is filled in by ICU using CLDR locale data.
+ *    ¹ The format form is the one used in formatted strings that include a
+ *    day; the stand-alone form is used when not including days, e.g., in
+ *    calendar headers. The two forms differ at least in some Slavic languages,
+ *    e.g. Russian: "22 марта 2013 г." vs. "Март 2013".
+ *
+ * 3) A pattern is a string specifying which date-time components to include,
+ *    in which order, with which separators, in which grammatical case. For
+ *    example, "EEEE, d MMMM y" specifies the full localized weekday name,
+ *    followed by comma and space, followed by the day, followed by space,
+ *    followed by the full month name in format form, followed by space,
+ *    followed by the full year. It
+ *    still does not specify localized strings for textual components and the
+ *    numbering system - these are determined by ICU using CLDR locale data or
+ *    possibly API parameters.
+ *
+ * All actual formatting in ICU4C is done with patterns; styles and skeletons
+ * have to be mapped to patterns before processing.
+ *
+ * The options of Intl.DateTimeFormat most closely correspond to ICU skeletons.
+ * This implementation therefore converts DateTimeFormat options to ICU
+ * skeletons, and then lets ICU map skeletons to actual ICU patterns. The
+ * pattern may not directly correspond to what the skeleton requests, as the
+ * mapper (UDateTimePatternGenerator) is constrained by the available locale
+ * data for the locale.
+ *
+ * An ICU pattern represents the information of the following DateTimeFormat
+ * internal properties described in the specification, which therefore don't
+ * exist separately in the implementation:
+ * - [[weekday]], [[era]], [[year]], [[month]], [[day]], [[hour]], [[minute]],
+ *   [[second]], [[timeZoneName]]
+ * - [[hour12]]
+ * - [[hourCycle]]
+ * - [[hourNo0]]
+ * When needed for the resolvedOptions method, the resolveICUPattern function
+ * queries the UDateFormat's internal pattern and then maps the it back to the
+ * specified properties of the object returned by resolvedOptions.
+ *
+ * ICU date-time skeletons and patterns aren't fully documented in the ICU
+ * documentation (see http://bugs.icu-project.org/trac/ticket/9627). The best
+ * documentation at this point is in UTR 35:
+ * http://unicode.org/reports/tr35/tr35-dates.html#Date_Format_Patterns
+ *
+ * Future support for ICU4X
+ * ========================
+ * This implementation exposes a components bag, and internally handles the
+ * complexity of working with skeletons and patterns to generate the correct
+ * results. In the future, if and when we switch to ICU4X, the complexities of
+ * manipulating patterns will be able to be removed, as ICU4X will directly know
+ * how to apply the components bag.
+ */
+class DateTimeFormat final {
+ public:
+  /**
+   * The hour cycle for components.
+   */
+  enum class HourCycle {
+    H11,
+    H12,
+    H23,
+    H24,
+  };
+
+  /**
+   * The style for dates or times.
+   */
+  enum class Style {
+    Full,
+    Long,
+    Medium,
+    Short,
+  };
+
+  /**
+   * A bag of options to determine the length of the time and date styles. The
+   * hour cycle can be overridden.
+   */
+  struct StyleBag {
+    Maybe<Style> date = Nothing();
+    Maybe<Style> time = Nothing();
+    Maybe<HourCycle> hourCycle = Nothing();
+    Maybe<bool> hour12 = Nothing();
+  };
+
+  /**
+   * How to to display numeric components such as the year and the day.
+   */
+  enum class Numeric {
+    Numeric,
+    TwoDigit,
+  };
+
+  /**
+   * How to display the text components, such as the weekday or day period.
+   */
+  enum class Text {
+    Long,
+    Short,
+    Narrow,
+  };
+
+  /**
+   * How to display the month.
+   */
+  enum class Month {
+    Numeric,
+    TwoDigit,
+    Long,
+    Short,
+    Narrow,
+  };
+
+  /**
+   * How to display the time zone name.
+   */
+  enum class TimeZoneName {
+    Long,
+    Short,
+    ShortOffset,
+    LongOffset,
+    ShortGeneric,
+    LongGeneric,
+  };
+
+  /**
+   * Get static strings representing the enums. These match ECMA-402's resolved
+   * options.
+   * https://tc39.es/ecma402/#sec-intl.datetimeformat.prototype.resolvedoptions
+   */
+  static const char* ToString(DateTimeFormat::HourCycle aHourCycle);
+  static const char* ToString(DateTimeFormat::Style aStyle);
+  static const char* ToString(DateTimeFormat::Numeric aNumeric);
+  static const char* ToString(DateTimeFormat::Text aText);
+  static const char* ToString(DateTimeFormat::Month aMonth);
+  static const char* ToString(DateTimeFormat::TimeZoneName aTimeZoneName);
+
+  /**
+   * A components bag specifies the components used to display a DateTime. Each
+   * component can be styled individually, and ICU will attempt to create a best
+   * match for a given locale.
+   */
+  struct ComponentsBag {
+    Maybe<Text> era = Nothing();
+    Maybe<Numeric> year = Nothing();
+    Maybe<Month> month = Nothing();
+    Maybe<Numeric> day = Nothing();
+    Maybe<Text> weekday = Nothing();
+    Maybe<Numeric> hour = Nothing();
+    Maybe<Numeric> minute = Nothing();
+    Maybe<Numeric> second = Nothing();
+    Maybe<TimeZoneName> timeZoneName = Nothing();
+    Maybe<bool> hour12 = Nothing();
+    Maybe<HourCycle> hourCycle = Nothing();
+    Maybe<Text> dayPeriod = Nothing();
+    Maybe<uint8_t> fractionalSecondDigits = Nothing();
+  };
+
+  // Do not allow copy as this class owns the ICU resource. Move is not
+  // currently implemented, but a custom move operator could be created if
+  // needed.
+  DateTimeFormat(const DateTimeFormat&) = delete;
+  DateTimeFormat& operator=(const DateTimeFormat&) = delete;
+
+  // mozilla::Vector can avoid heap allocations for small transient buffers.
+  using PatternVector = Vector<char16_t, 128>;
+  using SkeletonVector = Vector<char16_t, 16>;
+
+  /**
+   * Create a DateTimeFormat from styles.
+   *
+   * The "style" model uses different options for formatting a date or time
+   * based on how the result will be styled, rather than picking specific
+   * fields or lengths.
+   *
+   * Takes an optional time zone which will override the user's default
+   * time zone. This is a UTF-16 string that takes the form "GMT±hh:mm", or
+   * an IANA time zone identifier, e.g. "America/Chicago".
+   */
+  static Result<UniquePtr<DateTimeFormat>, ICUError> TryCreateFromStyle(
+      Span<const char> aLocale, const StyleBag& aStyleBag,
+      DateTimePatternGenerator* aDateTimePatternGenerator,
+      Maybe<Span<const char16_t>> aTimeZoneOverride = Nothing{});
+
+ private:
+  /**
+   * Create a DateTimeFormat from a UTF-16 skeleton.
+   *
+   * A skeleton is an unordered list of fields that are used to find an
+   * appropriate date time format pattern. Example skeletons would be "yMd",
+   * "yMMMd", "EBhm". If the skeleton includes string literals or other
+   * information, it will be discarded when matching against skeletons.
+   *
+   * Takes an optional time zone which will override the user's default
+   * time zone. This is a string that takes the form "GMT±hh:mm", or
+   * an IANA time zone identifier, e.g. "America/Chicago".
+   */
+  static Result<UniquePtr<DateTimeFormat>, ICUError> TryCreateFromSkeleton(
+      Span<const char> aLocale, Span<const char16_t> aSkeleton,
+      DateTimePatternGenerator* aDateTimePatternGenerator,
+      Maybe<DateTimeFormat::HourCycle> aHourCycle,
+      Maybe<Span<const char16_t>> aTimeZoneOverride);
+
+ public:
+  /**
+   * Create a DateTimeFormat from a ComponentsBag.
+   *
+   * See the ComponentsBag for additional documentation.
+   *
+   * Takes an optional time zone which will override the user's default
+   * time zone. This is a string that takes the form "GMT±hh:mm", or
+   * an IANA time zone identifier, e.g. "America/Chicago".
+   */
+  static Result<UniquePtr<DateTimeFormat>, ICUError> TryCreateFromComponents(
+      Span<const char> aLocale, const ComponentsBag& bag,
+      DateTimePatternGenerator* aDateTimePatternGenerator,
+      Maybe<Span<const char16_t>> aTimeZoneOverride = Nothing{});
+
+  /**
+   * Create a DateTimeFormat from a raw pattern.
+   *
+   * Warning: This method should not be added to new code. In the near future we
+   * plan to remove it.
+   */
+  static Result<UniquePtr<DateTimeFormat>, ICUError> TryCreateFromPattern(
+      Span<const char> aLocale, Span<const char16_t> aPattern,
+      Maybe<Span<const char16_t>> aTimeZoneOverride = Nothing{});
+
+  /**
+   * Use the format settings to format a date time into a string. The non-null
+   * terminated string will be placed into the provided buffer. The idea behind
+   * this API is that the constructor is expensive, and then the format
+   * operation is cheap.
+   *
+   * aUnixEpoch is the number of milliseconds since 1 January 1970, UTC.
+   */
+  template <typename B>
+  ICUResult TryFormat(double aUnixEpoch, B& aBuffer) const {
+    static_assert(
+        std::is_same_v<typename B::CharType, unsigned char> ||
+            std::is_same_v<typename B::CharType, char> ||
+            std::is_same_v<typename B::CharType, char16_t>,
+        "The only buffer CharTypes supported by DateTimeFormat are char "
+        "(for UTF-8 support) and char16_t (for UTF-16 support).");
+
+    if constexpr (std::is_same_v<typename B::CharType, char> ||
+                  std::is_same_v<typename B::CharType, unsigned char>) {
+      // The output buffer is UTF-8, but ICU uses UTF-16 internally.
+
+      // Write the formatted date into the u16Buffer.
+      PatternVector u16Vec;
+
+      auto result = FillBufferWithICUCall(
+          u16Vec, [this, &aUnixEpoch](UChar* target, int32_t length,
+                                      UErrorCode* status) {
+            return udat_format(mDateFormat, aUnixEpoch, target, length,
+                               /* UFieldPosition* */ nullptr, status);
+          });
+      if (result.isErr()) {
+        return result;
+      }
+
+#if DATE_TIME_FORMAT_REPLACE_SPECIAL_SPACES
+      for (auto& c : u16Vec) {
+        if (IsSpecialSpace(c)) {
+          c = ' ';
+        }
+      }
+#endif
+
+      if (!FillBuffer(u16Vec, aBuffer)) {
+        return Err(ICUError::OutOfMemory);
+      }
+      return Ok{};
+    } else {
+      static_assert(std::is_same_v<typename B::CharType, char16_t>);
+
+      // The output buffer is UTF-16. ICU can output directly into this buffer.
+      auto result = FillBufferWithICUCall(
+          aBuffer, [&](UChar* target, int32_t length, UErrorCode* status) {
+            return udat_format(mDateFormat, aUnixEpoch, target, length, nullptr,
+                               status);
+          });
+      if (result.isErr()) {
+        return result;
+      }
+
+#if DATE_TIME_FORMAT_REPLACE_SPECIAL_SPACES
+      for (auto& c : Span(aBuffer.data(), aBuffer.length())) {
+        if (IsSpecialSpace(c)) {
+          c = ' ';
+        }
+      }
+#endif
+
+      return Ok{};
+    }
+  };
+
+  /**
+   * Format the Unix epoch time into a DateTimePartVector.
+   *
+   * The caller has to create the buffer and the vector and pass to this method.
+   * The formatted string will be stored in the buffer and formatted parts in
+   * the vector.
+   *
+   * aUnixEpoch is the number of milliseconds since 1 January 1970, UTC.
+   *
+   * See:
+   * https://tc39.es/ecma402/#sec-formatdatetimetoparts
+   */
+  template <typename B>
+  ICUResult TryFormatToParts(double aUnixEpoch, B& aBuffer,
+                             DateTimePartVector& aParts) const {
+    static_assert(std::is_same_v<typename B::CharType, char16_t>,
+                  "Only char16_t is supported (for UTF-16 support) now.");
+
+    UErrorCode status = U_ZERO_ERROR;
+    UFieldPositionIterator* fpositer = ufieldpositer_open(&status);
+    if (U_FAILURE(status)) {
+      return Err(ToICUError(status));
+    }
+
+    auto result = FillBufferWithICUCall(
+        aBuffer, [this, aUnixEpoch, fpositer](UChar* chars, int32_t size,
+                                              UErrorCode* status) {
+          return udat_formatForFields(mDateFormat, aUnixEpoch, chars, size,
+                                      fpositer, status);
+        });
+    if (result.isErr()) {
+      ufieldpositer_close(fpositer);
+      return result.propagateErr();
+    }
+
+#if DATE_TIME_FORMAT_REPLACE_SPECIAL_SPACES
+    for (auto& c : Span(aBuffer.data(), aBuffer.length())) {
+      if (IsSpecialSpace(c)) {
+        c = ' ';
+      }
+    }
+#endif
+
+    return TryFormatToParts(fpositer, aBuffer.length(), aParts);
+  }
+
+  /**
+   * Copies the pattern for the current DateTimeFormat to a buffer.
+   *
+   * Warning: This method should not be added to new code. In the near future we
+   * plan to remove it.
+   */
+  template <typename B>
+  ICUResult GetPattern(B& aBuffer) const {
+    return FillBufferWithICUCall(
+        aBuffer, [&](UChar* target, int32_t length, UErrorCode* status) {
+          return udat_toPattern(mDateFormat, /* localized*/ false, target,
+                                length, status);
+        });
+  }
+
+  /**
+   * Copies the skeleton that was used to generate the current DateTimeFormat to
+   * the given buffer. If no skeleton was used, then a skeleton is generated
+   * from the resolved pattern. Note that going from skeleton -> resolved
+   * pattern -> skeleton is not a 1:1 mapping, as the resolved pattern can
+   * contain different symbols than the requested skeleton.
+   *
+   * Warning: This method should not be added to new code. In the near future we
+   * plan to remove it.
+   */
+  template <typename B>
+  ICUResult GetOriginalSkeleton(B& aBuffer) {
+    static_assert(std::is_same_v<typename B::CharType, char16_t>);
+    if (mOriginalSkeleton.length() == 0) {
+      // Generate a skeleton from the resolved pattern, there was no originally
+      // cached skeleton.
+      PatternVector pattern{};
+      VectorToBufferAdaptor buffer(pattern);
+      MOZ_TRY(GetPattern(buffer));
+
+      VectorToBufferAdaptor skeleton(mOriginalSkeleton);
+      MOZ_TRY(DateTimePatternGenerator::GetSkeleton(pattern, skeleton));
+    }
+
+    if (!FillBuffer(mOriginalSkeleton, aBuffer)) {
+      return Err(ICUError::OutOfMemory);
+    }
+    return Ok();
+  }
+  /**
+   * Set the start time of the Gregorian calendar. This is useful for
+   * ensuring the consistent use of a proleptic Gregorian calendar for ECMA-402.
+   * https://en.wikipedia.org/wiki/Proleptic_Gregorian_calendar
+   */
+  void SetStartTimeIfGregorian(double aTime);
+
+  /**
+   * Determines the resolved components for the current DateTimeFormat.
+   *
+   * When a DateTimeFormat is created, even from a components bag, the resolved
+   * formatter may tweak the resolved components depending on the configuration
+   * and the locale.
+   *
+   * For the implementation, with ICU4C, this takes a string pattern and maps it
+   * back to a ComponentsBag.
+   */
+  Result<ComponentsBag, ICUError> ResolveComponents();
+
+  ~DateTimeFormat();
+
+  /**
+   * Clones the Calendar from a DateTimeFormat, and sets its time with the
+   * relative milliseconds since 1 January 1970, UTC.
+   */
+  Result<UniquePtr<Calendar>, ICUError> CloneCalendar(double aUnixEpoch) const;
+
+  /**
+   * Return the hour cycle used in the input pattern or Nothing if none was
+   * found.
+   */
+  static Maybe<DateTimeFormat::HourCycle> HourCycleFromPattern(
+      Span<const char16_t> aPattern);
+
+  using HourCyclesVector = Vector<HourCycle, 4>;
+
+  /**
+   * Returns the allowed hour cycles for the input locale.
+   *
+   * NOTE: This function currently takes a language subtag and an optional
+   * region subtag. This is a restriction until bug 1719746 has migrated
+   * language tag processing into the unified Intl component. After bug 1719746,
+   * this function should be changed to accept a single locale tag.
+   */
+  static Result<HourCyclesVector, ICUError> GetAllowedHourCycles(
+      Span<const char> aLanguage, Maybe<Span<const char>> aRegion);
+
+  /**
+   * Returns an iterator over all supported date-time formatter locales.
+   *
+   * The returned strings are ICU locale identifiers and NOT BCP 47 language
+   * tags.
+   *
+   * Also see <https://unicode-org.github.io/icu/userguide/locale>.
+   */
+  static auto GetAvailableLocales() {
+    return AvailableLocalesEnumeration<udat_countAvailable,
+                                       udat_getAvailable>();
+  }
+
+ private:
+  explicit DateTimeFormat(UDateFormat* aDateFormat);
+
+  ICUResult CacheSkeleton(Span<const char16_t> aSkeleton);
+
+  ICUResult TryFormatToParts(UFieldPositionIterator* aFieldPositionIterator,
+                             size_t aSpanSize,
+                             DateTimePartVector& aParts) const;
+  /**
+   * Replaces all hour pattern characters in |patternOrSkeleton| to use the
+   * matching hour representation for |hourCycle|.
+   */
+  static void ReplaceHourSymbol(Span<char16_t> aPatternOrSkeleton,
+                                DateTimeFormat::HourCycle aHourCycle);
+
+  /**
+   * Find a matching pattern using the requested hour-12 options.
+   *
+   * This function is needed to work around the following two issues.
+   * - https://unicode-org.atlassian.net/browse/ICU-21023
+   * - https://unicode-org.atlassian.net/browse/CLDR-13425
+   *
+   * We're currently using a relatively simple workaround, which doesn't give
+   * the most accurate results. For example:
+   *
+   * ```
+   * var dtf = new Intl.DateTimeFormat("en", {
+   *   timeZone: "UTC",
+   *   dateStyle: "long",
+   *   timeStyle: "long",
+   *   hourCycle: "h12",
+   * });
+   * print(dtf.format(new Date("2020-01-01T00:00Z")));
+   * ```
+   *
+   * Returns the pattern "MMMM d, y 'at' h:mm:ss a z", but when going through
+   * |DateTimePatternGenerator::GetSkeleton| and then
+   * |DateTimePatternGenerator::GetBestPattern| to find an equivalent pattern
+   * for "h23", we'll end up with the pattern "MMMM d, y, HH:mm:ss z", so the
+   * combinator element " 'at' " was lost in the process.
+   */
+  static ICUResult FindPatternWithHourCycle(
+      DateTimePatternGenerator& aDateTimePatternGenerator,
+      DateTimeFormat::PatternVector& aPattern, bool aHour12,
+      DateTimeFormat::SkeletonVector& aSkeleton);
+
+  UDateFormat* mDateFormat = nullptr;
+
+  SkeletonVector mOriginalSkeleton;
+};
+
+}  // namespace mozilla::intl
+
+#endif
diff --git a/intl/components/src/DateTimeFormatUtils.cpp b/intl/components/src/DateTimeFormatUtils.cpp
new file mode 100644
index 0000000000..fd0649461e
--- /dev/null
+++ b/intl/components/src/DateTimeFormatUtils.cpp
@@ -0,0 +1,104 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "mozilla/Assertions.h"
+
+#include "DateTimeFormatUtils.h"
+
+namespace mozilla::intl {
+
+DateTimePartType ConvertUFormatFieldToPartType(UDateFormatField fieldName) {
+  // See intl/icu/source/i18n/unicode/udat.h for a detailed field list.  This
+  // switch is deliberately exhaustive: cases might have to be added/removed
+  // if this code is compiled with a different ICU with more
+  // UDateFormatField enum initializers.  Please guard such cases with
+  // appropriate ICU version-testing #ifdefs, should cross-version divergence
+  // occur.
+  switch (fieldName) {
+    case UDAT_ERA_FIELD:
+      return DateTimePartType::Era;
+
+    case UDAT_YEAR_FIELD:
+    case UDAT_YEAR_WOY_FIELD:
+    case UDAT_EXTENDED_YEAR_FIELD:
+      return DateTimePartType::Year;
+
+    case UDAT_YEAR_NAME_FIELD:
+      return DateTimePartType::YearName;
+
+    case UDAT_MONTH_FIELD:
+    case UDAT_STANDALONE_MONTH_FIELD:
+      return DateTimePartType::Month;
+
+    case UDAT_DATE_FIELD:
+    case UDAT_JULIAN_DAY_FIELD:
+      return DateTimePartType::Day;
+
+    case UDAT_HOUR_OF_DAY1_FIELD:
+    case UDAT_HOUR_OF_DAY0_FIELD:
+    case UDAT_HOUR1_FIELD:
+    case UDAT_HOUR0_FIELD:
+      return DateTimePartType::Hour;
+
+    case UDAT_MINUTE_FIELD:
+      return DateTimePartType::Minute;
+
+    case UDAT_SECOND_FIELD:
+      return DateTimePartType::Second;
+
+    case UDAT_DAY_OF_WEEK_FIELD:
+    case UDAT_STANDALONE_DAY_FIELD:
+    case UDAT_DOW_LOCAL_FIELD:
+    case UDAT_DAY_OF_WEEK_IN_MONTH_FIELD:
+      return DateTimePartType::Weekday;
+
+    case UDAT_AM_PM_FIELD:
+    case UDAT_FLEXIBLE_DAY_PERIOD_FIELD:
+      return DateTimePartType::DayPeriod;
+
+    case UDAT_TIMEZONE_FIELD:
+    case UDAT_TIMEZONE_GENERIC_FIELD:
+    case UDAT_TIMEZONE_LOCALIZED_GMT_OFFSET_FIELD:
+      return DateTimePartType::TimeZoneName;
+
+    case UDAT_FRACTIONAL_SECOND_FIELD:
+      return DateTimePartType::FractionalSecondDigits;
+
+#ifndef U_HIDE_INTERNAL_API
+    case UDAT_RELATED_YEAR_FIELD:
+      return DateTimePartType::RelatedYear;
+#endif
+
+    case UDAT_DAY_OF_YEAR_FIELD:
+    case UDAT_WEEK_OF_YEAR_FIELD:
+    case UDAT_WEEK_OF_MONTH_FIELD:
+    case UDAT_MILLISECONDS_IN_DAY_FIELD:
+    case UDAT_TIMEZONE_RFC_FIELD:
+    case UDAT_QUARTER_FIELD:
+    case UDAT_STANDALONE_QUARTER_FIELD:
+    case UDAT_TIMEZONE_SPECIAL_FIELD:
+    case UDAT_TIMEZONE_ISO_FIELD:
+    case UDAT_TIMEZONE_ISO_LOCAL_FIELD:
+    case UDAT_AM_PM_MIDNIGHT_NOON_FIELD:
+#ifndef U_HIDE_INTERNAL_API
+    case UDAT_TIME_SEPARATOR_FIELD:
+#endif
+      // These fields are all unsupported.
+      return DateTimePartType::Unknown;
+
+#ifndef U_HIDE_DEPRECATED_API
+    case UDAT_FIELD_COUNT:
+      MOZ_ASSERT_UNREACHABLE(
+          "format field sentinel value returned by "
+          "iterator!");
+#endif
+  }
+
+  MOZ_ASSERT_UNREACHABLE(
+      "unenumerated, undocumented format field returned "
+      "by iterator");
+  return DateTimePartType::Unknown;
+}
+
+}  // namespace mozilla::intl
diff --git a/intl/components/src/DateTimeFormatUtils.h b/intl/components/src/DateTimeFormatUtils.h
new file mode 100644
index 0000000000..89187b9871
--- /dev/null
+++ b/intl/components/src/DateTimeFormatUtils.h
@@ -0,0 +1,14 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+#ifndef intl_components_DateTimeFormatUtils_h_
+#define intl_components_DateTimeFormatUtils_h_
+#include "unicode/udat.h"
+
+#include "mozilla/intl/DateTimePart.h"
+
+namespace mozilla::intl {
+DateTimePartType ConvertUFormatFieldToPartType(UDateFormatField fieldName);
+}  // namespace mozilla::intl
+
+#endif
diff --git a/intl/components/src/DateTimePart.h b/intl/components/src/DateTimePart.h
new file mode 100644
index 0000000000..4de2c22996
--- /dev/null
+++ b/intl/components/src/DateTimePart.h
@@ -0,0 +1,84 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+#ifndef intl_components_DateTimePart_h_
+#define intl_components_DateTimePart_h_
+
+#include <cstddef>
+#include <cstdint>
+
+#include "mozilla/Vector.h"
+
+namespace mozilla::intl {
+
+enum class DateTimePartType : int16_t {
+  Literal,
+  Weekday,
+  Era,
+  Year,
+  YearName,
+  RelatedYear,
+  Month,
+  Day,
+  DayPeriod,
+  Hour,
+  Minute,
+  Second,
+  FractionalSecondDigits,
+  TimeZoneName,
+  Unknown
+};
+
+enum class DateTimePartSource : int16_t { Shared, StartRange, EndRange };
+
+/**
+ * The 'Part' object defined in FormatDateTimeToParts and
+ * FormatDateTimeRangeToParts
+ *
+ * Each part consists of three properties: ||Type||, ||Value|| and ||Source||,
+ * with the ||Source|| property is set to DateTimePartSource::Shared by default.
+ * (Note: From the spec, the part from FormatDateTimeToParts doesn't have the
+ * ||Source|| property, so if the caller is FormatDateTimeToParts, it should
+ * ignore the ||Source|| property).
+ *
+ * To store DateTimePart more efficiently, it doesn't store the ||Value|| of
+ * type string in this struct. Instead, it stores the end index of the string
+ * in the buffer(which is passed to DateTimeFormat::TryFormatToParts() or
+ * can be got by calling AutoFormattedDateInterval::ToSpan()). The begin index
+ * of the ||Value|| is the mEndIndex of the previous part.
+ *
+ *  Buffer
+ *  0               i                j
+ * +---------------+---------------+---------------+
+ * | Part[0].Value | Part[1].Value | Part[2].Value | ....
+ * +---------------+---------------+---------------+
+ *
+ *     Part[0].mEndIndex is i. Part[0].Value is stored in the Buffer[0..i].
+ *     Part[1].mEndIndex is j. Part[1].Value is stored in the Buffer[i..j].
+ *
+ * See:
+ * https://tc39.es/ecma402/#sec-formatdatetimetoparts
+ * https://tc39.es/ecma402/#sec-formatdatetimerangetoparts
+ */
+struct DateTimePart {
+  DateTimePart(DateTimePartType type, size_t endIndex,
+               DateTimePartSource source)
+      : mEndIndex(endIndex), mType(type), mSource(source) {}
+
+  // See the above comments for details, mEndIndex is placed first for reducing
+  // padding.
+  size_t mEndIndex;
+  DateTimePartType mType;
+  DateTimePartSource mSource;
+};
+
+// The common parts are 'month', 'literal', 'day', 'literal', 'year', 'literal',
+// 'hour', 'literal', 'minute', 'literal', which are 10 parts, for DateTimeRange
+// the number will be doubled, so choosing 32 as the initial length to prevent
+// heap allocation.
+constexpr size_t INITIAL_DATETIME_PART_VECTOR_SIZE = 32;
+using DateTimePartVector =
+    mozilla::Vector<DateTimePart, INITIAL_DATETIME_PART_VECTOR_SIZE>;
+
+}  // namespace mozilla::intl
+#endif
diff --git a/intl/components/src/DateTimePatternGenerator.cpp b/intl/components/src/DateTimePatternGenerator.cpp
new file mode 100644
index 0000000000..4362061172
--- /dev/null
+++ b/intl/components/src/DateTimePatternGenerator.cpp
@@ -0,0 +1,49 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+#include "mozilla/intl/DateTimePatternGenerator.h"
+
+namespace mozilla::intl {
+
+DateTimePatternGenerator::~DateTimePatternGenerator() {
+  // The mGenerator will not exist when the DateTimePatternGenerator is being
+  // moved.
+  if (mGenerator) {
+    udatpg_close(mGenerator.GetMut());
+  }
+}
+
+/* static */
+Result<UniquePtr<DateTimePatternGenerator>, ICUError>
+DateTimePatternGenerator::TryCreate(const char* aLocale) {
+  UErrorCode status = U_ZERO_ERROR;
+  UDateTimePatternGenerator* generator =
+      udatpg_open(IcuLocale(aLocale), &status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+  return MakeUnique<DateTimePatternGenerator>(generator);
+};
+
+DateTimePatternGenerator::DateTimePatternGenerator(
+    DateTimePatternGenerator&& other) noexcept
+    : mGenerator(other.mGenerator.GetMut()) {
+  other.mGenerator = nullptr;
+}
+
+DateTimePatternGenerator& DateTimePatternGenerator::operator=(
+    DateTimePatternGenerator&& other) noexcept {
+  if (this == &other) {
+    return *this;
+  }
+
+  if (mGenerator) {
+    udatpg_close(mGenerator.GetMut());
+  }
+  mGenerator = other.mGenerator.GetMut();
+  other.mGenerator = nullptr;
+
+  return *this;
+}
+
+}  // namespace mozilla::intl
diff --git a/intl/components/src/DateTimePatternGenerator.h b/intl/components/src/DateTimePatternGenerator.h
new file mode 100644
index 0000000000..d9d6de3928
--- /dev/null
+++ b/intl/components/src/DateTimePatternGenerator.h
@@ -0,0 +1,161 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+#ifndef intl_components_DateTimePatternGenerator_h_
+#define intl_components_DateTimePatternGenerator_h_
+
+#include "unicode/udatpg.h"
+#include "mozilla/EnumSet.h"
+#include "mozilla/Result.h"
+#include "mozilla/Span.h"
+#include "mozilla/UniquePtr.h"
+#include "mozilla/intl/ICU4CGlue.h"
+#include "mozilla/intl/ICUError.h"
+
+namespace mozilla::intl {
+
+class DisplayNames;
+
+/**
+ * The DateTimePatternGenerator is the machinery used to work with DateTime
+ * pattern manipulation. It is expensive to create one, and so generally it is
+ * created once and then cached. It may be needed to be passed in as an argument
+ * for different mozilla::intl APIs.
+ */
+class DateTimePatternGenerator final {
+ public:
+  explicit DateTimePatternGenerator(UDateTimePatternGenerator* aGenerator)
+      : mGenerator(aGenerator) {
+    MOZ_ASSERT(aGenerator);
+  };
+
+  // Transfer ownership of the UDateTimePatternGenerator in the move
+  // constructor.
+  DateTimePatternGenerator(DateTimePatternGenerator&& other) noexcept;
+
+  // Transfer ownership of the UEnumeration in the move assignment operator.
+  DateTimePatternGenerator& operator=(
+      DateTimePatternGenerator&& other) noexcept;
+
+  // Disallow copy.
+  DateTimePatternGenerator(const DateTimePatternGenerator&) = delete;
+  DateTimePatternGenerator& operator=(const DateTimePatternGenerator&) = delete;
+
+  ~DateTimePatternGenerator();
+
+  static Result<UniquePtr<DateTimePatternGenerator>, ICUError> TryCreate(
+      const char* aLocale);
+
+  enum class PatternMatchOption {
+    /**
+     * Adjust the 'hour' field in the resolved pattern to match the input
+     * skeleton width.
+     */
+    HourField,
+
+    /**
+     * Adjust the 'minute' field in the resolved pattern to match the input
+     * skeleton width.
+     */
+    MinuteField,
+
+    /**
+     * Adjust the 'second' field in the resolved pattern to match the input
+     * skeleton width.
+     */
+    SecondField,
+  };
+
+  /**
+   * Given a skeleton (a string with unordered datetime fields), get a best
+   * pattern that will fit for that locale. This pattern will be filled into the
+   * buffer. e.g. The skeleton "yMd" would return the pattern "M/d/y" for en-US,
+   * or "dd/MM/y" for en-GB.
+   */
+  template <typename B>
+  ICUResult GetBestPattern(Span<const char16_t> aSkeleton, B& aBuffer,
+                           EnumSet<PatternMatchOption> options = {}) {
+    return FillBufferWithICUCall(
+        aBuffer, [&](UChar* target, int32_t length, UErrorCode* status) {
+          return udatpg_getBestPatternWithOptions(
+              mGenerator.GetMut(), aSkeleton.data(),
+              static_cast<int32_t>(aSkeleton.Length()),
+              toUDateTimePatternMatchOptions(options), target, length, status);
+        });
+  }
+
+  /**
+   * Get a skeleton (a string with unordered datetime fields) from a pattern.
+   * For example, both "MMM-dd" and "dd/MMM" produce the skeleton "MMMdd".
+   */
+  template <typename B>
+  static ICUResult GetSkeleton(Span<const char16_t> aPattern, B& aBuffer) {
+    // At one time udatpg_getSkeleton required a UDateTimePatternGenerator*, but
+    // now it is valid to pass in a nullptr.
+    return FillBufferWithICUCall(
+        aBuffer, [&](UChar* target, int32_t length, UErrorCode* status) {
+          return udatpg_getSkeleton(nullptr, aPattern.data(),
+                                    static_cast<int32_t>(aPattern.Length()),
+                                    target, length, status);
+        });
+  }
+
+  /**
+   * Get a pattern of the form "{1} {0}" to combine separate date and time
+   * patterns into a single pattern. The "{0}" part is the placeholder for the
+   * time pattern and "{1}" is the placeholder for the date pattern.
+   *
+   * See dateTimeFormat from
+   * https://unicode.org/reports/tr35/tr35-dates.html#dateTimeFormat
+   *
+   * Note:
+   * In CLDR, it's called Date-Time Combined Format
+   * https://cldr.unicode.org/translation/date-time/datetime-patterns#h.x7ca7qwzh4m
+   *
+   * The naming 'placeholder pattern' is from ICU4X.
+   * https://unicode-org.github.io/icu4x-docs/doc/icu_pattern/index.html
+   */
+  Span<const char16_t> GetPlaceholderPattern() const {
+    int32_t length;
+    const char16_t* combined =
+        udatpg_getDateTimeFormat(mGenerator.GetConst(), &length);
+    return Span{combined, static_cast<size_t>(length)};
+  }
+
+ private:
+  // Allow other mozilla::intl components to access the underlying
+  // UDateTimePatternGenerator.
+  friend class DisplayNames;
+
+  UDateTimePatternGenerator* GetUDateTimePatternGenerator() {
+    return mGenerator.GetMut();
+  }
+
+  ICUPointer<UDateTimePatternGenerator> mGenerator =
+      ICUPointer<UDateTimePatternGenerator>(nullptr);
+
+  static UDateTimePatternMatchOptions toUDateTimePatternMatchOptions(
+      EnumSet<PatternMatchOption> options) {
+    struct OptionMap {
+      PatternMatchOption from;
+      UDateTimePatternMatchOptions to;
+    } static constexpr map[] = {
+        {PatternMatchOption::HourField, UDATPG_MATCH_HOUR_FIELD_LENGTH},
+#ifndef U_HIDE_INTERNAL_API
+        {PatternMatchOption::MinuteField, UDATPG_MATCH_MINUTE_FIELD_LENGTH},
+        {PatternMatchOption::SecondField, UDATPG_MATCH_SECOND_FIELD_LENGTH},
+#endif
+    };
+
+    UDateTimePatternMatchOptions result = UDATPG_MATCH_NO_OPTIONS;
+    for (const auto& entry : map) {
+      if (options.contains(entry.from)) {
+        result = UDateTimePatternMatchOptions(result | entry.to);
+      }
+    }
+    return result;
+  }
+};
+
+}  // namespace mozilla::intl
+#endif
diff --git a/intl/components/src/DisplayNames.cpp b/intl/components/src/DisplayNames.cpp
new file mode 100644
index 0000000000..252969ccbb
--- /dev/null
+++ b/intl/components/src/DisplayNames.cpp
@@ -0,0 +1,234 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+#include "mozilla/intl/DisplayNames.h"
+#include "ScopedICUObject.h"
+
+namespace mozilla::intl {
+
+DisplayNames::~DisplayNames() {
+  // The mDisplayNames will not exist when the DisplayNames is being
+  // moved.
+  if (auto* uldn = mULocaleDisplayNames.GetMut()) {
+    uldn_close(uldn);
+  }
+}
+
+DisplayNamesError DisplayNames::ToError(ICUError aError) const {
+  switch (aError) {
+    case ICUError::InternalError:
+    case ICUError::OverflowError:
+      return DisplayNamesError::InternalError;
+    case ICUError::OutOfMemory:
+      return DisplayNamesError::OutOfMemory;
+  }
+  MOZ_ASSERT_UNREACHABLE();
+  return DisplayNamesError::InternalError;
+}
+
+DisplayNamesError DisplayNames::ToError(
+    Locale::CanonicalizationError aError) const {
+  switch (aError) {
+    case Locale::CanonicalizationError::DuplicateVariant:
+      return DisplayNamesError::DuplicateVariantSubtag;
+    case Locale::CanonicalizationError::InternalError:
+      return DisplayNamesError::InternalError;
+    case Locale::CanonicalizationError::OutOfMemory:
+      return DisplayNamesError::OutOfMemory;
+  }
+  MOZ_ASSERT_UNREACHABLE();
+  return DisplayNamesError::InternalError;
+}
+
+/* static */
+Result<UniquePtr<DisplayNames>, ICUError> DisplayNames::TryCreate(
+    const char* aLocale, Options aOptions) {
+  UErrorCode status = U_ZERO_ERROR;
+  UDisplayContext contexts[] = {
+      // Use either standard or dialect names.
+      // For example either "English (GB)" or "British English".
+      aOptions.languageDisplay == DisplayNames::LanguageDisplay::Standard
+          ? UDISPCTX_STANDARD_NAMES
+          : UDISPCTX_DIALECT_NAMES,
+
+      // Assume the display names are used in a stand-alone context.
+      UDISPCTX_CAPITALIZATION_FOR_STANDALONE,
+
+      // Select either the long or short form. There's no separate narrow form
+      // available in ICU, therefore we equate "narrow"/"short" styles here.
+      aOptions.style == DisplayNames::Style::Long ? UDISPCTX_LENGTH_FULL
+                                                  : UDISPCTX_LENGTH_SHORT,
+
+      // Don't apply substitutes, because we need to apply our own fallbacks.
+      UDISPCTX_NO_SUBSTITUTE,
+  };
+
+  const char* locale = IcuLocale(aLocale);
+
+  ULocaleDisplayNames* uLocaleDisplayNames =
+      uldn_openForContext(locale, contexts, std::size(contexts), &status);
+
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+  return MakeUnique<DisplayNames>(uLocaleDisplayNames, MakeStringSpan(locale),
+                                  aOptions);
+};
+
+#ifdef DEBUG
+static bool IsStandaloneMonth(UDateFormatSymbolType symbolType) {
+  switch (symbolType) {
+    case UDAT_STANDALONE_MONTHS:
+    case UDAT_STANDALONE_SHORT_MONTHS:
+    case UDAT_STANDALONE_NARROW_MONTHS:
+      return true;
+
+    case UDAT_ERAS:
+    case UDAT_MONTHS:
+    case UDAT_SHORT_MONTHS:
+    case UDAT_WEEKDAYS:
+    case UDAT_SHORT_WEEKDAYS:
+    case UDAT_AM_PMS:
+    case UDAT_LOCALIZED_CHARS:
+    case UDAT_ERA_NAMES:
+    case UDAT_NARROW_MONTHS:
+    case UDAT_NARROW_WEEKDAYS:
+    case UDAT_STANDALONE_WEEKDAYS:
+    case UDAT_STANDALONE_SHORT_WEEKDAYS:
+    case UDAT_STANDALONE_NARROW_WEEKDAYS:
+    case UDAT_QUARTERS:
+    case UDAT_SHORT_QUARTERS:
+    case UDAT_STANDALONE_QUARTERS:
+    case UDAT_STANDALONE_SHORT_QUARTERS:
+    case UDAT_SHORTER_WEEKDAYS:
+    case UDAT_STANDALONE_SHORTER_WEEKDAYS:
+    case UDAT_CYCLIC_YEARS_WIDE:
+    case UDAT_CYCLIC_YEARS_ABBREVIATED:
+    case UDAT_CYCLIC_YEARS_NARROW:
+    case UDAT_ZODIAC_NAMES_WIDE:
+    case UDAT_ZODIAC_NAMES_ABBREVIATED:
+    case UDAT_ZODIAC_NAMES_NARROW:
+    case UDAT_NARROW_QUARTERS:
+    case UDAT_STANDALONE_NARROW_QUARTERS:
+      return false;
+  }
+
+  MOZ_ASSERT_UNREACHABLE("unenumerated, undocumented symbol type");
+  return false;
+}
+#endif
+
+Result<Ok, DisplayNamesError> DisplayNames::ComputeDateTimeDisplayNames(
+    UDateFormatSymbolType symbolType, mozilla::Span<const int32_t> indices,
+    Span<const char> aCalendar) {
+  if (!mDateTimeDisplayNames.empty()) {
+    // No need to re-compute the display names.
+    return Ok();
+  }
+  mozilla::intl::Locale tag;
+  // Do not use mLocale.AsSpan() as it includes the null terminator inside the
+  // span.
+  if (LocaleParser::TryParse(Span(mLocale.Elements(), mLocale.Length() - 1),
+                             tag)
+          .isErr()) {
+    return Err(DisplayNamesError::InvalidLanguageTag);
+  }
+
+  if (!aCalendar.empty()) {
+    // Add the calendar extension to the locale. This is only available via
+    // the MozExtension.
+    Vector<char, 32> extension;
+    Span<const char> prefix = MakeStringSpan("u-ca-");
+    if (!extension.append(prefix.data(), prefix.size()) ||
+        !extension.append(aCalendar.data(), aCalendar.size())) {
+      return Err(DisplayNamesError::OutOfMemory);
+    }
+    // This overwrites any other Unicode extensions, but should be okay to do
+    // here.
+    if (auto result = tag.SetUnicodeExtension(extension); result.isErr()) {
+      return Err(ToError(result.unwrapErr()));
+    }
+  }
+
+  constexpr char16_t* timeZone = nullptr;
+  constexpr int32_t timeZoneLength = 0;
+
+  constexpr char16_t* pattern = nullptr;
+  constexpr int32_t patternLength = 0;
+
+  Vector<char, DisplayNames::LocaleVecLength> localeWithCalendar;
+  VectorToBufferAdaptor buffer(localeWithCalendar);
+  if (auto result = tag.ToString(buffer); result.isErr()) {
+    return Err(ToError(result.unwrapErr()));
+  }
+  if (!localeWithCalendar.append('\0')) {
+    return Err(DisplayNamesError::OutOfMemory);
+  }
+
+  UErrorCode status = U_ZERO_ERROR;
+  UDateFormat* fmt = udat_open(
+      UDAT_DEFAULT, UDAT_DEFAULT,
+      IcuLocale(
+          // IcuLocale takes a Span that does not include the null terminator.
+          Span(localeWithCalendar.begin(), localeWithCalendar.length() - 1)),
+      timeZone, timeZoneLength, pattern, patternLength, &status);
+  if (U_FAILURE(status)) {
+    return Err(DisplayNamesError::InternalError);
+  }
+  ScopedICUObject<UDateFormat, udat_close> datToClose(fmt);
+
+  Vector<char16_t, DisplayNames::LocaleVecLength> name;
+  for (int32_t index : indices) {
+    auto result = FillBufferWithICUCall(name, [&](UChar* target, int32_t length,
+                                                  UErrorCode* status) {
+      return udat_getSymbols(fmt, symbolType, index, target, length, status);
+    });
+    if (result.isErr()) {
+      return Err(ToError(result.unwrapErr()));
+    }
+
+    // Everything except Undecimber should always have a non-empty name.
+    MOZ_ASSERT_IF(!IsStandaloneMonth(symbolType) || index != UCAL_UNDECIMBER,
+                  !name.empty());
+
+    if (!mDateTimeDisplayNames.emplaceBack(Span(name.begin(), name.length()))) {
+      return Err(DisplayNamesError::OutOfMemory);
+    }
+  }
+  return Ok();
+}
+
+Span<const char> DisplayNames::ToCodeString(Month aMonth) {
+  switch (aMonth) {
+    case Month::January:
+      return MakeStringSpan("1");
+    case Month::February:
+      return MakeStringSpan("2");
+    case Month::March:
+      return MakeStringSpan("3");
+    case Month::April:
+      return MakeStringSpan("4");
+    case Month::May:
+      return MakeStringSpan("5");
+    case Month::June:
+      return MakeStringSpan("6");
+    case Month::July:
+      return MakeStringSpan("7");
+    case Month::August:
+      return MakeStringSpan("8");
+    case Month::September:
+      return MakeStringSpan("9");
+    case Month::October:
+      return MakeStringSpan("10");
+    case Month::November:
+      return MakeStringSpan("11");
+    case Month::December:
+      return MakeStringSpan("12");
+    case Month::Undecimber:
+      return MakeStringSpan("13");
+  }
+  MOZ_ASSERT_UNREACHABLE();
+  return MakeStringSpan("1");
+};
+
+}  // namespace mozilla::intl
diff --git a/intl/components/src/DisplayNames.h b/intl/components/src/DisplayNames.h
new file mode 100644
index 0000000000..ae519f61ce
--- /dev/null
+++ b/intl/components/src/DisplayNames.h
@@ -0,0 +1,971 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+#ifndef intl_components_DisplayNames_h_
+#define intl_components_DisplayNames_h_
+
+#include <string>
+#include <string_view>
+#include "unicode/udat.h"
+#include "unicode/udatpg.h"
+#include "unicode/uldnames.h"
+#include "unicode/uloc.h"
+#include "unicode/ucurr.h"
+#include "mozilla/intl/Calendar.h"
+#include "mozilla/intl/DateTimePatternGenerator.h"
+#include "mozilla/intl/ICU4CGlue.h"
+#include "mozilla/intl/Locale.h"
+#include "mozilla/Buffer.h"
+#include "mozilla/Casting.h"
+#include "mozilla/PodOperations.h"
+#include "mozilla/Result.h"
+#include "mozilla/Span.h"
+#include "mozilla/TextUtils.h"
+#include "mozilla/UniquePtr.h"
+
+namespace mozilla::intl {
+/**
+ * Provide more granular errors for DisplayNames rather than use the generic
+ * ICUError type. This helps with providing more actionable feedback for
+ * errors with input validation.
+ *
+ * This type can't be nested in the DisplayNames class because it needs the
+ * UnusedZero and HasFreeLSB definitions.
+ */
+enum class DisplayNamesError {
+  // Since we claim UnusedZero<DisplayNamesError>::value and
+  // HasFreeLSB<Error>::value == true below, we must only use positive,
+  // even enum values.
+  InternalError = 2,
+  OutOfMemory = 4,
+  InvalidOption = 6,
+  DuplicateVariantSubtag = 8,
+  InvalidLanguageTag = 10,
+};
+}  // namespace mozilla::intl
+
+namespace mozilla::detail {
+// Ensure the efficient packing of the error types into the result. See
+// ICUError.h and the ICUError comments for more information.
+template <>
+struct UnusedZero<intl::DisplayNamesError>
+    : UnusedZeroEnum<intl::DisplayNamesError> {};
+
+template <>
+struct HasFreeLSB<intl::DisplayNamesError> {
+  static constexpr bool value = true;
+};
+}  // namespace mozilla::detail
+
+namespace mozilla::intl {
+
+// NOTE: The UTF-35 canonical "code" value for months and quarters are 1-based
+// integers, so some of the following enums are 1-based for consistency with
+// that. For simplicity, we make all of the following enums 1-based, but use
+// `EnumToIndex` (see below) to convert to zero based if indexing into internal
+// (non-ICU) tables.
+
+/**
+ * Month choices for display names.
+ */
+enum class Month : uint8_t {
+  January = 1,
+  February,
+  March,
+  April,
+  May,
+  June,
+  July,
+  August,
+  September,
+  October,
+  November,
+  December,
+  // Some calendar systems feature a 13th month.
+  // https://en.wikipedia.org/wiki/Undecimber
+  Undecimber
+};
+
+/**
+ * Quarter choices for display names.
+ */
+enum class Quarter : uint8_t {
+  Q1 = 1,
+  Q2,
+  Q3,
+  Q4,
+};
+
+/**
+ * Day period choices for display names.
+ */
+enum class DayPeriod : uint8_t {
+  AM = 1,
+  PM,
+};
+
+/**
+ * DateTimeField choices for display names.
+ */
+enum class DateTimeField : uint8_t {
+  Era = 1,
+  Year,
+  Quarter,
+  Month,
+  WeekOfYear,
+  Weekday,
+  Day,
+  DayPeriod,
+  Hour,
+  Minute,
+  Second,
+  TimeZoneName,
+};
+
+/**
+ * DisplayNames provide a way to get the localized names of various types of
+ * information such as the names of the day of the week, months, currency etc.
+ *
+ * This class backs SpiderMonkeys implementation of Intl.DisplayNames
+ * https://tc39.es/ecma402/#intl-displaynames-objects
+ */
+class DisplayNames final {
+ public:
+  /**
+   * The style of the display name, specified by the amount of space available
+   * for displaying the text.
+   */
+  enum class Style {
+    Narrow,
+    Short,
+    Long,
+    // Note: Abbreviated is not part of ECMA-402, but it is available for
+    // internal Mozilla usage.
+    Abbreviated,
+  };
+
+  /**
+   * Use either standard or dialect names for the "Language" type.
+   */
+  enum class LanguageDisplay {
+    Standard,
+    Dialect,
+  };
+
+  /**
+   * Determines the fallback behavior if no match is found.
+   */
+  enum class Fallback {
+    // The buffer will contain an empty string.
+    None,
+    // The buffer will contain the code, but typically in a canonicalized form.
+    Code
+  };
+
+  /**
+   * These options correlate to the ECMA-402 DisplayNames options. The defaults
+   * values must match the default initialized values of ECMA-402. The type
+   * option is omitted as the C++ API relies on directly calling the
+   * DisplayNames::Get* methods.
+   *
+   * https://tc39.es/ecma402/#intl-displaynames-objects
+   * https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/DisplayNames
+   */
+  struct Options {
+    Style style = Style::Long;
+    LanguageDisplay languageDisplay = LanguageDisplay::Standard;
+  };
+
+  DisplayNames(ULocaleDisplayNames* aDisplayNames, Span<const char> aLocale,
+               Options aOptions)
+      : mOptions(aOptions), mULocaleDisplayNames(aDisplayNames) {
+    MOZ_ASSERT(aDisplayNames);
+
+    // Copy the span and ensure null termination.
+    mLocale = Buffer<char>(aLocale.size() + 1);
+    PodCopy(mLocale.begin(), aLocale.data(), aLocale.size());
+    mLocale[aLocale.size()] = '\0';
+  }
+
+  /**
+   * Initialize a new DisplayNames for the provided locale and using the
+   * provided options.
+   *
+   * https://tc39.es/ecma402/#sec-Intl.DisplayNames
+   */
+  static Result<UniquePtr<DisplayNames>, ICUError> TryCreate(
+      const char* aLocale, Options aOptions);
+
+  // Not copyable or movable
+  DisplayNames(const DisplayNames&) = delete;
+  DisplayNames& operator=(const DisplayNames&) = delete;
+
+  ~DisplayNames();
+
+  /**
+   * Easily convert to a more specific DisplayNames error.
+   */
+  DisplayNamesError ToError(ICUError aError) const;
+
+  /**
+   * Easily convert to a more specific DisplayNames error.
+   */
+  DisplayNamesError ToError(Locale::CanonicalizationError aError) const;
+
+ private:
+  /**
+   * A helper function to handle the fallback behavior, where if there is a
+   * fallback the buffer is filled with the "code", often in canonicalized form.
+   */
+  template <typename B, typename Fn>
+  static Result<Ok, DisplayNamesError> HandleFallback(B& aBuffer,
+                                                      Fallback aFallback,
+                                                      Fn aGetFallbackSpan) {
+    if (aBuffer.length() == 0 &&
+        aFallback == mozilla::intl::DisplayNames::Fallback::Code) {
+      if (!FillBuffer(aGetFallbackSpan(), aBuffer)) {
+        return Err(DisplayNamesError::OutOfMemory);
+      }
+    }
+    return Ok();
+  }
+
+  /**
+   * This is a specialized form of the FillBufferWithICUCall for DisplayNames.
+   * Different APIs report that no display name is found with different
+   * statuses. This method signals no display name was found by setting the
+   * buffer to 0.
+   *
+   * The display name APIs such as `uldn_scriptDisplayName`,
+   * `uloc_getDisplayScript`, and `uldn_regionDisplayName` report
+   * U_ILLEGAL_ARGUMENT_ERROR when no display name was found. In order to
+   * accomodate fallbacking, return an empty string in this case.
+   */
+  template <typename B, typename F>
+  static ICUResult FillBufferWithICUDisplayNames(
+      B& aBuffer, UErrorCode aNoDisplayNameStatus, F aCallback) {
+    return FillBufferWithICUCall(
+        aBuffer, [&](UChar* target, int32_t length, UErrorCode* status) {
+          int32_t res = aCallback(target, length, status);
+
+          if (*status == aNoDisplayNameStatus) {
+            *status = U_ZERO_ERROR;
+            res = 0;
+          }
+          return res;
+        });
+  }
+
+  /**
+   * An internal helper to compute the list of display names for various
+   * DateTime options.
+   */
+  Result<Ok, DisplayNamesError> ComputeDateTimeDisplayNames(
+      UDateFormatSymbolType symbolType, mozilla::Span<const int32_t> indices,
+      Span<const char> aCalendar);
+
+  // The following are the stack-allocated sizes for various strings using the
+  // mozilla::Vector. The numbers should be large enough to fit the common
+  // cases, and when the strings are too large they will fall back to heap
+  // allocations.
+
+  // Fit BCP 47 locales such as "en-US", "zh-Hant". Locales can get quite long,
+  // but 32 should fit most smaller locales without a lot of extensions.
+  static constexpr size_t LocaleVecLength = 32;
+  // Fit calendar names such as "gregory", "buddhist", "islamic-civil".
+  // "islamic-umalqura" is 16 bytes + 1 for null termination, so round up to 32.
+  static constexpr size_t CalendarVecLength = 32;
+
+  /**
+   * Given an ASCII alpha, convert it to upper case.
+   */
+  static inline char16_t AsciiAlphaToUpperCase(char16_t aCh) {
+    MOZ_ASSERT(IsAsciiAlpha(aCh));
+    return AsciiToUpperCase(aCh);
+  };
+
+  /**
+   * Attempt to use enums to safely index into an array.
+   *
+   * Note: The enums we support here are all defined starting from 1.
+   */
+  template <typename T>
+  inline int32_t EnumToIndex(size_t aSize, T aEnum) {
+    size_t index = static_cast<size_t>(aEnum) - 1;
+    MOZ_RELEASE_ASSERT(index < aSize,
+                       "Enum indexing mismatch for display names.");
+    return index;
+  }
+
+  /**
+   * Convert the month to a numeric code as a string.
+   */
+  static Span<const char> ToCodeString(Month aMonth);
+
+ public:
+  /**
+   * Get the localized name of a language. Part of ECMA-402.
+   *
+   * Accepts:
+   *  languageCode ["-" scriptCode] ["-" regionCode ] *("-" variant )
+   *  Where the language code is:
+   *    1. A two letters ISO 639-1 language code
+   *         https://en.wikipedia.org/wiki/List_of_ISO_639-1_codes
+   *    2. A three letters ISO 639-2 language code
+   *         https://en.wikipedia.org/wiki/List_of_ISO_639-2_codes
+   *
+   * Examples:
+   *  "es-ES" => "European Spanish" (en-US), "español de España" (es-ES)
+   *  "zh-Hant" => "Traditional Chinese" (en-US), "chino tradicional" (es-ES)
+   */
+  template <typename B>
+  Result<Ok, DisplayNamesError> GetLanguage(
+      B& aBuffer, Span<const char> aLanguage,
+      Fallback aFallback = Fallback::None) const {
+    static_assert(std::is_same<typename B::CharType, char16_t>::value);
+    mozilla::intl::Locale tag;
+    if (LocaleParser::TryParseBaseName(aLanguage, tag).isErr()) {
+      return Err(DisplayNamesError::InvalidOption);
+    }
+
+    {
+      // ICU always canonicalizes the input locale, but since we know that ICU's
+      // canonicalization is incomplete, we need to perform our own
+      // canonicalization to ensure consistent result.
+      auto result = tag.CanonicalizeBaseName();
+      if (result.isErr()) {
+        return Err(ToError(result.unwrapErr()));
+      }
+    }
+
+    Vector<char, DisplayNames::LocaleVecLength> tagVec;
+    {
+      VectorToBufferAdaptor tagBuffer(tagVec);
+      auto result = tag.ToString(tagBuffer);
+      if (result.isErr()) {
+        return Err(ToError(result.unwrapErr()));
+      }
+      if (!tagVec.append('\0')) {
+        // The tag should be null terminated.
+        return Err(DisplayNamesError::OutOfMemory);
+      }
+    }
+
+    auto result = FillBufferWithICUDisplayNames(
+        aBuffer, U_ILLEGAL_ARGUMENT_ERROR,
+        [&](UChar* target, int32_t length, UErrorCode* status) {
+          return uldn_localeDisplayName(mULocaleDisplayNames.GetConst(),
+                                        tagVec.begin(), target, length, status);
+        });
+    if (result.isErr()) {
+      return Err(ToError(result.unwrapErr()));
+    }
+
+    return HandleFallback(aBuffer, aFallback, [&] {
+      // Remove the null terminator.
+      return Span(tagVec.begin(), tagVec.length() - 1);
+    });
+  };
+
+  /**
+   * Get the localized name of a region. Part of ECMA-402.
+   *
+   * Accepts:
+   *  1. an ISO-3166 two letters:
+   *      https://www.iso.org/iso-3166-country-codes.html
+   *  2. region code, or a three digits UN M49 Geographic Regions.
+   *      https://unstats.un.org/unsd/methodology/m49/
+   *
+   * Examples
+   *  "US"  => "United States" (en-US), "Estados Unidos", (es-ES)
+   *  "158" => "Taiwan" (en-US), "Taiwán", (es-ES)
+   */
+  template <typename B>
+  Result<Ok, DisplayNamesError> GetRegion(
+      B& aBuffer, Span<const char> aCode,
+      Fallback aFallback = Fallback::None) const {
+    static_assert(std::is_same<typename B::CharType, char16_t>::value);
+
+    mozilla::intl::RegionSubtag region;
+    if (!IsStructurallyValidRegionTag(aCode)) {
+      return Err(DisplayNamesError::InvalidOption);
+    }
+    region.Set(aCode);
+
+    mozilla::intl::Locale tag;
+    tag.SetLanguage("und");
+    tag.SetRegion(region);
+
+    {
+      // ICU always canonicalizes the input locale, but since we know that ICU's
+      // canonicalization is incomplete, we need to perform our own
+      // canonicalization to ensure consistent result.
+      auto result = tag.CanonicalizeBaseName();
+      if (result.isErr()) {
+        return Err(ToError(result.unwrapErr()));
+      }
+    }
+
+    MOZ_ASSERT(tag.Region().Present());
+
+    // Note: ICU requires the region subtag to be in canonical case.
+    const mozilla::intl::RegionSubtag& canonicalRegion = tag.Region();
+
+    char regionChars[mozilla::intl::LanguageTagLimits::RegionLength + 1] = {};
+    std::copy_n(canonicalRegion.Span().data(), canonicalRegion.Length(),
+                regionChars);
+
+    auto result = FillBufferWithICUDisplayNames(
+        aBuffer, U_ILLEGAL_ARGUMENT_ERROR,
+        [&](UChar* chars, uint32_t size, UErrorCode* status) {
+          return uldn_regionDisplayName(
+              mULocaleDisplayNames.GetConst(), regionChars, chars,
+              AssertedCast<int32_t, uint32_t>(size), status);
+        });
+
+    if (result.isErr()) {
+      return Err(ToError(result.unwrapErr()));
+    }
+
+    return HandleFallback(aBuffer, aFallback, [&] {
+      region.ToUpperCase();
+      return region.Span();
+    });
+  }
+
+  /**
+   * Get the localized name of a currency. Part of ECMA-402.
+   *
+   * Accepts:
+   *   A 3-letter ISO 4217 currency code.
+   *   https://en.wikipedia.org/wiki/ISO_4217
+   *
+   * Examples:
+   *   "EUR" => "Euro" (en-US), "euro" (es_ES), "欧元", (zh)
+   *   "JPY" => "Japanese Yen" (en-US), "yen" (es_ES), "日元", (zh)
+   */
+  template <typename B>
+  Result<Ok, DisplayNamesError> GetCurrency(
+      B& aBuffer, Span<const char> aCurrency,
+      Fallback aFallback = Fallback::None) const {
+    static_assert(std::is_same<typename B::CharType, char16_t>::value);
+    if (aCurrency.size() != 3) {
+      return Err(DisplayNamesError::InvalidOption);
+    }
+
+    if (!mozilla::IsAsciiAlpha(aCurrency[0]) ||
+        !mozilla::IsAsciiAlpha(aCurrency[1]) ||
+        !mozilla::IsAsciiAlpha(aCurrency[2])) {
+      return Err(DisplayNamesError::InvalidOption);
+    }
+
+    // Normally this type of operation wouldn't be safe, but ASCII characters
+    // all take 1 byte in UTF-8 encoding, and can be zero padded to be valid
+    // UTF-16. Currency codes are all three ASCII letters.
+    char16_t currency[] = {static_cast<char16_t>(aCurrency[0]),
+                           static_cast<char16_t>(aCurrency[1]),
+                           static_cast<char16_t>(aCurrency[2]), u'\0'};
+
+    UCurrNameStyle style;
+    switch (mOptions.style) {
+      case Style::Long:
+        style = UCURR_LONG_NAME;
+        break;
+      case Style::Abbreviated:
+      case Style::Short:
+        style = UCURR_SYMBOL_NAME;
+        break;
+      case Style::Narrow:
+        style = UCURR_NARROW_SYMBOL_NAME;
+        break;
+    }
+
+    int32_t length = 0;
+    UErrorCode status = U_ZERO_ERROR;
+    const char16_t* name = ucurr_getName(currency, IcuLocale(mLocale), style,
+                                         nullptr, &length, &status);
+    if (U_FAILURE(status)) {
+      return Err(DisplayNamesError::InternalError);
+    }
+
+    if (status == U_USING_DEFAULT_WARNING) {
+      // A resource bundle lookup returned a result from the root locale.
+      if (aFallback == DisplayNames::Fallback::Code) {
+        // Return the canonicalized input when no localized currency name was
+        // found. Canonical case for currency is upper case.
+        if (!aBuffer.reserve(3)) {
+          return Err(DisplayNamesError::OutOfMemory);
+        }
+        aBuffer.data()[0] = AsciiAlphaToUpperCase(currency[0]);
+        aBuffer.data()[1] = AsciiAlphaToUpperCase(currency[1]);
+        aBuffer.data()[2] = AsciiAlphaToUpperCase(currency[2]);
+        aBuffer.written(3);
+      } else if (aBuffer.length() != 0) {
+        // Ensure an empty string is in the buffer when there is no fallback.
+        aBuffer.written(0);
+      }
+      return Ok();
+    }
+
+    if (!FillBuffer(Span(name, length), aBuffer)) {
+      return Err(DisplayNamesError::OutOfMemory);
+    }
+
+    return Ok();
+  }
+
+  /**
+   * Get the localized name of a script. Part of ECMA-402.
+   *
+   * Accepts:
+   *   ECMA-402 expects the ISO-15924 four letters script code.
+   *   https://unicode.org/iso15924/iso15924-codes.html
+   *   e.g. "Latn"
+   *
+   * Examples:
+   *   "Cher" => "Cherokee" (en-US), "cherokee" (es-ES)
+   *   "Latn" => "Latin" (en-US), "latino" (es-ES)
+   */
+  template <typename B>
+  Result<Ok, DisplayNamesError> GetScript(
+      B& aBuffer, Span<const char> aScript,
+      Fallback aFallback = Fallback::None) const {
+    static_assert(std::is_same<typename B::CharType, char16_t>::value);
+    mozilla::intl::ScriptSubtag script;
+    if (!IsStructurallyValidScriptTag(aScript)) {
+      return Err(DisplayNamesError::InvalidOption);
+    }
+    script.Set(aScript);
+
+    mozilla::intl::Locale tag;
+    tag.SetLanguage("und");
+
+    tag.SetScript(script);
+
+    {
+      // ICU always canonicalizes the input locale, but since we know that ICU's
+      // canonicalization is incomplete, we need to perform our own
+      // canonicalization to ensure consistent result.
+      auto result = tag.CanonicalizeBaseName();
+      if (result.isErr()) {
+        return Err(ToError(result.unwrapErr()));
+      }
+    }
+
+    MOZ_ASSERT(tag.Script().Present());
+    mozilla::Vector<char, DisplayNames::LocaleVecLength> tagString;
+    VectorToBufferAdaptor buffer(tagString);
+
+    switch (mOptions.style) {
+      case Style::Long: {
+        // |uldn_scriptDisplayName| doesn't use the stand-alone form for script
+        // subtags, so we're using |uloc_getDisplayScript| instead. (This only
+        // applies to the long form.)
+        //
+        // ICU bug: https://unicode-org.atlassian.net/browse/ICU-9301
+
+        // |uloc_getDisplayScript| expects a full locale identifier as its
+        // input.
+        if (auto result = tag.ToString(buffer); result.isErr()) {
+          return Err(ToError(result.unwrapErr()));
+        }
+
+        // Null terminate the tag string.
+        if (!tagString.append('\0')) {
+          return Err(DisplayNamesError::OutOfMemory);
+        }
+
+        auto result = FillBufferWithICUDisplayNames(
+            aBuffer, U_USING_DEFAULT_WARNING,
+            [&](UChar* target, int32_t length, UErrorCode* status) {
+              return uloc_getDisplayScript(tagString.begin(),
+                                           IcuLocale(mLocale), target, length,
+                                           status);
+            });
+
+        if (result.isErr()) {
+          return Err(ToError(result.unwrapErr()));
+        }
+        break;
+      }
+      case Style::Abbreviated:
+      case Style::Short:
+      case Style::Narrow: {
+        // Note: ICU requires the script subtag to be in canonical case.
+        const mozilla::intl::ScriptSubtag& canonicalScript = tag.Script();
+
+        char scriptChars[mozilla::intl::LanguageTagLimits::ScriptLength + 1] =
+            {};
+        MOZ_ASSERT(canonicalScript.Length() <=
+                   mozilla::intl::LanguageTagLimits::ScriptLength + 1);
+        std::copy_n(canonicalScript.Span().data(), canonicalScript.Length(),
+                    scriptChars);
+
+        auto result = FillBufferWithICUDisplayNames(
+            aBuffer, U_ILLEGAL_ARGUMENT_ERROR,
+            [&](UChar* target, int32_t length, UErrorCode* status) {
+              return uldn_scriptDisplayName(mULocaleDisplayNames.GetConst(),
+                                            scriptChars, target, length,
+                                            status);
+            });
+
+        if (result.isErr()) {
+          return Err(ToError(result.unwrapErr()));
+        }
+        break;
+      }
+    }
+
+    return HandleFallback(aBuffer, aFallback, [&] {
+      script.ToTitleCase();
+      return script.Span();
+    });
+  };
+
+  /**
+   * Get the localized name of a calendar.
+   * Part of Intl.DisplayNames V2. https://tc39.es/intl-displaynames-v2/
+   * Accepts:
+   *   Unicode calendar key:
+   *   https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/Locale/calendar#unicode_calendar_keys
+   */
+  template <typename B>
+  Result<Ok, DisplayNamesError> GetCalendar(
+      B& aBuffer, Span<const char> aCalendar,
+      Fallback aFallback = Fallback::None) const {
+    if (aCalendar.empty() || !IsAscii(aCalendar)) {
+      return Err(DisplayNamesError::InvalidOption);
+    }
+
+    if (LocaleParser::CanParseUnicodeExtensionType(aCalendar).isErr()) {
+      return Err(DisplayNamesError::InvalidOption);
+    }
+
+    // Convert into canonical case before searching for replacements.
+    Vector<char, DisplayNames::CalendarVecLength> lowerCaseCalendar;
+    for (size_t i = 0; i < aCalendar.size(); i++) {
+      if (!lowerCaseCalendar.append(AsciiToLowerCase(aCalendar[i]))) {
+        return Err(DisplayNamesError::OutOfMemory);
+      }
+    }
+    if (!lowerCaseCalendar.append('\0')) {
+      return Err(DisplayNamesError::OutOfMemory);
+    }
+
+    Span<const char> canonicalCalendar = mozilla::Span(
+        lowerCaseCalendar.begin(), lowerCaseCalendar.length() - 1);
+
+    // Search if there's a replacement for the Unicode calendar keyword.
+    {
+      Span<const char> key = mozilla::MakeStringSpan("ca");
+      Span<const char> type = canonicalCalendar;
+      if (const char* replacement =
+              mozilla::intl::Locale::ReplaceUnicodeExtensionType(key, type)) {
+        canonicalCalendar = MakeStringSpan(replacement);
+      }
+    }
+
+    // The input calendar name is user-controlled, so be extra cautious before
+    // passing arbitrarily large strings to ICU.
+    static constexpr size_t maximumCalendarLength = 100;
+
+    if (canonicalCalendar.size() <= maximumCalendarLength) {
+      // |uldn_keyValueDisplayName| expects old-style keyword values.
+      if (const char* legacyCalendar =
+              uloc_toLegacyType("calendar", canonicalCalendar.Elements())) {
+        auto result = FillBufferWithICUDisplayNames(
+            aBuffer, U_ILLEGAL_ARGUMENT_ERROR,
+            [&](UChar* chars, uint32_t size, UErrorCode* status) {
+              // |uldn_keyValueDisplayName| expects old-style keyword values.
+              return uldn_keyValueDisplayName(mULocaleDisplayNames.GetConst(),
+                                              "calendar", legacyCalendar, chars,
+                                              size, status);
+            });
+        if (result.isErr()) {
+          return Err(ToError(result.unwrapErr()));
+        }
+      } else {
+        aBuffer.written(0);
+      }
+    } else {
+      aBuffer.written(0);
+    }
+
+    return HandleFallback(aBuffer, aFallback,
+                          [&] { return canonicalCalendar; });
+  }
+
+  /**
+   * Get the localized name of a weekday. This is a MozExtension, and not
+   * currently part of ECMA-402.
+   */
+  template <typename B>
+  Result<Ok, DisplayNamesError> GetWeekday(
+      B& aBuffer, Weekday aWeekday, Span<const char> aCalendar,
+      Fallback aFallback = Fallback::None) {
+    // SpiderMonkey static casts the enum, so ensure it is correctly in range.
+    MOZ_ASSERT(aWeekday >= Weekday::Monday && aWeekday <= Weekday::Sunday);
+
+    UDateFormatSymbolType symbolType;
+    switch (mOptions.style) {
+      case DisplayNames::Style::Long:
+        symbolType = UDAT_STANDALONE_WEEKDAYS;
+        break;
+
+      case DisplayNames::Style::Abbreviated:
+        // ICU "short" is CLDR "abbreviated" format.
+        symbolType = UDAT_STANDALONE_SHORT_WEEKDAYS;
+        break;
+
+      case DisplayNames::Style::Short:
+        // ICU "shorter" is CLDR "short" format.
+        symbolType = UDAT_STANDALONE_SHORTER_WEEKDAYS;
+        break;
+
+      case DisplayNames::Style::Narrow:
+        symbolType = UDAT_STANDALONE_NARROW_WEEKDAYS;
+        break;
+    }
+
+    static constexpr int32_t indices[] = {
+        UCAL_MONDAY, UCAL_TUESDAY,  UCAL_WEDNESDAY, UCAL_THURSDAY,
+        UCAL_FRIDAY, UCAL_SATURDAY, UCAL_SUNDAY};
+
+    if (auto result = ComputeDateTimeDisplayNames(
+            symbolType, mozilla::Span(indices), aCalendar);
+        result.isErr()) {
+      return result.propagateErr();
+    }
+    MOZ_ASSERT(mDateTimeDisplayNames.length() == std::size(indices));
+
+    auto& name =
+        mDateTimeDisplayNames[EnumToIndex(std::size(indices), aWeekday)];
+    if (!FillBuffer(name.AsSpan(), aBuffer)) {
+      return Err(DisplayNamesError::OutOfMemory);
+    }
+
+    // There is no need to fallback, as invalid options are
+    // DisplayNamesError::InvalidOption.
+    return Ok();
+  }
+
+  /**
+   * Get the localized name of a month. This is a MozExtension, and not
+   * currently part of ECMA-402.
+   */
+  template <typename B>
+  Result<Ok, DisplayNamesError> GetMonth(B& aBuffer, Month aMonth,
+                                         Span<const char> aCalendar,
+                                         Fallback aFallback = Fallback::None) {
+    // SpiderMonkey static casts the enum, so ensure it is correctly in range.
+    MOZ_ASSERT(aMonth >= Month::January && aMonth <= Month::Undecimber);
+
+    UDateFormatSymbolType symbolType;
+    switch (mOptions.style) {
+      case DisplayNames::Style::Long:
+        symbolType = UDAT_STANDALONE_MONTHS;
+        break;
+
+      case DisplayNames::Style::Abbreviated:
+      case DisplayNames::Style::Short:
+        symbolType = UDAT_STANDALONE_SHORT_MONTHS;
+        break;
+
+      case DisplayNames::Style::Narrow:
+        symbolType = UDAT_STANDALONE_NARROW_MONTHS;
+        break;
+    }
+
+    static constexpr int32_t indices[] = {
+        UCAL_JANUARY,   UCAL_FEBRUARY, UCAL_MARCH,    UCAL_APRIL,
+        UCAL_MAY,       UCAL_JUNE,     UCAL_JULY,     UCAL_AUGUST,
+        UCAL_SEPTEMBER, UCAL_OCTOBER,  UCAL_NOVEMBER, UCAL_DECEMBER,
+        UCAL_UNDECIMBER};
+
+    if (auto result = ComputeDateTimeDisplayNames(
+            symbolType, mozilla::Span(indices), aCalendar);
+        result.isErr()) {
+      return result.propagateErr();
+    }
+    MOZ_ASSERT(mDateTimeDisplayNames.length() == std::size(indices));
+    auto& name = mDateTimeDisplayNames[EnumToIndex(std::size(indices), aMonth)];
+    if (!FillBuffer(Span(name.AsSpan()), aBuffer)) {
+      return Err(DisplayNamesError::OutOfMemory);
+    }
+
+    return HandleFallback(aBuffer, aFallback,
+                          [&] { return ToCodeString(aMonth); });
+  }
+
+  /**
+   * Get the localized name of a quarter. This is a MozExtension, and not
+   * currently part of ECMA-402.
+   */
+  template <typename B>
+  Result<Ok, DisplayNamesError> GetQuarter(
+      B& aBuffer, Quarter aQuarter, Span<const char> aCalendar,
+      Fallback aFallback = Fallback::None) {
+    // SpiderMonkey static casts the enum, so ensure it is correctly in range.
+    MOZ_ASSERT(aQuarter >= Quarter::Q1 && aQuarter <= Quarter::Q4);
+
+    UDateFormatSymbolType symbolType;
+    switch (mOptions.style) {
+      case DisplayNames::Style::Long:
+        symbolType = UDAT_STANDALONE_QUARTERS;
+        break;
+
+      case DisplayNames::Style::Abbreviated:
+      case DisplayNames::Style::Short:
+        symbolType = UDAT_STANDALONE_SHORT_QUARTERS;
+        break;
+
+      case DisplayNames::Style::Narrow:
+        symbolType = UDAT_STANDALONE_NARROW_QUARTERS;
+        break;
+    }
+
+    // ICU doesn't provide an enum for quarters.
+    static constexpr int32_t indices[] = {0, 1, 2, 3};
+
+    if (auto result = ComputeDateTimeDisplayNames(
+            symbolType, mozilla::Span(indices), aCalendar);
+        result.isErr()) {
+      return result.propagateErr();
+    }
+    MOZ_ASSERT(mDateTimeDisplayNames.length() == std::size(indices));
+
+    auto& name =
+        mDateTimeDisplayNames[EnumToIndex(std::size(indices), aQuarter)];
+    if (!FillBuffer(Span(name.AsSpan()), aBuffer)) {
+      return Err(DisplayNamesError::OutOfMemory);
+    }
+
+    // There is no need to fallback, as invalid options are
+    // DisplayNamesError::InvalidOption.
+    return Ok();
+  }
+
+  /**
+   * Get the localized name of a day period. This is a MozExtension, and not
+   * currently part of ECMA-402.
+   */
+  template <typename B>
+  Result<Ok, DisplayNamesError> GetDayPeriod(
+      B& aBuffer, DayPeriod aDayPeriod, Span<const char> aCalendar,
+      Fallback aFallback = Fallback::None) {
+    UDateFormatSymbolType symbolType = UDAT_AM_PMS;
+
+    static constexpr int32_t indices[] = {UCAL_AM, UCAL_PM};
+
+    if (auto result = ComputeDateTimeDisplayNames(
+            symbolType, mozilla::Span(indices), aCalendar);
+        result.isErr()) {
+      return result.propagateErr();
+    }
+    MOZ_ASSERT(mDateTimeDisplayNames.length() == std::size(indices));
+
+    auto& name =
+        mDateTimeDisplayNames[EnumToIndex(std::size(indices), aDayPeriod)];
+    if (!FillBuffer(name.AsSpan(), aBuffer)) {
+      return Err(DisplayNamesError::OutOfMemory);
+    }
+
+    // There is no need to fallback, as invalid options are
+    // DisplayNamesError::InvalidOption.
+    return Ok();
+  }
+
+  /**
+   * Get the localized name of a date time field.
+   * Part of Intl.DisplayNames V2. https://tc39.es/intl-displaynames-v2/
+   * Accepts:
+   *    "era", "year", "quarter", "month", "weekOfYear", "weekday", "day",
+   *    "dayPeriod", "hour", "minute", "second", "timeZoneName"
+   * Examples:
+   *   "weekday" => "day of the week"
+   *   "dayPeriod" => "AM/PM"
+   */
+  template <typename B>
+  Result<Ok, DisplayNamesError> GetDateTimeField(
+      B& aBuffer, DateTimeField aField,
+      DateTimePatternGenerator& aDateTimePatternGen,
+      Fallback aFallback = Fallback::None) {
+    UDateTimePatternField field;
+    switch (aField) {
+      case DateTimeField::Era:
+        field = UDATPG_ERA_FIELD;
+        break;
+      case DateTimeField::Year:
+        field = UDATPG_YEAR_FIELD;
+        break;
+      case DateTimeField::Quarter:
+        field = UDATPG_QUARTER_FIELD;
+        break;
+      case DateTimeField::Month:
+        field = UDATPG_MONTH_FIELD;
+        break;
+      case DateTimeField::WeekOfYear:
+        field = UDATPG_WEEK_OF_YEAR_FIELD;
+        break;
+      case DateTimeField::Weekday:
+        field = UDATPG_WEEKDAY_FIELD;
+        break;
+      case DateTimeField::Day:
+        field = UDATPG_DAY_FIELD;
+        break;
+      case DateTimeField::DayPeriod:
+        field = UDATPG_DAYPERIOD_FIELD;
+        break;
+      case DateTimeField::Hour:
+        field = UDATPG_HOUR_FIELD;
+        break;
+      case DateTimeField::Minute:
+        field = UDATPG_MINUTE_FIELD;
+        break;
+      case DateTimeField::Second:
+        field = UDATPG_SECOND_FIELD;
+        break;
+      case DateTimeField::TimeZoneName:
+        field = UDATPG_ZONE_FIELD;
+        break;
+    }
+
+    UDateTimePGDisplayWidth width;
+    switch (mOptions.style) {
+      case DisplayNames::Style::Long:
+        width = UDATPG_WIDE;
+        break;
+      case DisplayNames::Style::Abbreviated:
+      case DisplayNames::Style::Short:
+        width = UDATPG_ABBREVIATED;
+        break;
+      case DisplayNames::Style::Narrow:
+        width = UDATPG_NARROW;
+        break;
+    }
+
+    auto result = FillBufferWithICUCall(
+        aBuffer, [&](UChar* target, int32_t length, UErrorCode* status) {
+          return udatpg_getFieldDisplayName(
+              aDateTimePatternGen.GetUDateTimePatternGenerator(), field, width,
+              target, length, status);
+        });
+
+    if (result.isErr()) {
+      return Err(ToError(result.unwrapErr()));
+    }
+    // There is no need to fallback, as invalid options are
+    // DisplayNamesError::InvalidOption.
+    return Ok();
+  }
+
+  Options mOptions;
+  Buffer<char> mLocale;
+  Vector<Buffer<char16_t>> mDateTimeDisplayNames;
+  ICUPointer<ULocaleDisplayNames> mULocaleDisplayNames =
+      ICUPointer<ULocaleDisplayNames>(nullptr);
+};
+
+}  // namespace mozilla::intl
+
+#endif
diff --git a/intl/components/src/FormatBuffer.h b/intl/components/src/FormatBuffer.h
new file mode 100644
index 0000000000..774e74d2ba
--- /dev/null
+++ b/intl/components/src/FormatBuffer.h
@@ -0,0 +1,77 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef intl_components_FormatBuffer_h
+#define intl_components_FormatBuffer_h
+
+/**
+ * This file contains public adaptors for the mozilla::intl Buffer template
+ * argument. Adaptors that can automatically be deduced are kept as private
+ * in ICU4CGlue.h. There is also the SpiderMonkey specific adaptor
+ * js::intl::FormatBuffer in js/src/builtin/intl/FormatBuffer.h.
+ */
+
+#include "nsTString.h"
+
+namespace mozilla::intl {
+
+/**
+ * mozilla::intl APIs require sizeable buffers. This class abstracts over
+ * the nsTSubstring.
+ */
+template <typename T>
+class nsTStringToBufferAdapter {
+ public:
+  using CharType = T;
+
+  // Do not allow copy or move. Move could be added in the future if needed.
+  nsTStringToBufferAdapter(const nsTStringToBufferAdapter&) = delete;
+  nsTStringToBufferAdapter& operator=(const nsTStringToBufferAdapter&) = delete;
+
+  explicit nsTStringToBufferAdapter(nsTSubstring<CharType>& aString)
+      : mString(aString) {}
+
+  /**
+   * Ensures the buffer has enough space to accommodate |size| elements.
+   */
+  [[nodiscard]] bool reserve(size_t size) {
+    return mString.SetLength(size, fallible);
+  }
+
+  /**
+   * Returns the raw data inside the buffer.
+   */
+  CharType* data() { return mString.BeginWriting(); }
+
+  /**
+   * Returns the count of elements written into the buffer.
+   */
+  size_t length() const { return mString.Length(); }
+
+  /**
+   * Returns the buffer's overall capacity.
+   */
+  size_t capacity() const {
+    // nsString's Capacity() method is protected, so just return the length.
+    return mString.Length();
+  }
+
+  /**
+   * Resizes the buffer to the given amount of written elements.
+   */
+  void written(size_t amount) {
+    MOZ_ASSERT(amount <= mString.Length());
+    // This sets |mString|'s internal size so that it matches how much was
+    // written. This is necessary because the write happens across FFI
+    // boundaries.
+    mString.SetLength(amount);
+  }
+
+ private:
+  nsTSubstring<CharType>& mString;
+};
+
+}  // namespace mozilla::intl
+
+#endif /* intl_components_FormatBuffer_h */
diff --git a/intl/components/src/GeneralCategory.h b/intl/components/src/GeneralCategory.h
new file mode 100644
index 0000000000..99603ce4b9
--- /dev/null
+++ b/intl/components/src/GeneralCategory.h
@@ -0,0 +1,52 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+#ifndef intl_components_GeneralCategory_h_
+#define intl_components_GeneralCategory_h_
+
+#include <cstdint>
+
+namespace mozilla::intl {
+
+// See https://www.unicode.org/reports/tr44/#General_Category_Values
+// for details of these values.
+
+// The values here must match the values used by ICU's UCharCategory.
+
+enum class GeneralCategory : uint8_t {
+  Unassigned = 0,
+  Uppercase_Letter = 1,
+  Lowercase_Letter = 2,
+  Titlecase_Letter = 3,
+  Modifier_Letter = 4,
+  Other_Letter = 5,
+  Nonspacing_Mark = 6,
+  Enclosing_Mark = 7,
+  Spacing_Mark = 8,
+  Decimal_Number = 9,
+  Letter_Number = 10,
+  Other_Number = 11,
+  Space_Separator = 12,
+  Line_Separator = 13,
+  Paragraph_Separator = 14,
+  Control = 15,
+  Format = 16,
+  Private_Use = 17,
+  Surrogate = 18,
+  Dash_Punctuation = 19,
+  Open_Punctuation = 20,
+  Close_Punctuation = 21,
+  Connector_Punctuation = 22,
+  Other_Punctuation = 23,
+  Math_Symbol = 24,
+  Currency_Symbol = 25,
+  Modifier_Symbol = 26,
+  Other_Symbol = 27,
+  Initial_Punctuation = 28,
+  Final_Punctuation = 29,
+  GeneralCategoryCount
+};
+
+}  // namespace mozilla::intl
+
+#endif
diff --git a/intl/components/src/ICU4CGlue.cpp b/intl/components/src/ICU4CGlue.cpp
new file mode 100644
index 0000000000..6b9e0c0c58
--- /dev/null
+++ b/intl/components/src/ICU4CGlue.cpp
@@ -0,0 +1,44 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "mozilla/intl/ICU4CGlue.h"
+#include "unicode/uformattedvalue.h"
+
+namespace mozilla::intl {
+
+// Starting with ICU 59, UChar defaults to char16_t.
+static_assert(std::is_same_v<UChar, char16_t>,
+              "Gecko doesn't support redefining UChar to a different type");
+
+ICUError ToICUError(UErrorCode status) {
+  MOZ_ASSERT(!U_SUCCESS(status));
+  switch (status) {
+    case U_MEMORY_ALLOCATION_ERROR:
+      return ICUError::OutOfMemory;
+    default:
+      return ICUError::InternalError;
+  }
+}
+
+ICUResult ToICUResult(UErrorCode status) {
+  if (U_SUCCESS(status)) {
+    return Ok();
+  }
+  return Err(ToICUError(status));
+}
+
+// static
+Result<Span<const char16_t>, ICUError> FormattedResult::ToSpanImpl(
+    const UFormattedValue* value) {
+  UErrorCode status = U_ZERO_ERROR;
+  int32_t strLength;
+  const char16_t* str = ufmtval_getString(value, &strLength, &status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+
+  return Span{str, AssertedCast<size_t>(strLength)};
+}
+
+}  // namespace mozilla::intl
diff --git a/intl/components/src/ICU4CGlue.h b/intl/components/src/ICU4CGlue.h
new file mode 100644
index 0000000000..aad04a6ef6
--- /dev/null
+++ b/intl/components/src/ICU4CGlue.h
@@ -0,0 +1,723 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef intl_components_ICUUtils_h
+#define intl_components_ICUUtils_h
+
+#include "unicode/uenum.h"
+#include "unicode/utypes.h"
+#include "mozilla/Buffer.h"
+#include "mozilla/DebugOnly.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/Result.h"
+#include "mozilla/Span.h"
+#include "mozilla/Utf8.h"
+#include "mozilla/Vector.h"
+#include "mozilla/intl/ICUError.h"
+
+// When building standalone js shell, it will include headers from
+// intl/components if JS_HAS_INTL_API is true (the default value), but js shell
+// won't include headers from XPCOM, so don't include nsTArray.h when building
+// standalone js shell.
+#ifndef JS_STANDALONE
+#  include "nsTArray.h"
+#endif
+
+#include <cstring>
+#include <iterator>
+#include <stddef.h>
+#include <stdint.h>
+#include <string>
+#include <string_view>
+
+struct UFormattedValue;
+namespace mozilla::intl {
+
+template <typename CharType>
+static inline CharType* AssertNullTerminatedString(Span<CharType> aSpan) {
+  // Intentionally check one past the last character, because we expect that the
+  // NUL character isn't part of the string.
+  MOZ_ASSERT(*(aSpan.data() + aSpan.size()) == '\0');
+
+  // Also ensure there aren't any other NUL characters within the string.
+  MOZ_ASSERT(std::char_traits<std::remove_const_t<CharType>>::length(
+                 aSpan.data()) == aSpan.size());
+
+  return aSpan.data();
+}
+
+static inline const char* AssertNullTerminatedString(std::string_view aView) {
+  // Intentionally check one past the last character, because we expect that the
+  // NUL character isn't part of the string.
+  MOZ_ASSERT(*(aView.data() + aView.size()) == '\0');
+
+  // Also ensure there aren't any other NUL characters within the string.
+  MOZ_ASSERT(std::strlen(aView.data()) == aView.size());
+
+  return aView.data();
+}
+
+/**
+ * Map the "und" locale to an empty string, which ICU uses internally.
+ */
+static inline const char* IcuLocale(const char* aLocale) {
+  // Return the empty string if the input is exactly equal to the string "und".
+  const char* locale = aLocale;
+  if (!std::strcmp(locale, "und")) {
+    locale = "";  // ICU root locale
+  }
+  return locale;
+}
+
+/**
+ * Ensure a locale is null-terminated, and map the "und" locale to an empty
+ * string, which ICU uses internally.
+ */
+static inline const char* IcuLocale(Span<const char> aLocale) {
+  return IcuLocale(AssertNullTerminatedString(aLocale));
+}
+
+/**
+ * Ensure a locale in the buffer is null-terminated, and map the "und" locale to
+ * an empty string, which ICU uses internally.
+ */
+static inline const char* IcuLocale(const Buffer<char>& aLocale) {
+  return IcuLocale(Span(aLocale.begin(), aLocale.Length() - 1));
+}
+
+using ICUResult = Result<Ok, ICUError>;
+
+/**
+ * Convert a UErrorCode to ICUError. This will correctly apply the OutOfMemory
+ * case.
+ */
+ICUError ToICUError(UErrorCode status);
+
+/**
+ * Convert a UErrorCode to ICUResult. This will correctly apply the OutOfMemory
+ * case.
+ */
+ICUResult ToICUResult(UErrorCode status);
+
+/**
+ * The ICU status can complain about a string not being terminated, but this
+ * is fine for this API, as it deals with the mozilla::Span that has a pointer
+ * and a length.
+ */
+static inline bool ICUSuccessForStringSpan(UErrorCode status) {
+  return U_SUCCESS(status) || status == U_STRING_NOT_TERMINATED_WARNING;
+}
+
+/**
+ * This class enforces that the unified mozilla::intl methods match the
+ * const-ness of the underlying ICU4C API calls. const ICU4C APIs take a const
+ * pointer, while mutable ones take a non-const pointer.
+ *
+ * For const ICU4C calls use:
+ *   ICUPointer::GetConst().
+ *
+ * For non-const ICU4C calls use:
+ *   ICUPointer::GetMut().
+ *
+ * This will propagate the `const` specifier from the ICU4C API call to the
+ * unified method, and it will be enforced by the compiler. This helps ensures
+ * a consistence and correct implementation.
+ */
+template <typename T>
+class ICUPointer {
+ public:
+  explicit ICUPointer(T* aPointer) : mPointer(aPointer) {}
+
+  // Only allow moves of ICUPointers, no copies.
+  ICUPointer(ICUPointer&& other) noexcept = default;
+  ICUPointer& operator=(ICUPointer&& other) noexcept = default;
+
+  // Implicitly take ownership of a raw pointer through copy assignment.
+  ICUPointer& operator=(T* aPointer) noexcept {
+    mPointer = aPointer;
+    return *this;
+  };
+
+  const T* GetConst() const { return const_cast<const T*>(mPointer); }
+  T* GetMut() { return mPointer; }
+
+  explicit operator bool() const { return !!mPointer; }
+
+ private:
+  T* mPointer;
+};
+
+/**
+ * Calling into ICU with the C-API can be a bit tricky. This function wraps up
+ * the relatively risky operations involving pointers, lengths, and buffers into
+ * a simpler call. This function accepts a lambda that performs the ICU call,
+ * and returns the length of characters in the buffer. When using a temporary
+ * stack-based buffer, the calls can often be done in one trip. However, if
+ * additional memory is needed, this function will call the C-API twice, in
+ * order to first get the size of the result, and then second to copy the result
+ * over to the buffer.
+ */
+template <typename ICUStringFunction, typename Buffer>
+static ICUResult FillBufferWithICUCall(Buffer& buffer,
+                                       const ICUStringFunction& strFn) {
+  static_assert(std::is_same_v<typename Buffer::CharType, char16_t> ||
+                std::is_same_v<typename Buffer::CharType, char> ||
+                std::is_same_v<typename Buffer::CharType, uint8_t>);
+
+  UErrorCode status = U_ZERO_ERROR;
+  int32_t length = strFn(buffer.data(), buffer.capacity(), &status);
+  if (status == U_BUFFER_OVERFLOW_ERROR) {
+    MOZ_ASSERT(length >= 0);
+
+    if (!buffer.reserve(length)) {
+      return Err(ICUError::OutOfMemory);
+    }
+
+    status = U_ZERO_ERROR;
+    mozilla::DebugOnly<int32_t> length2 = strFn(buffer.data(), length, &status);
+    MOZ_ASSERT(length == length2);
+  }
+  if (!ICUSuccessForStringSpan(status)) {
+    return Err(ToICUError(status));
+  }
+
+  buffer.written(length);
+
+  return Ok{};
+}
+
+/**
+ * Adaptor for mozilla::Vector to implement the Buffer interface.
+ */
+template <typename T, size_t N>
+class VectorToBufferAdaptor {
+  mozilla::Vector<T, N>& vector;
+
+ public:
+  using CharType = T;
+
+  explicit VectorToBufferAdaptor(mozilla::Vector<T, N>& vector)
+      : vector(vector) {}
+
+  T* data() { return vector.begin(); }
+
+  size_t capacity() const { return vector.capacity(); }
+
+  bool reserve(size_t length) { return vector.reserve(length); }
+
+  void written(size_t length) {
+    mozilla::DebugOnly<bool> result = vector.resizeUninitialized(length);
+    MOZ_ASSERT(result);
+  }
+};
+
+/**
+ * An overload of FillBufferWithICUCall that accepts a mozilla::Vector rather
+ * than a Buffer.
+ */
+template <typename ICUStringFunction, size_t InlineSize, typename CharType>
+static ICUResult FillBufferWithICUCall(Vector<CharType, InlineSize>& vector,
+                                       const ICUStringFunction& strFn) {
+  VectorToBufferAdaptor buffer(vector);
+  return FillBufferWithICUCall(buffer, strFn);
+}
+
+#ifndef JS_STANDALONE
+/**
+ * mozilla::intl APIs require sizeable buffers. This class abstracts over
+ * the nsTArray.
+ */
+template <typename T>
+class nsTArrayToBufferAdapter {
+ public:
+  using CharType = T;
+
+  // Do not allow copy or move. Move could be added in the future if needed.
+  nsTArrayToBufferAdapter(const nsTArrayToBufferAdapter&) = delete;
+  nsTArrayToBufferAdapter& operator=(const nsTArrayToBufferAdapter&) = delete;
+
+  explicit nsTArrayToBufferAdapter(nsTArray<CharType>& aArray)
+      : mArray(aArray) {}
+
+  /**
+   * Ensures the buffer has enough space to accommodate |size| elements.
+   */
+  [[nodiscard]] bool reserve(size_t size) {
+    // Use fallible behavior here.
+    return mArray.SetCapacity(size, fallible);
+  }
+
+  /**
+   * Returns the raw data inside the buffer.
+   */
+  CharType* data() { return mArray.Elements(); }
+
+  /**
+   * Returns the count of elements written into the buffer.
+   */
+  size_t length() const { return mArray.Length(); }
+
+  /**
+   * Returns the buffer's overall capacity.
+   */
+  size_t capacity() const { return mArray.Capacity(); }
+
+  /**
+   * Resizes the buffer to the given amount of written elements.
+   */
+  void written(size_t amount) {
+    MOZ_ASSERT(amount <= mArray.Capacity());
+    // This sets |mArray|'s internal size so that it matches how much was
+    // written. This is necessary because the write happens across FFI
+    // boundaries.
+    mArray.SetLengthAndRetainStorage(amount);
+  }
+
+ private:
+  nsTArray<CharType>& mArray;
+};
+
+template <typename T, size_t N>
+class AutoTArrayToBufferAdapter : public nsTArrayToBufferAdapter<T> {
+  using nsTArrayToBufferAdapter<T>::nsTArrayToBufferAdapter;
+};
+
+/**
+ * An overload of FillBufferWithICUCall that accepts a nsTArray.
+ */
+template <typename ICUStringFunction, typename CharType>
+static ICUResult FillBufferWithICUCall(nsTArray<CharType>& array,
+                                       const ICUStringFunction& strFn) {
+  nsTArrayToBufferAdapter<CharType> buffer(array);
+  return FillBufferWithICUCall(buffer, strFn);
+}
+
+template <typename ICUStringFunction, typename CharType, size_t N>
+static ICUResult FillBufferWithICUCall(AutoTArray<CharType, N>& array,
+                                       const ICUStringFunction& strFn) {
+  AutoTArrayToBufferAdapter<CharType, N> buffer(array);
+  return FillBufferWithICUCall(buffer, strFn);
+}
+#endif
+
+/**
+ * Fill a UTF-8 or a UTF-16 buffer with a UTF-16 span. ICU4C mostly uses UTF-16
+ * internally, but different consumers may have different situations with their
+ * buffers.
+ */
+template <typename Buffer>
+[[nodiscard]] bool FillBuffer(Span<const char16_t> utf16Span,
+                              Buffer& targetBuffer) {
+  static_assert(std::is_same_v<typename Buffer::CharType, char> ||
+                std::is_same_v<typename Buffer::CharType, unsigned char> ||
+                std::is_same_v<typename Buffer::CharType, char16_t>);
+
+  if constexpr (std::is_same_v<typename Buffer::CharType, char> ||
+                std::is_same_v<typename Buffer::CharType, unsigned char>) {
+    if (utf16Span.Length() & mozilla::tl::MulOverflowMask<3>::value) {
+      // Tripling the size of the buffer overflows the size_t.
+      return false;
+    }
+
+    if (!targetBuffer.reserve(3 * utf16Span.Length())) {
+      return false;
+    }
+
+    size_t amount = ConvertUtf16toUtf8(
+        utf16Span, Span(reinterpret_cast<char*>(targetBuffer.data()),
+                        targetBuffer.capacity()));
+
+    targetBuffer.written(amount);
+  }
+  if constexpr (std::is_same_v<typename Buffer::CharType, char16_t>) {
+    size_t amount = utf16Span.Length();
+    if (!targetBuffer.reserve(amount)) {
+      return false;
+    }
+    for (size_t i = 0; i < amount; i++) {
+      targetBuffer.data()[i] = utf16Span[i];
+    }
+    targetBuffer.written(amount);
+  }
+
+  return true;
+}
+
+/**
+ * Fill a UTF-8 or a UTF-16 buffer with a UTF-8 span. ICU4C mostly uses UTF-16
+ * internally, but different consumers may have different situations with their
+ * buffers.
+ */
+template <typename Buffer>
+[[nodiscard]] bool FillBuffer(Span<const char> utf8Span, Buffer& targetBuffer) {
+  static_assert(std::is_same_v<typename Buffer::CharType, char> ||
+                std::is_same_v<typename Buffer::CharType, unsigned char> ||
+                std::is_same_v<typename Buffer::CharType, char16_t>);
+
+  if constexpr (std::is_same_v<typename Buffer::CharType, char> ||
+                std::is_same_v<typename Buffer::CharType, unsigned char>) {
+    size_t amount = utf8Span.Length();
+    if (!targetBuffer.reserve(amount)) {
+      return false;
+    }
+    for (size_t i = 0; i < amount; i++) {
+      targetBuffer.data()[i] =
+          // Static cast in case of a mismatch between `unsigned char` and
+          // `char`
+          static_cast<typename Buffer::CharType>(utf8Span[i]);
+    }
+    targetBuffer.written(amount);
+  }
+  if constexpr (std::is_same_v<typename Buffer::CharType, char16_t>) {
+    if (!targetBuffer.reserve(utf8Span.Length() + 1)) {
+      return false;
+    }
+
+    size_t amount = ConvertUtf8toUtf16(
+        utf8Span, Span(targetBuffer.data(), targetBuffer.capacity()));
+
+    targetBuffer.written(amount);
+  }
+
+  return true;
+}
+
+/**
+ * It is convenient for callers to be able to pass in UTF-8 strings to the API.
+ * This function can be used to convert that to a stack-allocated UTF-16
+ * mozilla::Vector that can then be passed into ICU calls. The string will be
+ * null terminated.
+ */
+template <size_t StackSize>
+[[nodiscard]] static bool FillUTF16Vector(
+    Span<const char> utf8Span,
+    mozilla::Vector<char16_t, StackSize>& utf16TargetVec) {
+  // Per ConvertUtf8toUtf16: The length of aDest must be at least one greater
+  // than the length of aSource. This additional length will be used for null
+  // termination.
+  if (!utf16TargetVec.reserve(utf8Span.Length() + 1)) {
+    return false;
+  }
+
+  // ConvertUtf8toUtf16 fills the buffer with the data, but the length of the
+  // vector is unchanged.
+  size_t length = ConvertUtf8toUtf16(
+      utf8Span, Span(utf16TargetVec.begin(), utf16TargetVec.capacity()));
+
+  // Assert that the last element is free for writing a null terminator.
+  MOZ_ASSERT(length < utf16TargetVec.capacity());
+  utf16TargetVec.begin()[length] = '\0';
+
+  // The call to resizeUninitialized notifies the vector of how much was written
+  // exclusive of the null terminated character.
+  return utf16TargetVec.resizeUninitialized(length);
+}
+
+/**
+ * An iterable class that wraps calls to the ICU UEnumeration C API.
+ *
+ * Usage:
+ *
+ *  // Make sure the range expression is non-temporary, otherwise there is a
+ *  // risk of undefined behavior:
+ *  auto result = Calendar::GetBcp47KeywordValuesForLocale("en-US");
+ *
+ *  for (auto name : result.unwrap()) {
+ *    MOZ_ASSERT(name.unwrap(), "An iterable value exists".);
+ *  }
+ */
+template <typename CharType, typename T, T(Mapper)(const CharType*, int32_t)>
+class Enumeration {
+ public:
+  class Iterator;
+  friend class Iterator;
+
+  // Transfer ownership of the UEnumeration in the move constructor.
+  Enumeration(Enumeration&& other) noexcept
+      : mUEnumeration(other.mUEnumeration) {
+    other.mUEnumeration = nullptr;
+  }
+
+  // Transfer ownership of the UEnumeration in the move assignment operator.
+  Enumeration& operator=(Enumeration&& other) noexcept {
+    if (this == &other) {
+      return *this;
+    }
+    if (mUEnumeration) {
+      uenum_close(mUEnumeration);
+    }
+    mUEnumeration = other.mUEnumeration;
+    other.mUEnumeration = nullptr;
+    return *this;
+  }
+
+  class Iterator {
+    Enumeration& mEnumeration;
+    // `Nothing` signifies that no enumeration has been loaded through ICU yet.
+    Maybe<int32_t> mIteration = Nothing{};
+    const CharType* mNext = nullptr;
+    int32_t mNextLength = 0;
+
+   public:
+    using value_type = const CharType*;
+    using reference = T;
+    using iterator_category = std::input_iterator_tag;
+
+    explicit Iterator(Enumeration& aEnumeration, bool aIsBegin)
+        : mEnumeration(aEnumeration) {
+      if (aIsBegin) {
+        AdvanceUEnum();
+      }
+    }
+
+    Iterator& operator++() {
+      AdvanceUEnum();
+      return *this;
+    }
+
+    Iterator operator++(int) {
+      Iterator retval = *this;
+      ++(*this);
+      return retval;
+    }
+
+    bool operator==(Iterator other) const {
+      return mIteration == other.mIteration;
+    }
+
+    bool operator!=(Iterator other) const { return !(*this == other); }
+
+    T operator*() const {
+      // Map the iterated value to something new.
+      return Mapper(mNext, mNextLength);
+    }
+
+   private:
+    void AdvanceUEnum() {
+      if (mIteration.isNothing()) {
+        mIteration = Some(-1);
+      }
+      UErrorCode status = U_ZERO_ERROR;
+      if constexpr (std::is_same_v<CharType, char16_t>) {
+        mNext = uenum_unext(mEnumeration.mUEnumeration, &mNextLength, &status);
+      } else {
+        static_assert(std::is_same_v<CharType, char>,
+                      "Only char16_t and char are supported by "
+                      "mozilla::intl::Enumeration.");
+        mNext = uenum_next(mEnumeration.mUEnumeration, &mNextLength, &status);
+      }
+      if (U_FAILURE(status)) {
+        mNext = nullptr;
+      }
+
+      if (mNext) {
+        (*mIteration)++;
+      } else {
+        // The iterator is complete.
+        mIteration = Nothing{};
+      }
+    }
+  };
+
+  Iterator begin() { return Iterator(*this, true); }
+  Iterator end() { return Iterator(*this, false); }
+
+  explicit Enumeration(UEnumeration* aUEnumeration)
+      : mUEnumeration(aUEnumeration) {}
+
+  ~Enumeration() {
+    if (mUEnumeration) {
+      // Only close when the object is being destructed, not moved.
+      uenum_close(mUEnumeration);
+    }
+  }
+
+ private:
+  UEnumeration* mUEnumeration = nullptr;
+};
+
+template <typename CharType>
+Result<Span<const CharType>, InternalError> SpanMapper(const CharType* string,
+                                                       int32_t length) {
+  // Return the raw value from this Iterator.
+  if (string == nullptr) {
+    return Err(InternalError{});
+  }
+  MOZ_ASSERT(length >= 0);
+  return Span<const CharType>(string, static_cast<size_t>(length));
+}
+
+template <typename CharType>
+using SpanResult = Result<Span<const CharType>, InternalError>;
+
+template <typename CharType>
+using SpanEnumeration = Enumeration<CharType, SpanResult<CharType>, SpanMapper>;
+
+/**
+ * An iterable class that wraps calls to ICU's available locales API.
+ */
+template <int32_t(CountAvailable)(), const char*(GetAvailable)(int32_t)>
+class AvailableLocalesEnumeration final {
+  // The overall count of available locales.
+  int32_t mLocalesCount = 0;
+
+ public:
+  AvailableLocalesEnumeration() { mLocalesCount = CountAvailable(); }
+
+  class Iterator {
+   public:
+    // std::iterator traits.
+    using iterator_category = std::input_iterator_tag;
+    using value_type = const char*;
+    using difference_type = ptrdiff_t;
+    using pointer = value_type*;
+    using reference = value_type&;
+
+   private:
+    // The current position in the list of available locales.
+    int32_t mLocalesPos = 0;
+
+   public:
+    explicit Iterator(int32_t aLocalesPos) : mLocalesPos(aLocalesPos) {}
+
+    Iterator& operator++() {
+      mLocalesPos++;
+      return *this;
+    }
+
+    Iterator operator++(int) {
+      Iterator result = *this;
+      ++(*this);
+      return result;
+    }
+
+    bool operator==(const Iterator& aOther) const {
+      return mLocalesPos == aOther.mLocalesPos;
+    }
+
+    bool operator!=(const Iterator& aOther) const { return !(*this == aOther); }
+
+    value_type operator*() const { return GetAvailable(mLocalesPos); }
+  };
+
+  // std::iterator begin() and end() methods.
+
+  /**
+   * Return an iterator pointing to the first available locale.
+   */
+  Iterator begin() const { return Iterator(0); }
+
+  /**
+   * Return an iterator pointing to one past the last available locale.
+   */
+  Iterator end() const { return Iterator(mLocalesCount); }
+};
+
+/**
+ * A helper class to wrap calling ICU function in cpp file so we don't have to
+ * include the ICU header here.
+ */
+class FormattedResult {
+ protected:
+  static Result<Span<const char16_t>, ICUError> ToSpanImpl(
+      const UFormattedValue* value);
+};
+
+/**
+ * A RAII class to hold the formatted value of format result.
+ *
+ * The caller will need to create this AutoFormattedResult on the stack, with
+ * the following parameters:
+ * 1. Native ICU type.
+ * 2. An ICU function which opens the result.
+ * 3. An ICU function which can get the result as UFormattedValue.
+ * 4. An ICU function which closes the result.
+ *
+ * After the object is created, caller needs to call IsValid() method to check
+ * if the native object has been created properly, and then passes this
+ * object to other format interfaces.
+ * The format result will be stored in this object, the caller can use ToSpan()
+ * method to get the formatted string.
+ *
+ * The methods GetFormatted() and Value() are private methods since they expose
+ * native ICU types. If the caller wants to call these methods, the caller needs
+ * to register itself as a friend class in AutoFormattedResult.
+ *
+ * The formatted value and the native ICU object will be released once this
+ * class is destructed.
+ */
+template <typename T, T*(Open)(UErrorCode*),
+          const UFormattedValue*(GetValue)(const T*, UErrorCode*),
+          void(Close)(T*)>
+class MOZ_RAII AutoFormattedResult : FormattedResult {
+ public:
+  AutoFormattedResult() {
+    mFormatted = Open(&mError);
+    if (U_FAILURE(mError)) {
+      mFormatted = nullptr;
+    }
+  }
+  ~AutoFormattedResult() {
+    if (mFormatted) {
+      Close(mFormatted);
+    }
+  }
+
+  AutoFormattedResult(const AutoFormattedResult& other) = delete;
+  AutoFormattedResult& operator=(const AutoFormattedResult& other) = delete;
+
+  AutoFormattedResult(AutoFormattedResult&& other) = delete;
+  AutoFormattedResult& operator=(AutoFormattedResult&& other) = delete;
+
+  /**
+   * Check if the native UFormattedDateInterval was created successfully.
+   */
+  bool IsValid() const { return !!mFormatted; }
+
+  /**
+   *  Get error code if IsValid() returns false.
+   */
+  ICUError GetError() const { return ToICUError(mError); }
+
+  /**
+   * Get the formatted result.
+   */
+  Result<Span<const char16_t>, ICUError> ToSpan() const {
+    if (!IsValid()) {
+      return Err(GetError());
+    }
+
+    const UFormattedValue* value = Value();
+    if (!value) {
+      return Err(ICUError::InternalError);
+    }
+
+    return ToSpanImpl(value);
+  }
+
+ private:
+  friend class DateIntervalFormat;
+  friend class ListFormat;
+  T* GetFormatted() const { return mFormatted; }
+
+  const UFormattedValue* Value() const {
+    if (!IsValid()) {
+      return nullptr;
+    }
+
+    UErrorCode status = U_ZERO_ERROR;
+    const UFormattedValue* value = GetValue(mFormatted, &status);
+    if (U_FAILURE(status)) {
+      return nullptr;
+    }
+
+    return value;
+  };
+
+  T* mFormatted = nullptr;
+  UErrorCode mError = U_ZERO_ERROR;
+};
+}  // namespace mozilla::intl
+
+#endif /* intl_components_ICUUtils_h */
diff --git a/intl/components/src/ICU4CLibrary.cpp b/intl/components/src/ICU4CLibrary.cpp
new file mode 100644
index 0000000000..d13bc40ad4
--- /dev/null
+++ b/intl/components/src/ICU4CLibrary.cpp
@@ -0,0 +1,41 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "mozilla/intl/ICU4CLibrary.h"
+
+#include "unicode/putil.h"
+#include "unicode/uclean.h"
+#include "unicode/utypes.h"
+#include "unicode/uversion.h"
+
+namespace mozilla::intl {
+
+ICUResult ICU4CLibrary::Initialize() {
+#if !MOZ_SYSTEM_ICU
+  // Explicitly set the data directory to its default value, but only when we're
+  // sure that we use our in-tree ICU copy. See bug 1527879 and ICU bug
+  // report <https://unicode-org.atlassian.net/browse/ICU-20491>.
+  u_setDataDirectory("");
+#endif
+
+  UErrorCode status = U_ZERO_ERROR;
+  u_init(&status);
+  return ToICUResult(status);
+}
+
+void ICU4CLibrary::Cleanup() { u_cleanup(); }
+
+ICUResult ICU4CLibrary::SetMemoryFunctions(MemoryFunctions aMemoryFunctions) {
+  UErrorCode status = U_ZERO_ERROR;
+  u_setMemoryFunctions(/* context = */ nullptr, aMemoryFunctions.mAllocFn,
+                       aMemoryFunctions.mReallocFn, aMemoryFunctions.mFreeFn,
+                       &status);
+  return ToICUResult(status);
+}
+
+Span<const char> ICU4CLibrary::GetVersion() {
+  return MakeStringSpan(U_ICU_VERSION);
+}
+
+}  // namespace mozilla::intl
diff --git a/intl/components/src/ICU4CLibrary.h b/intl/components/src/ICU4CLibrary.h
new file mode 100644
index 0000000000..67cd1e205f
--- /dev/null
+++ b/intl/components/src/ICU4CLibrary.h
@@ -0,0 +1,74 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef intl_components_ICU4CLibrary_h
+#define intl_components_ICU4CLibrary_h
+
+#include "mozilla/intl/ICU4CGlue.h"
+#include "mozilla/Span.h"
+
+#include <stddef.h>
+
+namespace mozilla::intl {
+/**
+ * Wrapper around non-portable, ICU4C specific functions.
+ */
+class ICU4CLibrary final {
+ public:
+  ICU4CLibrary() = delete;
+
+  /**
+   * Initializes the ICU4C library.
+   *
+   * Note: This function should only be called once.
+   */
+  static ICUResult Initialize();
+
+  /**
+   * Releases any memory held by ICU. Any open ICU objects and resources are
+   * left in an undefined state after this operation.
+   *
+   * NOTE: This function is not thread-safe.
+   */
+  static void Cleanup();
+
+  struct MemoryFunctions {
+    // These are equivalent to ICU's |UMemAllocFn|, |UMemReallocFn|, and
+    // |UMemFreeFn| types. The first argument (called |context| in the ICU
+    // docs) will always be nullptr and should be ignored.
+    using AllocFn = void* (*)(const void*, size_t);
+    using ReallocFn = void* (*)(const void*, void*, size_t);
+    using FreeFn = void (*)(const void*, void*);
+
+    /**
+     * Function called when allocating memory.
+     */
+    AllocFn mAllocFn = nullptr;
+
+    /**
+     * Function called when reallocating memory.
+     */
+    ReallocFn mReallocFn = nullptr;
+
+    /**
+     * Function called when freeing memory.
+     */
+    FreeFn mFreeFn = nullptr;
+  };
+
+  /**
+   * Sets the ICU memory functions.
+   *
+   * This function can only be called before the initial call to Initialize()!
+   */
+  static ICUResult SetMemoryFunctions(MemoryFunctions aMemoryFunctions);
+
+  /**
+   * Return the ICU version number.
+   */
+  static Span<const char> GetVersion();
+};
+}  // namespace mozilla::intl
+
+#endif
diff --git a/intl/components/src/ICU4XGeckoDataProvider.cpp b/intl/components/src/ICU4XGeckoDataProvider.cpp
new file mode 100644
index 0000000000..8bc72f0730
--- /dev/null
+++ b/intl/components/src/ICU4XGeckoDataProvider.cpp
@@ -0,0 +1,35 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "ICU4XGeckoDataProvider.h"
+
+#include "ICU4XDataProvider.h"
+#include "mozilla/Assertions.h"
+
+#include <mutex>
+
+namespace mozilla::intl {
+
+static capi::ICU4XDataProvider* sICU4XDataProvider = nullptr;
+
+capi::ICU4XDataProvider* GetDataProvider() {
+  static std::once_flag sOnce;
+
+  std::call_once(sOnce, []() {
+    sICU4XDataProvider = capi::ICU4XDataProvider_create_compiled();
+  });
+
+  return sICU4XDataProvider;
+}
+
+void CleanupDataProvider() {
+  if (!sICU4XDataProvider) {
+    return;
+  }
+
+  capi::ICU4XDataProvider_destroy(sICU4XDataProvider);
+  sICU4XDataProvider = nullptr;
+}
+
+}  // namespace mozilla::intl
diff --git a/intl/components/src/ICU4XGeckoDataProvider.h b/intl/components/src/ICU4XGeckoDataProvider.h
new file mode 100644
index 0000000000..10ea58dc4d
--- /dev/null
+++ b/intl/components/src/ICU4XGeckoDataProvider.h
@@ -0,0 +1,21 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef intl_components_ICU4XGeckoDataProvider_h
+#define intl_components_ICU4XGeckoDataProvider_h
+
+/**
+ * This component is a C/C++ API to get ICU4X data provider.
+ */
+
+namespace capi {
+struct ICU4XDataProvider;
+}
+
+namespace mozilla::intl {
+capi::ICU4XDataProvider* GetDataProvider();
+void CleanupDataProvider();
+}  // namespace mozilla::intl
+
+#endif
diff --git a/intl/components/src/ICUError.h b/intl/components/src/ICUError.h
new file mode 100644
index 0000000000..c3ef236210
--- /dev/null
+++ b/intl/components/src/ICUError.h
@@ -0,0 +1,118 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef intl_components_ICUError_h
+#define intl_components_ICUError_h
+
+#include "mozilla/Attributes.h"
+#include "mozilla/Result.h"
+
+#include <cstdint>
+#include <type_traits>
+
+namespace mozilla::intl {
+
+/**
+ * General purpose error type for operations that can result in an ICU error.
+ */
+enum class ICUError : uint8_t {
+  // Since we claim UnusedZero<ICUError>::value and
+  // HasFreeLSB<ICUError>::value == true below, we must only use positive,
+  // even enum values.
+
+  OutOfMemory = 2,
+  InternalError = 4,
+  OverflowError = 6,
+};
+
+/**
+ * Error type when a method call can only result in an internal ICU error.
+ */
+struct InternalError {
+  // Since we claim UnusedZero<InternalError>::value and
+  // HasFreeLSB<InternalError>::value == true below, we must only use positive,
+  // even enum values.
+  enum class ErrorKind : uint8_t { Unspecified = 2 };
+
+  const ErrorKind kind = ErrorKind::Unspecified;
+
+  constexpr InternalError() = default;
+
+ private:
+  friend struct mozilla::detail::UnusedZero<InternalError>;
+
+  constexpr MOZ_IMPLICIT InternalError(ErrorKind aKind) : kind(aKind) {}
+};
+
+}  // namespace mozilla::intl
+
+namespace mozilla::detail {
+
+// Provide specializations for UnusedZero and HasFreeLSB to enable more
+// efficient packing for mozilla::Result. This also avoids having to include
+// the ResultVariant.h header.
+//
+// UnusedZero specialization:
+//
+// The UnusedZero specialization makes it possible to use CompactPair as the
+// underlying storage type for Result. For this optimization to work, it is
+// necessary that a distinct null-value is present for the error type. The
+// null-value represents the success case and must be different from all actual
+// error values.
+// This optimization can be easily enabled when the error type is a scoped enum.
+// No enum value must use zero as its value and UnusedZero must be specialized
+// through the helper struct UnusedZeroEnum.
+// For non-enum error types, a more complicated setup is necessary. The
+// UnusedZero specialization must implement all necessary interface methods
+// (i.e. `Inspect`, `Unwrap`, and `Store`) as well as all necessary constants
+// and types (i.e. `StorageType`, `value`, and `nullValue`).
+//
+// HasFreeLSB specialization:
+//
+// When the value and the error type are both providing specializations for
+// HasFreeLSB, Result uses an optimization to store both types within a single
+// storage location. This optimization uses the least significant bit as a tag
+// bit to mark the error case. And because the least significant bit is used for
+// tagging, it can't be used by the error type. That means for example when the
+// error type is an enum, all enum values must be even, because odd integer
+// values have the least significant bit set.
+// The actual HasFreeLSB specialization just needs to define `value` as a static
+// constant with the value `true`.
+
+template <>
+struct UnusedZero<mozilla::intl::ICUError>
+    : UnusedZeroEnum<mozilla::intl::ICUError> {};
+
+template <>
+struct UnusedZero<mozilla::intl::InternalError> {
+  using Error = mozilla::intl::InternalError;
+  using StorageType = std::underlying_type_t<Error::ErrorKind>;
+
+  static constexpr bool value = true;
+  static constexpr StorageType nullValue = 0;
+
+  static constexpr Error Inspect(const StorageType& aValue) {
+    return static_cast<Error::ErrorKind>(aValue);
+  }
+  static constexpr Error Unwrap(StorageType aValue) {
+    return static_cast<Error::ErrorKind>(aValue);
+  }
+  static constexpr StorageType Store(Error aValue) {
+    return static_cast<StorageType>(aValue.kind);
+  }
+};
+
+template <>
+struct HasFreeLSB<mozilla::intl::ICUError> {
+  static constexpr bool value = true;
+};
+
+template <>
+struct HasFreeLSB<mozilla::intl::InternalError> {
+  static constexpr bool value = true;
+};
+
+}  // namespace mozilla::detail
+
+#endif
diff --git a/intl/components/src/IDNA.cpp b/intl/components/src/IDNA.cpp
new file mode 100644
index 0000000000..9b5303f4e8
--- /dev/null
+++ b/intl/components/src/IDNA.cpp
@@ -0,0 +1,26 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "mozilla/intl/IDNA.h"
+
+namespace mozilla::intl {
+
+// static
+Result<UniquePtr<IDNA>, ICUError> IDNA::TryCreate(ProcessingType aProcessing) {
+  uint32_t IDNAOptions = UIDNA_CHECK_BIDI | UIDNA_CHECK_CONTEXTJ;
+  if (aProcessing == ProcessingType::NonTransitional) {
+    IDNAOptions |= UIDNA_NONTRANSITIONAL_TO_UNICODE;
+  }
+
+  UErrorCode status = U_ZERO_ERROR;
+  UIDNA* idna = uidna_openUTS46(IDNAOptions, &status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+
+  return UniquePtr<IDNA>(new IDNA(idna));
+}
+
+IDNA::~IDNA() { uidna_close(mIDNA.GetMut()); }
+}  // namespace mozilla::intl
diff --git a/intl/components/src/IDNA.h b/intl/components/src/IDNA.h
new file mode 100644
index 0000000000..77db2bc2ac
--- /dev/null
+++ b/intl/components/src/IDNA.h
@@ -0,0 +1,138 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+#ifndef intl_components_IDNA_h_
+#define intl_components_IDNA_h_
+
+#include "mozilla/Try.h"
+#include "mozilla/intl/ICU4CGlue.h"
+
+#include "unicode/uidna.h"
+
+namespace mozilla::intl {
+
+/**
+ * This component is a Mozilla-focused API for the Internationalizing Domain
+ * Names in Applications (IDNA).
+ *
+ * See UTS #46 for details.
+ * http://unicode.org/reports/tr46/
+ */
+class IDNA final {
+ public:
+  ~IDNA();
+
+  /**
+   * UTS #46 specifies two specific types of processing: Transitional Processing
+   * and NonTransitional Processing.
+   *
+   * See http://unicode.org/reports/tr46/#Compatibility_Processing
+   */
+  enum class ProcessingType {
+    Transitional,
+    NonTransitional,
+  };
+
+  /**
+   * Create an IDNA object, with specifying the type of processing by enum
+   * ProcessingType.
+   *
+   * Currently the implementation enables CheckBidi flag and CheckJoiners by
+   * default.
+   *
+   * See UTS #46, '4 Processing' for details.
+   * http://unicode.org/reports/tr46/#Processing
+   */
+  static Result<UniquePtr<IDNA>, ICUError> TryCreate(
+      ProcessingType aProcessing);
+
+  /**
+   * This class contains the error code information of IDNA processing.
+   */
+  class Info final {
+   public:
+    /**
+     * Check if there's any error.
+     */
+    bool HasErrors() const { return mErrorCode != 0; }
+
+    /**
+     * If the domain name label starts with "xn--", then the label contains
+     * Punycode. This checks if the domain name label has invalid Punycode.
+     *
+     * See https://www.rfc-editor.org/rfc/rfc3492.html
+     */
+    bool HasInvalidPunycode() const {
+      return (mErrorCode & UIDNA_ERROR_PUNYCODE) != 0;
+    }
+
+    /* The label was successfully ACE (Punycode) decoded but the resulting
+     * string had severe validation errors. For example,
+     * it might contain characters that are not allowed in ACE labels,
+     * or it might not be normalized.
+     */
+    bool HasInvalidAceLabel() const {
+      return (mErrorCode & UIDNA_ERROR_INVALID_ACE_LABEL) != 0;
+    }
+
+    /**
+     * Checks if the domain name label has any invalid hyphen characters.
+     *
+     * See CheckHyphens flag for details in UTS #46[1].
+     * - The label must not contain a U+002D HYPHEN-MINUS character in both the
+     *   third and fourth positions.
+     * - The label must neither begin nor end with a U+002D HYPHEN-MINUS
+     *   character.
+     *
+     * [1]: http://unicode.org/reports/tr46/#Validity_Criteria
+     */
+    bool HasInvalidHyphen() const {
+      uint32_t hyphenErrors = UIDNA_ERROR_LEADING_HYPHEN |
+                              UIDNA_ERROR_TRAILING_HYPHEN |
+                              UIDNA_ERROR_HYPHEN_3_4;
+      return (mErrorCode & hyphenErrors) != 0;
+    }
+
+    bool HasErrorsIgnoringInvalidHyphen() const {
+      uint32_t hyphenErrors = UIDNA_ERROR_LEADING_HYPHEN |
+                              UIDNA_ERROR_TRAILING_HYPHEN |
+                              UIDNA_ERROR_HYPHEN_3_4;
+      return (mErrorCode & ~hyphenErrors) != 0;
+    }
+
+   private:
+    friend class IDNA;
+    explicit Info(const UIDNAInfo* aUinfo) : mErrorCode(aUinfo->errors) {}
+
+    uint32_t mErrorCode = 0;
+  };
+
+  /**
+   * Converts a domain name label to its Unicode form for human-readable
+   * display, and writes the Unicode form into buffer, and returns IDNA::Info
+   * object.
+   * The IDNA::Info object contains the detail information about the processing
+   * result of IDNA call, caller should check the result by calling
+   * IDNA::Info::HasErrors() as well.
+   */
+  template <typename Buffer>
+  Result<Info, ICUError> LabelToUnicode(Span<const char16_t> aLabel,
+                                        Buffer& aBuffer) {
+    UIDNAInfo uinfo = UIDNA_INFO_INITIALIZER;
+    MOZ_TRY(FillBufferWithICUCall(
+        aBuffer, [&](UChar* target, int32_t length, UErrorCode* status) {
+          return uidna_labelToUnicode(mIDNA.GetConst(), aLabel.data(),
+                                      aLabel.size(), target, length, &uinfo,
+                                      status);
+        }));
+
+    return Info{&uinfo};
+  }
+
+ private:
+  explicit IDNA(UIDNA* aIDNA) : mIDNA(aIDNA) {}
+
+  ICUPointer<UIDNA> mIDNA = ICUPointer<UIDNA>(nullptr);
+};
+}  // namespace mozilla::intl
+#endif  // intl_components_IDNA_h_
diff --git a/intl/components/src/ListFormat.cpp b/intl/components/src/ListFormat.cpp
new file mode 100644
index 0000000000..6d1e10826a
--- /dev/null
+++ b/intl/components/src/ListFormat.cpp
@@ -0,0 +1,132 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+#include "mozilla/intl/ListFormat.h"
+
+#include "ScopedICUObject.h"
+
+namespace mozilla::intl {
+
+/*static*/ Result<UniquePtr<ListFormat>, ICUError> ListFormat::TryCreate(
+    mozilla::Span<const char> aLocale, const Options& aOptions) {
+  UListFormatterType utype = ToUListFormatterType(aOptions.mType);
+  UListFormatterWidth uwidth = ToUListFormatterWidth(aOptions.mStyle);
+
+  UErrorCode status = U_ZERO_ERROR;
+  UListFormatter* fmt =
+      ulistfmt_openForType(IcuLocale(aLocale), utype, uwidth, &status);
+  if (U_FAILURE(status)) {
+    return Err(ICUError::InternalError);
+  }
+
+  return UniquePtr<ListFormat>(new ListFormat(fmt));
+}
+
+ListFormat::~ListFormat() {
+  if (mListFormatter) {
+    ulistfmt_close(mListFormatter.GetMut());
+  }
+}
+
+/* static */ UListFormatterType ListFormat::ToUListFormatterType(Type type) {
+  switch (type) {
+    case Type::Conjunction:
+      return ULISTFMT_TYPE_AND;
+    case Type::Disjunction:
+      return ULISTFMT_TYPE_OR;
+    case Type::Unit:
+      return ULISTFMT_TYPE_UNITS;
+  }
+  MOZ_ASSERT_UNREACHABLE();
+  return ULISTFMT_TYPE_AND;
+}
+
+/* static */ UListFormatterWidth ListFormat::ToUListFormatterWidth(
+    Style style) {
+  switch (style) {
+    case Style::Long:
+      return ULISTFMT_WIDTH_WIDE;
+    case Style::Short:
+      return ULISTFMT_WIDTH_SHORT;
+    case Style::Narrow:
+      return ULISTFMT_WIDTH_NARROW;
+  }
+  MOZ_ASSERT_UNREACHABLE();
+  return ULISTFMT_WIDTH_WIDE;
+}
+
+ICUResult ListFormat::FormattedToParts(const UFormattedValue* formattedValue,
+                                       size_t formattedSize,
+                                       PartVector& parts) {
+  size_t lastEndIndex = 0;
+
+  auto AppendPart = [&](PartType type, size_t endIndex) {
+    if (!parts.emplaceBack(type, endIndex)) {
+      return false;
+    }
+
+    lastEndIndex = endIndex;
+    return true;
+  };
+
+  UErrorCode status = U_ZERO_ERROR;
+  UConstrainedFieldPosition* fpos = ucfpos_open(&status);
+  if (U_FAILURE(status)) {
+    return Err(ICUError::InternalError);
+  }
+  ScopedICUObject<UConstrainedFieldPosition, ucfpos_close> toCloseFpos(fpos);
+
+  // We're only interested in ULISTFMT_ELEMENT_FIELD fields.
+  ucfpos_constrainField(fpos, UFIELD_CATEGORY_LIST, ULISTFMT_ELEMENT_FIELD,
+                        &status);
+  if (U_FAILURE(status)) {
+    return Err(ICUError::InternalError);
+  }
+
+  while (true) {
+    bool hasMore = ufmtval_nextPosition(formattedValue, fpos, &status);
+    if (U_FAILURE(status)) {
+      return Err(ICUError::InternalError);
+    }
+    if (!hasMore) {
+      break;
+    }
+
+    int32_t beginIndexInt, endIndexInt;
+    ucfpos_getIndexes(fpos, &beginIndexInt, &endIndexInt, &status);
+    if (U_FAILURE(status)) {
+      return Err(ICUError::InternalError);
+    }
+
+    MOZ_ASSERT(beginIndexInt <= endIndexInt,
+               "field iterator returning invalid range");
+
+    size_t beginIndex = AssertedCast<size_t>(beginIndexInt);
+    size_t endIndex = AssertedCast<size_t>(endIndexInt);
+
+    // Indices are guaranteed to be returned in order (from left to right).
+    MOZ_ASSERT(lastEndIndex <= beginIndex,
+               "field iteration didn't return fields in order start to "
+               "finish as expected");
+
+    if (lastEndIndex < beginIndex) {
+      if (!AppendPart(PartType::Literal, beginIndex)) {
+        return Err(ICUError::InternalError);
+      }
+    }
+
+    if (!AppendPart(PartType::Element, endIndex)) {
+      return Err(ICUError::InternalError);
+    }
+  }
+
+  // Append any final literal.
+  if (lastEndIndex < formattedSize) {
+    if (!AppendPart(PartType::Literal, formattedSize)) {
+      return Err(ICUError::InternalError);
+    }
+  }
+
+  return Ok();
+}
+}  // namespace mozilla::intl
diff --git a/intl/components/src/ListFormat.h b/intl/components/src/ListFormat.h
new file mode 100644
index 0000000000..f6476c1334
--- /dev/null
+++ b/intl/components/src/ListFormat.h
@@ -0,0 +1,223 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+#ifndef intl_components_ListFormat_h_
+#define intl_components_ListFormat_h_
+
+#include "mozilla/CheckedInt.h"
+#include "mozilla/intl/ICU4CGlue.h"
+#include "mozilla/PodOperations.h"
+#include "mozilla/Try.h"
+#include "mozilla/Vector.h"
+#include "unicode/ulistformatter.h"
+
+struct UListFormatter;
+
+namespace mozilla::intl {
+
+static constexpr size_t DEFAULT_LIST_LENGTH = 8;
+
+/**
+ * This component is a Mozilla-focused API for the list formatting provided by
+ * ICU. It implements the API provided by the ECMA-402 Intl.ListFormat object.
+ *
+ * https://tc39.es/ecma402/#listformat-objects
+ */
+class ListFormat final {
+ public:
+  /**
+   * The [[Type]] and [[Style]] properties of ListFormat instances.
+   *
+   * https://tc39.es/ecma402/#sec-properties-of-intl-listformat-instances
+   */
+  // [[Type]]
+  enum class Type { Conjunction, Disjunction, Unit };
+  // [[Style]]
+  enum class Style { Long, Short, Narrow };
+
+  /**
+   * The 'options' object to create Intl.ListFormat instance.
+   *
+   * https://tc39.es/ecma402/#sec-Intl.ListFormat
+   */
+  struct Options {
+    // "conjunction" is the default fallback value.
+    Type mType = Type::Conjunction;
+
+    // "long" is the default fallback value.
+    Style mStyle = Style::Long;
+  };
+
+  /**
+   * Create a ListFormat object for the provided locale and options.
+   *
+   * https://tc39.es/ecma402/#sec-Intl.ListFormat
+   */
+  static Result<UniquePtr<ListFormat>, ICUError> TryCreate(
+      mozilla::Span<const char> aLocale, const Options& aOptions);
+
+  ~ListFormat();
+
+  /**
+   * The list of String values for FormatList and FormatListToParts.
+   *
+   * https://tc39.es/ecma402/#sec-formatlist
+   * https://tc39.es/ecma402/#sec-formatlisttoparts
+   */
+  using StringList =
+      mozilla::Vector<mozilla::Span<const char16_t>, DEFAULT_LIST_LENGTH>;
+
+  /**
+   * Format the list according and write the result in buffer.
+   *
+   * https://tc39.es/ecma402/#sec-Intl.ListFormat.prototype.format
+   * https://tc39.es/ecma402/#sec-formatlist
+   */
+  template <typename Buffer>
+  ICUResult Format(const StringList& list, Buffer& buffer) const {
+    static_assert(std::is_same_v<typename Buffer::CharType, char16_t>,
+                  "Currently only UTF-16 buffers are supported.");
+
+    mozilla::Vector<const char16_t*, DEFAULT_LIST_LENGTH> u16strings;
+    mozilla::Vector<int32_t, DEFAULT_LIST_LENGTH> u16stringLens;
+    MOZ_TRY(ConvertStringListToVectors(list, u16strings, u16stringLens));
+
+    int32_t u16stringCount = mozilla::AssertedCast<int32_t>(list.length());
+    MOZ_TRY(FillBufferWithICUCall(
+        buffer, [this, &u16strings, &u16stringLens, u16stringCount](
+                    char16_t* chars, int32_t size, UErrorCode* status) {
+          return ulistfmt_format(mListFormatter.GetConst(), u16strings.begin(),
+                                 u16stringLens.begin(), u16stringCount, chars,
+                                 size, status);
+        }));
+
+    return Ok{};
+  }
+
+  /**
+   * The corresponding list of parts according to the effective locale and the
+   * formatting options of ListFormat.
+   * Each part has a [[Type]] field, which must be "element" or "literal", and a
+   * [[Value]] field.
+   *
+   * To store Part more efficiently, it doesn't store the ||Value|| of type
+   * string in this struct. Instead, it stores the end index of the string in
+   * the buffer(which is passed to ListFormat::FormatToParts()). The begin index
+   * of the ||Value|| is the index of the previous part.
+   *
+   *  Buffer
+   *  0               i                j
+   * +---------------+---------------+---------------+
+   * | Part[0].Value | Part[1].Value | Part[2].Value | ....
+   * +---------------+---------------+---------------+
+   *
+   *     Part[0].index is i. Part[0].Value is stored in the Buffer[0..i].
+   *     Part[1].index is j. Part[1].Value is stored in the Buffer[i..j].
+   *
+   * See https://tc39.es/ecma402/#sec-createpartsfromlist
+   */
+  enum class PartType {
+    Element,
+    Literal,
+  };
+  // The 2nd field is the end index to the buffer as mentioned above.
+  using Part = std::pair<PartType, size_t>;
+  using PartVector = mozilla::Vector<Part, DEFAULT_LIST_LENGTH>;
+
+  /**
+   * Format the list to a list of parts, and store the formatted result of
+   * UTF-16 string into buffer, and formatted parts into the vector 'parts'.
+   *
+   * See:
+   * https://tc39.es/ecma402/#sec-Intl.ListFormat.prototype.formatToParts
+   * https://tc39.es/ecma402/#sec-formatlisttoparts
+   */
+  template <typename Buffer>
+  ICUResult FormatToParts(const StringList& list, Buffer& buffer,
+                          PartVector& parts) {
+    static_assert(std::is_same_v<typename Buffer::CharType, char16_t>,
+                  "Currently only UTF-16 buffers are supported.");
+
+    mozilla::Vector<const char16_t*, DEFAULT_LIST_LENGTH> u16strings;
+    mozilla::Vector<int32_t, DEFAULT_LIST_LENGTH> u16stringLens;
+    MOZ_TRY(ConvertStringListToVectors(list, u16strings, u16stringLens));
+
+    AutoFormattedList formatted;
+    UErrorCode status = U_ZERO_ERROR;
+    ulistfmt_formatStringsToResult(
+        mListFormatter.GetConst(), u16strings.begin(), u16stringLens.begin(),
+        int32_t(list.length()), formatted.GetFormatted(), &status);
+    if (U_FAILURE(status)) {
+      return Err(ToICUError(status));
+    }
+
+    auto spanResult = formatted.ToSpan();
+    if (spanResult.isErr()) {
+      return spanResult.propagateErr();
+    }
+    auto formattedSpan = spanResult.unwrap();
+    if (!FillBuffer(formattedSpan, buffer)) {
+      return Err(ICUError::OutOfMemory);
+    }
+
+    const UFormattedValue* value = formatted.Value();
+    if (!value) {
+      return Err(ICUError::InternalError);
+    }
+    return FormattedToParts(value, buffer.length(), parts);
+  }
+
+ private:
+  ListFormat() = delete;
+  explicit ListFormat(UListFormatter* fmt) : mListFormatter(fmt) {}
+  ListFormat(const ListFormat&) = delete;
+  ListFormat& operator=(const ListFormat&) = delete;
+
+  ICUPointer<UListFormatter> mListFormatter =
+      ICUPointer<UListFormatter>(nullptr);
+
+  // Convert StringList to an array of type 'const char16_t*' and an array of
+  // int32 for ICU-API.
+  ICUResult ConvertStringListToVectors(
+      const StringList& list,
+      mozilla::Vector<const char16_t*, DEFAULT_LIST_LENGTH>& u16strings,
+      mozilla::Vector<int32_t, DEFAULT_LIST_LENGTH>& u16stringLens) const {
+    // Keep a conservative running count of overall length.
+    mozilla::CheckedInt<int32_t> stringLengthTotal(0);
+    for (const auto& string : list) {
+      if (!u16strings.append(string.data())) {
+        return Err(ICUError::InternalError);
+      }
+
+      int32_t len = mozilla::AssertedCast<int32_t>(string.size());
+      if (!u16stringLens.append(len)) {
+        return Err(ICUError::InternalError);
+      }
+
+      stringLengthTotal += len;
+    }
+
+    // Add space for N unrealistically large conjunctions.
+    constexpr int32_t MaxConjunctionLen = 100;
+    stringLengthTotal += CheckedInt<int32_t>(list.length()) * MaxConjunctionLen;
+    // If the overestimate exceeds ICU length limits, don't try to format.
+    if (!stringLengthTotal.isValid()) {
+      return Err(ICUError::OverflowError);
+    }
+
+    return Ok{};
+  }
+
+  using AutoFormattedList =
+      AutoFormattedResult<UFormattedList, ulistfmt_openResult,
+                          ulistfmt_resultAsValue, ulistfmt_closeResult>;
+
+  ICUResult FormattedToParts(const UFormattedValue* formattedValue,
+                             size_t formattedSize, PartVector& parts);
+
+  static UListFormatterType ToUListFormatterType(Type type);
+  static UListFormatterWidth ToUListFormatterWidth(Style style);
+};
+
+}  // namespace mozilla::intl
+#endif  // intl_components_ListFormat_h_
diff --git a/intl/components/src/Locale.cpp b/intl/components/src/Locale.cpp
new file mode 100644
index 0000000000..9a043518cf
--- /dev/null
+++ b/intl/components/src/Locale.cpp
@@ -0,0 +1,1471 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "mozilla/intl/Locale.h"
+
+#include "mozilla/Assertions.h"
+#include "mozilla/DebugOnly.h"
+#include "mozilla/MathAlgorithms.h"
+#include "mozilla/Span.h"
+#include "mozilla/TextUtils.h"
+#include "mozilla/Variant.h"
+
+#include "ICU4CGlue.h"
+
+#include <algorithm>
+#include <iterator>
+#include <stddef.h>
+#include <stdint.h>
+#include <string>
+#include <string.h>
+#include <type_traits>
+#include <utility>
+
+#include "unicode/uloc.h"
+#include "unicode/utypes.h"
+
+namespace mozilla::intl {
+
+using namespace intl::LanguageTagLimits;
+
+template <typename CharT>
+bool IsStructurallyValidLanguageTag(Span<const CharT> aLanguage) {
+  // unicode_language_subtag = alpha{2,3} | alpha{5,8};
+  size_t length = aLanguage.size();
+  const CharT* str = aLanguage.data();
+  return ((2 <= length && length <= 3) || (5 <= length && length <= 8)) &&
+         std::all_of(str, str + length, IsAsciiAlpha<CharT>);
+}
+
+template bool IsStructurallyValidLanguageTag(Span<const char> aLanguage);
+template bool IsStructurallyValidLanguageTag(Span<const Latin1Char> aLanguage);
+template bool IsStructurallyValidLanguageTag(Span<const char16_t> aLanguage);
+
+template <typename CharT>
+bool IsStructurallyValidScriptTag(Span<const CharT> aScript) {
+  // unicode_script_subtag = alpha{4} ;
+  size_t length = aScript.size();
+  const CharT* str = aScript.data();
+  return length == 4 && std::all_of(str, str + length, IsAsciiAlpha<CharT>);
+}
+
+template bool IsStructurallyValidScriptTag(Span<const char> aScript);
+template bool IsStructurallyValidScriptTag(Span<const Latin1Char> aScript);
+template bool IsStructurallyValidScriptTag(Span<const char16_t> aScript);
+
+template <typename CharT>
+bool IsStructurallyValidRegionTag(Span<const CharT> aRegion) {
+  // unicode_region_subtag = (alpha{2} | digit{3}) ;
+  size_t length = aRegion.size();
+  const CharT* str = aRegion.data();
+  return (length == 2 && std::all_of(str, str + length, IsAsciiAlpha<CharT>)) ||
+         (length == 3 && std::all_of(str, str + length, IsAsciiDigit<CharT>));
+}
+
+template bool IsStructurallyValidRegionTag(Span<const char> aRegion);
+template bool IsStructurallyValidRegionTag(Span<const Latin1Char> aRegion);
+template bool IsStructurallyValidRegionTag(Span<const char16_t> aRegion);
+
+#ifdef DEBUG
+bool IsStructurallyValidVariantTag(Span<const char> aVariant) {
+  // unicode_variant_subtag = (alphanum{5,8} | digit alphanum{3}) ;
+  size_t length = aVariant.size();
+  const char* str = aVariant.data();
+  return ((5 <= length && length <= 8) ||
+          (length == 4 && IsAsciiDigit(str[0]))) &&
+         std::all_of(str, str + length, IsAsciiAlphanumeric<char>);
+}
+
+bool IsStructurallyValidUnicodeExtensionTag(Span<const char> aExtension) {
+  return LocaleParser::CanParseUnicodeExtension(aExtension).isOk();
+}
+
+static bool IsStructurallyValidExtensionTag(Span<const char> aExtension) {
+  // other_extensions = sep [alphanum-[tTuUxX]] (sep alphanum{2,8})+ ;
+  // NB: Allow any extension, including Unicode and Transform here, because
+  // this function is only used for an assertion.
+
+  size_t length = aExtension.size();
+  const char* str = aExtension.data();
+  const char* const end = aExtension.data() + length;
+  if (length <= 2) {
+    return false;
+  }
+  if (!IsAsciiAlphanumeric(str[0]) || str[0] == 'x' || str[0] == 'X') {
+    return false;
+  }
+  str++;
+  if (*str++ != '-') {
+    return false;
+  }
+  while (true) {
+    const char* sep =
+        reinterpret_cast<const char*>(memchr(str, '-', end - str));
+    size_t len = (sep ? sep : end) - str;
+    if (len < 2 || len > 8 ||
+        !std::all_of(str, str + len, IsAsciiAlphanumeric<char>)) {
+      return false;
+    }
+    if (!sep) {
+      return true;
+    }
+    str = sep + 1;
+  }
+}
+
+bool IsStructurallyValidPrivateUseTag(Span<const char> aPrivateUse) {
+  // pu_extensions = sep [xX] (sep alphanum{1,8})+ ;
+
+  size_t length = aPrivateUse.size();
+  const char* str = aPrivateUse.data();
+  const char* const end = aPrivateUse.data() + length;
+  if (length <= 2) {
+    return false;
+  }
+  if (str[0] != 'x' && str[0] != 'X') {
+    return false;
+  }
+  str++;
+  if (*str++ != '-') {
+    return false;
+  }
+  while (true) {
+    const char* sep =
+        reinterpret_cast<const char*>(memchr(str, '-', end - str));
+    size_t len = (sep ? sep : end) - str;
+    if (len == 0 || len > 8 ||
+        !std::all_of(str, str + len, IsAsciiAlphanumeric<char>)) {
+      return false;
+    }
+    if (!sep) {
+      return true;
+    }
+    str = sep + 1;
+  }
+}
+#endif
+
+ptrdiff_t Locale::UnicodeExtensionIndex() const {
+  // The extension subtags aren't necessarily sorted, so we can't use binary
+  // search here.
+  auto p = std::find_if(
+      mExtensions.begin(), mExtensions.end(),
+      [](const auto& ext) { return ext[0] == 'u' || ext[0] == 'U'; });
+  if (p != mExtensions.end()) {
+    return std::distance(mExtensions.begin(), p);
+  }
+  return -1;
+}
+
+Maybe<Span<const char>> Locale::GetUnicodeExtension() const {
+  ptrdiff_t index = UnicodeExtensionIndex();
+  if (index >= 0) {
+    return Some(MakeStringSpan(mExtensions[index].get()));
+  }
+  return Nothing();
+}
+
+ICUResult Locale::SetUnicodeExtension(Span<const char> aExtension) {
+  MOZ_ASSERT(IsStructurallyValidUnicodeExtensionTag(aExtension));
+
+  auto duplicated = DuplicateStringToUniqueChars(aExtension);
+
+  // Replace the existing Unicode extension subtag or append a new one.
+  ptrdiff_t index = UnicodeExtensionIndex();
+  if (index >= 0) {
+    mExtensions[index] = std::move(duplicated);
+    return Ok();
+  }
+  if (!mExtensions.append(std::move(duplicated))) {
+    return Err(ICUError::OutOfMemory);
+  }
+  return Ok();
+}
+
+void Locale::ClearUnicodeExtension() {
+  ptrdiff_t index = UnicodeExtensionIndex();
+  if (index >= 0) {
+    mExtensions.erase(mExtensions.begin() + index);
+  }
+}
+
+template <size_t InitialCapacity>
+static bool SortAlphabetically(Vector<UniqueChars, InitialCapacity>& aSubtags) {
+  size_t length = aSubtags.length();
+
+  // Zero or one element lists are already sorted.
+  if (length < 2) {
+    return true;
+  }
+
+  // Handle two element lists inline.
+  if (length == 2) {
+    if (strcmp(aSubtags[0].get(), aSubtags[1].get()) > 0) {
+      aSubtags[0].swap(aSubtags[1]);
+    }
+    return true;
+  }
+
+  Vector<char*, 8> scratch;
+  if (!scratch.resizeUninitialized(length)) {
+    return false;
+  }
+  for (size_t i = 0; i < length; i++) {
+    scratch[i] = aSubtags[i].release();
+  }
+
+  std::stable_sort(
+      scratch.begin(), scratch.end(),
+      [](const char* a, const char* b) { return strcmp(a, b) < 0; });
+
+  for (size_t i = 0; i < length; i++) {
+    aSubtags[i] = UniqueChars(scratch[i]);
+  }
+  return true;
+}
+
+Result<Ok, Locale::CanonicalizationError> Locale::CanonicalizeBaseName() {
+  // Per 6.2.3 CanonicalizeUnicodeLocaleId, the very first step is to
+  // canonicalize the syntax by normalizing the case and ordering all subtags.
+  // The canonical syntax form is specified in UTS 35, 3.2.1.
+
+  // Language codes need to be in lower case. "JA" -> "ja"
+  mLanguage.ToLowerCase();
+  MOZ_ASSERT(IsStructurallyValidLanguageTag(Language().Span()));
+
+  // The first character of a script code needs to be capitalized.
+  // "hans" -> "Hans"
+  mScript.ToTitleCase();
+  MOZ_ASSERT(Script().Missing() ||
+             IsStructurallyValidScriptTag(Script().Span()));
+
+  // Region codes need to be in upper case. "bu" -> "BU"
+  mRegion.ToUpperCase();
+  MOZ_ASSERT(Region().Missing() ||
+             IsStructurallyValidRegionTag(Region().Span()));
+
+  // The canonical case for variant subtags is lowercase.
+  for (UniqueChars& variant : mVariants) {
+    char* variantChars = variant.get();
+    size_t variantLength = strlen(variantChars);
+    AsciiToLowerCase(variantChars, variantLength, variantChars);
+
+    MOZ_ASSERT(IsStructurallyValidVariantTag({variantChars, variantLength}));
+  }
+
+  // Extensions and privateuse subtags are case normalized in the
+  // |canonicalizeExtensions| method.
+
+  // The second step in UTS 35, 3.2.1, is to order all subtags.
+
+  if (mVariants.length() > 1) {
+    // 1. Any variants are in alphabetical order.
+    if (!SortAlphabetically(mVariants)) {
+      return Err(CanonicalizationError::OutOfMemory);
+    }
+
+    // Reject the Locale identifier if a duplicate variant was found, e.g.
+    // "en-variant-Variant".
+    const UniqueChars* duplicate = std::adjacent_find(
+        mVariants.begin(), mVariants.end(), [](const auto& a, const auto& b) {
+          return strcmp(a.get(), b.get()) == 0;
+        });
+    if (duplicate != mVariants.end()) {
+      return Err(CanonicalizationError::DuplicateVariant);
+    }
+  }
+
+  // 2. Any extensions are in alphabetical order by their singleton.
+  // 3. All attributes are sorted in alphabetical order.
+  // 4. All keywords and tfields are sorted by alphabetical order of their keys,
+  //    within their respective extensions.
+  // 5. Any type or tfield value "true" is removed.
+  // - A subsequent call to canonicalizeExtensions() will perform these steps.
+
+  // 6.2.3 CanonicalizeUnicodeLocaleId, step 2 transforms the locale identifier
+  // into its canonical form per UTS 3.2.1.
+
+  // 1. Use the bcp47 data to replace keys, types, tfields, and tvalues by their
+  // canonical forms.
+  // - A subsequent call to canonicalizeExtensions() will perform this step.
+
+  // 2. Replace aliases in the unicode_language_id and tlang (if any).
+  // - tlang is handled in canonicalizeExtensions().
+
+  // Replace deprecated language, region, and variant subtags with their
+  // preferred mappings.
+
+  if (!UpdateLegacyMappings()) {
+    return Err(CanonicalizationError::OutOfMemory);
+  }
+
+  // Replace deprecated language subtags with their preferred values.
+  if (!LanguageMapping(mLanguage) && ComplexLanguageMapping(mLanguage)) {
+    PerformComplexLanguageMappings();
+  }
+
+  // Replace deprecated script subtags with their preferred values.
+  if (Script().Present()) {
+    ScriptMapping(mScript);
+  }
+
+  // Replace deprecated region subtags with their preferred values.
+  if (Region().Present()) {
+    if (!RegionMapping(mRegion) && ComplexRegionMapping(mRegion)) {
+      PerformComplexRegionMappings();
+    }
+  }
+
+  // Replace deprecated variant subtags with their preferred values.
+  if (!PerformVariantMappings()) {
+    return Err(CanonicalizationError::OutOfMemory);
+  }
+
+  // No extension replacements are currently present.
+  // Private use sequences are left as is.
+
+  // 3. Replace aliases in special key values.
+  // - A subsequent call to canonicalizeExtensions() will perform this step.
+
+  return Ok();
+}
+
+#ifdef DEBUG
+static bool IsAsciiLowercaseAlphanumericOrDash(Span<const char> aSpan) {
+  const char* ptr = aSpan.data();
+  size_t length = aSpan.size();
+  return std::all_of(ptr, ptr + length, [](auto c) {
+    return IsAsciiLowercaseAlpha(c) || IsAsciiDigit(c) || c == '-';
+  });
+}
+#endif
+
+Result<Ok, Locale::CanonicalizationError> Locale::CanonicalizeExtensions() {
+  // The canonical case for all extension subtags is lowercase.
+  for (UniqueChars& extension : mExtensions) {
+    char* extensionChars = extension.get();
+    size_t extensionLength = strlen(extensionChars);
+    AsciiToLowerCase(extensionChars, extensionLength, extensionChars);
+
+    MOZ_ASSERT(
+        IsStructurallyValidExtensionTag({extensionChars, extensionLength}));
+  }
+
+  // Any extensions are in alphabetical order by their singleton.
+  // "u-ca-chinese-t-zh-latn" -> "t-zh-latn-u-ca-chinese"
+  if (!SortAlphabetically(mExtensions)) {
+    return Err(CanonicalizationError::OutOfMemory);
+  }
+
+  for (UniqueChars& extension : mExtensions) {
+    if (extension[0] == 'u') {
+      MOZ_TRY(CanonicalizeUnicodeExtension(extension));
+    } else if (extension[0] == 't') {
+      MOZ_TRY(CanonicalizeTransformExtension(extension));
+    }
+
+    MOZ_ASSERT(
+        IsAsciiLowercaseAlphanumericOrDash(MakeStringSpan(extension.get())));
+  }
+
+  // The canonical case for privateuse subtags is lowercase.
+  if (char* privateuse = mPrivateUse.get()) {
+    size_t privateuseLength = strlen(privateuse);
+    AsciiToLowerCase(privateuse, privateuseLength, privateuse);
+
+    MOZ_ASSERT(
+        IsStructurallyValidPrivateUseTag({privateuse, privateuseLength}));
+  }
+  return Ok();
+}
+
+template <size_t N>
+static inline bool AppendSpan(Vector<char, N>& vector, Span<const char> aSpan) {
+  return vector.append(aSpan.data(), aSpan.size());
+}
+
+/**
+ * CanonicalizeUnicodeExtension( attributes, keywords )
+ *
+ * Canonical syntax per
+ * <https://unicode.org/reports/tr35/#Canonical_Unicode_Locale_Identifiers>:
+ *
+ * - All attributes and keywords are in lowercase.
+ *   - Note: The parser already converted keywords to lowercase.
+ * - All attributes are sorted in alphabetical order.
+ * - All keywords are sorted by alphabetical order of their keys.
+ * - Any type value "true" is removed.
+ *
+ * Canonical form:
+ * - All keys and types use the canonical form (from the name attribute;
+ *   see Section 3.6.4 U Extension Data Files).
+ */
+Result<Ok, Locale::CanonicalizationError> Locale::CanonicalizeUnicodeExtension(
+    UniqueChars& aUnicodeExtension) {
+  Span<const char> extension = MakeStringSpan(aUnicodeExtension.get());
+  MOZ_ASSERT(extension[0] == 'u');
+  MOZ_ASSERT(extension[1] == '-');
+  MOZ_ASSERT(IsStructurallyValidExtensionTag(extension));
+
+  LocaleParser::AttributesVector attributes;
+  LocaleParser::KeywordsVector keywords;
+
+  using Attribute = LocaleParser::AttributesVector::ElementType;
+  using Keyword = LocaleParser::KeywordsVector::ElementType;
+
+  if (LocaleParser::ParseUnicodeExtension(extension, attributes, keywords)
+          .isErr()) {
+    MOZ_ASSERT_UNREACHABLE("unexpected invalid Unicode extension subtag");
+    return Err(CanonicalizationError::InternalError);
+  }
+
+  auto attributesLess = [extension](const Attribute& a, const Attribute& b) {
+    auto astr = extension.Subspan(a.Begin(), a.Length());
+    auto bstr = extension.Subspan(b.Begin(), b.Length());
+    return astr < bstr;
+  };
+
+  // All attributes are sorted in alphabetical order.
+  if (attributes.length() > 1) {
+    std::stable_sort(attributes.begin(), attributes.end(), attributesLess);
+  }
+
+  auto keywordsLess = [extension](const Keyword& a, const Keyword& b) {
+    auto astr = extension.Subspan(a.Begin(), UnicodeKeyLength);
+    auto bstr = extension.Subspan(b.Begin(), UnicodeKeyLength);
+    return astr < bstr;
+  };
+
+  // All keywords are sorted by alphabetical order of keys.
+  if (keywords.length() > 1) {
+    // Using a stable sort algorithm, guarantees that two keywords using the
+    // same key are never reordered. That means for example
+    // when we have the input "u-nu-thai-kf-false-nu-latn", we are guaranteed to
+    // get the result "u-kf-false-nu-thai-nu-latn", i.e. "nu-thai" still occurs
+    // before "nu-latn".
+    // This is required so that deduplication below preserves the first keyword
+    // for a given key and discards the rest.
+    std::stable_sort(keywords.begin(), keywords.end(), keywordsLess);
+  }
+
+  Vector<char, 32> sb;
+  if (!sb.append('u')) {
+    return Err(CanonicalizationError::OutOfMemory);
+  }
+
+  // Append all Unicode extension attributes.
+  for (size_t i = 0; i < attributes.length(); i++) {
+    const auto& attribute = attributes[i];
+    auto span = extension.Subspan(attribute.Begin(), attribute.Length());
+
+    // Skip duplicate attributes.
+    if (i > 0) {
+      const auto& lastAttribute = attributes[i - 1];
+      if (span ==
+          extension.Subspan(lastAttribute.Begin(), lastAttribute.Length())) {
+        continue;
+      }
+      MOZ_ASSERT(attributesLess(lastAttribute, attribute));
+    }
+
+    if (!sb.append('-')) {
+      return Err(CanonicalizationError::OutOfMemory);
+    }
+    if (!AppendSpan(sb, span)) {
+      return Err(CanonicalizationError::OutOfMemory);
+    }
+  }
+
+  static constexpr size_t UnicodeKeyWithSepLength = UnicodeKeyLength + 1;
+
+  using StringSpan = Span<const char>;
+
+  static constexpr StringSpan True = MakeStringSpan("true");
+
+  // Append all Unicode extension keywords.
+  for (size_t i = 0; i < keywords.length(); i++) {
+    const auto& keyword = keywords[i];
+
+    // Skip duplicate keywords.
+    if (i > 0) {
+      const auto& lastKeyword = keywords[i - 1];
+      if (extension.Subspan(keyword.Begin(), UnicodeKeyLength) ==
+          extension.Subspan(lastKeyword.Begin(), UnicodeKeyLength)) {
+        continue;
+      }
+      MOZ_ASSERT(keywordsLess(lastKeyword, keyword));
+    }
+
+    if (!sb.append('-')) {
+      return Err(CanonicalizationError::OutOfMemory);
+    }
+
+    StringSpan span = extension.Subspan(keyword.Begin(), keyword.Length());
+    if (span.size() == UnicodeKeyLength) {
+      // Keyword without type value.
+      if (!AppendSpan(sb, span)) {
+        return Err(CanonicalizationError::OutOfMemory);
+      }
+    } else {
+      StringSpan key = span.To(UnicodeKeyLength);
+      StringSpan type = span.From(UnicodeKeyWithSepLength);
+
+      // Search if there's a replacement for the current Unicode keyword.
+      if (const char* replacement = ReplaceUnicodeExtensionType(key, type)) {
+        StringSpan repl = MakeStringSpan(replacement);
+        if (repl == True) {
+          // Elide the type "true" if present in the replacement.
+          if (!AppendSpan(sb, key)) {
+            return Err(CanonicalizationError::OutOfMemory);
+          }
+        } else {
+          // Otherwise append the Unicode key (including the separator) and the
+          // replaced type.
+          if (!AppendSpan(sb, span.To(UnicodeKeyWithSepLength))) {
+            return Err(CanonicalizationError::OutOfMemory);
+          }
+          if (!AppendSpan(sb, repl)) {
+            return Err(CanonicalizationError::OutOfMemory);
+          }
+        }
+      } else {
+        if (type == True) {
+          // Elide the Unicode extension type "true".
+          if (!AppendSpan(sb, key)) {
+            return Err(CanonicalizationError::OutOfMemory);
+          }
+        } else {
+          // Otherwise append the complete Unicode extension keyword.
+          if (!AppendSpan(sb, span)) {
+            return Err(CanonicalizationError::OutOfMemory);
+          }
+        }
+      }
+    }
+  }
+
+  // We can keep the previous extension when canonicalization didn't modify it.
+  if (static_cast<Span<const char>>(sb) != extension) {
+    // Otherwise replace the previous extension with the canonical extension.
+    UniqueChars canonical = DuplicateStringToUniqueChars(sb);
+    if (!canonical) {
+      return Err(CanonicalizationError::OutOfMemory);
+    }
+    aUnicodeExtension = std::move(canonical);
+  }
+
+  return Ok();
+}
+
+template <class Buffer>
+static bool LocaleToString(const Locale& aTag, Buffer& aBuffer) {
+  auto appendSubtag = [&aBuffer](const auto& subtag) {
+    auto span = subtag.Span();
+    MOZ_ASSERT(!span.empty());
+    return aBuffer.append(span.data(), span.size());
+  };
+
+  auto appendSubtagSpan = [&aBuffer](Span<const char> subtag) {
+    MOZ_ASSERT(!subtag.empty());
+    return aBuffer.append(subtag.data(), subtag.size());
+  };
+
+  auto appendSubtags = [&aBuffer, &appendSubtagSpan](const auto& subtags) {
+    for (const auto& subtag : subtags) {
+      if (!aBuffer.append('-') || !appendSubtagSpan(subtag)) {
+        return false;
+      }
+    }
+    return true;
+  };
+
+  // Append the language subtag.
+  if (!appendSubtag(aTag.Language())) {
+    return false;
+  }
+
+  // Append the script subtag if present.
+  if (aTag.Script().Present()) {
+    if (!aBuffer.append('-') || !appendSubtag(aTag.Script())) {
+      return false;
+    }
+  }
+
+  // Append the region subtag if present.
+  if (aTag.Region().Present()) {
+    if (!aBuffer.append('-') || !appendSubtag(aTag.Region())) {
+      return false;
+    }
+  }
+
+  // Append the variant subtags if present.
+  if (!appendSubtags(aTag.Variants())) {
+    return false;
+  }
+
+  // Append the extensions subtags if present.
+  if (!appendSubtags(aTag.Extensions())) {
+    return false;
+  }
+
+  // Append the private-use subtag if present.
+  if (auto privateuse = aTag.PrivateUse()) {
+    if (!aBuffer.append('-') || !appendSubtagSpan(privateuse.value())) {
+      return false;
+    }
+  }
+
+  return true;
+}
+
+/**
+ * CanonicalizeTransformExtension
+ *
+ * Canonical form per <https://unicode.org/reports/tr35/#BCP47_T_Extension>:
+ *
+ * - These subtags are all in lowercase (that is the canonical casing for these
+ *   subtags), [...].
+ *
+ * And per
+ * <https://unicode.org/reports/tr35/#Canonical_Unicode_Locale_Identifiers>:
+ *
+ * - All keywords and tfields are sorted by alphabetical order of their keys,
+ *   within their respective extensions.
+ */
+Result<Ok, Locale::CanonicalizationError>
+Locale::CanonicalizeTransformExtension(UniqueChars& aTransformExtension) {
+  Span<const char> extension = MakeStringSpan(aTransformExtension.get());
+  MOZ_ASSERT(extension[0] == 't');
+  MOZ_ASSERT(extension[1] == '-');
+  MOZ_ASSERT(IsStructurallyValidExtensionTag(extension));
+
+  Locale tag;
+  LocaleParser::TFieldVector fields;
+
+  using TField = LocaleParser::TFieldVector::ElementType;
+
+  if (LocaleParser::ParseTransformExtension(extension, tag, fields).isErr()) {
+    MOZ_ASSERT_UNREACHABLE("unexpected invalid transform extension subtag");
+    return Err(CanonicalizationError::InternalError);
+  }
+
+  auto tfieldLess = [extension](const TField& a, const TField& b) {
+    auto astr = extension.Subspan(a.Begin(), TransformKeyLength);
+    auto bstr = extension.Subspan(b.Begin(), TransformKeyLength);
+    return astr < bstr;
+  };
+
+  // All tfields are sorted by alphabetical order of their keys.
+  if (fields.length() > 1) {
+    std::stable_sort(fields.begin(), fields.end(), tfieldLess);
+  }
+
+  Vector<char, 32> sb;
+  if (!sb.append('t')) {
+    return Err(CanonicalizationError::OutOfMemory);
+  }
+
+  // Append the language subtag if present.
+  //
+  // Replace aliases in tlang per
+  // <https://unicode.org/reports/tr35/#Canonical_Unicode_Locale_Identifiers>.
+  if (tag.Language().Present()) {
+    if (!sb.append('-')) {
+      return Err(CanonicalizationError::OutOfMemory);
+    }
+
+    MOZ_TRY(tag.CanonicalizeBaseName());
+
+    // The canonical case for Transform extensions is lowercase per
+    // <https://unicode.org/reports/tr35/#BCP47_T_Extension>. Convert the two
+    // subtags which don't use lowercase for their canonical syntax.
+    tag.mScript.ToLowerCase();
+    tag.mRegion.ToLowerCase();
+
+    if (!LocaleToString(tag, sb)) {
+      return Err(CanonicalizationError::OutOfMemory);
+    }
+  }
+
+  static constexpr size_t TransformKeyWithSepLength = TransformKeyLength + 1;
+
+  using StringSpan = Span<const char>;
+
+  // Append all fields.
+  //
+  // UTS 35, 3.2.1 specifies:
+  // - Any type or tfield value "true" is removed.
+  //
+  // But the `tvalue` subtag is mandatory in `tfield: tkey tvalue`, so ignore
+  // this apparently invalid part of the UTS 35 specification and simply
+  // append all `tfield` subtags.
+  for (const auto& field : fields) {
+    if (!sb.append('-')) {
+      return Err(CanonicalizationError::OutOfMemory);
+    }
+
+    StringSpan span = extension.Subspan(field.Begin(), field.Length());
+    StringSpan key = span.To(TransformKeyLength);
+    StringSpan value = span.From(TransformKeyWithSepLength);
+
+    // Search if there's a replacement for the current transform keyword.
+    if (const char* replacement = ReplaceTransformExtensionType(key, value)) {
+      if (!AppendSpan(sb, span.To(TransformKeyWithSepLength))) {
+        return Err(CanonicalizationError::OutOfMemory);
+      }
+      if (!AppendSpan(sb, MakeStringSpan(replacement))) {
+        return Err(CanonicalizationError::OutOfMemory);
+      }
+    } else {
+      if (!AppendSpan(sb, span)) {
+        return Err(CanonicalizationError::OutOfMemory);
+      }
+    }
+  }
+
+  // We can keep the previous extension when canonicalization didn't modify it.
+  if (static_cast<Span<const char>>(sb) != extension) {
+    // Otherwise replace the previous extension with the canonical extension.
+    UniqueChars canonical = DuplicateStringToUniqueChars(sb);
+    if (!canonical) {
+      return Err(CanonicalizationError::OutOfMemory);
+    }
+    aTransformExtension = std::move(canonical);
+  }
+
+  return Ok();
+}
+
+// Zero-terminated ICU Locale ID.
+using LocaleId =
+    Vector<char, LanguageLength + 1 + ScriptLength + 1 + RegionLength + 1>;
+
+enum class LikelySubtags : bool { Add, Remove };
+
+// Return true iff the locale is already maximized resp. minimized.
+static bool HasLikelySubtags(LikelySubtags aLikelySubtags, const Locale& aTag) {
+  // The locale is already maximized if the language, script, and region
+  // subtags are present and no placeholder subtags ("und", "Zzzz", "ZZ") are
+  // used.
+  if (aLikelySubtags == LikelySubtags::Add) {
+    return !aTag.Language().EqualTo("und") &&
+           (aTag.Script().Present() && !aTag.Script().EqualTo("Zzzz")) &&
+           (aTag.Region().Present() && !aTag.Region().EqualTo("ZZ"));
+  }
+
+  // The locale is already minimized if it only contains a language
+  // subtag whose value is not the placeholder value "und".
+  return !aTag.Language().EqualTo("und") && aTag.Script().Missing() &&
+         aTag.Region().Missing();
+}
+
+// Create an ICU locale ID from the given locale.
+static bool CreateLocaleForLikelySubtags(const Locale& aTag,
+                                         LocaleId& aLocale) {
+  MOZ_ASSERT(aLocale.length() == 0);
+
+  auto appendSubtag = [&aLocale](const auto& subtag) {
+    auto span = subtag.Span();
+    MOZ_ASSERT(!span.empty());
+    return aLocale.append(span.data(), span.size());
+  };
+
+  // Append the language subtag.
+  if (!appendSubtag(aTag.Language())) {
+    return false;
+  }
+
+  // Append the script subtag if present.
+  if (aTag.Script().Present()) {
+    if (!aLocale.append('_') || !appendSubtag(aTag.Script())) {
+      return false;
+    }
+  }
+
+  // Append the region subtag if present.
+  if (aTag.Region().Present()) {
+    if (!aLocale.append('_') || !appendSubtag(aTag.Region())) {
+      return false;
+    }
+  }
+
+  // Zero-terminated for use with ICU.
+  return aLocale.append('\0');
+}
+
+static ICUError ParserErrorToICUError(LocaleParser::ParserError aErr) {
+  using ParserError = LocaleParser::ParserError;
+
+  switch (aErr) {
+    case ParserError::NotParseable:
+      return ICUError::InternalError;
+    case ParserError::OutOfMemory:
+      return ICUError::OutOfMemory;
+  }
+  MOZ_CRASH("Unexpected parser error");
+}
+
+static ICUError CanonicalizationErrorToICUError(
+    Locale::CanonicalizationError aErr) {
+  using CanonicalizationError = Locale::CanonicalizationError;
+
+  switch (aErr) {
+    case CanonicalizationError::DuplicateVariant:
+    case CanonicalizationError::InternalError:
+      return ICUError::InternalError;
+    case CanonicalizationError::OutOfMemory:
+      return ICUError::OutOfMemory;
+  }
+  MOZ_CRASH("Unexpected canonicalization error");
+}
+
+// Assign the language, script, and region subtags from an ICU locale ID.
+//
+// ICU provides |uloc_getLanguage|, |uloc_getScript|, and |uloc_getCountry| to
+// retrieve these subtags, but unfortunately these functions are rather slow, so
+// we use our own implementation.
+static ICUResult AssignFromLocaleId(LocaleId& aLocaleId, Locale& aTag) {
+  // Replace the ICU locale ID separator.
+  std::replace(aLocaleId.begin(), aLocaleId.end(), '_', '-');
+
+  // ICU replaces "und" with the empty string, which means "und" becomes "" and
+  // "und-Latn" becomes "-Latn". Handle this case separately.
+  if (aLocaleId.empty() || aLocaleId[0] == '-') {
+    static constexpr auto und = MakeStringSpan("und");
+    constexpr size_t length = und.size();
+
+    // Insert "und" in front of the locale ID.
+    if (!aLocaleId.growBy(length)) {
+      return Err(ICUError::OutOfMemory);
+    }
+    memmove(aLocaleId.begin() + length, aLocaleId.begin(), aLocaleId.length());
+    memmove(aLocaleId.begin(), und.data(), length);
+  }
+
+  // Retrieve the language, script, and region subtags from the locale ID
+  Locale localeTag;
+  MOZ_TRY(LocaleParser::TryParseBaseName(aLocaleId, localeTag)
+              .mapErr(ParserErrorToICUError));
+
+  aTag.SetLanguage(localeTag.Language());
+  aTag.SetScript(localeTag.Script());
+  aTag.SetRegion(localeTag.Region());
+
+  return Ok();
+}
+
+template <decltype(uloc_addLikelySubtags) likelySubtagsFn>
+static ICUResult CallLikelySubtags(const LocaleId& aLocaleId,
+                                   LocaleId& aResult) {
+  // Locale ID must be zero-terminated before passing it to ICU.
+  MOZ_ASSERT(aLocaleId.back() == '\0');
+  MOZ_ASSERT(aResult.length() == 0);
+
+  // Ensure there's enough room for the result.
+  MOZ_ALWAYS_TRUE(aResult.resize(LocaleId::InlineLength));
+
+  return FillBufferWithICUCall(
+      aResult, [&aLocaleId](char* chars, int32_t size, UErrorCode* status) {
+        return likelySubtagsFn(aLocaleId.begin(), chars, size, status);
+      });
+}
+
+// The canonical way to compute the Unicode BCP 47 locale identifier with likely
+// subtags is as follows:
+//
+// 1. Call uloc_forLanguageTag() to transform the locale identifer into an ICU
+//    locale ID.
+// 2. Call uloc_addLikelySubtags() to add the likely subtags to the locale ID.
+// 3. Call uloc_toLanguageTag() to transform the resulting locale ID back into
+//    a Unicode BCP 47 locale identifier.
+//
+// Since uloc_forLanguageTag() and uloc_toLanguageTag() are both kind of slow
+// and we know, by construction, that the input Unicode BCP 47 locale identifier
+// only contains valid language, script, and region subtags, we can avoid both
+// calls if we implement them ourselves, see CreateLocaleForLikelySubtags() and
+// AssignFromLocaleId(). (Where "slow" means about 50% of the execution time of
+// |Intl.Locale.prototype.maximize|.)
+static ICUResult LikelySubtags(LikelySubtags aLikelySubtags, Locale& aTag) {
+  // Return early if the input is already maximized/minimized.
+  if (HasLikelySubtags(aLikelySubtags, aTag)) {
+    return Ok();
+  }
+
+  // Create the locale ID for the input argument.
+  LocaleId locale;
+  if (!CreateLocaleForLikelySubtags(aTag, locale)) {
+    return Err(ICUError::OutOfMemory);
+  }
+
+  // Either add or remove likely subtags to/from the locale ID.
+  LocaleId localeLikelySubtags;
+  if (aLikelySubtags == LikelySubtags::Add) {
+    MOZ_TRY(
+        CallLikelySubtags<uloc_addLikelySubtags>(locale, localeLikelySubtags));
+  } else {
+    MOZ_TRY(
+        CallLikelySubtags<uloc_minimizeSubtags>(locale, localeLikelySubtags));
+  }
+
+  // Assign the language, script, and region subtags from the locale ID.
+  MOZ_TRY(AssignFromLocaleId(localeLikelySubtags, aTag));
+
+  // Update mappings in case ICU returned a non-canonical locale.
+  MOZ_TRY(aTag.CanonicalizeBaseName().mapErr(CanonicalizationErrorToICUError));
+
+  return Ok();
+}
+
+ICUResult Locale::AddLikelySubtags() {
+  return LikelySubtags(LikelySubtags::Add, *this);
+}
+
+ICUResult Locale::RemoveLikelySubtags() {
+  return LikelySubtags(LikelySubtags::Remove, *this);
+}
+
+UniqueChars Locale::DuplicateStringToUniqueChars(const char* aStr) {
+  size_t length = strlen(aStr) + 1;
+  auto duplicate = MakeUnique<char[]>(length);
+  memcpy(duplicate.get(), aStr, length);
+  return duplicate;
+}
+
+UniqueChars Locale::DuplicateStringToUniqueChars(Span<const char> aStr) {
+  size_t length = aStr.size();
+  auto duplicate = MakeUnique<char[]>(length + 1);
+  memcpy(duplicate.get(), aStr.data(), length);
+  duplicate[length] = '\0';
+  return duplicate;
+}
+
+size_t Locale::ToStringCapacity() const {
+  // This is a bit awkward, the buffer class currently does not support
+  // being resized, so we need to calculate the required size up front and
+  // reserve it all at once.
+  auto lengthSubtag = [](const auto& subtag) {
+    auto span = subtag.Span();
+    MOZ_ASSERT(!span.empty());
+    return span.size();
+  };
+
+  auto lengthSubtagZ = [](const char* subtag) {
+    size_t length = strlen(subtag);
+    MOZ_ASSERT(length > 0);
+    return length;
+  };
+
+  auto lengthSubtagsZ = [&lengthSubtagZ](const auto& subtags) {
+    size_t length = 0;
+    for (const auto& subtag : subtags) {
+      length += lengthSubtagZ(subtag.get()) + 1;
+    }
+    return length;
+  };
+
+  // First calculate required capacity
+  size_t capacity = 0;
+
+  capacity += lengthSubtag(mLanguage);
+
+  if (mScript.Present()) {
+    capacity += lengthSubtag(mScript) + 1;
+  }
+
+  if (mRegion.Present()) {
+    capacity += lengthSubtag(mRegion) + 1;
+  }
+
+  capacity += lengthSubtagsZ(mVariants);
+
+  capacity += lengthSubtagsZ(mExtensions);
+
+  if (mPrivateUse.get()) {
+    capacity += lengthSubtagZ(mPrivateUse.get()) + 1;
+  }
+
+  return capacity;
+}
+
+size_t Locale::ToStringAppend(char* aBuffer) const {
+  // Current write position inside buffer.
+  size_t offset = 0;
+
+  auto appendHyphen = [&offset, &aBuffer]() {
+    aBuffer[offset] = '-';
+    offset += 1;
+  };
+
+  auto appendSubtag = [&offset, &aBuffer](const auto& subtag) {
+    auto span = subtag.Span();
+    memcpy(aBuffer + offset, span.data(), span.size());
+    offset += span.size();
+  };
+
+  auto appendSubtagZ = [&offset, &aBuffer](const char* subtag) {
+    size_t length = strlen(subtag);
+    memcpy(aBuffer + offset, subtag, length);
+    offset += length;
+  };
+
+  auto appendSubtagsZ = [&appendHyphen, &appendSubtagZ](const auto& subtags) {
+    for (const auto& subtag : subtags) {
+      appendHyphen();
+      appendSubtagZ(subtag.get());
+    }
+  };
+
+  // Append the language subtag.
+  appendSubtag(mLanguage);
+
+  // Append the script subtag if present.
+  if (mScript.Present()) {
+    appendHyphen();
+    appendSubtag(mScript);
+  }
+
+  // Append the region subtag if present.
+  if (mRegion.Present()) {
+    appendHyphen();
+    appendSubtag(mRegion);
+  }
+
+  // Append the variant subtags if present.
+  appendSubtagsZ(mVariants);
+
+  // Append the extensions subtags if present.
+  appendSubtagsZ(mExtensions);
+
+  // Append the private-use subtag if present.
+  if (mPrivateUse.get()) {
+    appendHyphen();
+    appendSubtagZ(mPrivateUse.get());
+  }
+
+  return offset;
+}
+
+LocaleParser::Token LocaleParser::NextToken() {
+  MOZ_ASSERT(mIndex <= mLength + 1, "called after 'None' token was read");
+
+  TokenKind kind = TokenKind::None;
+  size_t tokenLength = 0;
+  for (size_t i = mIndex; i < mLength; i++) {
+    // UTS 35, section 3.1.
+    // alpha = [A-Z a-z] ;
+    // digit = [0-9] ;
+    char c = CharAt(i);
+    if (IsAsciiAlpha(c)) {
+      kind |= TokenKind::Alpha;
+    } else if (IsAsciiDigit(c)) {
+      kind |= TokenKind::Digit;
+    } else if (c == '-' && i > mIndex && i + 1 < mLength) {
+      break;
+    } else {
+      return {TokenKind::Error, 0, 0};
+    }
+    tokenLength += 1;
+  }
+
+  Token token{kind, mIndex, tokenLength};
+  mIndex += tokenLength + 1;
+  return token;
+}
+
+UniqueChars LocaleParser::Chars(size_t aIndex, size_t aLength) const {
+  // Add +1 to null-terminate the string.
+  auto chars = MakeUnique<char[]>(aLength + 1);
+  char* dest = chars.get();
+  std::copy_n(mLocale + aIndex, aLength, dest);
+  dest[aLength] = '\0';
+  return chars;
+}
+
+// Parse the `unicode_language_id` production.
+//
+// unicode_language_id = unicode_language_subtag
+//                       (sep unicode_script_subtag)?
+//                       (sep unicode_region_subtag)?
+//                       (sep unicode_variant_subtag)* ;
+//
+// sep                 = "-"
+//
+// Note: Unicode CLDR locale identifier backward compatibility extensions
+//       removed from `unicode_language_id`.
+//
+// |tok| is the current token from |ts|.
+//
+// All subtags will be added unaltered to |tag|, without canonicalizing their
+// case or, in the case of variant subtags, detecting and rejecting duplicate
+// variants. Users must subsequently |CanonicalizeBaseName| to perform these
+// actions.
+//
+// Do not use this function directly: use |ParseBaseName| or
+// |ParseTlangFromTransformExtension| instead.
+Result<Ok, LocaleParser::ParserError> LocaleParser::InternalParseBaseName(
+    LocaleParser& aLocaleParser, Locale& aTag, Token& aTok) {
+  if (aLocaleParser.IsLanguage(aTok)) {
+    aLocaleParser.CopyChars(aTok, aTag.mLanguage);
+
+    aTok = aLocaleParser.NextToken();
+  } else {
+    // The language subtag is mandatory.
+    return Err(ParserError::NotParseable);
+  }
+
+  if (aLocaleParser.IsScript(aTok)) {
+    aLocaleParser.CopyChars(aTok, aTag.mScript);
+
+    aTok = aLocaleParser.NextToken();
+  }
+
+  if (aLocaleParser.IsRegion(aTok)) {
+    aLocaleParser.CopyChars(aTok, aTag.mRegion);
+
+    aTok = aLocaleParser.NextToken();
+  }
+
+  auto& variants = aTag.mVariants;
+  MOZ_ASSERT(variants.length() == 0);
+  while (aLocaleParser.IsVariant(aTok)) {
+    auto variant = aLocaleParser.Chars(aTok);
+    if (!variants.append(std::move(variant))) {
+      return Err(ParserError::OutOfMemory);
+    }
+
+    aTok = aLocaleParser.NextToken();
+  }
+
+  return Ok();
+}
+
+Result<Ok, LocaleParser::ParserError> LocaleParser::TryParse(
+    mozilla::Span<const char> aLocale, Locale& aTag) {
+  // |aTag| must be a new, empty Locale.
+  MOZ_ASSERT(aTag.Language().Missing());
+  MOZ_ASSERT(aTag.Script().Missing());
+  MOZ_ASSERT(aTag.Region().Missing());
+  MOZ_ASSERT(aTag.Variants().empty());
+  MOZ_ASSERT(aTag.Extensions().empty());
+  MOZ_ASSERT(aTag.PrivateUse().isNothing());
+
+  // unicode_locale_id = unicode_language_id
+  //                     extensions*
+  //                     pu_extensions? ;
+
+  LocaleParser ts(aLocale);
+  Token tok = ts.NextToken();
+
+  MOZ_TRY(ParseBaseName(ts, aTag, tok));
+
+  // extensions = unicode_locale_extensions
+  //            | transformed_extensions
+  //            | other_extensions ;
+
+  // Bit set of seen singletons.
+  uint64_t seenSingletons = 0;
+
+  auto& extensions = aTag.mExtensions;
+  while (ts.IsExtensionStart(tok)) {
+    char singleton = ts.SingletonKey(tok);
+
+    // Reject the input if a duplicate singleton was found.
+    uint64_t hash = 1ULL << (AsciiAlphanumericToNumber(singleton) + 1);
+    if (seenSingletons & hash) {
+      return Err(ParserError::NotParseable);
+    }
+    seenSingletons |= hash;
+
+    Token start = tok;
+    tok = ts.NextToken();
+
+    // We'll check for missing non-singleton subtags after this block by
+    // comparing |startValue| with the then-current position.
+    size_t startValue = tok.Index();
+
+    if (singleton == 'u') {
+      while (ts.IsUnicodeExtensionPart(tok)) {
+        tok = ts.NextToken();
+      }
+    } else if (singleton == 't') {
+      // transformed_extensions = sep [tT]
+      //                          ((sep tlang (sep tfield)*)
+      //                           | (sep tfield)+) ;
+
+      // tlang = unicode_language_subtag
+      //         (sep unicode_script_subtag)?
+      //         (sep unicode_region_subtag)?
+      //         (sep unicode_variant_subtag)* ;
+      if (ts.IsLanguage(tok)) {
+        tok = ts.NextToken();
+
+        if (ts.IsScript(tok)) {
+          tok = ts.NextToken();
+        }
+
+        if (ts.IsRegion(tok)) {
+          tok = ts.NextToken();
+        }
+
+        while (ts.IsVariant(tok)) {
+          tok = ts.NextToken();
+        }
+      }
+
+      // tfield = tkey tvalue;
+      while (ts.IsTransformExtensionKey(tok)) {
+        tok = ts.NextToken();
+
+        size_t startTValue = tok.Index();
+        while (ts.IsTransformExtensionPart(tok)) {
+          tok = ts.NextToken();
+        }
+
+        // `tfield` requires at least one `tvalue`.
+        if (tok.Index() <= startTValue) {
+          return Err(ParserError::NotParseable);
+        }
+      }
+    } else {
+      while (ts.IsOtherExtensionPart(tok)) {
+        tok = ts.NextToken();
+      }
+    }
+
+    // Singletons must be followed by a non-singleton subtag, "en-a-b" is not
+    // allowed.
+    if (tok.Index() <= startValue) {
+      return Err(ParserError::NotParseable);
+    }
+
+    UniqueChars extension = ts.Extension(start, tok);
+    if (!extensions.append(std::move(extension))) {
+      return Err(ParserError::OutOfMemory);
+    }
+  }
+
+  // Trailing `pu_extension` component of the `unicode_locale_id` production.
+  if (ts.IsPrivateUseStart(tok)) {
+    Token start = tok;
+    tok = ts.NextToken();
+
+    size_t startValue = tok.Index();
+    while (ts.IsPrivateUsePart(tok)) {
+      tok = ts.NextToken();
+    }
+
+    // There must be at least one subtag after the "-x-".
+    if (tok.Index() <= startValue) {
+      return Err(ParserError::NotParseable);
+    }
+
+    UniqueChars privateUse = ts.Extension(start, tok);
+    aTag.mPrivateUse = std::move(privateUse);
+  }
+
+  if (!tok.IsNone()) {
+    return Err(ParserError::NotParseable);
+  }
+
+  return Ok();
+}
+
+Result<Ok, LocaleParser::ParserError> LocaleParser::TryParseBaseName(
+    Span<const char> aLocale, Locale& aTag) {
+  // |aTag| must be a new, empty Locale.
+  MOZ_ASSERT(aTag.Language().Missing());
+  MOZ_ASSERT(aTag.Script().Missing());
+  MOZ_ASSERT(aTag.Region().Missing());
+  MOZ_ASSERT(aTag.Variants().empty());
+  MOZ_ASSERT(aTag.Extensions().empty());
+  MOZ_ASSERT(aTag.PrivateUse().isNothing());
+
+  LocaleParser ts(aLocale);
+  Token tok = ts.NextToken();
+
+  MOZ_TRY(ParseBaseName(ts, aTag, tok));
+  if (!tok.IsNone()) {
+    return Err(ParserError::NotParseable);
+  }
+
+  return Ok();
+}
+
+// Parse |aExtension|, which must be a valid `transformed_extensions` subtag,
+// and fill |aTag| and |aFields| from the `tlang` and `tfield` components.
+Result<Ok, LocaleParser::ParserError> LocaleParser::ParseTransformExtension(
+    Span<const char> aExtension, Locale& aTag, TFieldVector& aFields) {
+  LocaleParser ts(aExtension);
+  Token tok = ts.NextToken();
+
+  if (!ts.IsExtensionStart(tok) || ts.SingletonKey(tok) != 't') {
+    return Err(ParserError::NotParseable);
+  }
+
+  tok = ts.NextToken();
+
+  if (tok.IsNone()) {
+    return Err(ParserError::NotParseable);
+  }
+
+  if (ts.IsLanguage(tok)) {
+    // We're parsing a possible `tlang` in a known-valid transform extension, so
+    // use the special-purpose function that takes advantage of this to compute
+    // lowercased |tag| contents in an optimal manner.
+    MOZ_TRY(ParseTlangInTransformExtension(ts, aTag, tok));
+
+    // After `tlang` we must have a `tfield` and its `tkey`, or we're at the end
+    // of the transform extension.
+    MOZ_ASSERT(ts.IsTransformExtensionKey(tok) || tok.IsNone());
+  } else {
+    // If there's no `tlang` subtag, at least one `tfield` must be present.
+    MOZ_ASSERT(ts.IsTransformExtensionKey(tok));
+  }
+
+  // Trailing `tfield` subtags. (Any other trailing subtags are an error,
+  // because we're guaranteed to only see a valid tranform extension here.)
+  while (ts.IsTransformExtensionKey(tok)) {
+    size_t begin = tok.Index();
+    tok = ts.NextToken();
+
+    size_t startTValue = tok.Index();
+    while (ts.IsTransformExtensionPart(tok)) {
+      tok = ts.NextToken();
+    }
+
+    // `tfield` requires at least one `tvalue`.
+    if (tok.Index() <= startTValue) {
+      return Err(ParserError::NotParseable);
+    }
+
+    size_t length = tok.Index() - 1 - begin;
+    if (!aFields.emplaceBack(begin, length)) {
+      return Err(ParserError::OutOfMemory);
+    }
+  }
+
+  if (!tok.IsNone()) {
+    return Err(ParserError::NotParseable);
+  }
+
+  return Ok();
+}
+
+// Parse |aExtension|, which must be a valid `unicode_locale_extensions` subtag,
+// and fill |aAttributes| and |aKeywords| from the `attribute` and `keyword`
+// components.
+Result<Ok, LocaleParser::ParserError> LocaleParser::ParseUnicodeExtension(
+    Span<const char> aExtension, AttributesVector& aAttributes,
+    KeywordsVector& aKeywords) {
+  LocaleParser ts(aExtension);
+  Token tok = ts.NextToken();
+
+  // unicode_locale_extensions = sep [uU] ((sep keyword)+ |
+  //                                       (sep attribute)+ (sep keyword)*) ;
+
+  if (!ts.IsExtensionStart(tok) || ts.SingletonKey(tok) != 'u') {
+    return Err(ParserError::NotParseable);
+  }
+
+  tok = ts.NextToken();
+
+  if (tok.IsNone()) {
+    return Err(ParserError::NotParseable);
+  }
+
+  while (ts.IsUnicodeExtensionAttribute(tok)) {
+    if (!aAttributes.emplaceBack(tok.Index(), tok.Length())) {
+      return Err(ParserError::OutOfMemory);
+    }
+
+    tok = ts.NextToken();
+  }
+
+  // keyword = key (sep type)? ;
+  while (ts.IsUnicodeExtensionKey(tok)) {
+    size_t begin = tok.Index();
+    tok = ts.NextToken();
+
+    while (ts.IsUnicodeExtensionType(tok)) {
+      tok = ts.NextToken();
+    }
+
+    if (tok.IsError()) {
+      return Err(ParserError::NotParseable);
+    }
+
+    size_t length = tok.Index() - 1 - begin;
+    if (!aKeywords.emplaceBack(begin, length)) {
+      return Err(ParserError::OutOfMemory);
+    }
+  }
+
+  if (!tok.IsNone()) {
+    return Err(ParserError::NotParseable);
+  }
+
+  return Ok();
+}
+
+Result<Ok, LocaleParser::ParserError> LocaleParser::CanParseUnicodeExtension(
+    Span<const char> aExtension) {
+  LocaleParser ts(aExtension);
+  Token tok = ts.NextToken();
+
+  // unicode_locale_extensions = sep [uU] ((sep keyword)+ |
+  //                                       (sep attribute)+ (sep keyword)*) ;
+
+  if (!ts.IsExtensionStart(tok) || ts.SingletonKey(tok) != 'u') {
+    return Err(ParserError::NotParseable);
+  }
+
+  tok = ts.NextToken();
+
+  if (tok.IsNone()) {
+    return Err(ParserError::NotParseable);
+  }
+
+  while (ts.IsUnicodeExtensionAttribute(tok)) {
+    tok = ts.NextToken();
+  }
+
+  // keyword = key (sep type)? ;
+  while (ts.IsUnicodeExtensionKey(tok)) {
+    tok = ts.NextToken();
+
+    while (ts.IsUnicodeExtensionType(tok)) {
+      tok = ts.NextToken();
+    }
+
+    if (tok.IsError()) {
+      return Err(ParserError::NotParseable);
+    }
+  }
+
+  if (!tok.IsNone()) {
+    return Err(ParserError::OutOfMemory);
+  }
+
+  return Ok();
+}
+
+Result<Ok, LocaleParser::ParserError>
+LocaleParser::CanParseUnicodeExtensionType(Span<const char> aUnicodeType) {
+  MOZ_ASSERT(!aUnicodeType.empty(), "caller must exclude empty strings");
+
+  LocaleParser ts(aUnicodeType);
+  Token tok = ts.NextToken();
+
+  while (ts.IsUnicodeExtensionType(tok)) {
+    tok = ts.NextToken();
+  }
+
+  if (!tok.IsNone()) {
+    return Err(ParserError::NotParseable);
+  }
+
+  return Ok();
+}
+
+}  // namespace mozilla::intl
diff --git a/intl/components/src/Locale.h b/intl/components/src/Locale.h
new file mode 100644
index 0000000000..1f4e06f543
--- /dev/null
+++ b/intl/components/src/Locale.h
@@ -0,0 +1,773 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* Structured representation of Unicode locale IDs used with Intl functions. */
+
+#ifndef intl_components_Locale_h
+#define intl_components_Locale_h
+
+#include "mozilla/Assertions.h"
+#include "mozilla/intl/ICUError.h"
+#include "mozilla/intl/ICU4CGlue.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/Span.h"
+#include "mozilla/TextUtils.h"
+#include "mozilla/Try.h"
+#include "mozilla/TypedEnumBits.h"
+#include "mozilla/Variant.h"
+#include "mozilla/Vector.h"
+
+#include <algorithm>
+#include <stddef.h>
+#include <stdint.h>
+#include <string.h>
+#include <utility>
+
+#include "unicode/uloc.h"
+
+namespace mozilla::intl {
+
+/**
+ * Return true if |language| is a valid language subtag.
+ */
+template <typename CharT>
+bool IsStructurallyValidLanguageTag(mozilla::Span<const CharT> aLanguage);
+
+/**
+ * Return true if |script| is a valid script subtag.
+ */
+template <typename CharT>
+bool IsStructurallyValidScriptTag(mozilla::Span<const CharT> aScript);
+
+/**
+ * Return true if |region| is a valid region subtag.
+ */
+template <typename CharT>
+bool IsStructurallyValidRegionTag(mozilla::Span<const CharT> aRegion);
+
+#ifdef DEBUG
+/**
+ * Return true if |variant| is a valid variant subtag.
+ */
+bool IsStructurallyValidVariantTag(mozilla::Span<const char> aVariant);
+
+/**
+ * Return true if |extension| is a valid Unicode extension subtag.
+ */
+bool IsStructurallyValidUnicodeExtensionTag(
+    mozilla::Span<const char> aExtension);
+
+/**
+ * Return true if |privateUse| is a valid private-use subtag.
+ */
+bool IsStructurallyValidPrivateUseTag(mozilla::Span<const char> aPrivateUse);
+
+#endif
+
+template <typename CharT>
+char AsciiToLowerCase(CharT aChar) {
+  MOZ_ASSERT(mozilla::IsAscii(aChar));
+  return mozilla::IsAsciiUppercaseAlpha(aChar) ? (aChar + 0x20) : aChar;
+}
+
+template <typename CharT>
+char AsciiToUpperCase(CharT aChar) {
+  MOZ_ASSERT(mozilla::IsAscii(aChar));
+  return mozilla::IsAsciiLowercaseAlpha(aChar) ? (aChar - 0x20) : aChar;
+}
+
+template <typename CharT>
+void AsciiToLowerCase(CharT* aChars, size_t aLength, char* aDest) {
+  char (&fn)(CharT) = AsciiToLowerCase;
+  std::transform(aChars, aChars + aLength, aDest, fn);
+}
+
+template <typename CharT>
+void AsciiToUpperCase(CharT* aChars, size_t aLength, char* aDest) {
+  char (&fn)(CharT) = AsciiToUpperCase;
+  std::transform(aChars, aChars + aLength, aDest, fn);
+}
+
+template <typename CharT>
+void AsciiToTitleCase(CharT* aChars, size_t aLength, char* aDest) {
+  if (aLength > 0) {
+    AsciiToUpperCase(aChars, 1, aDest);
+    AsciiToLowerCase(aChars + 1, aLength - 1, aDest + 1);
+  }
+}
+
+// Constants for language subtag lengths.
+namespace LanguageTagLimits {
+
+// unicode_language_subtag = alpha{2,3} | alpha{5,8} ;
+static constexpr size_t LanguageLength = 8;
+
+// unicode_script_subtag = alpha{4} ;
+static constexpr size_t ScriptLength = 4;
+
+// unicode_region_subtag = (alpha{2} | digit{3}) ;
+static constexpr size_t RegionLength = 3;
+static constexpr size_t AlphaRegionLength = 2;
+static constexpr size_t DigitRegionLength = 3;
+
+// key = alphanum alpha ;
+static constexpr size_t UnicodeKeyLength = 2;
+
+// tkey = alpha digit ;
+static constexpr size_t TransformKeyLength = 2;
+
+}  // namespace LanguageTagLimits
+
+// Fixed size language subtag which is stored inline in Locale.
+template <size_t SubtagLength>
+class LanguageTagSubtag final {
+  uint8_t mLength = 0;
+  char mChars[SubtagLength] = {};  // zero initialize
+
+ public:
+  LanguageTagSubtag() = default;
+
+  LanguageTagSubtag(const LanguageTagSubtag& aOther) {
+    std::copy_n(aOther.mChars, SubtagLength, mChars);
+    mLength = aOther.mLength;
+  }
+
+  LanguageTagSubtag& operator=(const LanguageTagSubtag& aOther) {
+    std::copy_n(aOther.mChars, SubtagLength, mChars);
+    mLength = aOther.mLength;
+    return *this;
+  }
+
+  size_t Length() const { return mLength; }
+  bool Missing() const { return mLength == 0; }
+  bool Present() const { return mLength > 0; }
+
+  mozilla::Span<const char> Span() const { return {mChars, mLength}; }
+
+  template <typename CharT>
+  void Set(mozilla::Span<const CharT> str) {
+    MOZ_ASSERT(str.size() <= SubtagLength);
+    std::copy_n(str.data(), str.size(), mChars);
+    mLength = str.size();
+  }
+
+  // The toXYZCase() methods are using |SubtagLength| instead of |length()|,
+  // because current compilers (tested GCC and Clang) can't infer the maximum
+  // string length - even when using hints like |std::min| - and instead are
+  // emitting SIMD optimized code. Using a fixed sized length avoids emitting
+  // the SIMD code. (Emitting SIMD code doesn't make sense here, because the
+  // SIMD code only kicks in for long strings.) A fixed length will
+  // additionally ensure the compiler unrolls the loop in the case conversion
+  // code.
+
+  void ToLowerCase() { AsciiToLowerCase(mChars, SubtagLength, mChars); }
+
+  void ToUpperCase() { AsciiToUpperCase(mChars, SubtagLength, mChars); }
+
+  void ToTitleCase() { AsciiToTitleCase(mChars, SubtagLength, mChars); }
+
+  template <size_t N>
+  bool EqualTo(const char (&str)[N]) const {
+    static_assert(N - 1 <= SubtagLength,
+                  "subtag literals must not exceed the maximum subtag length");
+
+    return mLength == N - 1 && memcmp(mChars, str, N - 1) == 0;
+  }
+};
+
+using LanguageSubtag = LanguageTagSubtag<LanguageTagLimits::LanguageLength>;
+using ScriptSubtag = LanguageTagSubtag<LanguageTagLimits::ScriptLength>;
+using RegionSubtag = LanguageTagSubtag<LanguageTagLimits::RegionLength>;
+
+using Latin1Char = unsigned char;
+using UniqueChars = UniquePtr<char[]>;
+
+/**
+ * Object representing a Unicode BCP 47 locale identifier.
+ *
+ * All subtags are already in canonicalized case.
+ */
+class MOZ_STACK_CLASS Locale final {
+  LanguageSubtag mLanguage = {};
+  ScriptSubtag mScript = {};
+  RegionSubtag mRegion = {};
+
+  using VariantsVector = Vector<UniqueChars, 2>;
+  using ExtensionsVector = Vector<UniqueChars, 2>;
+
+  VariantsVector mVariants;
+  ExtensionsVector mExtensions;
+  UniqueChars mPrivateUse = nullptr;
+
+  friend class LocaleParser;
+
+ public:
+  enum class CanonicalizationError : uint8_t {
+    DuplicateVariant,
+    InternalError,
+    OutOfMemory,
+  };
+
+ private:
+  Result<Ok, CanonicalizationError> CanonicalizeUnicodeExtension(
+      UniqueChars& unicodeExtension);
+
+  Result<Ok, CanonicalizationError> CanonicalizeTransformExtension(
+      UniqueChars& transformExtension);
+
+ public:
+  static bool LanguageMapping(LanguageSubtag& aLanguage);
+  static bool ComplexLanguageMapping(const LanguageSubtag& aLanguage);
+
+ private:
+  static bool ScriptMapping(ScriptSubtag& aScript);
+  static bool RegionMapping(RegionSubtag& aRegion);
+  static bool ComplexRegionMapping(const RegionSubtag& aRegion);
+
+  void PerformComplexLanguageMappings();
+  void PerformComplexRegionMappings();
+  [[nodiscard]] bool PerformVariantMappings();
+
+  [[nodiscard]] bool UpdateLegacyMappings();
+
+  static bool SignLanguageMapping(LanguageSubtag& aLanguage,
+                                  const RegionSubtag& aRegion);
+
+  static const char* ReplaceTransformExtensionType(
+      mozilla::Span<const char> aKey, mozilla::Span<const char> aType);
+
+ public:
+  /**
+   * Given a Unicode key and type, return the null-terminated preferred
+   * replacement for that type if there is one, or null if there is none, e.g.
+   * in effect
+   * |ReplaceUnicodeExtensionType("ca", "islamicc") == "islamic-civil"|
+   * and
+   * |ReplaceUnicodeExtensionType("ca", "islamic-civil") == nullptr|.
+   */
+  static const char* ReplaceUnicodeExtensionType(
+      mozilla::Span<const char> aKey, mozilla::Span<const char> aType);
+
+ public:
+  Locale() = default;
+  Locale(const Locale&) = delete;
+  Locale& operator=(const Locale&) = delete;
+  Locale(Locale&&) = default;
+  Locale& operator=(Locale&&) = default;
+
+  template <class Vec>
+  class SubtagIterator {
+    using Iter = decltype(std::declval<const Vec>().begin());
+
+    Iter mIter;
+
+   public:
+    explicit SubtagIterator(Iter iter) : mIter(iter) {}
+
+    // std::iterator traits.
+    using iterator_category = std::input_iterator_tag;
+    using value_type = Span<const char>;
+    using difference_type = ptrdiff_t;
+    using pointer = value_type*;
+    using reference = value_type&;
+
+    SubtagIterator& operator++() {
+      mIter++;
+      return *this;
+    }
+
+    SubtagIterator operator++(int) {
+      SubtagIterator result = *this;
+      ++(*this);
+      return result;
+    }
+
+    bool operator==(const SubtagIterator& aOther) const {
+      return mIter == aOther.mIter;
+    }
+
+    bool operator!=(const SubtagIterator& aOther) const {
+      return !(*this == aOther);
+    }
+
+    value_type operator*() const { return MakeStringSpan(mIter->get()); }
+  };
+
+  template <size_t N>
+  class SubtagEnumeration {
+    using Vec = Vector<UniqueChars, N>;
+
+    const Vec& mVector;
+
+   public:
+    explicit SubtagEnumeration(const Vec& aVector) : mVector(aVector) {}
+
+    size_t length() const { return mVector.length(); }
+    bool empty() const { return mVector.empty(); }
+
+    auto begin() const { return SubtagIterator<Vec>(mVector.begin()); }
+    auto end() const { return SubtagIterator<Vec>(mVector.end()); }
+
+    Span<const char> operator[](size_t aIndex) const {
+      return MakeStringSpan(mVector[aIndex].get());
+    }
+  };
+
+  const LanguageSubtag& Language() const { return mLanguage; }
+  const ScriptSubtag& Script() const { return mScript; }
+  const RegionSubtag& Region() const { return mRegion; }
+  auto Variants() const { return SubtagEnumeration(mVariants); }
+  auto Extensions() const { return SubtagEnumeration(mExtensions); }
+  Maybe<Span<const char>> PrivateUse() const {
+    if (const char* p = mPrivateUse.get()) {
+      return Some(MakeStringSpan(p));
+    }
+    return Nothing();
+  }
+
+  /**
+   * Return the Unicode extension subtag or Nothing if not present.
+   */
+  Maybe<Span<const char>> GetUnicodeExtension() const;
+
+ private:
+  ptrdiff_t UnicodeExtensionIndex() const;
+
+ public:
+  /**
+   * Set the language subtag. The input must be a valid language subtag.
+   */
+  template <size_t N>
+  void SetLanguage(const char (&aLanguage)[N]) {
+    mozilla::Span<const char> span(aLanguage, N - 1);
+    MOZ_ASSERT(IsStructurallyValidLanguageTag(span));
+    mLanguage.Set(span);
+  }
+
+  /**
+   * Set the language subtag. The input must be a valid language subtag.
+   */
+  void SetLanguage(const LanguageSubtag& aLanguage) {
+    MOZ_ASSERT(IsStructurallyValidLanguageTag(aLanguage.Span()));
+    mLanguage.Set(aLanguage.Span());
+  }
+
+  /**
+   * Set the script subtag. The input must be a valid script subtag.
+   */
+  template <size_t N>
+  void SetScript(const char (&aScript)[N]) {
+    mozilla::Span<const char> span(aScript, N - 1);
+    MOZ_ASSERT(IsStructurallyValidScriptTag(span));
+    mScript.Set(span);
+  }
+
+  /**
+   * Set the script subtag. The input must be a valid script subtag or the empty
+   * string.
+   */
+  void SetScript(const ScriptSubtag& aScript) {
+    MOZ_ASSERT(aScript.Missing() ||
+               IsStructurallyValidScriptTag(aScript.Span()));
+    mScript.Set(aScript.Span());
+  }
+
+  /**
+   * Set the region subtag. The input must be a valid region subtag.
+   */
+  template <size_t N>
+  void SetRegion(const char (&aRegion)[N]) {
+    mozilla::Span<const char> span(aRegion, N - 1);
+    MOZ_ASSERT(IsStructurallyValidRegionTag(span));
+    mRegion.Set(span);
+  }
+
+  /**
+   * Set the region subtag. The input must be a valid region subtag or the empty
+   * empty string.
+   */
+  void SetRegion(const RegionSubtag& aRegion) {
+    MOZ_ASSERT(aRegion.Missing() ||
+               IsStructurallyValidRegionTag(aRegion.Span()));
+    mRegion.Set(aRegion.Span());
+  }
+
+  /**
+   * Removes all variant subtags.
+   */
+  void ClearVariants() { mVariants.clearAndFree(); }
+
+  /**
+   * Set the Unicode extension subtag. The input must be a valid Unicode
+   * extension subtag.
+   */
+  ICUResult SetUnicodeExtension(Span<const char> aExtension);
+
+  /**
+   * Remove any Unicode extension subtag if present.
+   */
+  void ClearUnicodeExtension();
+
+  /** Canonicalize the base-name (language, script, region, variant) subtags. */
+  Result<Ok, CanonicalizationError> CanonicalizeBaseName();
+
+  /**
+   * Canonicalize all extension subtags.
+   */
+  Result<Ok, CanonicalizationError> CanonicalizeExtensions();
+
+  /**
+   * Canonicalizes the given structurally valid Unicode BCP 47 locale
+   * identifier, including regularized case of subtags. For example, the
+   * locale Zh-haNS-bu-variant2-Variant1-u-ca-chinese-t-Zh-laTN-x-PRIVATE,
+   * where
+   *
+   *     Zh             ; 2*3ALPHA
+   *     -haNS          ; ["-" script]
+   *     -bu            ; ["-" region]
+   *     -variant2      ; *("-" variant)
+   *     -Variant1
+   *     -u-ca-chinese  ; *("-" extension)
+   *     -t-Zh-laTN
+   *     -x-PRIVATE     ; ["-" privateuse]
+   *
+   * becomes zh-Hans-MM-variant1-variant2-t-zh-latn-u-ca-chinese-x-private
+   *
+   * Spec: ECMAScript Internationalization API Specification, 6.2.3.
+   */
+  Result<Ok, CanonicalizationError> Canonicalize() {
+    MOZ_TRY(CanonicalizeBaseName());
+    return CanonicalizeExtensions();
+  }
+
+  /**
+   * Fill the buffer with a string representation of the locale.
+   */
+  template <typename B>
+  ICUResult ToString(B& aBuffer) const {
+    static_assert(std::is_same_v<typename B::CharType, char>);
+
+    size_t capacity = ToStringCapacity();
+
+    // Attempt to reserve needed capacity
+    if (!aBuffer.reserve(capacity)) {
+      return Err(ICUError::OutOfMemory);
+    }
+
+    size_t offset = ToStringAppend(aBuffer.data());
+
+    MOZ_ASSERT(capacity == offset);
+    aBuffer.written(offset);
+
+    return Ok();
+  }
+
+  /**
+   * Add likely-subtags to the locale.
+   *
+   * Spec: <https://www.unicode.org/reports/tr35/#Likely_Subtags>
+   */
+  ICUResult AddLikelySubtags();
+
+  /**
+   * Remove likely-subtags from the locale.
+   *
+   * Spec: <https://www.unicode.org/reports/tr35/#Likely_Subtags>
+   */
+  ICUResult RemoveLikelySubtags();
+
+  /**
+   * Returns the default locale as an ICU locale identifier. The returned string
+   * is NOT a valid BCP 47 locale!
+   *
+   * Also see <https://unicode-org.github.io/icu/userguide/locale>.
+   */
+  static const char* GetDefaultLocale() { return uloc_getDefault(); }
+
+  /**
+   * Returns an iterator over all supported locales.
+   *
+   * The returned strings are ICU locale identifiers and NOT BCP 47 language
+   * tags.
+   *
+   * Also see <https://unicode-org.github.io/icu/userguide/locale>.
+   */
+  static auto GetAvailableLocales() {
+    return AvailableLocalesEnumeration<uloc_countAvailable,
+                                       uloc_getAvailable>();
+  }
+
+ private:
+  static UniqueChars DuplicateStringToUniqueChars(const char* aStr);
+  static UniqueChars DuplicateStringToUniqueChars(Span<const char> aStr);
+  size_t ToStringCapacity() const;
+  size_t ToStringAppend(char* aBuffer) const;
+};
+
+/**
+ * Parser for Unicode BCP 47 locale identifiers.
+ *
+ * <https://unicode.org/reports/tr35/#Unicode_Language_and_Locale_Identifiers>
+ */
+class MOZ_STACK_CLASS LocaleParser final {
+ public:
+  enum class ParserError : uint8_t {
+    // Input was not parseable as a locale, subtag or extension.
+    NotParseable,
+    // Unable to allocate memory for the parser to operate.
+    OutOfMemory,
+  };
+
+  // Exposed as |public| for |MOZ_MAKE_ENUM_CLASS_BITWISE_OPERATORS|.
+  enum class TokenKind : uint8_t {
+    None = 0b000,
+    Alpha = 0b001,
+    Digit = 0b010,
+    AlphaDigit = 0b011,
+    Error = 0b100
+  };
+
+ private:
+  class Token final {
+    size_t mIndex;
+    size_t mLength;
+    TokenKind mKind;
+
+   public:
+    Token(TokenKind aKind, size_t aIndex, size_t aLength)
+        : mIndex(aIndex), mLength(aLength), mKind(aKind) {}
+
+    TokenKind Kind() const { return mKind; }
+    size_t Index() const { return mIndex; }
+    size_t Length() const { return mLength; }
+
+    bool IsError() const { return mKind == TokenKind::Error; }
+    bool IsNone() const { return mKind == TokenKind::None; }
+    bool IsAlpha() const { return mKind == TokenKind::Alpha; }
+    bool IsDigit() const { return mKind == TokenKind::Digit; }
+    bool IsAlphaDigit() const { return mKind == TokenKind::AlphaDigit; }
+  };
+
+  const char* mLocale;
+  size_t mLength;
+  size_t mIndex = 0;
+
+  explicit LocaleParser(Span<const char> aLocale)
+      : mLocale(aLocale.data()), mLength(aLocale.size()) {}
+
+  char CharAt(size_t aIndex) const { return mLocale[aIndex]; }
+
+  // Copy the token characters into |subtag|.
+  template <size_t N>
+  void CopyChars(const Token& aTok, LanguageTagSubtag<N>& aSubtag) const {
+    aSubtag.Set(mozilla::Span(mLocale + aTok.Index(), aTok.Length()));
+  }
+
+  // Create a string copy of |length| characters starting at |index|.
+  UniqueChars Chars(size_t aIndex, size_t aLength) const;
+
+  // Create a string copy of the token characters.
+  UniqueChars Chars(const Token& aTok) const {
+    return Chars(aTok.Index(), aTok.Length());
+  }
+
+  UniqueChars Extension(const Token& aStart, const Token& aEnd) const {
+    MOZ_ASSERT(aStart.Index() < aEnd.Index());
+
+    size_t length = aEnd.Index() - 1 - aStart.Index();
+    return Chars(aStart.Index(), length);
+  }
+
+  Token NextToken();
+
+  // unicode_language_subtag = alpha{2,3} | alpha{5,8} ;
+  //
+  // Four character language subtags are not allowed in Unicode BCP 47 locale
+  // identifiers. Also see the comparison to Unicode CLDR locale identifiers in
+  // <https://unicode.org/reports/tr35/#BCP_47_Conformance>.
+  bool IsLanguage(const Token& aTok) const {
+    return aTok.IsAlpha() && ((2 <= aTok.Length() && aTok.Length() <= 3) ||
+                              (5 <= aTok.Length() && aTok.Length() <= 8));
+  }
+
+  // unicode_script_subtag = alpha{4} ;
+  bool IsScript(const Token& aTok) const {
+    return aTok.IsAlpha() && aTok.Length() == 4;
+  }
+
+  // unicode_region_subtag = (alpha{2} | digit{3}) ;
+  bool IsRegion(const Token& aTok) const {
+    return (aTok.IsAlpha() && aTok.Length() == 2) ||
+           (aTok.IsDigit() && aTok.Length() == 3);
+  }
+
+  // unicode_variant_subtag = (alphanum{5,8} | digit alphanum{3}) ;
+  bool IsVariant(const Token& aTok) const {
+    return (5 <= aTok.Length() && aTok.Length() <= 8) ||
+           (aTok.Length() == 4 && mozilla::IsAsciiDigit(CharAt(aTok.Index())));
+  }
+
+  // Returns the code unit of the first character at the given singleton token.
+  // Always returns the lower case form of an alphabetical character.
+  char SingletonKey(const Token& aTok) const {
+    MOZ_ASSERT(aTok.Length() == 1);
+    return AsciiToLowerCase(CharAt(aTok.Index()));
+  }
+
+  // extensions = unicode_locale_extensions |
+  //              transformed_extensions |
+  //              other_extensions ;
+  //
+  // unicode_locale_extensions = sep [uU] ((sep keyword)+ |
+  //                                       (sep attribute)+ (sep keyword)*) ;
+  //
+  // transformed_extensions = sep [tT] ((sep tlang (sep tfield)*) |
+  //                                    (sep tfield)+) ;
+  //
+  // other_extensions = sep [alphanum-[tTuUxX]] (sep alphanum{2,8})+ ;
+  bool IsExtensionStart(const Token& aTok) const {
+    return aTok.Length() == 1 && SingletonKey(aTok) != 'x';
+  }
+
+  // other_extensions = sep [alphanum-[tTuUxX]] (sep alphanum{2,8})+ ;
+  bool IsOtherExtensionPart(const Token& aTok) const {
+    return 2 <= aTok.Length() && aTok.Length() <= 8;
+  }
+
+  // unicode_locale_extensions = sep [uU] ((sep keyword)+ |
+  //                                       (sep attribute)+ (sep keyword)*) ;
+  // keyword = key (sep type)? ;
+  bool IsUnicodeExtensionPart(const Token& aTok) const {
+    return IsUnicodeExtensionKey(aTok) || IsUnicodeExtensionType(aTok) ||
+           IsUnicodeExtensionAttribute(aTok);
+  }
+
+  // attribute = alphanum{3,8} ;
+  bool IsUnicodeExtensionAttribute(const Token& aTok) const {
+    return 3 <= aTok.Length() && aTok.Length() <= 8;
+  }
+
+  // key = alphanum alpha ;
+  bool IsUnicodeExtensionKey(const Token& aTok) const {
+    return aTok.Length() == 2 &&
+           mozilla::IsAsciiAlpha(CharAt(aTok.Index() + 1));
+  }
+
+  // type = alphanum{3,8} (sep alphanum{3,8})* ;
+  bool IsUnicodeExtensionType(const Token& aTok) const {
+    return 3 <= aTok.Length() && aTok.Length() <= 8;
+  }
+
+  // tkey = alpha digit ;
+  bool IsTransformExtensionKey(const Token& aTok) const {
+    return aTok.Length() == 2 && mozilla::IsAsciiAlpha(CharAt(aTok.Index())) &&
+           mozilla::IsAsciiDigit(CharAt(aTok.Index() + 1));
+  }
+
+  // tvalue = (sep alphanum{3,8})+ ;
+  bool IsTransformExtensionPart(const Token& aTok) const {
+    return 3 <= aTok.Length() && aTok.Length() <= 8;
+  }
+
+  // pu_extensions = sep [xX] (sep alphanum{1,8})+ ;
+  bool IsPrivateUseStart(const Token& aTok) const {
+    return aTok.Length() == 1 && SingletonKey(aTok) == 'x';
+  }
+
+  // pu_extensions = sep [xX] (sep alphanum{1,8})+ ;
+  bool IsPrivateUsePart(const Token& aTok) const {
+    return 1 <= aTok.Length() && aTok.Length() <= 8;
+  }
+
+  // Helper function for use in |ParseBaseName| and
+  // |ParseTlangInTransformExtension|.  Do not use this directly!
+  static Result<Ok, ParserError> InternalParseBaseName(
+      LocaleParser& aLocaleParser, Locale& aTag, Token& aTok);
+
+  // Parse the `unicode_language_id` production, i.e. the
+  // language/script/region/variants portion of a locale, into |aTag|.
+  // |aTok| must be the current token.
+  static Result<Ok, ParserError> ParseBaseName(LocaleParser& aLocaleParser,
+                                               Locale& aTag, Token& aTok) {
+    return InternalParseBaseName(aLocaleParser, aTag, aTok);
+  }
+
+  // Parse the `tlang` production within a parsed 't' transform extension.
+  // The precise requirements for "previously parsed" are:
+  //
+  //   * the input begins from current token |tok| with a valid `tlang`
+  //   * the `tlang` is wholly lowercase (*not* canonical case)
+  //   * variant subtags in the `tlang` may contain duplicates and be
+  //     unordered
+  //
+  // Return an error on internal failure. Otherwise, return a success value. If
+  // there was no `tlang`, then |tag.language().missing()|. But if there was a
+  // `tlang`, then |tag| is filled with subtags exactly as they appeared in the
+  // parse input.
+  static Result<Ok, ParserError> ParseTlangInTransformExtension(
+      LocaleParser& aLocaleParser, Locale& aTag, Token& aTok) {
+    MOZ_ASSERT(aLocaleParser.IsLanguage(aTok));
+    return InternalParseBaseName(aLocaleParser, aTag, aTok);
+  }
+
+  friend class Locale;
+
+  class Range final {
+    size_t mBegin;
+    size_t mLength;
+
+   public:
+    Range(size_t aBegin, size_t aLength) : mBegin(aBegin), mLength(aLength) {}
+
+    size_t Begin() const { return mBegin; }
+    size_t Length() const { return mLength; }
+  };
+
+  using TFieldVector = Vector<Range, 8>;
+  using AttributesVector = Vector<Range, 8>;
+  using KeywordsVector = Vector<Range, 8>;
+
+  // Parse |extension|, which must be a validated, fully lowercase
+  // `transformed_extensions` subtag, and fill |tag| and |fields| from the
+  // `tlang` and `tfield` components. Data in |tag| is lowercase, consistent
+  // with |extension|.
+  static Result<Ok, ParserError> ParseTransformExtension(
+      mozilla::Span<const char> aExtension, Locale& aTag,
+      TFieldVector& aFields);
+
+  // Parse |extension|, which must be a validated, fully lowercase
+  // `unicode_locale_extensions` subtag, and fill |attributes| and |keywords|
+  // from the `attribute` and `keyword` components.
+  static Result<Ok, ParserError> ParseUnicodeExtension(
+      mozilla::Span<const char> aExtension, AttributesVector& aAttributes,
+      KeywordsVector& aKeywords);
+
+ public:
+  // Parse the input string as a locale.
+  //
+  // NOTE: |aTag| must be a new, empty Locale.
+  static Result<Ok, ParserError> TryParse(Span<const char> aLocale,
+                                          Locale& aTag);
+
+  // Parse the input string as the base-name parts (language, script, region,
+  // variants) of a locale.
+  //
+  // NOTE: |aTag| must be a new, empty Locale.
+  static Result<Ok, ParserError> TryParseBaseName(Span<const char> aLocale,
+                                                  Locale& aTag);
+
+  // Return Ok() iff |extension| can be parsed as a Unicode extension subtag.
+  static Result<Ok, ParserError> CanParseUnicodeExtension(
+      Span<const char> aExtension);
+
+  // Return Ok() iff |unicodeType| can be parsed as a Unicode extension type.
+  static Result<Ok, ParserError> CanParseUnicodeExtensionType(
+      Span<const char> aUnicodeType);
+};
+
+MOZ_MAKE_ENUM_CLASS_BITWISE_OPERATORS(LocaleParser::TokenKind)
+
+}  // namespace mozilla::intl
+
+#endif /* intl_components_Locale_h */
diff --git a/intl/components/src/LocaleCanonicalizer.cpp b/intl/components/src/LocaleCanonicalizer.cpp
new file mode 100644
index 0000000000..8a83874390
--- /dev/null
+++ b/intl/components/src/LocaleCanonicalizer.cpp
@@ -0,0 +1,36 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "mozilla/intl/LocaleCanonicalizer.h"
+#include <cstdio>
+#include "unicode/uloc.h"
+
+namespace mozilla::intl {
+
+/* static */
+ICUResult LocaleCanonicalizer::CanonicalizeICULevel1(
+    const char* aLocaleIn, LocaleCanonicalizer::Vector& aLocaleOut) {
+  auto result = FillBufferWithICUCall(
+      aLocaleOut,
+      [&aLocaleIn](char* target, int32_t length, UErrorCode* status) {
+        return uloc_canonicalize(aLocaleIn, target, length, status);
+      });
+
+  if (result.isErr()) {
+    return Err(result.unwrapErr());
+  }
+
+  // This step is not included in the normal ICU4C canonicalization step, but
+  // consumers were expecting the results to actually be ASCII. It seemed safer
+  // to include it.
+  for (auto byte : aLocaleOut) {
+    if (static_cast<unsigned char>(byte) > 127) {
+      return Err(ICUError::InternalError);
+    }
+  }
+
+  return Ok();
+}
+
+}  // namespace mozilla::intl
diff --git a/intl/components/src/LocaleCanonicalizer.h b/intl/components/src/LocaleCanonicalizer.h
new file mode 100644
index 0000000000..bd17c9dfd4
--- /dev/null
+++ b/intl/components/src/LocaleCanonicalizer.h
@@ -0,0 +1,43 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+#ifndef intl_components_LocaleCanonicalizer_h_
+#define intl_components_LocaleCanonicalizer_h_
+
+#include "mozilla/intl/ICU4CGlue.h"
+#include "mozilla/Span.h"
+#include "mozilla/Vector.h"
+
+namespace mozilla::intl {
+
+/**
+ * 32 is somewhat an arbitrary size, but it should fit most locales on the
+ * stack to avoid heap allocations.
+ */
+constexpr size_t INITIAL_LOCALE_CANONICALIZER_BUFFER_SIZE = 32;
+
+/**
+ * Eventually this class will unify the behaviors of Locale Canonicalization.
+ * See Bug 1723586.
+ */
+class LocaleCanonicalizer {
+ public:
+  using Vector =
+      mozilla::Vector<char, INITIAL_LOCALE_CANONICALIZER_BUFFER_SIZE>;
+
+  /**
+   * This static method will canonicalize a locale string, per the Level 1
+   * canonicalization steps outlined in:
+   * http://userguide.icu-project.org/locale#TOC-Canonicalization
+   *
+   * For instance it will turn the string "en-US" to "en_US". It guarantees that
+   * the string span targeted will be in the ASCII range. The canonicalization
+   * process on ICU is somewhat permissive in what it accepts as input, but only
+   * ASCII locales are technically correct.
+   */
+  static ICUResult CanonicalizeICULevel1(
+      const char* aLocale, LocaleCanonicalizer::Vector& aLocaleOut);
+};
+
+}  // namespace mozilla::intl
+#endif
diff --git a/intl/components/src/LocaleGenerated.cpp b/intl/components/src/LocaleGenerated.cpp
new file mode 100644
index 0000000000..427a78de72
--- /dev/null
+++ b/intl/components/src/LocaleGenerated.cpp
@@ -0,0 +1,1208 @@
+// Generated by make_intl_data.py. DO NOT EDIT.
+// Version: CLDR-43
+// URL: https://unicode.org/Public/cldr/43/cldr-common-43.0.zip
+
+#include "mozilla/Assertions.h"
+#include "mozilla/Span.h"
+#include "mozilla/TextUtils.h"
+
+#include <algorithm>
+#include <cstdint>
+#include <cstring>
+#include <iterator>
+#include <string>
+#include <type_traits>
+
+#include "mozilla/intl/Locale.h"
+
+using namespace mozilla::intl::LanguageTagLimits;
+
+template <size_t Length, size_t TagLength, size_t SubtagLength>
+static inline bool HasReplacement(
+    const char (&subtags)[Length][TagLength],
+    const mozilla::intl::LanguageTagSubtag<SubtagLength>& subtag) {
+  MOZ_ASSERT(subtag.Length() == TagLength - 1,
+             "subtag must have the same length as the list of subtags");
+
+  const char* ptr = subtag.Span().data();
+  return std::binary_search(std::begin(subtags), std::end(subtags), ptr,
+                            [](const char* a, const char* b) {
+                              return memcmp(a, b, TagLength - 1) < 0;
+                            });
+}
+
+template <size_t Length, size_t TagLength, size_t SubtagLength>
+static inline const char* SearchReplacement(
+    const char (&subtags)[Length][TagLength], const char* (&aliases)[Length],
+    const mozilla::intl::LanguageTagSubtag<SubtagLength>& subtag) {
+  MOZ_ASSERT(subtag.Length() == TagLength - 1,
+             "subtag must have the same length as the list of subtags");
+
+  const char* ptr = subtag.Span().data();
+  auto p = std::lower_bound(std::begin(subtags), std::end(subtags), ptr,
+                            [](const char* a, const char* b) {
+                              return memcmp(a, b, TagLength - 1) < 0;
+                            });
+  if (p != std::end(subtags) && memcmp(*p, ptr, TagLength - 1) == 0) {
+    return aliases[std::distance(std::begin(subtags), p)];
+  }
+  return nullptr;
+}
+
+#ifdef DEBUG
+static bool IsAsciiLowercaseAlphanumeric(char c) {
+  return mozilla::IsAsciiLowercaseAlpha(c) || mozilla::IsAsciiDigit(c);
+}
+
+static bool IsAsciiLowercaseAlphanumericOrDash(char c) {
+  return IsAsciiLowercaseAlphanumeric(c) || c == '-';
+}
+
+static bool IsCanonicallyCasedLanguageTag(mozilla::Span<const char> span) {
+  return std::all_of(span.begin(), span.end(),
+                     mozilla::IsAsciiLowercaseAlpha<char>);
+}
+
+static bool IsCanonicallyCasedScriptTag(mozilla::Span<const char> span) {
+  return mozilla::IsAsciiUppercaseAlpha(span[0]) &&
+         std::all_of(span.begin() + 1, span.end(),
+                     mozilla::IsAsciiLowercaseAlpha<char>);
+}
+
+static bool IsCanonicallyCasedRegionTag(mozilla::Span<const char> span) {
+  return std::all_of(span.begin(), span.end(),
+                     mozilla::IsAsciiUppercaseAlpha<char>) ||
+         std::all_of(span.begin(), span.end(), mozilla::IsAsciiDigit<char>);
+}
+
+static bool IsCanonicallyCasedVariantTag(mozilla::Span<const char> span) {
+  return std::all_of(span.begin(), span.end(), IsAsciiLowercaseAlphanumeric);
+}
+
+static bool IsCanonicallyCasedUnicodeKey(mozilla::Span<const char> key) {
+  return std::all_of(key.begin(), key.end(), IsAsciiLowercaseAlphanumeric);
+}
+
+static bool IsCanonicallyCasedUnicodeType(mozilla::Span<const char> type) {
+  return std::all_of(type.begin(), type.end(),
+                     IsAsciiLowercaseAlphanumericOrDash);
+}
+
+static bool IsCanonicallyCasedTransformKey(mozilla::Span<const char> key) {
+  return std::all_of(key.begin(), key.end(), IsAsciiLowercaseAlphanumeric);
+}
+
+static bool IsCanonicallyCasedTransformType(mozilla::Span<const char> type) {
+  return std::all_of(type.begin(), type.end(),
+                     IsAsciiLowercaseAlphanumericOrDash);
+}
+#endif
+
+// Mappings from language subtags to preferred values.
+// Derived from CLDR Supplemental Data, version 43.
+// https://unicode.org/Public/cldr/43/cldr-common-43.0.zip
+bool mozilla::intl::Locale::LanguageMapping(LanguageSubtag& language) {
+  MOZ_ASSERT(IsStructurallyValidLanguageTag(language.Span()));
+  MOZ_ASSERT(IsCanonicallyCasedLanguageTag(language.Span()));
+
+  if (language.Length() == 2) {
+    static const char languages[8][3] = {
+      "bh", "in", "iw", "ji", "jw", "mo", "tl", "tw",
+    };
+    static const char* aliases[8] = {
+      "bho", "id", "he", "yi", "jv", "ro", "fil", "ak",
+    };
+
+    if (const char* replacement = SearchReplacement(languages, aliases, language)) {
+      language.Set(mozilla::MakeStringSpan(replacement));
+      return true;
+    }
+    return false;
+  }
+
+  if (language.Length() == 3) {
+    static const char languages[408][4] = {
+      "aam", "aar", "abk", "adp", "afr", "agp", "ais", "ajt", "aju", "aka",
+      "alb", "als", "amh", "ara", "arb", "arg", "arm", "asd", "asm", "aue",
+      "ava", "ave", "aym", "ayr", "ayx", "aze", "azj", "bak", "bam", "baq",
+      "baz", "bcc", "bcl", "bel", "ben", "bgm", "bhk", "bic", "bih", "bis",
+      "bjd", "bjq", "bkb", "blg", "bod", "bos", "bre", "btb", "bul", "bur",
+      "bxk", "bxr", "cat", "ccq", "ces", "cha", "che", "chi", "chu", "chv",
+      "cjr", "cka", "cld", "cmk", "cmn", "cor", "cos", "coy", "cqu", "cre",
+      "cwd", "cym", "cze", "daf", "dan", "dap", "deu", "dgo", "dhd", "dik",
+      "diq", "dit", "div", "djl", "dkl", "drh", "drr", "dud", "duj", "dut",
+      "dwl", "dzo", "ekk", "ell", "elp", "emk", "eng", "epo", "esk", "est",
+      "eus", "ewe", "fao", "fas", "fat", "fij", "fin", "fra", "fre", "fry",
+      "fuc", "ful", "gav", "gaz", "gbc", "gbo", "geo", "ger", "gfx", "ggn",
+      "ggo", "ggr", "gio", "gla", "gle", "glg", "gli", "glv", "gno", "gre",
+      "grn", "gti", "gug", "guj", "guv", "gya", "hat", "hau", "hdn", "hea",
+      "heb", "her", "him", "hin", "hmo", "hrr", "hrv", "hun", "hye", "ibi",
+      "ibo", "ice", "ido", "iii", "ike", "iku", "ile", "ill", "ilw", "ina",
+      "ind", "ipk", "isl", "ita", "izi", "jar", "jav", "jeg", "jpn", "kal",
+      "kan", "kas", "kat", "kau", "kaz", "kdv", "kgc", "kgd", "kgh", "khk",
+      "khm", "kik", "kin", "kir", "kmr", "knc", "kng", "knn", "koj", "kom",
+      "kon", "kor", "kpp", "kpv", "krm", "ktr", "kua", "kur", "kvs", "kwq",
+      "kxe", "kxl", "kzh", "kzj", "kzt", "lak", "lao", "lat", "lav", "lbk",
+      "leg", "lii", "lim", "lin", "lit", "llo", "lmm", "ltz", "lub", "lug",
+      "lvs", "mac", "mah", "mal", "mao", "mar", "may", "meg", "mgx", "mhr",
+      "mkd", "mlg", "mlt", "mnk", "mnt", "mof", "mol", "mon", "mri", "msa",
+      "mst", "mup", "mwd", "mwj", "mya", "myd", "myt", "nad", "nau", "nav",
+      "nbf", "nbl", "nbx", "ncp", "nde", "ndo", "nep", "nld", "nln", "nlr",
+      "nno", "nns", "nnx", "nob", "noo", "nor", "npi", "nts", "nxu", "nya",
+      "oci", "ojg", "oji", "ori", "orm", "ory", "oss", "oun", "pan", "pat",
+      "pbu", "pcr", "per", "pes", "pli", "plt", "pmc", "pmu", "pnb", "pol",
+      "por", "ppa", "ppr", "pry", "pus", "puz", "que", "quz", "rmr", "rmy",
+      "roh", "ron", "rum", "run", "rus", "sag", "san", "sap", "sca", "scc",
+      "scr", "sgl", "sin", "skk", "slk", "slo", "slv", "smd", "sme", "smo",
+      "sna", "snb", "snd", "som", "sot", "spa", "spy", "sqi", "src", "srd",
+      "srp", "ssw", "sul", "sum", "sun", "swa", "swe", "swh", "tah", "tam",
+      "tat", "tdu", "tel", "tgg", "tgk", "tgl", "tha", "thc", "thw", "thx",
+      "tib", "tid", "tie", "tir", "tkk", "tlw", "tmp", "tne", "ton", "tsf",
+      "tsn", "tso", "ttq", "tuk", "tur", "twi", "uig", "ukr", "umu", "unp",
+      "uok", "urd", "uzb", "uzn", "ven", "vie", "vol", "wel", "wgw", "wit",
+      "wiw", "wln", "wol", "xba", "xho", "xia", "xkh", "xpe", "xrq", "xsj",
+      "xsl", "ybd", "ydd", "yen", "yid", "yiy", "yma", "ymt", "yor", "yos",
+      "yuu", "zai", "zha", "zho", "zir", "zsm", "zul", "zyb",
+    };
+    static const char* aliases[408] = {
+      "aas",  "aa",  "ab",  "dz",  "af", "apf", "ami", "aeb", "jrb",  "ak",
+       "sq",  "sq",  "am",  "ar",  "ar",  "an",  "hy", "snz",  "as", "ktz",
+       "av",  "ae",  "ay",  "ay", "nun",  "az",  "az",  "ba",  "bm",  "eu",
+      "nvo", "bal", "bik",  "be",  "bn", "bcg", "fbl", "bir", "bho",  "bi",
+      "drl", "bzc", "ebk", "iba",  "bo",  "bs",  "br", "beb",  "bg",  "my",
+      "luy", "bua",  "ca", "rki",  "cs",  "ch",  "ce",  "zh",  "cu",  "cv",
+      "mom", "cmr", "syr", "xch",  "zh",  "kw",  "co", "pij", "quh",  "cr",
+       "cr",  "cy",  "cs", "dnj",  "da", "njz",  "de", "doi", "mwr", "din",
+      "zza", "dif",  "dv", "dze", "aqd",  "mn", "kzk", "uth", "dwu",  "nl",
+      "dbt",  "dz",  "et",  "el", "amq", "man",  "en",  "eo",  "ik",  "et",
+       "eu",  "ee",  "fo",  "fa",  "ak",  "fj",  "fi",  "fr",  "fr",  "fy",
+       "ff",  "ff", "dev",  "om", "wny", "grb",  "ka",  "de", "vaj", "gvr",
+      "esg", "gtu", "aou",  "gd",  "ga",  "gl", "kzk",  "gv", "gon",  "el",
+       "gn", "nyc",  "gn",  "gu", "duz", "gba",  "ht",  "ha", "hai", "hmn",
+       "he",  "hz", "srx",  "hi",  "ho", "jal",  "hr",  "hu",  "hy", "opa",
+       "ig",  "is",  "io",  "ii",  "iu",  "iu",  "ie", "ilm", "gal",  "ia",
+       "id",  "ik",  "is",  "it", "eza", "jgk",  "jv", "oyb",  "ja",  "kl",
+       "kn",  "ks",  "ka",  "kr",  "kk", "zkd", "tdf", "ncq", "kml",  "mn",
+       "km",  "ki",  "rw",  "ky",  "ku",  "kr",  "kg", "kok", "kwv",  "kv",
+       "kg",  "ko", "jkm",  "kv", "bmf", "dtp",  "kj",  "ku", "gdj", "yam",
+      "tvd", "kru", "dgl", "dtp", "dtp", "ksp",  "lo",  "la",  "lv", "bnc",
+      "enl", "raq",  "li",  "ln",  "lt", "ngt", "rmx",  "lb",  "lu",  "lg",
+       "lv",  "mk",  "mh",  "ml",  "mi",  "mr",  "ms", "cir", "jbk", "chm",
+       "mk",  "mg",  "mt", "man", "wnn", "xnt",  "ro",  "mn",  "mi",  "ms",
+      "mry", "raj", "dmw", "vaj",  "my", "aog", "mry", "xny",  "na",  "nv",
+      "nru",  "nr", "ekc", "kdz",  "nd",  "ng",  "ne",  "nl", "azd", "nrk",
+       "nn", "nbr", "ngv",  "nb", "dtd",  "no",  "ne", "pij", "bpp",  "ny",
+       "oc",  "oj",  "oj",  "or",  "om",  "or",  "os", "vaj",  "pa", "kxr",
+       "ps", "adx",  "fa",  "fa",  "pi",  "mg", "huw", "phr", "lah",  "pl",
+       "pt", "bfy", "lcq", "prt",  "ps", "pub",  "qu",  "qu", "emx", "rom",
+       "rm",  "ro",  "ro",  "rn",  "ru",  "sg",  "sa", "aqt", "hle",  "sr",
+       "hr", "isk",  "si", "oyb",  "sk",  "sk",  "sl", "kmb",  "se",  "sm",
+       "sn", "iba",  "sd",  "so",  "st",  "es", "kln",  "sq",  "sc",  "sc",
+       "sr",  "ss", "sgd", "ulw",  "su",  "sw",  "sv",  "sw",  "ty",  "ta",
+       "tt", "dtp",  "te", "bjp",  "tg", "fil",  "th", "tpo", "ola", "oyb",
+       "bo", "itd", "ras",  "ti", "twm", "weo", "tyj", "kak",  "to", "taj",
+       "tn",  "ts", "tmh",  "tk",  "tr",  "ak",  "ug",  "uk", "del", "wro",
+      "ema",  "ur",  "uz",  "uz",  "ve",  "vi",  "vo",  "cy", "wgb", "nol",
+      "nwo",  "wa",  "wo", "cax",  "xh", "acn", "waw", "kpe", "dmw", "suj",
+      "den", "rki",  "yi", "ynq",  "yi", "yrm", "lrr", "mtm",  "yo", "zom",
+      "yug", "zap",  "za",  "zh", "scv",  "ms",  "zu",  "za",
+    };
+
+    if (const char* replacement = SearchReplacement(languages, aliases, language)) {
+      language.Set(mozilla::MakeStringSpan(replacement));
+      return true;
+    }
+    return false;
+  }
+
+  return false;
+}
+
+// Language subtags with complex mappings.
+// Derived from CLDR Supplemental Data, version 43.
+// https://unicode.org/Public/cldr/43/cldr-common-43.0.zip
+bool mozilla::intl::Locale::ComplexLanguageMapping(const LanguageSubtag& language) {
+  MOZ_ASSERT(IsStructurallyValidLanguageTag(language.Span()));
+  MOZ_ASSERT(IsCanonicallyCasedLanguageTag(language.Span()));
+
+  if (language.Length() == 2) {
+    return language.EqualTo("sh");
+  }
+
+  if (language.Length() == 3) {
+    static const char languages[6][4] = {
+      "cnr", "drw", "hbs", "prs", "swc", "tnf",
+    };
+
+    return HasReplacement(languages, language);
+  }
+
+  return false;
+}
+
+// Mappings from script subtags to preferred values.
+// Derived from CLDR Supplemental Data, version 43.
+// https://unicode.org/Public/cldr/43/cldr-common-43.0.zip
+bool mozilla::intl::Locale::ScriptMapping(ScriptSubtag& script) {
+  MOZ_ASSERT(IsStructurallyValidScriptTag(script.Span()));
+  MOZ_ASSERT(IsCanonicallyCasedScriptTag(script.Span()));
+
+  {
+    if (script.EqualTo("Qaai")) {
+      script.Set(mozilla::MakeStringSpan("Zinh"));
+      return true;
+    }
+    return false;
+  }
+}
+
+// Mappings from region subtags to preferred values.
+// Derived from CLDR Supplemental Data, version 43.
+// https://unicode.org/Public/cldr/43/cldr-common-43.0.zip
+bool mozilla::intl::Locale::RegionMapping(RegionSubtag& region) {
+  MOZ_ASSERT(IsStructurallyValidRegionTag(region.Span()));
+  MOZ_ASSERT(IsCanonicallyCasedRegionTag(region.Span()));
+
+  if (region.Length() == 2) {
+    static const char regions[23][3] = {
+      "BU", "CS", "CT", "DD", "DY", "FQ", "FX", "HV", "JT", "MI",
+      "NH", "NQ", "PU", "PZ", "QU", "RH", "TP", "UK", "VD", "WK",
+      "YD", "YU", "ZR",
+    };
+    static const char* aliases[23] = {
+      "MM", "RS", "KI", "DE", "BJ", "AQ", "FR", "BF", "UM", "UM",
+      "VU", "AQ", "UM", "PA", "EU", "ZW", "TL", "GB", "VN", "UM",
+      "YE", "RS", "CD",
+    };
+
+    if (const char* replacement = SearchReplacement(regions, aliases, region)) {
+      region.Set(mozilla::MakeStringSpan(replacement));
+      return true;
+    }
+    return false;
+  }
+
+  {
+    static const char regions[299][4] = {
+      "004", "008", "010", "012", "016", "020", "024", "028", "031", "032",
+      "036", "040", "044", "048", "050", "051", "052", "056", "060", "064",
+      "068", "070", "072", "074", "076", "084", "086", "090", "092", "096",
+      "100", "104", "108", "112", "116", "120", "124", "132", "136", "140",
+      "144", "148", "152", "156", "158", "162", "166", "170", "174", "175",
+      "178", "180", "184", "188", "191", "192", "196", "203", "204", "208",
+      "212", "214", "218", "222", "226", "230", "231", "232", "233", "234",
+      "238", "239", "242", "246", "248", "249", "250", "254", "258", "260",
+      "262", "266", "268", "270", "275", "276", "278", "280", "288", "292",
+      "296", "300", "304", "308", "312", "316", "320", "324", "328", "332",
+      "334", "336", "340", "344", "348", "352", "356", "360", "364", "368",
+      "372", "376", "380", "384", "388", "392", "398", "400", "404", "408",
+      "410", "414", "417", "418", "422", "426", "428", "430", "434", "438",
+      "440", "442", "446", "450", "454", "458", "462", "466", "470", "474",
+      "478", "480", "484", "492", "496", "498", "499", "500", "504", "508",
+      "512", "516", "520", "524", "528", "531", "533", "534", "535", "540",
+      "548", "554", "558", "562", "566", "570", "574", "578", "580", "581",
+      "583", "584", "585", "586", "591", "598", "600", "604", "608", "612",
+      "616", "620", "624", "626", "630", "634", "638", "642", "643", "646",
+      "652", "654", "659", "660", "662", "663", "666", "670", "674", "678",
+      "682", "686", "688", "690", "694", "702", "703", "704", "705", "706",
+      "710", "716", "720", "724", "728", "729", "732", "736", "740", "744",
+      "748", "752", "756", "760", "762", "764", "768", "772", "776", "780",
+      "784", "788", "792", "795", "796", "798", "800", "804", "807", "818",
+      "826", "830", "831", "832", "833", "834", "840", "850", "854", "858",
+      "860", "862", "876", "882", "886", "887", "891", "894", "958", "959",
+      "960", "962", "963", "964", "965", "966", "967", "968", "969", "970",
+      "971", "972", "973", "974", "975", "976", "977", "978", "979", "980",
+      "981", "982", "983", "984", "985", "986", "987", "988", "989", "990",
+      "991", "992", "993", "994", "995", "996", "997", "998", "999",
+    };
+    static const char* aliases[299] = {
+       "AF",  "AL",  "AQ",  "DZ",  "AS",  "AD",  "AO",  "AG",  "AZ",  "AR",
+       "AU",  "AT",  "BS",  "BH",  "BD",  "AM",  "BB",  "BE",  "BM",  "BT",
+       "BO",  "BA",  "BW",  "BV",  "BR",  "BZ",  "IO",  "SB",  "VG",  "BN",
+       "BG",  "MM",  "BI",  "BY",  "KH",  "CM",  "CA",  "CV",  "KY",  "CF",
+       "LK",  "TD",  "CL",  "CN",  "TW",  "CX",  "CC",  "CO",  "KM",  "YT",
+       "CG",  "CD",  "CK",  "CR",  "HR",  "CU",  "CY",  "CZ",  "BJ",  "DK",
+       "DM",  "DO",  "EC",  "SV",  "GQ",  "ET",  "ET",  "ER",  "EE",  "FO",
+       "FK",  "GS",  "FJ",  "FI",  "AX",  "FR",  "FR",  "GF",  "PF",  "TF",
+       "DJ",  "GA",  "GE",  "GM",  "PS",  "DE",  "DE",  "DE",  "GH",  "GI",
+       "KI",  "GR",  "GL",  "GD",  "GP",  "GU",  "GT",  "GN",  "GY",  "HT",
+       "HM",  "VA",  "HN",  "HK",  "HU",  "IS",  "IN",  "ID",  "IR",  "IQ",
+       "IE",  "IL",  "IT",  "CI",  "JM",  "JP",  "KZ",  "JO",  "KE",  "KP",
+       "KR",  "KW",  "KG",  "LA",  "LB",  "LS",  "LV",  "LR",  "LY",  "LI",
+       "LT",  "LU",  "MO",  "MG",  "MW",  "MY",  "MV",  "ML",  "MT",  "MQ",
+       "MR",  "MU",  "MX",  "MC",  "MN",  "MD",  "ME",  "MS",  "MA",  "MZ",
+       "OM",  "NA",  "NR",  "NP",  "NL",  "CW",  "AW",  "SX",  "BQ",  "NC",
+       "VU",  "NZ",  "NI",  "NE",  "NG",  "NU",  "NF",  "NO",  "MP",  "UM",
+       "FM",  "MH",  "PW",  "PK",  "PA",  "PG",  "PY",  "PE",  "PH",  "PN",
+       "PL",  "PT",  "GW",  "TL",  "PR",  "QA",  "RE",  "RO",  "RU",  "RW",
+       "BL",  "SH",  "KN",  "AI",  "LC",  "MF",  "PM",  "VC",  "SM",  "ST",
+       "SA",  "SN",  "RS",  "SC",  "SL",  "SG",  "SK",  "VN",  "SI",  "SO",
+       "ZA",  "ZW",  "YE",  "ES",  "SS",  "SD",  "EH",  "SD",  "SR",  "SJ",
+       "SZ",  "SE",  "CH",  "SY",  "TJ",  "TH",  "TG",  "TK",  "TO",  "TT",
+       "AE",  "TN",  "TR",  "TM",  "TC",  "TV",  "UG",  "UA",  "MK",  "EG",
+       "GB",  "JE",  "GG",  "JE",  "IM",  "TZ",  "US",  "VI",  "BF",  "UY",
+       "UZ",  "VE",  "WF",  "WS",  "YE",  "YE",  "RS",  "ZM",  "AA",  "QM",
+       "QN",  "QP",  "QQ",  "QR",  "QS",  "QT",  "EU",  "QV",  "QW",  "QX",
+       "QY",  "QZ",  "XA",  "XB",  "XC",  "XD",  "XE",  "XF",  "XG",  "XH",
+       "XI",  "XJ",  "XK",  "XL",  "XM",  "XN",  "XO",  "XP",  "XQ",  "XR",
+       "XS",  "XT",  "XU",  "XV",  "XW",  "XX",  "XY",  "XZ",  "ZZ",
+    };
+
+    if (const char* replacement = SearchReplacement(regions, aliases, region)) {
+      region.Set(mozilla::MakeStringSpan(replacement));
+      return true;
+    }
+    return false;
+  }
+}
+
+// Region subtags with complex mappings.
+// Derived from CLDR Supplemental Data, version 43.
+// https://unicode.org/Public/cldr/43/cldr-common-43.0.zip
+bool mozilla::intl::Locale::ComplexRegionMapping(const RegionSubtag& region) {
+  MOZ_ASSERT(IsStructurallyValidRegionTag(region.Span()));
+  MOZ_ASSERT(IsCanonicallyCasedRegionTag(region.Span()));
+
+  if (region.Length() == 2) {
+    return region.EqualTo("AN") ||
+           region.EqualTo("NT") ||
+           region.EqualTo("PC") ||
+           region.EqualTo("SU");
+  }
+
+  {
+    static const char regions[9][4] = {
+      "062", "172", "200", "530", "532", "536", "582", "810", "890",
+    };
+
+    return HasReplacement(regions, region);
+  }
+}
+
+// Language subtags with complex mappings.
+// Derived from CLDR Supplemental Data, version 43.
+// https://unicode.org/Public/cldr/43/cldr-common-43.0.zip
+void mozilla::intl::Locale::PerformComplexLanguageMappings() {
+  MOZ_ASSERT(IsStructurallyValidLanguageTag(Language().Span()));
+  MOZ_ASSERT(IsCanonicallyCasedLanguageTag(Language().Span()));
+
+  if (Language().EqualTo("cnr")) {
+    SetLanguage("sr");
+    if (Region().Missing()) {
+      SetRegion("ME");
+    }
+  }
+  else if (Language().EqualTo("drw") ||
+           Language().EqualTo("prs") ||
+           Language().EqualTo("tnf")) {
+    SetLanguage("fa");
+    if (Region().Missing()) {
+      SetRegion("AF");
+    }
+  }
+  else if (Language().EqualTo("hbs") ||
+           Language().EqualTo("sh")) {
+    SetLanguage("sr");
+    if (Script().Missing()) {
+      SetScript("Latn");
+    }
+  }
+  else if (Language().EqualTo("swc")) {
+    SetLanguage("sw");
+    if (Region().Missing()) {
+      SetRegion("CD");
+    }
+  }
+}
+
+// Region subtags with complex mappings.
+// Derived from CLDR Supplemental Data, version 43.
+// https://unicode.org/Public/cldr/43/cldr-common-43.0.zip
+void mozilla::intl::Locale::PerformComplexRegionMappings() {
+  MOZ_ASSERT(IsStructurallyValidLanguageTag(Language().Span()));
+  MOZ_ASSERT(IsCanonicallyCasedLanguageTag(Language().Span()));
+  MOZ_ASSERT(IsStructurallyValidRegionTag(Region().Span()));
+  MOZ_ASSERT(IsCanonicallyCasedRegionTag(Region().Span()));
+
+  if (Region().EqualTo("062")) {
+    if (Language().EqualTo("oui") ||
+        (Language().EqualTo("und") && Script().EqualTo("Ougr"))) {
+      SetRegion("143");
+    }
+    else {
+      SetRegion("034");
+    }
+  }
+  else if (Region().EqualTo("172")) {
+    if (Language().EqualTo("axm") ||
+        Language().EqualTo("hy") ||
+        Language().EqualTo("hyw") ||
+        Language().EqualTo("rmi") ||
+        (Language().EqualTo("und") && Script().EqualTo("Armn"))) {
+      SetRegion("AM");
+    }
+    else if (Language().EqualTo("az") ||
+             (Language().EqualTo("azb") && Script().EqualTo("Cyrl")) ||
+             (Language().EqualTo("azb") && Script().EqualTo("Latn")) ||
+             Language().EqualTo("bdk") ||
+             (Language().EqualTo("jdt") && Script().EqualTo("Latn")) ||
+             Language().EqualTo("kjj") ||
+             Language().EqualTo("kry") ||
+             (Language().EqualTo("rut") && Script().EqualTo("Latn")) ||
+             Language().EqualTo("tkr") ||
+             Language().EqualTo("tly") ||
+             Language().EqualTo("ttt")) {
+      SetRegion("AZ");
+    }
+    else if (Language().EqualTo("be") ||
+             (Language().EqualTo("rml") && Script().EqualTo("Cyrl"))) {
+      SetRegion("BY");
+    }
+    else if (Language().EqualTo("ab") ||
+             Language().EqualTo("bbl") ||
+             Language().EqualTo("bhn") ||
+             Language().EqualTo("jge") ||
+             Language().EqualTo("ka") ||
+             (Language().EqualTo("ku") && Script().EqualTo("Yezi")) ||
+             Language().EqualTo("oav") ||
+             Language().EqualTo("os") ||
+             Language().EqualTo("sva") ||
+             (Language().EqualTo("und") && Script().EqualTo("Geor")) ||
+             (Language().EqualTo("und") && Script().EqualTo("Yezi")) ||
+             Language().EqualTo("uum") ||
+             Language().EqualTo("xmf")) {
+      SetRegion("GE");
+    }
+    else if (Language().EqualTo("dng") ||
+             Language().EqualTo("ky")) {
+      SetRegion("KG");
+    }
+    else if ((Language().EqualTo("ili") && Script().EqualTo("Cyrl")) ||
+             Language().EqualTo("kk") ||
+             (Language().EqualTo("ug") && Script().EqualTo("Cyrl"))) {
+      SetRegion("KZ");
+    }
+    else if (Language().EqualTo("gag")) {
+      SetRegion("MD");
+    }
+    else if (Language().EqualTo("abh") ||
+             (Language().EqualTo("isk") && Script().EqualTo("Cyrl")) ||
+             Language().EqualTo("paq") ||
+             Language().EqualTo("sgh") ||
+             Language().EqualTo("tg") ||
+             (Language().EqualTo("wbl") && Script().EqualTo("Cyrl")) ||
+             Language().EqualTo("yai")) {
+      SetRegion("TJ");
+    }
+    else if (Language().EqualTo("chg") ||
+             Language().EqualTo("tk")) {
+      SetRegion("TM");
+    }
+    else if (Language().EqualTo("crh") ||
+             Language().EqualTo("got") ||
+             Language().EqualTo("jct") ||
+             Language().EqualTo("ji") ||
+             (Language().EqualTo("kdr") && Script().EqualTo("Cyrl")) ||
+             Language().EqualTo("rue") ||
+             Language().EqualTo("uk") ||
+             (Language().EqualTo("und") && Script().EqualTo("Goth"))) {
+      SetRegion("UA");
+    }
+    else if (Language().EqualTo("auz") ||
+             Language().EqualTo("kaa") ||
+             Language().EqualTo("sog") ||
+             (Language().EqualTo("und") && Script().EqualTo("Chrs")) ||
+             (Language().EqualTo("und") && Script().EqualTo("Sogd")) ||
+             (Language().EqualTo("und") && Script().EqualTo("Sogo")) ||
+             Language().EqualTo("uz") ||
+             Language().EqualTo("xco")) {
+      SetRegion("UZ");
+    }
+    else {
+      SetRegion("RU");
+    }
+  }
+  else if (Region().EqualTo("200")) {
+    if (Language().EqualTo("rmc") ||
+        Language().EqualTo("sk")) {
+      SetRegion("SK");
+    }
+    else {
+      SetRegion("CZ");
+    }
+  }
+  else if (Region().EqualTo("530") ||
+           Region().EqualTo("532") ||
+           Region().EqualTo("AN")) {
+    if (Language().EqualTo("vic")) {
+      SetRegion("SX");
+    }
+    else {
+      SetRegion("CW");
+    }
+  }
+  else if (Region().EqualTo("536") ||
+           Region().EqualTo("NT")) {
+    if (Language().EqualTo("acm") ||
+        Language().EqualTo("akk") ||
+        Language().EqualTo("ayp") ||
+        Language().EqualTo("bjm") ||
+        Language().EqualTo("ckb") ||
+        Language().EqualTo("kqd") ||
+        (Language().EqualTo("ku") && Script().EqualTo("Arab")) ||
+        Language().EqualTo("mid") ||
+        Language().EqualTo("sdb") ||
+        Language().EqualTo("sdf") ||
+        Language().EqualTo("syr") ||
+        (Language().EqualTo("und") && Script().EqualTo("Syrc")) ||
+        (Language().EqualTo("und") && Script().EqualTo("Xsux"))) {
+      SetRegion("IQ");
+    }
+    else {
+      SetRegion("SA");
+    }
+  }
+  else if (Region().EqualTo("582") ||
+           Region().EqualTo("PC")) {
+    if (Language().EqualTo("mh")) {
+      SetRegion("MH");
+    }
+    else if (Language().EqualTo("cal") ||
+             Language().EqualTo("tpv")) {
+      SetRegion("MP");
+    }
+    else if (Language().EqualTo("pau") ||
+             Language().EqualTo("sov") ||
+             Language().EqualTo("tox")) {
+      SetRegion("PW");
+    }
+    else {
+      SetRegion("FM");
+    }
+  }
+  else if (Region().EqualTo("810") ||
+           Region().EqualTo("SU")) {
+    if (Language().EqualTo("axm") ||
+        Language().EqualTo("hy") ||
+        Language().EqualTo("hyw") ||
+        Language().EqualTo("rmi") ||
+        (Language().EqualTo("und") && Script().EqualTo("Armn"))) {
+      SetRegion("AM");
+    }
+    else if (Language().EqualTo("az") ||
+             (Language().EqualTo("azb") && Script().EqualTo("Cyrl")) ||
+             (Language().EqualTo("azb") && Script().EqualTo("Latn")) ||
+             Language().EqualTo("bdk") ||
+             (Language().EqualTo("jdt") && Script().EqualTo("Latn")) ||
+             Language().EqualTo("kjj") ||
+             Language().EqualTo("kry") ||
+             (Language().EqualTo("rut") && Script().EqualTo("Latn")) ||
+             Language().EqualTo("tkr") ||
+             Language().EqualTo("tly") ||
+             Language().EqualTo("ttt")) {
+      SetRegion("AZ");
+    }
+    else if (Language().EqualTo("be") ||
+             (Language().EqualTo("rml") && Script().EqualTo("Cyrl"))) {
+      SetRegion("BY");
+    }
+    else if (Language().EqualTo("et") ||
+             Language().EqualTo("vro")) {
+      SetRegion("EE");
+    }
+    else if (Language().EqualTo("ab") ||
+             Language().EqualTo("bbl") ||
+             Language().EqualTo("bhn") ||
+             Language().EqualTo("jge") ||
+             Language().EqualTo("ka") ||
+             (Language().EqualTo("ku") && Script().EqualTo("Yezi")) ||
+             Language().EqualTo("oav") ||
+             Language().EqualTo("os") ||
+             Language().EqualTo("sva") ||
+             (Language().EqualTo("und") && Script().EqualTo("Geor")) ||
+             (Language().EqualTo("und") && Script().EqualTo("Yezi")) ||
+             Language().EqualTo("uum") ||
+             Language().EqualTo("xmf")) {
+      SetRegion("GE");
+    }
+    else if (Language().EqualTo("dng") ||
+             Language().EqualTo("ky")) {
+      SetRegion("KG");
+    }
+    else if ((Language().EqualTo("ili") && Script().EqualTo("Cyrl")) ||
+             Language().EqualTo("kk") ||
+             (Language().EqualTo("ug") && Script().EqualTo("Cyrl"))) {
+      SetRegion("KZ");
+    }
+    else if (Language().EqualTo("kdr") ||
+             Language().EqualTo("lt") ||
+             Language().EqualTo("olt") ||
+             Language().EqualTo("sgs")) {
+      SetRegion("LT");
+    }
+    else if (Language().EqualTo("liv") ||
+             Language().EqualTo("ltg") ||
+             Language().EqualTo("lv")) {
+      SetRegion("LV");
+    }
+    else if (Language().EqualTo("gag")) {
+      SetRegion("MD");
+    }
+    else if (Language().EqualTo("abh") ||
+             (Language().EqualTo("isk") && Script().EqualTo("Cyrl")) ||
+             Language().EqualTo("paq") ||
+             Language().EqualTo("sgh") ||
+             Language().EqualTo("tg") ||
+             (Language().EqualTo("wbl") && Script().EqualTo("Cyrl")) ||
+             Language().EqualTo("yai")) {
+      SetRegion("TJ");
+    }
+    else if (Language().EqualTo("chg") ||
+             Language().EqualTo("tk")) {
+      SetRegion("TM");
+    }
+    else if (Language().EqualTo("crh") ||
+             Language().EqualTo("got") ||
+             Language().EqualTo("jct") ||
+             Language().EqualTo("ji") ||
+             (Language().EqualTo("kdr") && Script().EqualTo("Cyrl")) ||
+             Language().EqualTo("rue") ||
+             Language().EqualTo("uk") ||
+             (Language().EqualTo("und") && Script().EqualTo("Goth"))) {
+      SetRegion("UA");
+    }
+    else if (Language().EqualTo("auz") ||
+             Language().EqualTo("kaa") ||
+             Language().EqualTo("sog") ||
+             (Language().EqualTo("und") && Script().EqualTo("Chrs")) ||
+             (Language().EqualTo("und") && Script().EqualTo("Sogd")) ||
+             (Language().EqualTo("und") && Script().EqualTo("Sogo")) ||
+             Language().EqualTo("uz") ||
+             Language().EqualTo("xco")) {
+      SetRegion("UZ");
+    }
+    else {
+      SetRegion("RU");
+    }
+  }
+  else if (Region().EqualTo("890")) {
+    if (Language().EqualTo("bs")) {
+      SetRegion("BA");
+    }
+    else if (Language().EqualTo("ckm") ||
+             Language().EqualTo("dlm") ||
+             Language().EqualTo("hr") ||
+             Language().EqualTo("ist") ||
+             Language().EqualTo("ruo")) {
+      SetRegion("HR");
+    }
+    else if (Language().EqualTo("mk")) {
+      SetRegion("MK");
+    }
+    else if (Language().EqualTo("sl")) {
+      SetRegion("SI");
+    }
+    else {
+      SetRegion("RS");
+    }
+  }
+}
+
+static const char* ToCharPointer(const char* str) {
+  return str;
+}
+
+static const char* ToCharPointer(const mozilla::intl::UniqueChars& str) {
+  return str.get();
+}
+
+template <typename T, typename U = T>
+static bool IsLessThan(const T& a, const U& b) {
+  return strcmp(ToCharPointer(a), ToCharPointer(b)) < 0;
+}
+
+// Mappings from variant subtags to preferred values.
+// Derived from CLDR Supplemental Data, version 43.
+// https://unicode.org/Public/cldr/43/cldr-common-43.0.zip
+bool mozilla::intl::Locale::PerformVariantMappings() {
+  // The variant subtags need to be sorted for binary search.
+  MOZ_ASSERT(std::is_sorted(mVariants.begin(), mVariants.end(),
+                            IsLessThan<decltype(mVariants)::ElementType>));
+
+  auto removeVariantAt = [&](size_t index) {
+    mVariants.erase(mVariants.begin() + index);
+  };
+
+  auto insertVariantSortedIfNotPresent = [&](const char* variant) {
+    auto* p = std::lower_bound(
+        mVariants.begin(), mVariants.end(), variant,
+        IsLessThan<decltype(mVariants)::ElementType, decltype(variant)>);
+
+    // Don't insert the replacement when already present.
+    if (p != mVariants.end() && strcmp(p->get(), variant) == 0) {
+      return true;
+    }
+
+    // Insert the preferred variant in sort order.
+    auto preferred = DuplicateStringToUniqueChars(variant);
+    return !!mVariants.insert(p, std::move(preferred));
+  };
+
+  for (size_t i = 0; i < mVariants.length();) {
+    const char* variant = mVariants[i].get();
+    MOZ_ASSERT(IsCanonicallyCasedVariantTag(mozilla::MakeStringSpan(variant)));
+
+    if (strcmp(variant, "arevela") == 0 ||
+        strcmp(variant, "arevmda") == 0 ||
+        strcmp(variant, "bokmal") == 0 ||
+        strcmp(variant, "hakka") == 0 ||
+        strcmp(variant, "lojban") == 0 ||
+        strcmp(variant, "nynorsk") == 0 ||
+        strcmp(variant, "saaho") == 0 ||
+        strcmp(variant, "xiang") == 0) {
+      removeVariantAt(i);
+    }
+    else if (strcmp(variant, "aaland") == 0) {
+      removeVariantAt(i);
+      SetRegion("AX");
+    }
+    else if (strcmp(variant, "heploc") == 0) {
+      removeVariantAt(i);
+      if (!insertVariantSortedIfNotPresent("alalc97")) {
+        return false;
+      }
+    }
+    else if (strcmp(variant, "polytoni") == 0) {
+      removeVariantAt(i);
+      if (!insertVariantSortedIfNotPresent("polyton")) {
+        return false;
+      }
+    }
+    else {
+      i++;
+    }
+  }
+  return true;
+}
+
+// Canonicalize legacy locale identifiers.
+// Derived from CLDR Supplemental Data, version 43.
+// https://unicode.org/Public/cldr/43/cldr-common-43.0.zip
+bool mozilla::intl::Locale::UpdateLegacyMappings() {
+  // We're mapping legacy tags to non-legacy form here.
+  // Other tags remain unchanged.
+  //
+  // Legacy tags are either sign language tags ("sgn") or have one or multiple
+  // variant subtags. Therefore we can quickly exclude most tags by checking
+  // these two subtags.
+
+  MOZ_ASSERT(IsCanonicallyCasedLanguageTag(Language().Span()));
+
+  if (!Language().EqualTo("sgn") && mVariants.length() == 0) {
+    return true;
+  }
+
+#ifdef DEBUG
+  for (const auto& variant : Variants()) {
+    MOZ_ASSERT(IsStructurallyValidVariantTag(variant));
+    MOZ_ASSERT(IsCanonicallyCasedVariantTag(variant));
+  }
+#endif
+
+  // The variant subtags need to be sorted for binary search.
+  MOZ_ASSERT(std::is_sorted(mVariants.begin(), mVariants.end(),
+                            IsLessThan<decltype(mVariants)::ElementType>));
+
+  auto findVariant = [this](const char* variant) {
+    auto* p = std::lower_bound(mVariants.begin(), mVariants.end(), variant,
+                               IsLessThan<decltype(mVariants)::ElementType,
+                                          decltype(variant)>);
+
+    if (p != mVariants.end() && strcmp(p->get(), variant) == 0) {
+      return p;
+    }
+    return static_cast<decltype(p)>(nullptr);
+  };
+
+  auto insertVariantSortedIfNotPresent = [&](const char* variant) {
+    auto* p = std::lower_bound(mVariants.begin(), mVariants.end(), variant,
+                               IsLessThan<decltype(mVariants)::ElementType,
+                                          decltype(variant)>);
+
+    // Don't insert the replacement when already present.
+    if (p != mVariants.end() && strcmp(p->get(), variant) == 0) {
+      return true;
+    }
+
+    // Insert the preferred variant in sort order.
+    auto preferred = DuplicateStringToUniqueChars(variant);
+    return !!mVariants.insert(p, std::move(preferred));
+  };
+
+  auto removeVariant = [&](auto* p) {
+    size_t index = std::distance(mVariants.begin(), p);
+    mVariants.erase(mVariants.begin() + index);
+  };
+
+  auto removeVariants = [&](auto* p, auto* q) {
+    size_t pIndex = std::distance(mVariants.begin(), p);
+    size_t qIndex = std::distance(mVariants.begin(), q);
+    MOZ_ASSERT(pIndex < qIndex, "variant subtags are sorted");
+
+    mVariants.erase(mVariants.begin() + qIndex);
+    mVariants.erase(mVariants.begin() + pIndex);
+  };
+
+  if (mVariants.length() >= 2) {
+    if (auto* hepburn = findVariant("hepburn")) {
+      if (auto* heploc = findVariant("heploc")) {
+        removeVariants(hepburn, heploc);
+
+        if (!insertVariantSortedIfNotPresent("alalc97")) {
+          return false;
+        }
+      }
+    }
+  }
+
+  if (Language().EqualTo("sgn")) {
+    if (Region().Present() && SignLanguageMapping(mLanguage, Region())) {
+      mRegion.Set(mozilla::MakeStringSpan(""));
+    }
+  }
+  else if (Language().EqualTo("aa") ||
+           Language().EqualTo("aar")) {
+    if (auto* saaho = findVariant("saaho")) {
+      removeVariant(saaho);
+      SetLanguage("ssy");
+    }
+  }
+  else if (Language().EqualTo("arm") ||
+           Language().EqualTo("hy") ||
+           Language().EqualTo("hye")) {
+    if (auto* arevmda = findVariant("arevmda")) {
+      removeVariant(arevmda);
+      SetLanguage("hyw");
+    }
+  }
+  else if (Language().EqualTo("art")) {
+    if (auto* lojban = findVariant("lojban")) {
+      removeVariant(lojban);
+      SetLanguage("jbo");
+    }
+  }
+  else if (Language().EqualTo("cel")) {
+    if (auto* gaulish = findVariant("gaulish")) {
+      removeVariant(gaulish);
+      SetLanguage("xtg");
+    }
+  }
+  else if (Language().EqualTo("chi") ||
+           Language().EqualTo("cmn") ||
+           Language().EqualTo("zh") ||
+           Language().EqualTo("zho")) {
+    if (auto* guoyu = findVariant("guoyu")) {
+      if (auto* hakka = findVariant("hakka")) {
+        removeVariants(guoyu, hakka);
+        SetLanguage("hak");
+        return true;
+      }
+    }
+    if (auto* guoyu = findVariant("guoyu")) {
+      if (auto* xiang = findVariant("xiang")) {
+        removeVariants(guoyu, xiang);
+        SetLanguage("hsn");
+        return true;
+      }
+    }
+    if (auto* guoyu = findVariant("guoyu")) {
+      removeVariant(guoyu);
+      SetLanguage("zh");
+    }
+    else if (auto* hakka = findVariant("hakka")) {
+      removeVariant(hakka);
+      SetLanguage("hak");
+    }
+    else if (auto* xiang = findVariant("xiang")) {
+      removeVariant(xiang);
+      SetLanguage("hsn");
+    }
+  }
+  else if (Language().EqualTo("no") ||
+           Language().EqualTo("nor")) {
+    if (auto* bokmal = findVariant("bokmal")) {
+      removeVariant(bokmal);
+      SetLanguage("nb");
+    }
+    else if (auto* nynorsk = findVariant("nynorsk")) {
+      removeVariant(nynorsk);
+      SetLanguage("nn");
+    }
+  }
+
+  return true;
+}
+
+// Mappings from legacy sign languages.
+// Derived from CLDR Supplemental Data, version 43.
+// https://unicode.org/Public/cldr/43/cldr-common-43.0.zip
+bool mozilla::intl::Locale::SignLanguageMapping(LanguageSubtag& language,
+                                                const RegionSubtag& region) {
+  MOZ_ASSERT(language.EqualTo("sgn"));
+  MOZ_ASSERT(IsStructurallyValidRegionTag(region.Span()));
+  MOZ_ASSERT(IsCanonicallyCasedRegionTag(region.Span()));
+
+  if (region.Length() == 2) {
+    static const char regions[22][3] = {
+      "BR", "CO", "DD", "DE", "DK", "ES", "FR", "FX", "GB", "GR",
+      "IE", "IT", "JP", "MX", "NI", "NL", "NO", "PT", "SE", "UK",
+      "US", "ZA",
+    };
+    static const char* aliases[22] = {
+      "bzs", "csn", "gsg", "gsg", "dsl", "ssp", "fsl", "fsl", "bfi", "gss",
+      "isg", "ise", "jsl", "mfs", "ncs", "dse", "nsi", "psr", "swl", "bfi",
+      "ase", "sfs",
+    };
+
+    if (const char* replacement = SearchReplacement(regions, aliases, region)) {
+      language.Set(mozilla::MakeStringSpan(replacement));
+      return true;
+    }
+    return false;
+  }
+
+  {
+    static const char regions[22][4] = {
+      "076", "170", "208", "249", "250", "276", "278", "280", "300", "372",
+      "380", "392", "484", "528", "558", "578", "620", "710", "724", "752",
+      "826", "840",
+    };
+    static const char* aliases[22] = {
+      "bzs", "csn", "dsl", "fsl", "fsl", "gsg", "gsg", "gsg", "gss", "isg",
+      "ise", "jsl", "mfs", "dse", "ncs", "nsi", "psr", "sfs", "ssp", "swl",
+      "bfi", "ase",
+    };
+
+    if (const char* replacement = SearchReplacement(regions, aliases, region)) {
+      language.Set(mozilla::MakeStringSpan(replacement));
+      return true;
+    }
+    return false;
+  }
+}
+
+template <size_t Length>
+static inline bool IsUnicodeKey(mozilla::Span<const char> key, const char (&str)[Length]) {
+  static_assert(Length == UnicodeKeyLength + 1,
+                "Unicode extension key is two characters long");
+  return memcmp(key.data(), str, Length - 1) == 0;
+}
+
+template <size_t Length>
+static inline bool IsUnicodeType(mozilla::Span<const char> type, const char (&str)[Length]) {
+  static_assert(Length > UnicodeKeyLength + 1,
+                "Unicode extension type contains more than two characters");
+  return type.size() == (Length - 1) &&
+         memcmp(type.data(), str, Length - 1) == 0;
+}
+
+static int32_t CompareUnicodeType(const char* a, mozilla::Span<const char> b) {
+  MOZ_ASSERT(!std::char_traits<char>::find(b.data(), b.size(), '\0'),
+             "unexpected null-character in string");
+
+  using UnsignedChar = unsigned char;
+  for (size_t i = 0; i < b.size(); i++) {
+    // |a| is zero-terminated and |b| doesn't contain a null-terminator. So if
+    // we've reached the end of |a|, the below if-statement will always be true.
+    // That ensures we don't read past the end of |a|.
+    if (int32_t r = UnsignedChar(a[i]) - UnsignedChar(b[i])) {
+      return r;
+    }
+  }
+
+  // Return zero if both strings are equal or a positive number if |b| is a
+  // prefix of |a|.
+  return int32_t(UnsignedChar(a[b.size()]));
+}
+
+template <size_t Length>
+static inline const char* SearchUnicodeReplacement(
+  const char* (&types)[Length], const char* (&aliases)[Length],
+  mozilla::Span<const char> type) {
+
+  auto p = std::lower_bound(std::begin(types), std::end(types), type,
+                            [](const auto& a, const auto& b) {
+                              return CompareUnicodeType(a, b) < 0;
+                            });
+  if (p != std::end(types) && CompareUnicodeType(*p, type) == 0) {
+    return aliases[std::distance(std::begin(types), p)];
+  }
+  return nullptr;
+}
+
+/**
+ * Mapping from deprecated BCP 47 Unicode extension types to their preferred
+ * values.
+ *
+ * Spec: https://www.unicode.org/reports/tr35/#Unicode_Locale_Extension_Data_Files
+ * Spec: https://www.unicode.org/reports/tr35/#t_Extension
+ */
+const char* mozilla::intl::Locale::ReplaceUnicodeExtensionType(
+    mozilla::Span<const char> key, mozilla::Span<const char> type) {
+  MOZ_ASSERT(key.size() == UnicodeKeyLength);
+  MOZ_ASSERT(IsCanonicallyCasedUnicodeKey(key));
+
+  MOZ_ASSERT(type.size() > UnicodeKeyLength);
+  MOZ_ASSERT(IsCanonicallyCasedUnicodeType(type));
+
+  if (IsUnicodeKey(key, "ca")) {
+    if (IsUnicodeType(type, "ethiopic-amete-alem")) {
+      return "ethioaa";
+    }
+    if (IsUnicodeType(type, "islamicc")) {
+      return "islamic-civil";
+    }
+  }
+  else if (IsUnicodeKey(key, "kb") ||
+           IsUnicodeKey(key, "kc") ||
+           IsUnicodeKey(key, "kh") ||
+           IsUnicodeKey(key, "kk") ||
+           IsUnicodeKey(key, "kn")) {
+    if (IsUnicodeType(type, "yes")) {
+      return "true";
+    }
+  }
+  else if (IsUnicodeKey(key, "ks")) {
+    if (IsUnicodeType(type, "primary")) {
+      return "level1";
+    }
+    if (IsUnicodeType(type, "tertiary")) {
+      return "level3";
+    }
+  }
+  else if (IsUnicodeKey(key, "ms")) {
+    if (IsUnicodeType(type, "imperial")) {
+      return "uksystem";
+    }
+  }
+  else if (IsUnicodeKey(key, "rg") ||
+           IsUnicodeKey(key, "sd")) {
+    static const char* types[144] = {
+         "cn11" ,  "cn12" ,  "cn13" ,  "cn14" ,  "cn15" ,  "cn21" ,  "cn22" ,
+         "cn23" ,  "cn31" ,  "cn32" ,  "cn33" ,  "cn34" ,  "cn35" ,  "cn36" ,
+         "cn37" ,  "cn41" ,  "cn42" ,  "cn43" ,  "cn44" ,  "cn45" ,  "cn46" ,
+         "cn50" ,  "cn51" ,  "cn52" ,  "cn53" ,  "cn54" ,  "cn61" ,  "cn62" ,
+         "cn63" ,  "cn64" ,  "cn65" ,  "cn71" ,  "cn91" ,  "cn92" , "cz10a" ,
+        "cz10b" , "cz10c" , "cz10d" , "cz10e" , "cz10f" , "cz611" , "cz612" ,
+        "cz613" , "cz614" , "cz615" , "cz621" , "cz622" , "cz623" , "cz624" ,
+        "cz626" , "cz627" ,  "czjc" ,  "czjm" ,  "czka" ,  "czkr" ,  "czli" ,
+         "czmo" ,  "czol" ,  "czpa" ,  "czpl" ,  "czpr" ,  "czst" ,  "czus" ,
+         "czvy" ,  "czzl" ,  "fi01" ,  "fra"  ,  "frb"  ,  "frbl" ,  "frc"  ,
+         "frcp" ,  "frd"  ,  "fre"  ,  "frf"  ,  "frg"  ,  "frgf" ,  "frgp" ,
+         "frh"  ,  "fri"  ,  "frj"  ,  "frk"  ,  "frl"  ,  "frm"  ,  "frmf" ,
+         "frmq" ,  "frn"  ,  "frnc" ,  "fro"  ,  "frp"  ,  "frpf" ,  "frpm" ,
+         "frq"  ,  "frr"  ,  "frre" ,  "frs"  ,  "frt"  ,  "frtf" ,  "fru"  ,
+         "frv"  ,  "frwf" ,  "fryt" ,  "laxn" ,  "lud"  ,  "lug"  ,  "lul"  ,
+        "mrnkc" ,  "nlaw" ,  "nlcw" ,  "nlsx" ,  "no23" ,  "nzn"  ,  "nzs"  ,
+         "omba" ,  "omsh" ,  "plds" ,  "plkp" ,  "pllb" ,  "plld" ,  "pllu" ,
+         "plma" ,  "plmz" ,  "plop" ,  "plpd" ,  "plpk" ,  "plpm" ,  "plsk" ,
+         "plsl" ,  "plwn" ,  "plwp" ,  "plzp" ,  "shta" , "tteto" , "ttrcm" ,
+        "ttwto" , "twkhq" , "twtnq" , "twtpq" , "twtxq" ,  "usas" ,  "usgu" ,
+         "usmp" ,  "uspr" ,  "usum" ,  "usvi" ,
+    };
+    static const char* aliases[144] = {
+         "cnbj" ,  "cntj" ,  "cnhe" ,  "cnsx" ,  "cnmn" ,  "cnln" ,  "cnjl" ,
+         "cnhl" ,  "cnsh" ,  "cnjs" ,  "cnzj" ,  "cnah" ,  "cnfj" ,  "cnjx" ,
+         "cnsd" ,  "cnha" ,  "cnhb" ,  "cnhn" ,  "cngd" ,  "cngx" ,  "cnhi" ,
+         "cncq" ,  "cnsc" ,  "cngz" ,  "cnyn" ,  "cnxz" ,  "cnsn" ,  "cngs" ,
+         "cnqh" ,  "cnnx" ,  "cnxj" , "twzzzz", "hkzzzz", "mozzzz", "cz110" ,
+        "cz111" , "cz112" , "cz113" , "cz114" , "cz115" , "cz663" , "cz632" ,
+        "cz633" , "cz634" , "cz635" , "cz641" , "cz642" , "cz643" , "cz644" ,
+        "cz646" , "cz647" ,  "cz31" ,  "cz64" ,  "cz41" ,  "cz52" ,  "cz51" ,
+         "cz80" ,  "cz71" ,  "cz53" ,  "cz32" ,  "cz10" ,  "cz20" ,  "cz42" ,
+         "cz63" ,  "cz72" , "axzzzz", "frges" , "frnaq" , "blzzzz", "frara" ,
+        "cpzzzz", "frbfc" , "frbre" , "frcvl" , "frges" , "gfzzzz", "gpzzzz",
+        "frcor" , "frbfc" , "fridf" , "frocc" , "frnaq" , "frges" , "mfzzzz",
+        "mqzzzz", "frocc" , "nczzzz", "frhdf" , "frnor" , "pfzzzz", "pmzzzz",
+        "frnor" , "frpdl" , "rezzzz", "frhdf" , "frnaq" , "tfzzzz", "frpac" ,
+        "frara" , "wfzzzz", "ytzzzz",  "laxs" ,  "lucl" ,  "luec" ,  "luca" ,
+         "mr13" , "awzzzz", "cwzzzz", "sxzzzz",  "no50" , "nzauk" , "nzcan" ,
+         "ombj" ,  "omsj" ,  "pl02" ,  "pl04" ,  "pl08" ,  "pl10" ,  "pl06" ,
+         "pl12" ,  "pl14" ,  "pl16" ,  "pl20" ,  "pl18" ,  "pl22" ,  "pl26" ,
+         "pl24" ,  "pl28" ,  "pl30" ,  "pl32" , "tazzzz", "tttob" , "ttmrc" ,
+        "tttob" , "twkhh" , "twtnn" , "twnwt" , "twtxg" , "aszzzz", "guzzzz",
+        "mpzzzz", "przzzz", "umzzzz", "vizzzz",
+    };
+    return SearchUnicodeReplacement(types, aliases, type);
+  }
+  else if (IsUnicodeKey(key, "tz")) {
+    static const char* types[30] = {
+         "aqams"  ,  "camtr"  ,  "cnckg"  ,  "cnhrb"  ,  "cnkhg"  ,   "cuba"  ,
+         "egypt"  ,   "eire"  ,   "est"   ,   "gaza"  ,   "gmt0"  , "hongkong",
+          "hst"   , "iceland" ,   "iran"  ,  "israel" , "jamaica" ,  "japan"  ,
+         "libya"  ,   "mst"   ,  "navajo" ,  "poland" , "portugal",   "prc"   ,
+          "roc"   ,   "rok"   ,  "turkey" ,   "uct"   , "usnavajo",   "zulu"  ,
+    };
+    static const char* aliases[30] = {
+         "nzakl"  ,  "cator"  ,  "cnsha"  ,  "cnsha"  ,  "cnurc"  ,  "cuhav"  ,
+         "egcai"  ,  "iedub"  ,  "utcw05" , "gazastrp",   "gmt"   ,  "hkhkg"  ,
+         "utcw10" ,  "isrey"  ,  "irthr"  , "jeruslm" ,  "jmkin"  ,  "jptyo"  ,
+         "lytip"  ,  "utcw07" ,  "usden"  ,  "plwaw"  ,  "ptlis"  ,  "cnsha"  ,
+         "twtpe"  ,  "krsel"  ,  "trist"  ,   "utc"   ,  "usden"  ,   "utc"   ,
+    };
+    return SearchUnicodeReplacement(types, aliases, type);
+  }
+  return nullptr;
+}
+
+template <size_t Length>
+static inline bool IsTransformKey(mozilla::Span<const char> key, const char (&str)[Length]) {
+  static_assert(Length == TransformKeyLength + 1,
+                "Transform extension key is two characters long");
+  return memcmp(key.data(), str, Length - 1) == 0;
+}
+
+template <size_t Length>
+static inline bool IsTransformType(mozilla::Span<const char> type, const char (&str)[Length]) {
+  static_assert(Length > TransformKeyLength + 1,
+                "Transform extension type contains more than two characters");
+  return type.size() == (Length - 1) &&
+         memcmp(type.data(), str, Length - 1) == 0;
+}
+
+/**
+ * Mapping from deprecated BCP 47 Transform extension types to their preferred
+ * values.
+ *
+ * Spec: https://www.unicode.org/reports/tr35/#Unicode_Locale_Extension_Data_Files
+ * Spec: https://www.unicode.org/reports/tr35/#t_Extension
+ */
+const char* mozilla::intl::Locale::ReplaceTransformExtensionType(
+    mozilla::Span<const char> key, mozilla::Span<const char> type) {
+  MOZ_ASSERT(key.size() == TransformKeyLength);
+  MOZ_ASSERT(IsCanonicallyCasedTransformKey(key));
+
+  MOZ_ASSERT(type.size() > TransformKeyLength);
+  MOZ_ASSERT(IsCanonicallyCasedTransformType(type));
+
+  if (IsTransformKey(key, "d0")) {
+    if (IsTransformType(type, "name")) {
+      return "charname";
+    }
+  }
+  else if (IsTransformKey(key, "m0")) {
+    if (IsTransformType(type, "beta-metsehaf")) {
+      return "betamets";
+    }
+    if (IsTransformType(type, "ies-jes")) {
+      return "iesjes";
+    }
+    if (IsTransformType(type, "names")) {
+      return "prprname";
+    }
+    if (IsTransformType(type, "tekie-alibekit")) {
+      return "tekieali";
+    }
+  }
+  return nullptr;
+}
diff --git a/intl/components/src/MeasureUnit.cpp b/intl/components/src/MeasureUnit.cpp
new file mode 100644
index 0000000000..3b932c9168
--- /dev/null
+++ b/intl/components/src/MeasureUnit.cpp
@@ -0,0 +1,110 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "mozilla/intl/MeasureUnit.h"
+
+#include "unicode/udata.h"
+#include "unicode/ures.h"
+#include "unicode/utypes.h"
+
+namespace mozilla::intl {
+
+void MeasureUnit::UResourceBundleDeleter::operator()(UResourceBundle* aPtr) {
+  ures_close(aPtr);
+}
+
+MeasureUnit::Enumeration::Enumeration(UniqueUResourceBundle aRootLocale,
+                                      UniqueUResourceBundle aUnits)
+    : mRootLocale(std::move(aRootLocale)), mUnits(std::move(aUnits)) {
+  mUnitsSize = ures_getSize(mUnits.get());
+}
+
+MeasureUnit::Enumeration::Iterator::value_type
+MeasureUnit::Enumeration::Iterator::operator*() const {
+  // Return an error result after an ICU error has occurred.
+  if (mHasError) {
+    return Err(InternalError{});
+  }
+
+  // Otherwise return the name of the current measurement unit.
+  const char* unitIdentifier = ures_getKey(mSubtype.get());
+  MOZ_ASSERT(unitIdentifier);
+  return MakeStringSpan(unitIdentifier);
+}
+
+void MeasureUnit::Enumeration::Iterator::advance() {
+  // Reject any attempts to modify this iterator after an error has occurred.
+  if (mHasError) {
+    return;
+  }
+
+  while (true) {
+    // Read the next measurement unit in the types table.
+    if (mTypePos < mTypeSize) {
+      UErrorCode status = U_ZERO_ERROR;
+      UResourceBundle* rawSubtype =
+          ures_getByIndex(mType.get(), mTypePos, nullptr, &status);
+      if (U_FAILURE(status)) {
+        mHasError = true;
+        return;
+      }
+
+      mTypePos += 1;
+      mSubtype.reset(rawSubtype);
+      return;
+    }
+
+    // Read the next measurement unit type in the "units" table.
+    if (mUnitsPos < mEnumeration.mUnitsSize) {
+      UErrorCode status = U_ZERO_ERROR;
+      UResourceBundle* rawType = ures_getByIndex(mEnumeration.mUnits.get(),
+                                                 mUnitsPos, nullptr, &status);
+      if (U_FAILURE(status)) {
+        mHasError = true;
+        return;
+      }
+
+      mUnitsPos += 1;
+      mType.reset(rawType);
+      mTypeSize = ures_getSize(rawType);
+      mTypePos = 0;
+      continue;
+    }
+
+    // All measurement units have been processed. Reset the two |mType*| fields
+    // to zero to match the end-iterator state and then return.
+    MOZ_ASSERT(mUnitsPos == mEnumeration.mUnitsSize);
+    mTypePos = 0;
+    mTypeSize = 0;
+    return;
+  }
+}
+
+Result<MeasureUnit::Enumeration, ICUError>
+MeasureUnit::Enumeration::TryCreate() {
+  // Look up the available measurement units in the resource bundle of the root
+  // locale.
+
+  static const char packageName[] =
+      U_ICUDATA_NAME U_TREE_SEPARATOR_STRING "unit";
+  static const char rootLocale[] = "";
+
+  UErrorCode status = U_ZERO_ERROR;
+  UResourceBundle* rawRes = ures_open(packageName, rootLocale, &status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+  UniqueUResourceBundle res(rawRes);
+
+  UResourceBundle* rawUnits =
+      ures_getByKey(res.get(), "units", nullptr, &status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+  UniqueUResourceBundle units(rawUnits);
+
+  return MeasureUnit::Enumeration(std::move(res), std::move(units));
+}
+
+}  // namespace mozilla::intl
diff --git a/intl/components/src/MeasureUnit.h b/intl/components/src/MeasureUnit.h
new file mode 100644
index 0000000000..8a8cf1629a
--- /dev/null
+++ b/intl/components/src/MeasureUnit.h
@@ -0,0 +1,155 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef intl_components_MeasureUnit_h_
+#define intl_components_MeasureUnit_h_
+
+#include "mozilla/Assertions.h"
+#include "mozilla/intl/ICU4CGlue.h"
+#include "mozilla/intl/ICUError.h"
+#include "mozilla/Result.h"
+#include "mozilla/Span.h"
+#include "mozilla/UniquePtr.h"
+
+#include <iterator>
+#include <stddef.h>
+#include <stdint.h>
+#include <utility>
+
+struct UResourceBundle;
+
+namespace mozilla::intl {
+
+/**
+ * This component is a Mozilla-focused API for working with measurement units in
+ * internationalization code. It is used in coordination with other operations
+ * such as number formatting.
+ */
+class MeasureUnit final {
+  class UResourceBundleDeleter {
+   public:
+    void operator()(UResourceBundle* aPtr);
+  };
+
+  using UniqueUResourceBundle =
+      UniquePtr<UResourceBundle, UResourceBundleDeleter>;
+
+ public:
+  MeasureUnit() = delete;
+
+  class Enumeration final {
+    // Resource bundle for the root locale.
+    UniqueUResourceBundle mRootLocale = nullptr;
+
+    // Resource bundle for the root locale's "units" resource table.
+    UniqueUResourceBundle mUnits = nullptr;
+
+    // The overall amount of available units.
+    int32_t mUnitsSize = 0;
+
+   public:
+    Enumeration(UniqueUResourceBundle aRootLocale,
+                UniqueUResourceBundle aUnits);
+
+    class Iterator {
+     public:
+      // std::iterator traits.
+      using iterator_category = std::input_iterator_tag;
+      using value_type = SpanResult<char>;
+      using difference_type = ptrdiff_t;
+      using pointer = value_type*;
+      using reference = value_type&;
+
+     private:
+      const Enumeration& mEnumeration;
+
+      // Resource bundle to a measurement type within the "units" table.
+      //
+      // Measurement types describe various categories, like "area", "length",
+      // or "mass".
+      UniqueUResourceBundle mType = nullptr;
+
+      // Resource bundle to a specific subtype within the type table.
+      //
+      // Measurement subtypes describe concrete measure units, like "acre",
+      // "meter", or "kilogram".
+      UniqueUResourceBundle mSubtype = nullptr;
+
+      // The next position within the "units" table.
+      int32_t mUnitsPos = 0;
+
+      // The overall amount of types within the |mType| table.
+      int32_t mTypeSize = 0;
+
+      // The next position within the |mType| table.
+      int32_t mTypePos = 0;
+
+      // Flag set when an ICU error has occurred. All further operations on this
+      // iterator will return an error result when this flag is set.
+      bool mHasError = false;
+
+      void advance();
+
+     public:
+      Iterator(const Enumeration& aEnumeration, int32_t aUnitsPos)
+          : mEnumeration(aEnumeration), mUnitsPos(aUnitsPos) {
+        advance();
+      }
+
+      Iterator& operator++() {
+        advance();
+        return *this;
+      }
+
+      // The post-increment operator would return an invalid iterator, so it's
+      // not implemented.
+      Iterator operator++(int) = delete;
+
+      bool operator==(const Iterator& aOther) const {
+        // It's an error to compare an iterator against an iterator from a
+        // different enumeration.
+        MOZ_ASSERT(&mEnumeration == &aOther.mEnumeration);
+
+        return mUnitsPos == aOther.mUnitsPos && mTypeSize == aOther.mTypeSize &&
+               mTypePos == aOther.mTypePos && mHasError == aOther.mHasError;
+      }
+
+      bool operator!=(const Iterator& aOther) const {
+        return !(*this == aOther);
+      }
+
+      value_type operator*() const;
+    };
+
+    friend class Iterator;
+
+    // std::iterator begin() and end() methods.
+
+    /**
+     * Return an iterator pointing to the start of the "units" table.
+     */
+    Iterator begin() { return Iterator(*this, 0); }
+
+    /**
+     * Return an iterator pointing to the end of the "units" table.
+     */
+    Iterator end() { return Iterator(*this, mUnitsSize); }
+
+    /**
+     * Create a new measurement unit enumeration.
+     */
+    static Result<Enumeration, ICUError> TryCreate();
+  };
+
+  /**
+   * Return an enumeration over all available measurement units.
+   */
+  static Result<Enumeration, ICUError> GetAvailable() {
+    return Enumeration::TryCreate();
+  }
+};
+
+}  // namespace mozilla::intl
+
+#endif
diff --git a/intl/components/src/MeasureUnitGenerated.h b/intl/components/src/MeasureUnitGenerated.h
new file mode 100644
index 0000000000..8febc88649
--- /dev/null
+++ b/intl/components/src/MeasureUnitGenerated.h
@@ -0,0 +1,70 @@
+// Generated by make_intl_data.py. DO NOT EDIT.
+
+#ifndef intl_components_MeasureUnitGenerated_h
+#define intl_components_MeasureUnitGenerated_h
+
+namespace mozilla::intl {
+
+struct SimpleMeasureUnit {
+  const char* const type;
+  const char* const name;
+};
+
+/**
+ * The list of currently supported simple unit identifiers.
+ *
+ * The list must be kept in alphabetical order of |name|.
+ */
+inline constexpr SimpleMeasureUnit simpleMeasureUnits[] = {
+    // clang-format off
+  {"area", "acre"},
+  {"digital", "bit"},
+  {"digital", "byte"},
+  {"temperature", "celsius"},
+  {"length", "centimeter"},
+  {"duration", "day"},
+  {"angle", "degree"},
+  {"temperature", "fahrenheit"},
+  {"volume", "fluid-ounce"},
+  {"length", "foot"},
+  {"volume", "gallon"},
+  {"digital", "gigabit"},
+  {"digital", "gigabyte"},
+  {"mass", "gram"},
+  {"area", "hectare"},
+  {"duration", "hour"},
+  {"length", "inch"},
+  {"digital", "kilobit"},
+  {"digital", "kilobyte"},
+  {"mass", "kilogram"},
+  {"length", "kilometer"},
+  {"volume", "liter"},
+  {"digital", "megabit"},
+  {"digital", "megabyte"},
+  {"length", "meter"},
+  {"duration", "microsecond"},
+  {"length", "mile"},
+  {"length", "mile-scandinavian"},
+  {"volume", "milliliter"},
+  {"length", "millimeter"},
+  {"duration", "millisecond"},
+  {"duration", "minute"},
+  {"duration", "month"},
+  {"duration", "nanosecond"},
+  {"mass", "ounce"},
+  {"concentr", "percent"},
+  {"digital", "petabyte"},
+  {"mass", "pound"},
+  {"duration", "second"},
+  {"mass", "stone"},
+  {"digital", "terabit"},
+  {"digital", "terabyte"},
+  {"duration", "week"},
+  {"length", "yard"},
+  {"duration", "year"},
+    // clang-format on
+};
+
+}  // namespace mozilla::intl
+
+#endif
diff --git a/intl/components/src/NumberFormat.cpp b/intl/components/src/NumberFormat.cpp
new file mode 100644
index 0000000000..4d9dbc0cbf
--- /dev/null
+++ b/intl/components/src/NumberFormat.cpp
@@ -0,0 +1,155 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+#include "mozilla/Try.h"
+#include "mozilla/intl/NumberFormat.h"
+#include "NumberFormatFields.h"
+#include "NumberFormatterSkeleton.h"
+#include "ScopedICUObject.h"
+
+#include "unicode/unumberformatter.h"
+#include "unicode/upluralrules.h"
+
+namespace mozilla::intl {
+
+/*static*/ Result<UniquePtr<NumberFormat>, ICUError> NumberFormat::TryCreate(
+    std::string_view aLocale, const NumberFormatOptions& aOptions) {
+  UniquePtr<NumberFormat> nf = MakeUnique<NumberFormat>();
+  Result<Ok, ICUError> result = nf->initialize(aLocale, aOptions);
+  if (result.isOk()) {
+    return nf;
+  }
+
+  return Err(result.unwrapErr());
+}
+
+NumberFormat::~NumberFormat() {
+  if (mFormattedNumber) {
+    unumf_closeResult(mFormattedNumber);
+  }
+  if (mNumberFormatter) {
+    unumf_close(mNumberFormatter);
+  }
+}
+
+Result<Ok, ICUError> NumberFormat::initialize(
+    std::string_view aLocale, const NumberFormatOptions& aOptions) {
+  mFormatForUnit = aOptions.mUnit.isSome();
+  NumberFormatterSkeleton skeleton(aOptions);
+  mNumberFormatter = skeleton.toFormatter(aLocale);
+  if (mNumberFormatter) {
+    UErrorCode status = U_ZERO_ERROR;
+    mFormattedNumber = unumf_openResult(&status);
+    if (U_FAILURE(status)) {
+      return Err(ToICUError(status));
+    }
+    return Ok();
+  }
+  return Err(ICUError::InternalError);
+}
+
+Result<std::u16string_view, ICUError> NumberFormat::formatToParts(
+    double number, NumberPartVector& parts) const {
+  if (!formatInternal(number)) {
+    return Err(ICUError::InternalError);
+  }
+
+  bool isNegative = !std::isnan(number) && IsNegative(number);
+
+  return FormatResultToParts(mFormattedNumber, Some(number), isNegative,
+                             mFormatForUnit, parts);
+}
+
+Result<std::u16string_view, ICUError> NumberFormat::formatToParts(
+    int64_t number, NumberPartVector& parts) const {
+  if (!formatInternal(number)) {
+    return Err(ICUError::InternalError);
+  }
+
+  return FormatResultToParts(mFormattedNumber, Nothing(), number < 0,
+                             mFormatForUnit, parts);
+}
+
+Result<std::u16string_view, ICUError> NumberFormat::formatToParts(
+    std::string_view number, NumberPartVector& parts) const {
+  if (!formatInternal(number)) {
+    return Err(ICUError::InternalError);
+  }
+
+  // Non-finite numbers aren't currently supported here. If we ever need to
+  // support those, the |Maybe<double>| argument must be computed here.
+  MOZ_ASSERT(number != "Infinity");
+  MOZ_ASSERT(number != "+Infinity");
+  MOZ_ASSERT(number != "-Infinity");
+  MOZ_ASSERT(number != "NaN");
+
+  bool isNegative = !number.empty() && number[0] == '-';
+
+  return FormatResultToParts(mFormattedNumber, Nothing(), isNegative,
+                             mFormatForUnit, parts);
+}
+
+Result<int32_t, ICUError> NumberFormat::selectFormatted(
+    double number, char16_t* keyword, int32_t keywordSize,
+    UPluralRules* pluralRules) const {
+  MOZ_ASSERT(keyword && pluralRules);
+  UErrorCode status = U_ZERO_ERROR;
+
+  MOZ_TRY(format(number));
+
+  int32_t utf16KeywordLength = uplrules_selectFormatted(
+      pluralRules, mFormattedNumber, keyword, keywordSize, &status);
+
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+
+  return utf16KeywordLength;
+}
+
+bool NumberFormat::formatInternal(double number) const {
+  // ICU incorrectly formats NaN values with the sign bit set, as if they
+  // were negative.  Replace all NaNs with a single pattern with sign bit
+  // unset ("positive", that is) until ICU is fixed.
+  if (MOZ_UNLIKELY(std::isnan(number))) {
+    number = SpecificNaN<double>(0, 1);
+  }
+
+  UErrorCode status = U_ZERO_ERROR;
+  unumf_formatDouble(mNumberFormatter, number, mFormattedNumber, &status);
+  return U_SUCCESS(status);
+}
+
+bool NumberFormat::formatInternal(int64_t number) const {
+  UErrorCode status = U_ZERO_ERROR;
+  unumf_formatInt(mNumberFormatter, number, mFormattedNumber, &status);
+  return U_SUCCESS(status);
+}
+
+bool NumberFormat::formatInternal(std::string_view number) const {
+  UErrorCode status = U_ZERO_ERROR;
+  unumf_formatDecimal(mNumberFormatter, number.data(), number.size(),
+                      mFormattedNumber, &status);
+  return U_SUCCESS(status);
+}
+
+Result<std::u16string_view, ICUError> NumberFormat::formatResult() const {
+  UErrorCode status = U_ZERO_ERROR;
+
+  const UFormattedValue* formattedValue =
+      unumf_resultAsValue(mFormattedNumber, &status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+
+  int32_t utf16Length;
+  const char16_t* utf16Str =
+      ufmtval_getString(formattedValue, &utf16Length, &status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+
+  return std::u16string_view(utf16Str, static_cast<size_t>(utf16Length));
+}
+
+}  // namespace mozilla::intl
diff --git a/intl/components/src/NumberFormat.h b/intl/components/src/NumberFormat.h
new file mode 100644
index 0000000000..684b772e30
--- /dev/null
+++ b/intl/components/src/NumberFormat.h
@@ -0,0 +1,426 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+#ifndef intl_components_NumberFormat_h_
+#define intl_components_NumberFormat_h_
+#include <string_view>
+#include <utility>
+#include <vector>
+
+#include "mozilla/FloatingPoint.h"
+#include "mozilla/intl/ICU4CGlue.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/PodOperations.h"
+#include "mozilla/Result.h"
+#include "mozilla/Utf8.h"
+#include "mozilla/Vector.h"
+#include "mozilla/intl/ICUError.h"
+#include "mozilla/intl/NumberPart.h"
+
+#include "unicode/ustring.h"
+#include "unicode/unum.h"
+#include "unicode/unumberformatter.h"
+
+struct UPluralRules;
+
+namespace mozilla::intl {
+
+struct PluralRulesOptions;
+
+/**
+ * Configure NumberFormat options.
+ * The supported display styles are:
+ *   * Decimal (default)
+ *   * Currency (controlled by mCurrency)
+ *   * Unit (controlled by mUnit)
+ *   * Percent (controlled by mPercent)
+ *
+ * Only one of mCurrency, mUnit or mPercent should be set. If none are set,
+ * the number will formatted as a decimal.
+ *
+ * https://github.com/unicode-org/icu/blob/master/docs/userguide/format_parse/numbers/skeletons.md#unit
+ */
+struct MOZ_STACK_CLASS NumberFormatOptions {
+  /**
+   * Display a currency amount. |currency| must be a three-letter currency code.
+   *
+   * https://github.com/unicode-org/icu/blob/master/docs/userguide/format_parse/numbers/skeletons.md#unit
+   * https://github.com/unicode-org/icu/blob/master/docs/userguide/format_parse/numbers/skeletons.md#unit-width
+   */
+  enum class CurrencyDisplay {
+    Symbol,
+    Code,
+    Name,
+    NarrowSymbol,
+  };
+  Maybe<std::pair<std::string_view, CurrencyDisplay>> mCurrency;
+
+  /**
+   * Set the fraction digits settings. |min| can be zero, |max| must be
+   * larger-or-equal to |min|.
+   *
+   * https://github.com/unicode-org/icu/blob/master/docs/userguide/format_parse/numbers/skeletons.md#fraction-precision
+   */
+  Maybe<std::pair<uint32_t, uint32_t>> mFractionDigits;
+
+  /**
+   * Set the minimum number of integer digits. |min| must be a non-zero
+   * number.
+   *
+   * https://github.com/unicode-org/icu/blob/master/docs/userguide/format_parse/numbers/skeletons.md#integer-width
+   */
+  Maybe<uint32_t> mMinIntegerDigits;
+
+  /**
+   * Set the significant digits settings. |min| must be a non-zero number, |max|
+   * must be larger-or-equal to |min|.
+   *
+   * https://github.com/unicode-org/icu/blob/master/docs/userguide/format_parse/numbers/skeletons.md#significant-digits-precision
+   */
+  Maybe<std::pair<uint32_t, uint32_t>> mSignificantDigits;
+
+  /**
+   * Display a unit amount. |unit| must be a well-formed unit identifier.
+   *
+   * https://github.com/unicode-org/icu/blob/master/docs/userguide/format_parse/numbers/skeletons.md#unit
+   * https://github.com/unicode-org/icu/blob/master/docs/userguide/format_parse/numbers/skeletons.md#per-unit
+   * https://github.com/unicode-org/icu/blob/master/docs/userguide/format_parse/numbers/skeletons.md#unit-width
+   */
+  enum class UnitDisplay { Short, Narrow, Long };
+  Maybe<std::pair<std::string_view, UnitDisplay>> mUnit;
+
+  /**
+   * Display a percent number.
+   *
+   * https://github.com/unicode-org/icu/blob/master/docs/userguide/format_parse/numbers/skeletons.md#unit
+   * https://github.com/unicode-org/icu/blob/master/docs/userguide/format_parse/numbers/skeletons.md#scale
+   */
+  bool mPercent = false;
+
+  /**
+   * Set to true to strip trailing zeros after the decimal point for integer
+   * values.
+   *
+   * https://github.com/unicode-org/icu/blob/master/docs/userguide/format_parse/numbers/skeletons.md#trailing-zero-display
+   */
+  bool mStripTrailingZero = false;
+
+  /**
+   * Enable or disable grouping.
+   *
+   * https://github.com/unicode-org/icu/blob/master/docs/userguide/format_parse/numbers/skeletons.md#grouping
+   */
+  enum class Grouping {
+    Auto,
+    Always,
+    Min2,
+    Never,
+  } mGrouping = Grouping::Auto;
+
+  /**
+   * Set the notation style.
+   *
+   * https://github.com/unicode-org/icu/blob/master/docs/userguide/format_parse/numbers/skeletons.md#notation
+   */
+  enum class Notation {
+    Standard,
+    Scientific,
+    Engineering,
+    CompactShort,
+    CompactLong
+  } mNotation = Notation::Standard;
+
+  /**
+   * Set the sign-display.
+   *
+   * https://github.com/unicode-org/icu/blob/master/docs/userguide/format_parse/numbers/skeletons.md#sign-display
+   */
+  enum class SignDisplay {
+    Auto,
+    Never,
+    Always,
+    ExceptZero,
+    Negative,
+    Accounting,
+    AccountingAlways,
+    AccountingExceptZero,
+    AccountingNegative,
+  } mSignDisplay = SignDisplay::Auto;
+
+  /**
+   * Set the rounding increment, which must be a non-zero number.
+   *
+   * https://github.com/unicode-org/icu/blob/master/docs/userguide/format_parse/numbers/skeletons.md#precision
+   */
+  uint32_t mRoundingIncrement = 1;
+
+  /**
+   * Set the rounding mode.
+   *
+   * https://github.com/unicode-org/icu/blob/master/docs/userguide/format_parse/numbers/skeletons.md#rounding-mode
+   */
+  enum class RoundingMode {
+    Ceil,
+    Floor,
+    Expand,
+    Trunc,
+    HalfCeil,
+    HalfFloor,
+    HalfExpand,
+    HalfTrunc,
+    HalfEven,
+    HalfOdd,
+  } mRoundingMode = RoundingMode::HalfExpand;
+
+  /**
+   * Set the rounding priority. |mFractionDigits| and |mSignificantDigits| must
+   * both be set if the rounding priority isn't equal to "auto".
+   *
+   * https://github.com/unicode-org/icu/blob/master/docs/userguide/format_parse/numbers/skeletons.md#fraction-precision
+   */
+  enum class RoundingPriority {
+    Auto,
+    MorePrecision,
+    LessPrecision,
+  } mRoundingPriority = RoundingPriority::Auto;
+};
+
+/**
+ * According to http://userguide.icu-project.org/design, as long as we constrain
+ * ourselves to const APIs ICU is const-correct.
+ */
+
+/**
+ * A NumberFormat implementation that roughly mirrors the API provided by
+ * the ECMA-402 Intl.NumberFormat object.
+ *
+ * https://tc39.es/ecma402/#numberformat-objects
+ */
+class NumberFormat final {
+ public:
+  /**
+   * Initialize a new NumberFormat for the provided locale and using the
+   * provided options.
+   *
+   * https://tc39.es/ecma402/#sec-initializenumberformat
+   */
+  static Result<UniquePtr<NumberFormat>, ICUError> TryCreate(
+      std::string_view aLocale, const NumberFormatOptions& aOptions);
+
+  NumberFormat() = default;
+  NumberFormat(const NumberFormat&) = delete;
+  NumberFormat& operator=(const NumberFormat&) = delete;
+  ~NumberFormat();
+
+  /**
+   * Formats a double to a utf-16 string. The string view is valid until
+   * another number is formatted. Accessing the string view after this event
+   * is undefined behavior.
+   *
+   * https://tc39.es/ecma402/#sec-formatnumberstring
+   */
+  Result<std::u16string_view, ICUError> format(double number) const {
+    if (!formatInternal(number)) {
+      return Err(ICUError::InternalError);
+    }
+
+    return formatResult();
+  }
+
+  /**
+   * Formats a double to a utf-16 string, and fills the provided parts vector.
+   * The string view is valid until another number is formatted. Accessing the
+   * string view after this event is undefined behavior.
+   *
+   * This is utf-16 only because the only current use case is in
+   * SpiderMonkey. Supporting utf-8 would require recalculating the offsets
+   * in NumberPartVector from fixed width to variable width, which might be
+   * tricky to get right and is work that won't be necessary if we switch to
+   * ICU4X (see Bug 1707035).
+   *
+   * https://tc39.es/ecma402/#sec-partitionnumberpattern
+   */
+  Result<std::u16string_view, ICUError> formatToParts(
+      double number, NumberPartVector& parts) const;
+
+  /**
+   * Formats a double to the provider buffer (either utf-8 or utf-16)
+   *
+   * https://tc39.es/ecma402/#sec-formatnumberstring
+   */
+  template <typename B>
+  Result<Ok, ICUError> format(double number, B& buffer) const {
+    if (!formatInternal(number)) {
+      return Err(ICUError::InternalError);
+    }
+
+    return formatResult<typename B::CharType, B>(buffer);
+  }
+
+  /**
+   * Formats an int64_t to a utf-16 string. The string view is valid until
+   * another number is formatted. Accessing the string view after this event is
+   * undefined behavior.
+   *
+   * https://tc39.es/ecma402/#sec-formatnumberstring
+   */
+  Result<std::u16string_view, ICUError> format(int64_t number) const {
+    if (!formatInternal(number)) {
+      return Err(ICUError::InternalError);
+    }
+
+    return formatResult();
+  }
+
+  /**
+   * Formats a int64_t to a utf-16 string, and fills the provided parts vector.
+   * The string view is valid until another number is formatted. Accessing the
+   * string view after this event is undefined behavior.
+   *
+   * This is utf-16 only because the only current use case is in
+   * SpiderMonkey. Supporting utf-8 would require recalculating the offsets
+   * in NumberPartVector from fixed width to variable width, which might be
+   * tricky to get right and is work that won't be necessary if we switch to
+   * ICU4X (see Bug 1707035).
+   *
+   * https://tc39.es/ecma402/#sec-partitionnumberpattern
+   */
+  Result<std::u16string_view, ICUError> formatToParts(
+      int64_t number, NumberPartVector& parts) const;
+
+  /**
+   * Formats an int64_t to the provider buffer (either utf-8 or utf-16).
+   *
+   * https://tc39.es/ecma402/#sec-formatnumberstring
+   */
+  template <typename B>
+  Result<Ok, ICUError> format(int64_t number, B& buffer) const {
+    if (!formatInternal(number)) {
+      return Err(ICUError::InternalError);
+    }
+
+    return formatResult<typename B::CharType, B>(buffer);
+  }
+
+  /**
+   * Formats a string encoded decimal number to a utf-16 string. The string view
+   * is valid until another number is formatted. Accessing the string view
+   * after this event is undefined behavior.
+   *
+   * https://tc39.es/ecma402/#sec-formatnumberstring
+   */
+  Result<std::u16string_view, ICUError> format(std::string_view number) const {
+    if (!formatInternal(number)) {
+      return Err(ICUError::InternalError);
+    }
+
+    return formatResult();
+  }
+
+  /**
+   * Formats a string encoded decimal number to a utf-16 string, and fills the
+   * provided parts vector. The string view is valid until another number is
+   * formatted. Accessing the string view after this event is undefined
+   * behavior.
+   *
+   * This is utf-16 only because the only current use case is in
+   * SpiderMonkey. Supporting utf-8 would require recalculating the offsets
+   * in NumberPartVector from fixed width to variable width, which might be
+   * tricky to get right and is work that won't be necessary if we switch to
+   * ICU4X (see Bug 1707035).
+   *
+   * https://tc39.es/ecma402/#sec-partitionnumberpattern
+   */
+  Result<std::u16string_view, ICUError> formatToParts(
+      std::string_view number, NumberPartVector& parts) const;
+
+  /**
+   * Formats a string encoded decimal number to the provider buffer
+   * (either utf-8 or utf-16).
+   *
+   * https://tc39.es/ecma402/#sec-formatnumberstring
+   */
+  template <typename B>
+  Result<Ok, ICUError> format(std::string_view number, B& buffer) const {
+    if (!formatInternal(number)) {
+      return Err(ICUError::InternalError);
+    }
+
+    return formatResult<typename B::CharType, B>(buffer);
+  }
+
+  /**
+   * Formats the number and selects the keyword by using a provided
+   * UPluralRules object.
+   *
+   * https://tc39.es/ecma402/#sec-intl.pluralrules.prototype.select
+   *
+   * TODO(1713917) This is necessary because both PluralRules and
+   * NumberFormat have a shared dependency on the raw UFormattedNumber
+   * type. Once we transition to using ICU4X, the FFI calls should no
+   * longer require such shared dependencies. At that time, this
+   * functionality should be removed from NumberFormat and invoked
+   * solely from PluralRules.
+   */
+  Result<int32_t, ICUError> selectFormatted(double number, char16_t* keyword,
+                                            int32_t keywordSize,
+                                            UPluralRules* pluralRules) const;
+
+  /**
+   * Returns an iterator over all supported number formatter locales.
+   *
+   * The returned strings are ICU locale identifiers and NOT BCP 47 language
+   * tags.
+   *
+   * Also see <https://unicode-org.github.io/icu/userguide/locale>.
+   */
+  static auto GetAvailableLocales() {
+    return AvailableLocalesEnumeration<unum_countAvailable,
+                                       unum_getAvailable>();
+  }
+
+ private:
+  UNumberFormatter* mNumberFormatter = nullptr;
+  UFormattedNumber* mFormattedNumber = nullptr;
+  bool mFormatForUnit = false;
+
+  Result<Ok, ICUError> initialize(std::string_view aLocale,
+                                  const NumberFormatOptions& aOptions);
+
+  [[nodiscard]] bool formatInternal(double number) const;
+  [[nodiscard]] bool formatInternal(int64_t number) const;
+  [[nodiscard]] bool formatInternal(std::string_view number) const;
+
+  Result<std::u16string_view, ICUError> formatResult() const;
+
+  template <typename C, typename B>
+  Result<Ok, ICUError> formatResult(B& buffer) const {
+    // We only support buffers with char or char16_t.
+    static_assert(std::is_same_v<C, char> || std::is_same_v<C, char16_t>);
+
+    return formatResult().andThen(
+        [&buffer](std::u16string_view result) -> Result<Ok, ICUError> {
+          if constexpr (std::is_same_v<C, char>) {
+            if (!FillBuffer(Span(result.data(), result.size()), buffer)) {
+              return Err(ICUError::OutOfMemory);
+            }
+            return Ok();
+          } else {
+            // ICU provides APIs which accept a buffer, but they just copy from
+            // an internal buffer behind the scenes anyway.
+            if (!buffer.reserve(result.size())) {
+              return Err(ICUError::OutOfMemory);
+            }
+            PodCopy(static_cast<char16_t*>(buffer.data()), result.data(),
+                    result.size());
+            buffer.written(result.size());
+
+            return Ok();
+          }
+        });
+  }
+};
+
+}  // namespace mozilla::intl
+
+#endif
diff --git a/intl/components/src/NumberFormatFields.cpp b/intl/components/src/NumberFormatFields.cpp
new file mode 100644
index 0000000000..ae6fb531dd
--- /dev/null
+++ b/intl/components/src/NumberFormatFields.cpp
@@ -0,0 +1,396 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+#include "ICU4CGlue.h"
+#include "NumberFormatFields.h"
+#include "ScopedICUObject.h"
+
+#include "mozilla/FloatingPoint.h"
+#include "unicode/uformattedvalue.h"
+#include "unicode/unum.h"
+#include "unicode/unumberformatter.h"
+
+namespace mozilla::intl {
+
+bool NumberFormatFields::append(NumberPartType type, int32_t begin,
+                                int32_t end) {
+  MOZ_ASSERT(begin >= 0);
+  MOZ_ASSERT(end >= 0);
+  MOZ_ASSERT(begin < end, "erm, aren't fields always non-empty?");
+
+  return fields_.emplaceBack(uint32_t(begin), uint32_t(end), type);
+}
+
+bool NumberFormatFields::toPartsVector(size_t overallLength,
+                                       const NumberPartSourceMap& sourceMap,
+                                       NumberPartVector& parts) {
+  std::sort(fields_.begin(), fields_.end(),
+            [](const NumberFormatField& left, const NumberFormatField& right) {
+              // Sort first by begin index, then to place
+              // enclosing fields before nested fields.
+              return left.begin < right.begin ||
+                     (left.begin == right.begin && left.end > right.end);
+            });
+
+  // Then iterate over the sorted field list to generate a sequence of parts
+  // (what ECMA-402 actually exposes).  A part is a maximal character sequence
+  // entirely within no field or a single most-nested field.
+  //
+  // Diagrams may be helpful to illustrate how fields map to parts.  Consider
+  // formatting -19,766,580,028,249.41, the US national surplus (negative
+  // because it's actually a debt) on October 18, 2016.
+  //
+  //    var options =
+  //      { style: "currency", currency: "USD", currencyDisplay: "name" };
+  //    var usdFormatter = new Intl.NumberFormat("en-US", options);
+  //    usdFormatter.format(-19766580028249.41);
+  //
+  // The formatted result is "-19,766,580,028,249.41 US dollars".  ICU
+  // identifies these fields in the string:
+  //
+  //     UNUM_GROUPING_SEPARATOR_FIELD
+  //                   |
+  //   UNUM_SIGN_FIELD |  UNUM_DECIMAL_SEPARATOR_FIELD
+  //    |   __________/|   |
+  //    |  /   |   |   |   |
+  //   "-19,766,580,028,249.41 US dollars"
+  //     \________________/ |/ \_______/
+  //             |          |      |
+  //    UNUM_INTEGER_FIELD  |  UNUM_CURRENCY_FIELD
+  //                        |
+  //               UNUM_FRACTION_FIELD
+  //
+  // These fields map to parts as follows:
+  //
+  //         integer     decimal
+  //       _____|________  |
+  //      /  /| |\  |\  |\ |  literal
+  //     /| / | | \ | \ | \|  |
+  //   "-19,766,580,028,249.41 US dollars"
+  //    |  \___|___|___/    |/ \________/
+  //    |        |          |       |
+  //    |      group        |   currency
+  //    |                   |
+  //   minusSign        fraction
+  //
+  // The sign is a part.  Each comma is a part, splitting the integer field
+  // into parts for trillions/billions/&c. digits.  The decimal point is a
+  // part.  Cents are a part.  The space between cents and currency is a part
+  // (outside any field).  Last, the currency field is a part.
+
+  class PartGenerator {
+    // The fields in order from start to end, then least to most nested.
+    const FieldsVector& fields;
+
+    // Index of the current field, in |fields|, being considered to
+    // determine part boundaries.  |lastEnd <= fields[index].begin| is an
+    // invariant.
+    size_t index = 0;
+
+    // The end index of the last part produced, always less than or equal
+    // to |limit|, strictly increasing.
+    uint32_t lastEnd = 0;
+
+    // The length of the overall formatted string.
+    const uint32_t limit = 0;
+
+    NumberPartSourceMap sourceMap;
+
+    Vector<size_t, 4> enclosingFields;
+
+    void popEnclosingFieldsEndingAt(uint32_t end) {
+      MOZ_ASSERT_IF(enclosingFields.length() > 0,
+                    fields[enclosingFields.back()].end >= end);
+
+      while (enclosingFields.length() > 0 &&
+             fields[enclosingFields.back()].end == end) {
+        enclosingFields.popBack();
+      }
+    }
+
+    bool nextPartInternal(NumberPart* part) {
+      size_t len = fields.length();
+      MOZ_ASSERT(index <= len);
+
+      // If we're out of fields, all that remains are part(s) consisting
+      // of trailing portions of enclosing fields, and maybe a final
+      // literal part.
+      if (index == len) {
+        if (enclosingFields.length() > 0) {
+          const auto& enclosing = fields[enclosingFields.popCopy()];
+          *part = {enclosing.type, sourceMap.source(enclosing), enclosing.end};
+
+          // If additional enclosing fields end where this part ends,
+          // pop them as well.
+          popEnclosingFieldsEndingAt(part->endIndex);
+        } else {
+          *part = {NumberPartType::Literal, sourceMap.source(limit), limit};
+        }
+
+        return true;
+      }
+
+      // Otherwise we still have a field to process.
+      const NumberFormatField* current = &fields[index];
+      MOZ_ASSERT(lastEnd <= current->begin);
+      MOZ_ASSERT(current->begin < current->end);
+
+      // But first, deal with inter-field space.
+      if (lastEnd < current->begin) {
+        if (enclosingFields.length() > 0) {
+          // Space between fields, within an enclosing field, is part
+          // of that enclosing field, until the start of the current
+          // field or the end of the enclosing field, whichever is
+          // earlier.
+          const auto& enclosing = fields[enclosingFields.back()];
+          *part = {enclosing.type, sourceMap.source(enclosing),
+                   std::min(enclosing.end, current->begin)};
+          popEnclosingFieldsEndingAt(part->endIndex);
+        } else {
+          // If there's no enclosing field, the space is a literal.
+          *part = {NumberPartType::Literal, sourceMap.source(current->begin),
+                   current->begin};
+        }
+
+        return true;
+      }
+
+      // Otherwise, the part spans a prefix of the current field.  Find
+      // the most-nested field containing that prefix.
+      const NumberFormatField* next;
+      do {
+        current = &fields[index];
+
+        // If the current field is last, the part extends to its end.
+        if (++index == len) {
+          *part = {current->type, sourceMap.source(*current), current->end};
+          return true;
+        }
+
+        next = &fields[index];
+        MOZ_ASSERT(current->begin <= next->begin);
+        MOZ_ASSERT(current->begin < next->end);
+
+        // If the next field nests within the current field, push an
+        // enclosing field.  (If there are no nested fields, don't
+        // bother pushing a field that'd be immediately popped.)
+        if (current->end > next->begin) {
+          if (!enclosingFields.append(index - 1)) {
+            return false;
+          }
+        }
+
+        // Do so until the next field begins after this one.
+      } while (current->begin == next->begin);
+
+      if (current->end <= next->begin) {
+        // The next field begins after the current field ends.  Therefore
+        // the current part ends at the end of the current field.
+        *part = {current->type, sourceMap.source(*current), current->end};
+        popEnclosingFieldsEndingAt(part->endIndex);
+      } else {
+        // The current field encloses the next one.  The current part
+        // ends where the next field/part will start.
+        *part = {current->type, sourceMap.source(*current), next->begin};
+      }
+
+      return true;
+    }
+
+   public:
+    PartGenerator(const FieldsVector& vec, uint32_t limit,
+                  const NumberPartSourceMap& sourceMap)
+        : fields(vec), limit(limit), sourceMap(sourceMap) {}
+
+    bool nextPart(bool* hasPart, NumberPart* part) {
+      // There are no parts left if we've partitioned the entire string.
+      if (lastEnd == limit) {
+        MOZ_ASSERT(enclosingFields.length() == 0);
+        *hasPart = false;
+        return true;
+      }
+
+      if (!nextPartInternal(part)) {
+        return false;
+      }
+
+      *hasPart = true;
+      lastEnd = part->endIndex;
+      return true;
+    }
+  };
+
+  // Finally, generate the result array.
+  size_t lastEndIndex = 0;
+
+  PartGenerator gen(fields_, overallLength, sourceMap);
+  do {
+    bool hasPart;
+    NumberPart part;
+    if (!gen.nextPart(&hasPart, &part)) {
+      return false;
+    }
+
+    if (!hasPart) {
+      break;
+    }
+
+    MOZ_ASSERT(lastEndIndex < part.endIndex);
+
+    if (!parts.append(part)) {
+      return false;
+    }
+
+    lastEndIndex = part.endIndex;
+  } while (true);
+
+  MOZ_ASSERT(lastEndIndex == overallLength,
+             "result array must partition the entire string");
+
+  return lastEndIndex == overallLength;
+}
+
+Result<std::u16string_view, ICUError> FormatResultToParts(
+    const UFormattedNumber* value, Maybe<double> number, bool isNegative,
+    bool formatForUnit, NumberPartVector& parts) {
+  UErrorCode status = U_ZERO_ERROR;
+
+  const UFormattedValue* formattedValue = unumf_resultAsValue(value, &status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+
+  return FormatResultToParts(formattedValue, number, isNegative, formatForUnit,
+                             parts);
+}
+
+Result<std::u16string_view, ICUError> FormatResultToParts(
+    const UFormattedValue* value, Maybe<double> number, bool isNegative,
+    bool formatForUnit, NumberPartVector& parts) {
+  UErrorCode status = U_ZERO_ERROR;
+
+  int32_t utf16Length;
+  const char16_t* utf16Str = ufmtval_getString(value, &utf16Length, &status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+
+  UConstrainedFieldPosition* fpos = ucfpos_open(&status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+  ScopedICUObject<UConstrainedFieldPosition, ucfpos_close> toCloseFpos(fpos);
+
+  // We're only interested in UFIELD_CATEGORY_NUMBER fields.
+  ucfpos_constrainCategory(fpos, UFIELD_CATEGORY_NUMBER, &status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+
+  // Vacuum up fields in the overall formatted string.
+  NumberFormatFields fields;
+
+  while (true) {
+    bool hasMore = ufmtval_nextPosition(value, fpos, &status);
+    if (U_FAILURE(status)) {
+      return Err(ToICUError(status));
+    }
+    if (!hasMore) {
+      break;
+    }
+
+    int32_t fieldName = ucfpos_getField(fpos, &status);
+    if (U_FAILURE(status)) {
+      return Err(ToICUError(status));
+    }
+
+    int32_t beginIndex, endIndex;
+    ucfpos_getIndexes(fpos, &beginIndex, &endIndex, &status);
+    if (U_FAILURE(status)) {
+      return Err(ToICUError(status));
+    }
+
+    Maybe<NumberPartType> partType = GetPartTypeForNumberField(
+        UNumberFormatFields(fieldName), number, isNegative, formatForUnit);
+    if (!partType || !fields.append(*partType, beginIndex, endIndex)) {
+      return Err(ICUError::InternalError);
+    }
+  }
+
+  if (!fields.toPartsVector(utf16Length, parts)) {
+    return Err(ICUError::InternalError);
+  }
+
+  return std::u16string_view(utf16Str, static_cast<size_t>(utf16Length));
+}
+
+// See intl/icu/source/i18n/unicode/unum.h for a detailed field list.  This
+// list is deliberately exhaustive: cases might have to be added/removed if
+// this code is compiled with a different ICU with more UNumberFormatFields
+// enum initializers.  Please guard such cases with appropriate ICU
+// version-testing #ifdefs, should cross-version divergence occur.
+Maybe<NumberPartType> GetPartTypeForNumberField(UNumberFormatFields fieldName,
+                                                Maybe<double> number,
+                                                bool isNegative,
+                                                bool formatForUnit) {
+  switch (fieldName) {
+    case UNUM_INTEGER_FIELD:
+      if (number.isSome()) {
+        if (std::isnan(*number)) {
+          return Some(NumberPartType::Nan);
+        }
+        if (!std::isfinite(*number)) {
+          return Some(NumberPartType::Infinity);
+        }
+      }
+      return Some(NumberPartType::Integer);
+    case UNUM_FRACTION_FIELD:
+      return Some(NumberPartType::Fraction);
+    case UNUM_DECIMAL_SEPARATOR_FIELD:
+      return Some(NumberPartType::Decimal);
+    case UNUM_EXPONENT_SYMBOL_FIELD:
+      return Some(NumberPartType::ExponentSeparator);
+    case UNUM_EXPONENT_SIGN_FIELD:
+      return Some(NumberPartType::ExponentMinusSign);
+    case UNUM_EXPONENT_FIELD:
+      return Some(NumberPartType::ExponentInteger);
+    case UNUM_GROUPING_SEPARATOR_FIELD:
+      return Some(NumberPartType::Group);
+    case UNUM_CURRENCY_FIELD:
+      return Some(NumberPartType::Currency);
+    case UNUM_PERCENT_FIELD:
+      if (formatForUnit) {
+        return Some(NumberPartType::Unit);
+      }
+      return Some(NumberPartType::Percent);
+    case UNUM_PERMILL_FIELD:
+      MOZ_ASSERT_UNREACHABLE(
+          "unexpected permill field found, even though "
+          "we don't use any user-defined patterns that "
+          "would require a permill field");
+      break;
+    case UNUM_SIGN_FIELD:
+      if (isNegative) {
+        return Some(NumberPartType::MinusSign);
+      }
+      return Some(NumberPartType::PlusSign);
+    case UNUM_MEASURE_UNIT_FIELD:
+      return Some(NumberPartType::Unit);
+    case UNUM_COMPACT_FIELD:
+      return Some(NumberPartType::Compact);
+    case UNUM_APPROXIMATELY_SIGN_FIELD:
+      return Some(NumberPartType::ApproximatelySign);
+#ifndef U_HIDE_DEPRECATED_API
+    case UNUM_FIELD_COUNT:
+      MOZ_ASSERT_UNREACHABLE(
+          "format field sentinel value returned by iterator!");
+      break;
+#endif
+  }
+
+  MOZ_ASSERT_UNREACHABLE(
+      "unenumerated, undocumented format field returned by iterator");
+  return Nothing();
+}
+
+}  // namespace mozilla::intl
diff --git a/intl/components/src/NumberFormatFields.h b/intl/components/src/NumberFormatFields.h
new file mode 100644
index 0000000000..4f05d4e98b
--- /dev/null
+++ b/intl/components/src/NumberFormatFields.h
@@ -0,0 +1,91 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+#ifndef intl_components_NumberFormatFields_h_
+#define intl_components_NumberFormatFields_h_
+#include "mozilla/intl/ICUError.h"
+#include "mozilla/intl/NumberPart.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/Result.h"
+#include "mozilla/Vector.h"
+
+#include "unicode/unum.h"
+
+struct UFormattedNumber;
+struct UFormattedValue;
+
+namespace mozilla::intl {
+
+struct NumberFormatField {
+  uint32_t begin;
+  uint32_t end;
+  NumberPartType type;
+
+  // Needed for vector-resizing scratch space.
+  NumberFormatField() = default;
+
+  NumberFormatField(uint32_t begin, uint32_t end, NumberPartType type)
+      : begin(begin), end(end), type(type) {}
+};
+
+struct NumberPartSourceMap {
+  struct Range {
+    uint32_t begin = 0;
+    uint32_t end = 0;
+  };
+
+  // Begin and end position of the start range.
+  Range start;
+
+  // Begin and end position of the end range.
+  Range end;
+
+  NumberPartSource source(uint32_t endIndex) {
+    if (start.begin < endIndex && endIndex <= start.end) {
+      return NumberPartSource::Start;
+    }
+    if (end.begin < endIndex && endIndex <= end.end) {
+      return NumberPartSource::End;
+    }
+    return NumberPartSource::Shared;
+  }
+
+  NumberPartSource source(const NumberFormatField& field) {
+    return source(field.end);
+  }
+};
+
+class NumberFormatFields {
+  using FieldsVector = Vector<NumberFormatField, 16>;
+
+  FieldsVector fields_;
+
+ public:
+  [[nodiscard]] bool append(NumberPartType type, int32_t begin, int32_t end);
+
+  [[nodiscard]] bool toPartsVector(size_t overallLength,
+                                   NumberPartVector& parts) {
+    return toPartsVector(overallLength, {}, parts);
+  }
+
+  [[nodiscard]] bool toPartsVector(size_t overallLength,
+                                   const NumberPartSourceMap& sourceMap,
+                                   NumberPartVector& parts);
+};
+
+Result<std::u16string_view, ICUError> FormatResultToParts(
+    const UFormattedNumber* value, Maybe<double> number, bool isNegative,
+    bool formatForUnit, NumberPartVector& parts);
+
+Result<std::u16string_view, ICUError> FormatResultToParts(
+    const UFormattedValue* value, Maybe<double> number, bool isNegative,
+    bool formatForUnit, NumberPartVector& parts);
+
+Maybe<NumberPartType> GetPartTypeForNumberField(UNumberFormatFields fieldName,
+                                                Maybe<double> number,
+                                                bool isNegative,
+                                                bool formatForUnit);
+
+}  // namespace mozilla::intl
+
+#endif
diff --git a/intl/components/src/NumberFormatterSkeleton.cpp b/intl/components/src/NumberFormatterSkeleton.cpp
new file mode 100644
index 0000000000..951a7a70a5
--- /dev/null
+++ b/intl/components/src/NumberFormatterSkeleton.cpp
@@ -0,0 +1,472 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+#include "NumberFormatterSkeleton.h"
+#include "NumberFormat.h"
+
+#include "MeasureUnitGenerated.h"
+
+#include "mozilla/RangedPtr.h"
+
+#include <algorithm>
+#include <limits>
+
+#include "unicode/unumberrangeformatter.h"
+
+namespace mozilla::intl {
+
+NumberFormatterSkeleton::NumberFormatterSkeleton(
+    const NumberFormatOptions& options) {
+  if (options.mCurrency.isSome()) {
+    if (!currency(options.mCurrency->first) ||
+        !currencyDisplay(options.mCurrency->second)) {
+      return;
+    }
+  } else if (options.mUnit.isSome()) {
+    if (!unit(options.mUnit->first) || !unitDisplay(options.mUnit->second)) {
+      return;
+    }
+  } else if (options.mPercent) {
+    if (!percent()) {
+      return;
+    }
+  }
+
+  if (options.mRoundingIncrement != 1) {
+    auto fd = options.mFractionDigits.valueOr(std::pair{0, 0});
+    if (!roundingIncrement(options.mRoundingIncrement, fd.first, fd.second,
+                           options.mStripTrailingZero)) {
+      return;
+    }
+  } else if (options.mRoundingPriority ==
+             NumberFormatOptions::RoundingPriority::Auto) {
+    if (options.mFractionDigits.isSome()) {
+      if (!fractionDigits(options.mFractionDigits->first,
+                          options.mFractionDigits->second,
+                          options.mStripTrailingZero)) {
+        return;
+      }
+    }
+
+    if (options.mSignificantDigits.isSome()) {
+      if (!significantDigits(options.mSignificantDigits->first,
+                             options.mSignificantDigits->second,
+                             options.mStripTrailingZero)) {
+        return;
+      }
+    }
+  } else {
+    MOZ_ASSERT(options.mFractionDigits);
+    MOZ_ASSERT(options.mSignificantDigits);
+
+    bool relaxed = options.mRoundingPriority ==
+                   NumberFormatOptions::RoundingPriority::MorePrecision;
+    if (!fractionWithSignificantDigits(options.mFractionDigits->first,
+                                       options.mFractionDigits->second,
+                                       options.mSignificantDigits->first,
+                                       options.mSignificantDigits->second,
+                                       relaxed, options.mStripTrailingZero)) {
+      return;
+    }
+  }
+
+  if (options.mMinIntegerDigits.isSome()) {
+    if (!minIntegerDigits(*options.mMinIntegerDigits)) {
+      return;
+    }
+  }
+
+  if (!grouping(options.mGrouping)) {
+    return;
+  }
+
+  if (!notation(options.mNotation)) {
+    return;
+  }
+
+  if (!signDisplay(options.mSignDisplay)) {
+    return;
+  }
+
+  if (!roundingMode(options.mRoundingMode)) {
+    return;
+  }
+
+  mValidSkeleton = true;
+}
+
+bool NumberFormatterSkeleton::currency(std::string_view currency) {
+  MOZ_ASSERT(currency.size() == 3,
+             "IsWellFormedCurrencyCode permits only length-3 strings");
+
+  char16_t currencyChars[] = {static_cast<char16_t>(currency[0]),
+                              static_cast<char16_t>(currency[1]),
+                              static_cast<char16_t>(currency[2]), '\0'};
+  return append(u"currency/") && append(currencyChars) && append(' ');
+}
+
+bool NumberFormatterSkeleton::currencyDisplay(
+    NumberFormatOptions::CurrencyDisplay display) {
+  switch (display) {
+    case NumberFormatOptions::CurrencyDisplay::Code:
+      return appendToken(u"unit-width-iso-code");
+    case NumberFormatOptions::CurrencyDisplay::Name:
+      return appendToken(u"unit-width-full-name");
+    case NumberFormatOptions::CurrencyDisplay::Symbol:
+      // Default, no additional tokens needed.
+      return true;
+    case NumberFormatOptions::CurrencyDisplay::NarrowSymbol:
+      return appendToken(u"unit-width-narrow");
+  }
+  MOZ_ASSERT_UNREACHABLE("unexpected currency display type");
+  return false;
+}
+
+static const SimpleMeasureUnit& FindSimpleMeasureUnit(std::string_view name) {
+  const auto* measureUnit = std::lower_bound(
+      std::begin(simpleMeasureUnits), std::end(simpleMeasureUnits), name,
+      [](const auto& measureUnit, std::string_view name) {
+        return name.compare(measureUnit.name) > 0;
+      });
+  MOZ_ASSERT(measureUnit != std::end(simpleMeasureUnits),
+             "unexpected unit identifier: unit not found");
+  MOZ_ASSERT(measureUnit->name == name,
+             "unexpected unit identifier: wrong unit found");
+  return *measureUnit;
+}
+
+static constexpr size_t MaxUnitLength() {
+  size_t length = 0;
+  for (const auto& unit : simpleMeasureUnits) {
+    length = std::max(length, std::char_traits<char>::length(unit.name));
+  }
+  return length * 2 + std::char_traits<char>::length("-per-");
+}
+
+bool NumberFormatterSkeleton::unit(std::string_view unit) {
+  MOZ_RELEASE_ASSERT(unit.length() <= MaxUnitLength());
+
+  auto appendUnit = [this](const SimpleMeasureUnit& unit) {
+    return append(unit.type, strlen(unit.type)) && append('-') &&
+           append(unit.name, strlen(unit.name));
+  };
+
+  // |unit| can be a compound unit identifier, separated by "-per-".
+  static constexpr char separator[] = "-per-";
+  size_t separator_len = strlen(separator);
+  size_t offset = unit.find(separator);
+  if (offset != std::string_view::npos) {
+    const auto& numerator = FindSimpleMeasureUnit(unit.substr(0, offset));
+    const auto& denominator = FindSimpleMeasureUnit(
+        std::string_view(unit.data() + offset + separator_len,
+                         unit.length() - offset - separator_len));
+    return append(u"measure-unit/") && appendUnit(numerator) && append(' ') &&
+           append(u"per-measure-unit/") && appendUnit(denominator) &&
+           append(' ');
+  }
+
+  const auto& simple = FindSimpleMeasureUnit(unit);
+  return append(u"measure-unit/") && appendUnit(simple) && append(' ');
+}
+
+bool NumberFormatterSkeleton::unitDisplay(
+    NumberFormatOptions::UnitDisplay display) {
+  switch (display) {
+    case NumberFormatOptions::UnitDisplay::Short:
+      return appendToken(u"unit-width-short");
+    case NumberFormatOptions::UnitDisplay::Narrow:
+      return appendToken(u"unit-width-narrow");
+    case NumberFormatOptions::UnitDisplay::Long:
+      return appendToken(u"unit-width-full-name");
+  }
+  MOZ_ASSERT_UNREACHABLE("unexpected unit display type");
+  return false;
+}
+
+bool NumberFormatterSkeleton::percent() {
+  return appendToken(u"percent scale/100");
+}
+
+bool NumberFormatterSkeleton::fractionDigits(uint32_t min, uint32_t max,
+                                             bool stripTrailingZero) {
+  // Note: |min| can be zero here.
+  MOZ_ASSERT(min <= max);
+  if (!append('.') || !appendN('0', min) || !appendN('#', max - min)) {
+    return false;
+  }
+  if (stripTrailingZero) {
+    if (!append(u"/w")) {
+      return false;
+    }
+  }
+  return append(' ');
+}
+
+bool NumberFormatterSkeleton::fractionWithSignificantDigits(
+    uint32_t mnfd, uint32_t mxfd, uint32_t mnsd, uint32_t mxsd, bool relaxed,
+    bool stripTrailingZero) {
+  // Note: |mnfd| can be zero here.
+  MOZ_ASSERT(mnfd <= mxfd);
+  MOZ_ASSERT(mnsd > 0);
+  MOZ_ASSERT(mnsd <= mxsd);
+
+  if (!append('.') || !appendN('0', mnfd) || !appendN('#', mxfd - mnfd)) {
+    return false;
+  }
+  if (!append('/') || !appendN('@', mnsd) || !appendN('#', mxsd - mnsd)) {
+    return false;
+  }
+  if (!append(relaxed ? 'r' : 's')) {
+    return false;
+  }
+  if (stripTrailingZero) {
+    if (!append(u"/w")) {
+      return false;
+    }
+  }
+  return append(' ');
+}
+
+bool NumberFormatterSkeleton::minIntegerDigits(uint32_t min) {
+  MOZ_ASSERT(min > 0);
+  return append(u"integer-width/+") && appendN('0', min) && append(' ');
+}
+
+bool NumberFormatterSkeleton::significantDigits(uint32_t min, uint32_t max,
+                                                bool stripTrailingZero) {
+  MOZ_ASSERT(min > 0);
+  MOZ_ASSERT(min <= max);
+  if (!appendN('@', min) || !appendN('#', max - min)) {
+    return false;
+  }
+  if (stripTrailingZero) {
+    if (!append(u"/w")) {
+      return false;
+    }
+  }
+  return append(' ');
+}
+
+bool NumberFormatterSkeleton::grouping(NumberFormatOptions::Grouping grouping) {
+  switch (grouping) {
+    case NumberFormatOptions::Grouping::Auto:
+      // Default, no additional tokens needed.
+      return true;
+    case NumberFormatOptions::Grouping::Always:
+      return appendToken(u"group-on-aligned");
+    case NumberFormatOptions::Grouping::Min2:
+      return appendToken(u"group-min2");
+    case NumberFormatOptions::Grouping::Never:
+      return appendToken(u"group-off");
+  }
+  MOZ_ASSERT_UNREACHABLE("unexpected grouping mode");
+  return false;
+}
+
+bool NumberFormatterSkeleton::notation(NumberFormatOptions::Notation style) {
+  switch (style) {
+    case NumberFormatOptions::Notation::Standard:
+      // Default, no additional tokens needed.
+      return true;
+    case NumberFormatOptions::Notation::Scientific:
+      return appendToken(u"scientific");
+    case NumberFormatOptions::Notation::Engineering:
+      return appendToken(u"engineering");
+    case NumberFormatOptions::Notation::CompactShort:
+      return appendToken(u"compact-short");
+    case NumberFormatOptions::Notation::CompactLong:
+      return appendToken(u"compact-long");
+  }
+  MOZ_ASSERT_UNREACHABLE("unexpected notation style");
+  return false;
+}
+
+bool NumberFormatterSkeleton::signDisplay(
+    NumberFormatOptions::SignDisplay display) {
+  switch (display) {
+    case NumberFormatOptions::SignDisplay::Auto:
+      // Default, no additional tokens needed.
+      return true;
+    case NumberFormatOptions::SignDisplay::Always:
+      return appendToken(u"sign-always");
+    case NumberFormatOptions::SignDisplay::Never:
+      return appendToken(u"sign-never");
+    case NumberFormatOptions::SignDisplay::ExceptZero:
+      return appendToken(u"sign-except-zero");
+    case NumberFormatOptions::SignDisplay::Negative:
+      return appendToken(u"sign-negative");
+    case NumberFormatOptions::SignDisplay::Accounting:
+      return appendToken(u"sign-accounting");
+    case NumberFormatOptions::SignDisplay::AccountingAlways:
+      return appendToken(u"sign-accounting-always");
+    case NumberFormatOptions::SignDisplay::AccountingExceptZero:
+      return appendToken(u"sign-accounting-except-zero");
+    case NumberFormatOptions::SignDisplay::AccountingNegative:
+      return appendToken(u"sign-accounting-negative");
+  }
+  MOZ_ASSERT_UNREACHABLE("unexpected sign display type");
+  return false;
+}
+
+bool NumberFormatterSkeleton::roundingIncrement(uint32_t increment,
+                                                uint32_t mnfd, uint32_t mxfd,
+                                                bool stripTrailingZero) {
+  // Note: |mnfd| can be zero here.
+  MOZ_ASSERT(mnfd <= mxfd);
+  MOZ_ASSERT(increment > 1);
+
+  // Limit |mxfd| to 100.
+  constexpr size_t maxFracDigits = 100;
+  MOZ_RELEASE_ASSERT(mxfd <= maxFracDigits);
+
+  static constexpr char digits[] = "0123456789";
+
+  // We need enough space to print any uint32_t, which is possibly shifted by
+  // |mxfd| decimal places. And additionally we need to reserve space for "0.".
+  static_assert(std::numeric_limits<uint32_t>::digits10 + 1 < maxFracDigits);
+  constexpr size_t maxLength = maxFracDigits + 2;
+
+  char chars[maxLength];
+  RangedPtr<char> ptr(chars + maxLength, chars, maxLength);
+  const RangedPtr<char> end = ptr;
+
+  // Convert to a signed integer, so we don't have to worry about underflows.
+  int32_t maxFrac = int32_t(mxfd);
+
+  // Write |increment| from back to front.
+  while (increment != 0) {
+    *--ptr = digits[increment % 10];
+    increment /= 10;
+    maxFrac -= 1;
+
+    if (maxFrac == 0) {
+      *--ptr = '.';
+    }
+  }
+
+  // Write any remaining zeros from |mxfd| and prepend '0' if we last wrote the
+  // decimal point.
+  while (maxFrac >= 0) {
+    MOZ_ASSERT_IF(maxFrac == 0, *ptr == '.');
+
+    *--ptr = '0';
+    maxFrac -= 1;
+
+    if (maxFrac == 0) {
+      *--ptr = '.';
+    }
+  }
+
+  MOZ_ASSERT(ptr < end, "At least one character is written.");
+  MOZ_ASSERT(*ptr != '.', "First character is a digit.");
+
+  if (!append(u"precision-increment/") || !append(ptr.get(), end - ptr)) {
+    return false;
+  }
+  if (stripTrailingZero) {
+    if (!append(u"/w")) {
+      return false;
+    }
+  }
+  return append(' ');
+}
+
+bool NumberFormatterSkeleton::roundingMode(
+    NumberFormatOptions::RoundingMode rounding) {
+  switch (rounding) {
+    case NumberFormatOptions::RoundingMode::Ceil:
+      return appendToken(u"rounding-mode-ceiling");
+    case NumberFormatOptions::RoundingMode::Floor:
+      return appendToken(u"rounding-mode-floor");
+    case NumberFormatOptions::RoundingMode::Expand:
+      return appendToken(u"rounding-mode-up");
+    case NumberFormatOptions::RoundingMode::Trunc:
+      return appendToken(u"rounding-mode-down");
+    case NumberFormatOptions::RoundingMode::HalfCeil:
+      return appendToken(u"rounding-mode-half-ceiling");
+    case NumberFormatOptions::RoundingMode::HalfFloor:
+      return appendToken(u"rounding-mode-half-floor");
+    case NumberFormatOptions::RoundingMode::HalfExpand:
+      return appendToken(u"rounding-mode-half-up");
+    case NumberFormatOptions::RoundingMode::HalfTrunc:
+      return appendToken(u"rounding-mode-half-down");
+    case NumberFormatOptions::RoundingMode::HalfEven:
+      return appendToken(u"rounding-mode-half-even");
+    case NumberFormatOptions::RoundingMode::HalfOdd:
+      return appendToken(u"rounding-mode-half-odd");
+  }
+  MOZ_ASSERT_UNREACHABLE("unexpected rounding mode");
+  return false;
+}
+
+UNumberFormatter* NumberFormatterSkeleton::toFormatter(
+    std::string_view locale) {
+  if (!mValidSkeleton) {
+    return nullptr;
+  }
+
+  UErrorCode status = U_ZERO_ERROR;
+  UNumberFormatter* nf = unumf_openForSkeletonAndLocale(
+      mVector.begin(), mVector.length(), AssertNullTerminatedString(locale),
+      &status);
+  if (U_FAILURE(status)) {
+    return nullptr;
+  }
+  return nf;
+}
+
+static UNumberRangeCollapse ToUNumberRangeCollapse(
+    NumberRangeFormatOptions::RangeCollapse collapse) {
+  using RangeCollapse = NumberRangeFormatOptions::RangeCollapse;
+  switch (collapse) {
+    case RangeCollapse::Auto:
+      return UNUM_RANGE_COLLAPSE_AUTO;
+    case RangeCollapse::None:
+      return UNUM_RANGE_COLLAPSE_NONE;
+    case RangeCollapse::Unit:
+      return UNUM_RANGE_COLLAPSE_UNIT;
+    case RangeCollapse::All:
+      return UNUM_RANGE_COLLAPSE_ALL;
+  }
+  MOZ_ASSERT_UNREACHABLE("unexpected range collapse");
+  return UNUM_RANGE_COLLAPSE_NONE;
+}
+
+static UNumberRangeIdentityFallback ToUNumberRangeIdentityFallback(
+    NumberRangeFormatOptions::RangeIdentityFallback identity) {
+  using RangeIdentityFallback = NumberRangeFormatOptions::RangeIdentityFallback;
+  switch (identity) {
+    case RangeIdentityFallback::SingleValue:
+      return UNUM_IDENTITY_FALLBACK_SINGLE_VALUE;
+    case RangeIdentityFallback::ApproximatelyOrSingleValue:
+      return UNUM_IDENTITY_FALLBACK_APPROXIMATELY_OR_SINGLE_VALUE;
+    case RangeIdentityFallback::Approximately:
+      return UNUM_IDENTITY_FALLBACK_APPROXIMATELY;
+    case RangeIdentityFallback::Range:
+      return UNUM_IDENTITY_FALLBACK_RANGE;
+  }
+  MOZ_ASSERT_UNREACHABLE("unexpected range identity fallback");
+  return UNUM_IDENTITY_FALLBACK_RANGE;
+}
+
+UNumberRangeFormatter* NumberFormatterSkeleton::toRangeFormatter(
+    std::string_view locale, NumberRangeFormatOptions::RangeCollapse collapse,
+    NumberRangeFormatOptions::RangeIdentityFallback identity) {
+  if (!mValidSkeleton) {
+    return nullptr;
+  }
+
+  UParseError* perror = nullptr;
+  UErrorCode status = U_ZERO_ERROR;
+  UNumberRangeFormatter* nrf =
+      unumrf_openForSkeletonWithCollapseAndIdentityFallback(
+          mVector.begin(), mVector.length(), ToUNumberRangeCollapse(collapse),
+          ToUNumberRangeIdentityFallback(identity),
+          AssertNullTerminatedString(locale), perror, &status);
+  if (U_FAILURE(status)) {
+    return nullptr;
+  }
+  return nrf;
+}
+
+}  // namespace mozilla::intl
diff --git a/intl/components/src/NumberFormatterSkeleton.h b/intl/components/src/NumberFormatterSkeleton.h
new file mode 100644
index 0000000000..134e9e0860
--- /dev/null
+++ b/intl/components/src/NumberFormatterSkeleton.h
@@ -0,0 +1,110 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+#ifndef intl_components_NumberFormatterSkeleton_h_
+#define intl_components_NumberFormatterSkeleton_h_
+#include <string_view>
+#include "mozilla/intl/NumberFormat.h"
+#include "mozilla/intl/NumberRangeFormat.h"
+#include "mozilla/Vector.h"
+#include "unicode/unumberformatter.h"
+#include "unicode/utypes.h"
+
+struct UNumberRangeFormatter;
+
+namespace mozilla::intl {
+
+/**
+ * Class to create a number formatter skeleton.
+ *
+ * The skeleton syntax is documented at:
+ * https://github.com/unicode-org/icu/blob/master/docs/userguide/format_parse/numbers/skeletons.md
+ */
+class MOZ_STACK_CLASS NumberFormatterSkeleton final {
+ public:
+  explicit NumberFormatterSkeleton(const NumberFormatOptions& options);
+
+  /**
+   * Return a new UNumberFormatter based on this skeleton.
+   */
+  UNumberFormatter* toFormatter(std::string_view locale);
+
+  /**
+   * Return a new UNumberRangeFormatter based on this skeleton.
+   */
+  UNumberRangeFormatter* toRangeFormatter(
+      std::string_view locale, NumberRangeFormatOptions::RangeCollapse collapse,
+      NumberRangeFormatOptions::RangeIdentityFallback identity);
+
+ private:
+  static constexpr size_t DefaultVectorSize = 128;
+
+  mozilla::Vector<char16_t, DefaultVectorSize> mVector;
+  bool mValidSkeleton = false;
+
+  [[nodiscard]] bool append(char16_t c) { return mVector.append(c); }
+
+  [[nodiscard]] bool appendN(char16_t c, size_t times) {
+    return mVector.appendN(c, times);
+  }
+
+  template <size_t N>
+  [[nodiscard]] bool append(const char16_t (&chars)[N]) {
+    static_assert(N > 0,
+                  "should only be used with string literals or properly "
+                  "null-terminated arrays");
+    MOZ_ASSERT(chars[N - 1] == '\0',
+               "should only be used with string literals or properly "
+               "null-terminated arrays");
+    // Without trailing \0.
+    return mVector.append(chars, N - 1);
+  }
+
+  template <size_t N>
+  [[nodiscard]] bool appendToken(const char16_t (&token)[N]) {
+    return append(token) && append(' ');
+  }
+
+  [[nodiscard]] bool append(const char* chars, size_t length) {
+    return mVector.append(chars, length);
+  }
+
+  [[nodiscard]] bool currency(std::string_view currency);
+
+  [[nodiscard]] bool currencyDisplay(
+      NumberFormatOptions::CurrencyDisplay display);
+
+  [[nodiscard]] bool unit(std::string_view unit);
+
+  [[nodiscard]] bool unitDisplay(NumberFormatOptions::UnitDisplay display);
+
+  [[nodiscard]] bool percent();
+
+  [[nodiscard]] bool fractionDigits(uint32_t min, uint32_t max,
+                                    bool stripTrailingZero);
+
+  [[nodiscard]] bool fractionWithSignificantDigits(uint32_t mnfd, uint32_t mxfd,
+                                                   uint32_t mnsd, uint32_t mxsd,
+                                                   bool relaxed,
+                                                   bool stripTrailingZero);
+
+  [[nodiscard]] bool minIntegerDigits(uint32_t min);
+
+  [[nodiscard]] bool significantDigits(uint32_t min, uint32_t max,
+                                       bool stripTrailingZero);
+
+  [[nodiscard]] bool grouping(NumberFormatOptions::Grouping grouping);
+
+  [[nodiscard]] bool notation(NumberFormatOptions::Notation style);
+
+  [[nodiscard]] bool signDisplay(NumberFormatOptions::SignDisplay display);
+
+  [[nodiscard]] bool roundingIncrement(uint32_t increment, uint32_t mnfd,
+                                       uint32_t mxfd, bool stripTrailingZero);
+
+  [[nodiscard]] bool roundingMode(NumberFormatOptions::RoundingMode rounding);
+};
+
+}  // namespace mozilla::intl
+
+#endif
diff --git a/intl/components/src/NumberParser.cpp b/intl/components/src/NumberParser.cpp
new file mode 100644
index 0000000000..fb97393783
--- /dev/null
+++ b/intl/components/src/NumberParser.cpp
@@ -0,0 +1,45 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+#include "mozilla/intl/NumberParser.h"
+
+namespace mozilla::intl {
+
+/*static*/ Result<UniquePtr<NumberParser>, ICUError> NumberParser::TryCreate(
+    const char* aLocale, bool aUseGrouping) {
+  UniquePtr<NumberParser> nf = MakeUnique<NumberParser>();
+
+  UErrorCode status = U_ZERO_ERROR;
+  nf->mNumberFormat =
+      unum_open(UNUM_DECIMAL, nullptr, 0, aLocale, nullptr, &status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+
+  if (!aUseGrouping) {
+    unum_setAttribute(nf->mNumberFormat.GetMut(), UNUM_GROUPING_USED, UBool(0));
+  }
+
+  return nf;
+}
+
+NumberParser::~NumberParser() {
+  if (mNumberFormat) {
+    unum_close(mNumberFormat.GetMut());
+  }
+}
+
+Result<std::pair<double, int32_t>, ICUError> NumberParser::ParseDouble(
+    Span<const char16_t> aDouble) const {
+  UErrorCode status = U_ZERO_ERROR;
+  int32_t parsePos = 0;
+  double value = unum_parseDouble(mNumberFormat.GetConst(), aDouble.data(),
+                                  static_cast<int32_t>(aDouble.size()),
+                                  &parsePos, &status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+  return std::make_pair(value, parsePos);
+}
+
+}  // namespace mozilla::intl
diff --git a/intl/components/src/NumberParser.h b/intl/components/src/NumberParser.h
new file mode 100644
index 0000000000..97efec0836
--- /dev/null
+++ b/intl/components/src/NumberParser.h
@@ -0,0 +1,46 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+#ifndef intl_components_NumberParser_h_
+#define intl_components_NumberParser_h_
+
+#include "mozilla/intl/ICUError.h"
+#include "mozilla/intl/ICU4CGlue.h"
+#include "mozilla/Span.h"
+#include "mozilla/UniquePtr.h"
+
+#include "unicode/unum.h"
+
+namespace mozilla::intl {
+
+class NumberParser {
+ public:
+  /**
+   * Initialize a new NumberParser for the provided locale and using the
+   * provided options.
+   */
+  static Result<UniquePtr<NumberParser>, ICUError> TryCreate(
+      const char* aLocale, bool aUseGrouping);
+
+  NumberParser() : mNumberFormat(nullptr){};
+  NumberParser(const NumberParser&) = delete;
+  NumberParser& operator=(const NumberParser&) = delete;
+  ~NumberParser();
+
+  /**
+   * Attempts to parse a string representing a double, returning the parsed
+   * double and the parse position if successful, or an error.
+   *
+   * The parse position is the index into the input string where parsing
+   * stopped because an non-numeric character was encountered.
+   */
+  Result<std::pair<double, int32_t>, ICUError> ParseDouble(
+      Span<const char16_t> aDouble) const;
+
+ private:
+  ICUPointer<UNumberFormat> mNumberFormat;
+};
+
+}  // namespace mozilla::intl
+
+#endif
diff --git a/intl/components/src/NumberPart.h b/intl/components/src/NumberPart.h
new file mode 100644
index 0000000000..8639db5768
--- /dev/null
+++ b/intl/components/src/NumberPart.h
@@ -0,0 +1,53 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+#ifndef intl_components_NumberPart_h_
+#define intl_components_NumberPart_h_
+
+#include <cstddef>
+#include <cstdint>
+
+#include "mozilla/Vector.h"
+
+namespace mozilla::intl {
+
+enum class NumberPartType : int16_t {
+  ApproximatelySign,
+  Compact,
+  Currency,
+  Decimal,
+  ExponentInteger,
+  ExponentMinusSign,
+  ExponentSeparator,
+  Fraction,
+  Group,
+  Infinity,
+  Integer,
+  Literal,
+  MinusSign,
+  Nan,
+  Percent,
+  PlusSign,
+  Unit,
+};
+
+enum class NumberPartSource : int16_t { Shared, Start, End };
+
+// Because parts fully partition the formatted string, we only track the
+// index of the end of each part -- the beginning is implicitly the last
+// part's end.
+struct NumberPart {
+  NumberPartType type;
+  NumberPartSource source;
+  size_t endIndex;
+
+  bool operator==(const NumberPart& rhs) const {
+    return type == rhs.type && source == rhs.source && endIndex == rhs.endIndex;
+  }
+  bool operator!=(const NumberPart& rhs) const { return !(*this == rhs); }
+};
+
+using NumberPartVector = mozilla::Vector<NumberPart, 8>;
+
+}  // namespace mozilla::intl
+#endif
diff --git a/intl/components/src/NumberRangeFormat.cpp b/intl/components/src/NumberRangeFormat.cpp
new file mode 100644
index 0000000000..9de8982c72
--- /dev/null
+++ b/intl/components/src/NumberRangeFormat.cpp
@@ -0,0 +1,216 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "mozilla/intl/NumberRangeFormat.h"
+
+#include "mozilla/Try.h"
+#include "mozilla/intl/ICU4CGlue.h"
+#include "mozilla/intl/NumberFormat.h"
+#include "NumberFormatFields.h"
+#include "NumberFormatterSkeleton.h"
+#include "ScopedICUObject.h"
+
+#include "unicode/uformattedvalue.h"
+#include "unicode/unumberrangeformatter.h"
+#include "unicode/upluralrules.h"
+
+namespace mozilla::intl {
+
+/*static*/ Result<UniquePtr<NumberRangeFormat>, ICUError>
+NumberRangeFormat::TryCreate(std::string_view aLocale,
+                             const NumberRangeFormatOptions& aOptions) {
+  UniquePtr<NumberRangeFormat> nrf = MakeUnique<NumberRangeFormat>();
+  MOZ_TRY(nrf->initialize(aLocale, aOptions));
+  return nrf;
+}
+
+NumberRangeFormat::~NumberRangeFormat() {
+  if (mFormattedNumberRange) {
+    unumrf_closeResult(mFormattedNumberRange);
+  }
+  if (mNumberRangeFormatter) {
+    unumrf_close(mNumberRangeFormatter);
+  }
+}
+
+Result<Ok, ICUError> NumberRangeFormat::initialize(
+    std::string_view aLocale, const NumberRangeFormatOptions& aOptions) {
+  mFormatForUnit = aOptions.mUnit.isSome();
+
+  NumberFormatterSkeleton skeleton(aOptions);
+  mNumberRangeFormatter = skeleton.toRangeFormatter(
+      aLocale, aOptions.mRangeCollapse, aOptions.mRangeIdentityFallback);
+  if (mNumberRangeFormatter) {
+    UErrorCode status = U_ZERO_ERROR;
+    mFormattedNumberRange = unumrf_openResult(&status);
+    if (U_FAILURE(status)) {
+      return Err(ToICUError(status));
+    }
+    return Ok();
+  }
+  return Err(ICUError::InternalError);
+}
+
+Result<int32_t, ICUError> NumberRangeFormat::selectForRange(
+    double start, double end, char16_t* keyword, int32_t keywordSize,
+    const UPluralRules* pluralRules) const {
+  MOZ_ASSERT(keyword);
+  MOZ_ASSERT(pluralRules);
+
+  MOZ_TRY(format(start, end));
+
+  UErrorCode status = U_ZERO_ERROR;
+  int32_t utf16KeywordLength = uplrules_selectForRange(
+      pluralRules, mFormattedNumberRange, keyword, keywordSize, &status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+
+  return utf16KeywordLength;
+}
+
+bool NumberRangeFormat::formatInternal(double start, double end) const {
+  // ICU incorrectly formats NaN values with the sign bit set, as if they
+  // were negative.  Replace all NaNs with a single pattern with sign bit
+  // unset ("positive", that is) until ICU is fixed.
+  if (MOZ_UNLIKELY(std::isnan(start))) {
+    start = SpecificNaN<double>(0, 1);
+  }
+  if (MOZ_UNLIKELY(std::isnan(end))) {
+    end = SpecificNaN<double>(0, 1);
+  }
+
+  UErrorCode status = U_ZERO_ERROR;
+  unumrf_formatDoubleRange(mNumberRangeFormatter, start, end,
+                           mFormattedNumberRange, &status);
+  return U_SUCCESS(status);
+}
+
+bool NumberRangeFormat::formatInternal(std::string_view start,
+                                       std::string_view end) const {
+  UErrorCode status = U_ZERO_ERROR;
+  unumrf_formatDecimalRange(mNumberRangeFormatter, start.data(), start.size(),
+                            end.data(), end.size(), mFormattedNumberRange,
+                            &status);
+  return U_SUCCESS(status);
+}
+
+Result<std::u16string_view, ICUError> NumberRangeFormat::formatResult() const {
+  UErrorCode status = U_ZERO_ERROR;
+
+  const UFormattedValue* formattedValue =
+      unumrf_resultAsValue(mFormattedNumberRange, &status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+
+  int32_t utf16Length;
+  const char16_t* utf16Str =
+      ufmtval_getString(formattedValue, &utf16Length, &status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+
+  return std::u16string_view(utf16Str, static_cast<size_t>(utf16Length));
+}
+
+Result<std::u16string_view, ICUError> NumberRangeFormat::formatResultToParts(
+    Maybe<double> start, bool startIsNegative, Maybe<double> end,
+    bool endIsNegative, NumberPartVector& parts) const {
+  UErrorCode status = U_ZERO_ERROR;
+
+  const UFormattedValue* formattedValue =
+      unumrf_resultAsValue(mFormattedNumberRange, &status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+
+  int32_t utf16Length;
+  const char16_t* utf16Str =
+      ufmtval_getString(formattedValue, &utf16Length, &status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+
+  UConstrainedFieldPosition* fpos = ucfpos_open(&status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+  ScopedICUObject<UConstrainedFieldPosition, ucfpos_close> toCloseFpos(fpos);
+
+  Maybe<double> number = start;
+  bool isNegative = startIsNegative;
+
+  NumberPartSourceMap sourceMap;
+
+  // Vacuum up fields in the overall formatted string.
+  NumberFormatFields fields;
+
+  while (true) {
+    bool hasMore = ufmtval_nextPosition(formattedValue, fpos, &status);
+    if (U_FAILURE(status)) {
+      return Err(ToICUError(status));
+    }
+    if (!hasMore) {
+      break;
+    }
+
+    int32_t category = ucfpos_getCategory(fpos, &status);
+    if (U_FAILURE(status)) {
+      return Err(ToICUError(status));
+    }
+
+    int32_t fieldName = ucfpos_getField(fpos, &status);
+    if (U_FAILURE(status)) {
+      return Err(ToICUError(status));
+    }
+
+    int32_t beginIndex, endIndex;
+    ucfpos_getIndexes(fpos, &beginIndex, &endIndex, &status);
+    if (U_FAILURE(status)) {
+      return Err(ToICUError(status));
+    }
+
+    if (category == UFIELD_CATEGORY_NUMBER_RANGE_SPAN) {
+      // The special field category UFIELD_CATEGORY_NUMBER_RANGE_SPAN has only
+      // two allowed values (0 or 1), indicating the begin of the start resp.
+      // end number.
+      MOZ_ASSERT(fieldName == 0 || fieldName == 1,
+                 "span category has unexpected value");
+
+      if (fieldName == 0) {
+        number = start;
+        isNegative = startIsNegative;
+
+        sourceMap.start = {uint32_t(beginIndex), uint32_t(endIndex)};
+      } else {
+        number = end;
+        isNegative = endIsNegative;
+
+        sourceMap.end = {uint32_t(beginIndex), uint32_t(endIndex)};
+      }
+
+      continue;
+    }
+
+    // Ignore categories other than UFIELD_CATEGORY_NUMBER.
+    if (category != UFIELD_CATEGORY_NUMBER) {
+      continue;
+    }
+
+    Maybe<NumberPartType> partType = GetPartTypeForNumberField(
+        UNumberFormatFields(fieldName), number, isNegative, mFormatForUnit);
+    if (!partType || !fields.append(*partType, beginIndex, endIndex)) {
+      return Err(ToICUError(status));
+    }
+  }
+
+  if (!fields.toPartsVector(utf16Length, sourceMap, parts)) {
+    return Err(ToICUError(status));
+  }
+
+  return std::u16string_view(utf16Str, static_cast<size_t>(utf16Length));
+}
+
+}  // namespace mozilla::intl
diff --git a/intl/components/src/NumberRangeFormat.h b/intl/components/src/NumberRangeFormat.h
new file mode 100644
index 0000000000..40bb85d6d2
--- /dev/null
+++ b/intl/components/src/NumberRangeFormat.h
@@ -0,0 +1,237 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+#ifndef intl_components_NumberRangeFormat_h_
+#define intl_components_NumberRangeFormat_h_
+
+#include "mozilla/FloatingPoint.h"
+#include "mozilla/intl/ICUError.h"
+#include "mozilla/intl/NumberFormat.h"
+#include "mozilla/Result.h"
+#include "mozilla/UniquePtr.h"
+
+#include <stdint.h>
+#include <string_view>
+
+#include "unicode/utypes.h"
+
+struct UFormattedNumberRange;
+struct UNumberRangeFormatter;
+struct UPluralRules;
+
+namespace mozilla::intl {
+
+/**
+ * NumberRangeFormatOptions supports the same set of options as
+ * NumberFormatOptions and additionally allows to control how to display ranges.
+ */
+struct MOZ_STACK_CLASS NumberRangeFormatOptions : public NumberFormatOptions {
+  /**
+   * Controls if and how to collapse identical parts in a range.
+   */
+  enum class RangeCollapse {
+    /**
+     * Apply locale-specific heuristics.
+     */
+    Auto,
+
+    /**
+     * Never collapse identical parts.
+     */
+    None,
+
+    /**
+     * Collapse identical unit parts.
+     */
+    Unit,
+
+    /**
+     * Collapse all identical parts.
+     */
+    All,
+  } mRangeCollapse = RangeCollapse::Auto;
+
+  /**
+   * Controls how to display identical numbers.
+   */
+  enum class RangeIdentityFallback {
+    /**
+     * Display the range as a single value.
+     */
+    SingleValue,
+
+    /**
+     * Display the range as a single value if both numbers were equal before
+     * rounding. Otherwise display with a locale-sensitive approximation
+     * pattern.
+     */
+    ApproximatelyOrSingleValue,
+
+    /**
+     * Display with a locale-sensitive approximation pattern.
+     */
+    Approximately,
+
+    /**
+     * Display as a range expression.
+     */
+    Range,
+  } mRangeIdentityFallback = RangeIdentityFallback::SingleValue;
+};
+
+/**
+ * A NumberRangeFormat implementation that roughly mirrors the API provided by
+ * the ECMA-402 Intl.NumberFormat object for formatting number ranges.
+ *
+ * https://tc39.es/ecma402/#numberformat-objects
+ */
+class NumberRangeFormat final {
+ public:
+  /**
+   * Initialize a new NumberRangeFormat for the provided locale and using the
+   * provided options.
+   *
+   * https://tc39.es/ecma402/#sec-initializenumberformat
+   */
+  static Result<UniquePtr<NumberRangeFormat>, ICUError> TryCreate(
+      std::string_view aLocale, const NumberRangeFormatOptions& aOptions);
+
+  NumberRangeFormat() = default;
+  NumberRangeFormat(const NumberRangeFormat&) = delete;
+  NumberRangeFormat& operator=(const NumberRangeFormat&) = delete;
+
+  ~NumberRangeFormat();
+
+  /**
+   * Formats a double range to a utf-16 string. The string view is valid until
+   * another number range is formatted. Accessing the string view after this
+   * event is undefined behavior.
+   *
+   * https://tc39.es/ecma402/#sec-formatnumericrange
+   */
+  Result<std::u16string_view, ICUError> format(double start, double end) const {
+    if (!formatInternal(start, end)) {
+      return Err(ICUError::InternalError);
+    }
+
+    return formatResult();
+  }
+
+  /**
+   * Formats a double range to a utf-16 string, and fills the provided parts
+   * vector. The string view is valid until another number is formatted.
+   * Accessing the string view after this event is undefined behavior.
+   *
+   * https://tc39.es/ecma402/#sec-partitionnumberrangepattern
+   */
+  Result<std::u16string_view, ICUError> formatToParts(
+      double start, double end, NumberPartVector& parts) const {
+    if (!formatInternal(start, end)) {
+      return Err(ICUError::InternalError);
+    }
+
+    bool isNegativeStart = !std::isnan(start) && IsNegative(start);
+    bool isNegativeEnd = !std::isnan(end) && IsNegative(end);
+
+    return formatResultToParts(Some(start), isNegativeStart, Some(end),
+                               isNegativeEnd, parts);
+  }
+
+  /**
+   * Formats a decimal number range to a utf-16 string. The string view is valid
+   * until another number range is formatted. Accessing the string view after
+   * this event is undefined behavior.
+   *
+   * https://tc39.es/ecma402/#sec-formatnumericrange
+   */
+  Result<std::u16string_view, ICUError> format(std::string_view start,
+                                               std::string_view end) const {
+    if (!formatInternal(start, end)) {
+      return Err(ICUError::InternalError);
+    }
+
+    return formatResult();
+  }
+
+  /**
+   * Formats a string encoded decimal number range to a utf-16 string, and fills
+   * the provided parts vector. The string view is valid until another number is
+   * formatted. Accessing the string view after this event is undefined
+   * behavior.
+   *
+   * https://tc39.es/ecma402/#sec-partitionnumberrangepattern
+   */
+  Result<std::u16string_view, ICUError> formatToParts(
+      std::string_view start, std::string_view end,
+      NumberPartVector& parts) const {
+    if (!formatInternal(start, end)) {
+      return Err(ICUError::InternalError);
+    }
+
+    Maybe<double> numStart = Nothing();
+    if (start == "Infinity" || start == "+Infinity") {
+      numStart.emplace(PositiveInfinity<double>());
+    } else if (start == "-Infinity") {
+      numStart.emplace(NegativeInfinity<double>());
+    } else {
+      // Not currently expected, so we assert here.
+      MOZ_ASSERT(start != "NaN");
+    }
+
+    Maybe<double> numEnd = Nothing();
+    if (end == "Infinity" || end == "+Infinity") {
+      numEnd.emplace(PositiveInfinity<double>());
+    } else if (end == "-Infinity") {
+      numEnd.emplace(NegativeInfinity<double>());
+    } else {
+      // Not currently expected, so we assert here.
+      MOZ_ASSERT(end != "NaN");
+    }
+
+    bool isNegativeStart = !start.empty() && start[0] == '-';
+    bool isNegativeEnd = !end.empty() && end[0] == '-';
+
+    return formatResultToParts(numStart, isNegativeStart, numEnd, isNegativeEnd,
+                               parts);
+  }
+
+  /**
+   * Formats the number range and selects the keyword by using a provided
+   * UPluralRules object.
+   *
+   * https://tc39.es/ecma402/#sec-intl.pluralrules.prototype.selectrange
+   *
+   * TODO(1713917) This is necessary because both PluralRules and
+   * NumberRangeFormat have a shared dependency on the raw UFormattedNumberRange
+   * type. Once we transition to using ICU4X, the FFI calls should no
+   * longer require such shared dependencies. At that time, this
+   * functionality should be removed from NumberRangeFormat and invoked
+   * solely from PluralRules.
+   */
+  Result<int32_t, ICUError> selectForRange(
+      double start, double end, char16_t* keyword, int32_t keywordSize,
+      const UPluralRules* pluralRules) const;
+
+ private:
+  UNumberRangeFormatter* mNumberRangeFormatter = nullptr;
+  UFormattedNumberRange* mFormattedNumberRange = nullptr;
+  bool mFormatForUnit = false;
+
+  Result<Ok, ICUError> initialize(std::string_view aLocale,
+                                  const NumberRangeFormatOptions& aOptions);
+
+  [[nodiscard]] bool formatInternal(double start, double end) const;
+
+  [[nodiscard]] bool formatInternal(std::string_view start,
+                                    std::string_view end) const;
+
+  Result<std::u16string_view, ICUError> formatResult() const;
+
+  Result<std::u16string_view, ICUError> formatResultToParts(
+      Maybe<double> start, bool startIsNegative, Maybe<double> end,
+      bool endIsNegative, NumberPartVector& parts) const;
+};
+
+}  // namespace mozilla::intl
+
+#endif
diff --git a/intl/components/src/NumberingSystem.cpp b/intl/components/src/NumberingSystem.cpp
new file mode 100644
index 0000000000..b86484a5f7
--- /dev/null
+++ b/intl/components/src/NumberingSystem.cpp
@@ -0,0 +1,38 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "mozilla/intl/NumberingSystem.h"
+#include "mozilla/intl/ICU4CGlue.h"
+
+#include "unicode/unumsys.h"
+#include "unicode/utypes.h"
+
+namespace mozilla::intl {
+
+NumberingSystem::~NumberingSystem() {
+  MOZ_ASSERT(mNumberingSystem);
+  unumsys_close(mNumberingSystem);
+}
+
+Result<UniquePtr<NumberingSystem>, ICUError> NumberingSystem::TryCreate(
+    const char* aLocale) {
+  UErrorCode status = U_ZERO_ERROR;
+  UNumberingSystem* numbers = unumsys_open(IcuLocale(aLocale), &status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+
+  return MakeUnique<NumberingSystem>(numbers);
+}
+
+Result<Span<const char>, ICUError> NumberingSystem::GetName() {
+  const char* name = unumsys_getName(mNumberingSystem);
+  if (!name) {
+    return Err(ICUError::InternalError);
+  }
+
+  return MakeStringSpan(name);
+}
+
+}  // namespace mozilla::intl
diff --git a/intl/components/src/NumberingSystem.h b/intl/components/src/NumberingSystem.h
new file mode 100644
index 0000000000..a3d1903dd1
--- /dev/null
+++ b/intl/components/src/NumberingSystem.h
@@ -0,0 +1,56 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef intl_components_NumberingSystem_h_
+#define intl_components_NumberingSystem_h_
+
+#include "mozilla/intl/ICUError.h"
+#include "mozilla/Result.h"
+#include "mozilla/Span.h"
+#include "mozilla/UniquePtr.h"
+
+struct UNumberingSystem;
+
+namespace mozilla::intl {
+
+/**
+ * This component is a Mozilla-focused API for working with numbering systems in
+ * internationalization code. It is used in coordination with other operations
+ * such as number formatting.
+ */
+class NumberingSystem final {
+ public:
+  explicit NumberingSystem(UNumberingSystem* aNumberingSystem)
+      : mNumberingSystem(aNumberingSystem) {
+    MOZ_ASSERT(aNumberingSystem);
+  };
+
+  // Do not allow copy as this class owns the ICU resource. Move is not
+  // currently implemented, but a custom move operator could be created if
+  // needed.
+  NumberingSystem(const NumberingSystem&) = delete;
+  NumberingSystem& operator=(const NumberingSystem&) = delete;
+
+  ~NumberingSystem();
+
+  /**
+   * Create a NumberingSystem.
+   */
+  static Result<UniquePtr<NumberingSystem>, ICUError> TryCreate(
+      const char* aLocale);
+
+  /**
+   * Returns the name of this numbering system.
+   *
+   * The returned string has the same lifetime as this NumberingSystem object.
+   */
+  Result<Span<const char>, ICUError> GetName();
+
+ private:
+  UNumberingSystem* mNumberingSystem = nullptr;
+};
+
+}  // namespace mozilla::intl
+
+#endif
diff --git a/intl/components/src/PluralRules.cpp b/intl/components/src/PluralRules.cpp
new file mode 100644
index 0000000000..891ca45769
--- /dev/null
+++ b/intl/components/src/PluralRules.cpp
@@ -0,0 +1,180 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "mozilla/intl/PluralRules.h"
+
+#include "mozilla/intl/ICU4CGlue.h"
+#include "mozilla/intl/NumberFormat.h"
+#include "mozilla/intl/NumberRangeFormat.h"
+#include "mozilla/Utf8.h"
+#include "mozilla/PodOperations.h"
+#include "mozilla/Span.h"
+#include "ScopedICUObject.h"
+
+#include "unicode/unum.h"
+#include "unicode/upluralrules.h"
+#include "unicode/ustring.h"
+
+namespace mozilla::intl {
+
+PluralRules::PluralRules(UPluralRules*& aPluralRules,
+                         UniquePtr<NumberFormat>&& aNumberFormat,
+                         UniquePtr<NumberRangeFormat>&& aNumberRangeFormat)
+    : mPluralRules(aPluralRules),
+      mNumberFormat(std::move(aNumberFormat)),
+      mNumberRangeFormat(std::move(aNumberRangeFormat)) {
+  MOZ_ASSERT(aPluralRules);
+  aPluralRules = nullptr;
+}
+
+Result<UniquePtr<PluralRules>, ICUError> PluralRules::TryCreate(
+    const std::string_view aLocale, const PluralRulesOptions& aOptions) {
+  auto numberFormat =
+      NumberFormat::TryCreate(aLocale, aOptions.ToNumberFormatOptions());
+
+  if (numberFormat.isErr()) {
+    return Err(numberFormat.unwrapErr());
+  }
+
+  auto numberRangeFormat = NumberRangeFormat::TryCreate(
+      aLocale, aOptions.ToNumberRangeFormatOptions());
+
+  if (numberRangeFormat.isErr()) {
+    return Err(numberRangeFormat.unwrapErr());
+  }
+
+  UErrorCode status = U_ZERO_ERROR;
+  auto pluralType = aOptions.mPluralType == PluralRules::Type::Cardinal
+                        ? UPLURAL_TYPE_CARDINAL
+                        : UPLURAL_TYPE_ORDINAL;
+  UPluralRules* pluralRules = uplrules_openForType(
+      AssertNullTerminatedString(aLocale), pluralType, &status);
+
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+
+  return UniquePtr<PluralRules>(new PluralRules(
+      pluralRules, numberFormat.unwrap(), numberRangeFormat.unwrap()));
+}
+
+Result<PluralRules::Keyword, ICUError> PluralRules::Select(
+    const double aNumber) const {
+  char16_t keyword[MAX_KEYWORD_LENGTH];
+
+  auto lengthResult = mNumberFormat->selectFormatted(
+      aNumber, keyword, MAX_KEYWORD_LENGTH, mPluralRules);
+
+  if (lengthResult.isErr()) {
+    return Err(lengthResult.unwrapErr());
+  }
+
+  return KeywordFromUtf16(Span(keyword, lengthResult.unwrap()));
+}
+
+Result<PluralRules::Keyword, ICUError> PluralRules::SelectRange(
+    double aStart, double aEnd) const {
+  char16_t keyword[MAX_KEYWORD_LENGTH];
+
+  auto lengthResult = mNumberRangeFormat->selectForRange(
+      aStart, aEnd, keyword, MAX_KEYWORD_LENGTH, mPluralRules);
+
+  if (lengthResult.isErr()) {
+    return Err(lengthResult.unwrapErr());
+  }
+
+  return KeywordFromUtf16(Span(keyword, lengthResult.unwrap()));
+}
+
+Result<EnumSet<PluralRules::Keyword>, ICUError> PluralRules::Categories()
+    const {
+  UErrorCode status = U_ZERO_ERROR;
+  UEnumeration* enumeration = uplrules_getKeywords(mPluralRules, &status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+
+  ScopedICUObject<UEnumeration, uenum_close> closeEnum(enumeration);
+  EnumSet<PluralRules::Keyword> set;
+
+  while (true) {
+    int32_t keywordLength;
+    const char* keyword = uenum_next(enumeration, &keywordLength, &status);
+    if (U_FAILURE(status)) {
+      return Err(ToICUError(status));
+    }
+
+    if (!keyword) {
+      break;
+    }
+
+    set += KeywordFromAscii(Span(keyword, keywordLength));
+  }
+
+  return set;
+}
+
+PluralRules::Keyword PluralRules::KeywordFromUtf16(
+    Span<const char16_t> aKeyword) {
+  static constexpr auto kZero = MakeStringSpan(u"zero");
+  static constexpr auto kOne = MakeStringSpan(u"one");
+  static constexpr auto kTwo = MakeStringSpan(u"two");
+  static constexpr auto kFew = MakeStringSpan(u"few");
+  static constexpr auto kMany = MakeStringSpan(u"many");
+
+  if (aKeyword == kZero) {
+    return PluralRules::Keyword::Zero;
+  }
+  if (aKeyword == kOne) {
+    return PluralRules::Keyword::One;
+  }
+  if (aKeyword == kTwo) {
+    return PluralRules::Keyword::Two;
+  }
+  if (aKeyword == kFew) {
+    return PluralRules::Keyword::Few;
+  }
+  if (aKeyword == kMany) {
+    return PluralRules::Keyword::Many;
+  }
+
+  MOZ_ASSERT(aKeyword == MakeStringSpan(u"other"));
+  return PluralRules::Keyword::Other;
+}
+
+PluralRules::Keyword PluralRules::KeywordFromAscii(Span<const char> aKeyword) {
+  static constexpr auto kZero = MakeStringSpan("zero");
+  static constexpr auto kOne = MakeStringSpan("one");
+  static constexpr auto kTwo = MakeStringSpan("two");
+  static constexpr auto kFew = MakeStringSpan("few");
+  static constexpr auto kMany = MakeStringSpan("many");
+
+  if (aKeyword == kZero) {
+    return PluralRules::Keyword::Zero;
+  }
+  if (aKeyword == kOne) {
+    return PluralRules::Keyword::One;
+  }
+  if (aKeyword == kTwo) {
+    return PluralRules::Keyword::Two;
+  }
+  if (aKeyword == kFew) {
+    return PluralRules::Keyword::Few;
+  }
+  if (aKeyword == kMany) {
+    return PluralRules::Keyword::Many;
+  }
+
+  MOZ_ASSERT(aKeyword == MakeStringSpan("other"));
+  return PluralRules::Keyword::Other;
+}
+
+PluralRules::~PluralRules() {
+  if (mPluralRules) {
+    uplrules_close(mPluralRules);
+    mPluralRules = nullptr;
+  }
+}
+
+}  // namespace mozilla::intl
diff --git a/intl/components/src/PluralRules.h b/intl/components/src/PluralRules.h
new file mode 100644
index 0000000000..e8a309d7ec
--- /dev/null
+++ b/intl/components/src/PluralRules.h
@@ -0,0 +1,233 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef intl_components_PluralRules_h_
+#define intl_components_PluralRules_h_
+
+#include <string_view>
+#include <utility>
+
+#include "mozilla/intl/ICUError.h"
+#include "mozilla/intl/NumberFormat.h"
+#include "mozilla/intl/NumberRangeFormat.h"
+#include "mozilla/EnumSet.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/Result.h"
+#include "mozilla/Span.h"
+
+#include "unicode/utypes.h"
+
+namespace mozilla::intl {
+
+class PluralRules final {
+ public:
+  /**
+   * The set of keywords that a PluralRules object uses.
+   *
+   * https://tc39.es/ecma402/#sec-intl.pluralrules.prototype.resolvedoptions
+   */
+  enum class Keyword : uint8_t {
+    Few,
+    Many,
+    One,
+    Other,
+    Two,
+    Zero,
+  };
+
+  /**
+   * The two different types of PluralRules objects that can be created.
+   *
+   * https://tc39.es/ecma402/#sec-properties-of-intl-pluralrules-instances
+   */
+  enum class Type : uint8_t {
+    Cardinal,
+    Ordinal,
+  };
+
+  PluralRules(const PluralRules&) = delete;
+  PluralRules& operator=(const PluralRules&) = delete;
+
+  /**
+   * Attempts to construct a PluralRules with the given locale and options.
+   */
+  // TODO(1709880) use mozilla::Span instead of std::string_view.
+  static Result<UniquePtr<PluralRules>, ICUError> TryCreate(
+      std::string_view aLocale, const PluralRulesOptions& aOptions);
+
+  /**
+   * Returns the PluralRules keyword that corresponds to the |aNumber|.
+   *
+   * https://tc39.es/ecma402/#sec-intl.pluralrules.prototype.select
+   */
+  Result<PluralRules::Keyword, ICUError> Select(double aNumber) const;
+
+  /**
+   * Returns the PluralRules keyword that corresponds to the range from |aStart|
+   * to |aEnd|.
+   *
+   * https://tc39.es/ecma402/#sec-intl.pluralrules.prototype.selectrange
+   */
+  Result<PluralRules::Keyword, ICUError> SelectRange(double aStart,
+                                                     double aEnd) const;
+
+  /**
+   * Returns an EnumSet with the plural-rules categories that are supported by
+   * the locale that the PluralRules instance was created with.
+   */
+  Result<EnumSet<PluralRules::Keyword>, ICUError> Categories() const;
+
+  ~PluralRules();
+
+ private:
+  // The longest keyword is "other"
+  static const size_t MAX_KEYWORD_LENGTH = 5;
+
+  UPluralRules* mPluralRules = nullptr;
+  UniquePtr<NumberFormat> mNumberFormat;
+  UniquePtr<NumberRangeFormat> mNumberRangeFormat;
+
+  PluralRules(UPluralRules*&, UniquePtr<NumberFormat>&&,
+              UniquePtr<NumberRangeFormat>&&);
+
+  /**
+   * Returns the PluralRules::Keyword that matches the UTF-16 string.
+   * Strings must be [u"few", u"many", u"one", u"other", u"two", u"zero"]
+   */
+  static PluralRules::Keyword KeywordFromUtf16(Span<const char16_t> aKeyword);
+
+  /**
+   * Returns the PluralRules::Keyword that matches the ASCII string.
+   * Strings must be ["few", "many", "one", "other", "two", "zero"]
+   */
+  static PluralRules::Keyword KeywordFromAscii(Span<const char> aKeyword);
+};
+
+/**
+ * Options required for constructing a PluralRules object.
+ */
+struct MOZ_STACK_CLASS PluralRulesOptions {
+  /**
+   * Creates a NumberFormatOptions from the PluralRulesOptions.
+   */
+  NumberFormatOptions ToNumberFormatOptions() const {
+    NumberFormatOptions options;
+    options.mRoundingMode = NumberFormatOptions::RoundingMode::HalfExpand;
+
+    if (mFractionDigits.isSome()) {
+      options.mFractionDigits.emplace(mFractionDigits.ref());
+    }
+
+    if (mMinIntegerDigits.isSome()) {
+      options.mMinIntegerDigits.emplace(mMinIntegerDigits.ref());
+    }
+
+    if (mSignificantDigits.isSome()) {
+      options.mSignificantDigits.emplace(mSignificantDigits.ref());
+    }
+
+    options.mStripTrailingZero = mStripTrailingZero;
+
+    options.mRoundingIncrement = mRoundingIncrement;
+
+    options.mRoundingMode = NumberFormatOptions::RoundingMode(mRoundingMode);
+
+    options.mRoundingPriority =
+        NumberFormatOptions::RoundingPriority(mRoundingPriority);
+
+    return options;
+  }
+  /**
+   * Creates a NumberFormatOptions from the PluralRulesOptions.
+   */
+  NumberRangeFormatOptions ToNumberRangeFormatOptions() const {
+    NumberRangeFormatOptions options;
+    options.mRoundingMode = NumberRangeFormatOptions::RoundingMode::HalfExpand;
+    options.mRangeCollapse = NumberRangeFormatOptions::RangeCollapse::None;
+    options.mRangeIdentityFallback =
+        NumberRangeFormatOptions::RangeIdentityFallback::Range;
+
+    if (mFractionDigits.isSome()) {
+      options.mFractionDigits.emplace(mFractionDigits.ref());
+    }
+
+    if (mMinIntegerDigits.isSome()) {
+      options.mMinIntegerDigits.emplace(mMinIntegerDigits.ref());
+    }
+
+    if (mSignificantDigits.isSome()) {
+      options.mSignificantDigits.emplace(mSignificantDigits.ref());
+    }
+
+    options.mStripTrailingZero = mStripTrailingZero;
+
+    options.mRoundingIncrement = mRoundingIncrement;
+
+    options.mRoundingMode = NumberFormatOptions::RoundingMode(mRoundingMode);
+
+    options.mRoundingPriority =
+        NumberFormatOptions::RoundingPriority(mRoundingPriority);
+
+    return options;
+  }
+
+  /**
+   * Set the plural type between cardinal and ordinal.
+   *
+   * https://tc39.es/ecma402/#sec-intl.pluralrules.prototype.resolvedoptions
+   */
+  PluralRules::Type mPluralType = PluralRules::Type::Cardinal;
+
+  /**
+   * Set the minimum number of integer digits. |min| must be a non-zero
+   * number.
+   *
+   * https://tc39.es/ecma402/#sec-intl.pluralrules.prototype.resolvedoptions
+   */
+  Maybe<uint32_t> mMinIntegerDigits;
+
+  /**
+   * Set the fraction digits settings. |min| can be zero, |max| must be
+   * larger-or-equal to |min|.
+   *
+   * https://tc39.es/ecma402/#sec-intl.pluralrules.prototype.resolvedoptions
+   */
+  Maybe<std::pair<uint32_t, uint32_t>> mFractionDigits;
+
+  /**
+   * Set the significant digits settings. |min| must be a non-zero number, |max|
+   * must be larger-or-equal to |min|.
+   *
+   * https://tc39.es/ecma402/#sec-intl.pluralrules.prototype.resolvedoptions
+   */
+  Maybe<std::pair<uint32_t, uint32_t>> mSignificantDigits;
+
+  /**
+   * Set to true to strip trailing zeros after the decimal point for integer
+   * values.
+   */
+  bool mStripTrailingZero = false;
+
+  /**
+   * Set the rounding increment, which must be a non-zero number.
+   */
+  uint32_t mRoundingIncrement = 1;
+
+  /**
+   * Set the rounding mode.
+   */
+  using RoundingMode = NumberFormatOptions::RoundingMode;
+  RoundingMode mRoundingMode = RoundingMode::HalfExpand;
+
+  /**
+   * Set the rounding priority. |mFractionDigits| and |mSignificantDigits| must
+   * both be set if the rounding priority isn't equal to "auto".
+   */
+  using RoundingPriority = NumberFormatOptions::RoundingPriority;
+  RoundingPriority mRoundingPriority = RoundingPriority::Auto;
+};
+
+}  // namespace mozilla::intl
+
+#endif
diff --git a/intl/components/src/RelativeTimeFormat.cpp b/intl/components/src/RelativeTimeFormat.cpp
new file mode 100644
index 0000000000..da67f7587d
--- /dev/null
+++ b/intl/components/src/RelativeTimeFormat.cpp
@@ -0,0 +1,153 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+#include "mozilla/intl/RelativeTimeFormat.h"
+#include "mozilla/FloatingPoint.h"
+
+#include "unicode/unum.h"
+
+#include "NumberFormatFields.h"
+#include "ICU4CGlue.h"
+#include "ScopedICUObject.h"
+
+namespace mozilla::intl {
+
+/*static*/ Result<UniquePtr<RelativeTimeFormat>, ICUError>
+RelativeTimeFormat::TryCreate(const char* aLocale,
+                              const RelativeTimeFormatOptions& aOptions) {
+  UErrorCode status = U_ZERO_ERROR;
+
+  UFormattedRelativeDateTime* formattedRelativeDateTime =
+      ureldatefmt_openResult(&status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+  ScopedICUObject<UFormattedRelativeDateTime, ureldatefmt_closeResult>
+      closeFormattedRelativeDate(formattedRelativeDateTime);
+
+  UNumberFormat* nf =
+      unum_open(UNUM_DECIMAL, nullptr, 0, IcuLocale(aLocale), nullptr, &status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+  ScopedICUObject<UNumberFormat, unum_close> closeNumberFormatter(nf);
+
+  // Use the default values as if a new Intl.NumberFormat had been constructed.
+  unum_setAttribute(nf, UNUM_MIN_INTEGER_DIGITS, 1);
+  unum_setAttribute(nf, UNUM_MIN_FRACTION_DIGITS, 0);
+  unum_setAttribute(nf, UNUM_MAX_FRACTION_DIGITS, 3);
+  unum_setAttribute(nf, UNUM_GROUPING_USED, true);
+  unum_setAttribute(nf, UNUM_MINIMUM_GROUPING_DIGITS,
+                    UNUM_MINIMUM_GROUPING_DIGITS_AUTO);
+
+  UDateRelativeDateTimeFormatterStyle relDateTimeStyle;
+  switch (aOptions.style) {
+    case RelativeTimeFormatOptions::Style::Short:
+      relDateTimeStyle = UDAT_STYLE_SHORT;
+      break;
+    case RelativeTimeFormatOptions::Style::Narrow:
+      relDateTimeStyle = UDAT_STYLE_NARROW;
+      break;
+    case RelativeTimeFormatOptions::Style::Long:
+      relDateTimeStyle = UDAT_STYLE_LONG;
+      break;
+  }
+
+  URelativeDateTimeFormatter* formatter =
+      ureldatefmt_open(IcuLocale(aLocale), nf, relDateTimeStyle,
+                       UDISPCTX_CAPITALIZATION_FOR_STANDALONE, &status);
+
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+
+  // Ownership was transferred to mFormatter.
+  closeNumberFormatter.forget();
+
+  UniquePtr<RelativeTimeFormat> rtf = MakeUnique<RelativeTimeFormat>(
+      aOptions.numeric, formatter, formattedRelativeDateTime);
+
+  // Ownership was transferred to rtf.
+  closeFormattedRelativeDate.forget();
+  return rtf;
+}
+
+RelativeTimeFormat::RelativeTimeFormat(
+    RelativeTimeFormatOptions::Numeric aNumeric,
+    URelativeDateTimeFormatter* aFormatter,
+    UFormattedRelativeDateTime* aFormattedRelativeDateTime)
+    : mNumeric(aNumeric),
+      mFormatter(aFormatter),
+      mFormattedRelativeDateTime(aFormattedRelativeDateTime) {}
+
+RelativeTimeFormat::~RelativeTimeFormat() {
+  if (mFormattedRelativeDateTime) {
+    ureldatefmt_closeResult(mFormattedRelativeDateTime);
+    mFormattedRelativeDateTime = nullptr;
+  }
+
+  if (mFormatter) {
+    ureldatefmt_close(mFormatter);
+    mFormatter = nullptr;
+  }
+}
+
+URelativeDateTimeUnit RelativeTimeFormat::ToURelativeDateTimeUnit(
+    FormatUnit unit) const {
+  switch (unit) {
+    case FormatUnit::Second:
+      return UDAT_REL_UNIT_SECOND;
+    case FormatUnit::Minute:
+      return UDAT_REL_UNIT_MINUTE;
+    case FormatUnit::Hour:
+      return UDAT_REL_UNIT_HOUR;
+    case FormatUnit::Day:
+      return UDAT_REL_UNIT_DAY;
+    case FormatUnit::Week:
+      return UDAT_REL_UNIT_WEEK;
+    case FormatUnit::Month:
+      return UDAT_REL_UNIT_MONTH;
+    case FormatUnit::Quarter:
+      return UDAT_REL_UNIT_QUARTER;
+    case FormatUnit::Year:
+      return UDAT_REL_UNIT_YEAR;
+  };
+  MOZ_ASSERT_UNREACHABLE();
+  return UDAT_REL_UNIT_SECOND;
+}
+
+Result<Span<const char16_t>, ICUError> RelativeTimeFormat::formatToParts(
+    double aNumber, FormatUnit aUnit, NumberPartVector& aParts) const {
+  UErrorCode status = U_ZERO_ERROR;
+
+  if (mNumeric == RelativeTimeFormatOptions::Numeric::Auto) {
+    ureldatefmt_formatToResult(mFormatter, aNumber,
+                               ToURelativeDateTimeUnit(aUnit),
+                               mFormattedRelativeDateTime, &status);
+  } else {
+    ureldatefmt_formatNumericToResult(mFormatter, aNumber,
+                                      ToURelativeDateTimeUnit(aUnit),
+                                      mFormattedRelativeDateTime, &status);
+  }
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+
+  const UFormattedValue* formattedValue =
+      ureldatefmt_resultAsValue(mFormattedRelativeDateTime, &status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+
+  bool isNegative = !std::isnan(aNumber) && IsNegative(aNumber);
+
+  // Necessary until all of intl is using Span (Bug 1709880)
+  return FormatResultToParts(formattedValue, Nothing(), isNegative,
+                             false /*formatForUnit*/, aParts)
+      .andThen([](std::u16string_view result)
+                   -> Result<Span<const char16_t>, ICUError> {
+        return Span<const char16_t>(result.data(), result.length());
+      });
+}
+
+}  // namespace mozilla::intl
diff --git a/intl/components/src/RelativeTimeFormat.h b/intl/components/src/RelativeTimeFormat.h
new file mode 100644
index 0000000000..4d4cf2e60e
--- /dev/null
+++ b/intl/components/src/RelativeTimeFormat.h
@@ -0,0 +1,147 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+#ifndef intl_components_RelativeTimeFormat_h_
+#define intl_components_RelativeTimeFormat_h_
+
+#include "mozilla/Try.h"
+#include "mozilla/intl/ICU4CGlue.h"
+#include "mozilla/intl/ICUError.h"
+#include "mozilla/intl/NumberPart.h"
+
+#include "unicode/ureldatefmt.h"
+#include "unicode/utypes.h"
+
+namespace mozilla::intl {
+
+struct RelativeTimeFormatOptions {
+  enum class Style { Short, Narrow, Long };
+  Style style = Style::Long;
+
+  enum class Numeric {
+    /**
+     * Only strings with numeric components like `1 day ago`.
+     */
+    Always,
+    /**
+     * Natural-language strings like `yesterday` when possible,
+     * otherwise strings with numeric components as in `7 months ago`.
+     */
+    Auto,
+  };
+  Numeric numeric = Numeric::Always;
+};
+
+/**
+ * A RelativeTimeFormat implementation that roughly mirrors the API provided by
+ * the ECMA-402 Intl.RelativeTimeFormat object.
+ *
+ * https://tc39.es/ecma402/#relativetimeformat-objects
+ */
+class RelativeTimeFormat final {
+ public:
+  /**
+   *
+   * Initialize a new RelativeTimeFormat for the provided locale and using the
+   * provided options.
+   *
+   * https://tc39.es/ecma402/#sec-InitializeRelativeTimeFormat
+   */
+  static Result<UniquePtr<RelativeTimeFormat>, ICUError> TryCreate(
+      const char* aLocale, const RelativeTimeFormatOptions& aOptions);
+
+  RelativeTimeFormat() = default;
+
+  RelativeTimeFormat(RelativeTimeFormatOptions::Numeric aNumeric,
+                     URelativeDateTimeFormatter* aFormatter,
+                     UFormattedRelativeDateTime* aFormattedRelativeDateTime);
+
+  RelativeTimeFormat(const RelativeTimeFormat&) = delete;
+  RelativeTimeFormat& operator=(const RelativeTimeFormat&) = delete;
+  ~RelativeTimeFormat();
+
+  enum class FormatUnit {
+    Second,
+    Minute,
+    Hour,
+    Day,
+    Week,
+    Month,
+    Quarter,
+    Year
+  };
+
+  /**
+   * Formats a double to the provider buffer (either utf-8 or utf-16)
+   *
+   * https://tc39.es/ecma402/#sec-FormatRelativeTime
+   */
+  template <typename B>
+  Result<Ok, ICUError> format(double aNumber, FormatUnit aUnit,
+                              B& aBuffer) const {
+    static_assert(
+        std::is_same_v<typename B::CharType, char> ||
+            std::is_same_v<typename B::CharType, char16_t>,
+        "The only buffer CharTypes supported by RelativeTimeFormat are char "
+        "(for UTF-8 support) and char16_t (for UTF-16 support).");
+
+    auto fmt = mNumeric == RelativeTimeFormatOptions::Numeric::Auto
+                   ? ureldatefmt_format
+                   : ureldatefmt_formatNumeric;
+
+    if constexpr (std::is_same_v<typename B::CharType, char>) {
+      mozilla::Vector<char16_t, StackU16VectorSize> u16Vec;
+
+      MOZ_TRY(FillBufferWithICUCall(
+          u16Vec, [this, aNumber, aUnit, fmt](UChar* target, int32_t length,
+                                              UErrorCode* status) {
+            return fmt(mFormatter, aNumber, ToURelativeDateTimeUnit(aUnit),
+                       target, length, status);
+          }));
+
+      if (!FillBuffer(u16Vec, aBuffer)) {
+        return Err(ICUError::OutOfMemory);
+      }
+      return Ok{};
+    } else {
+      static_assert(std::is_same_v<typename B::CharType, char16_t>);
+
+      return FillBufferWithICUCall(
+          aBuffer, [this, aNumber, aUnit, fmt](UChar* target, int32_t length,
+                                               UErrorCode* status) {
+            return fmt(mFormatter, aNumber, ToURelativeDateTimeUnit(aUnit),
+                       target, length, status);
+          });
+    }
+  }
+
+  /**
+   * Formats the relative time to a utf-16 string, and fills the provided parts
+   * vector. The string view is valid until another time is formatted.
+   * Accessing the string view after this event is undefined behavior.
+   *
+   * This is utf-16 only because the only current use case is in
+   * SpiderMonkey. Supporting utf-8 would require recalculating the offsets
+   * in NumberPartVector from fixed width to variable width, which might be
+   * tricky to get right and is work that won't be necessary if we switch to
+   * ICU4X (see Bug 1723120).
+   *
+   * https://tc39.es/ecma402/#sec-FormatRelativeTimeToParts
+   */
+  Result<Span<const char16_t>, ICUError> formatToParts(
+      double aNumber, FormatUnit aUnit, NumberPartVector& aParts) const;
+
+ private:
+  RelativeTimeFormatOptions::Numeric mNumeric =
+      RelativeTimeFormatOptions::Numeric::Always;
+  URelativeDateTimeFormatter* mFormatter = nullptr;
+  UFormattedRelativeDateTime* mFormattedRelativeDateTime = nullptr;
+
+  static constexpr size_t StackU16VectorSize = 128;
+
+  URelativeDateTimeUnit ToURelativeDateTimeUnit(FormatUnit unit) const;
+};
+
+}  // namespace mozilla::intl
+
+#endif
diff --git a/intl/components/src/ScopedICUObject.h b/intl/components/src/ScopedICUObject.h
new file mode 100644
index 0000000000..1aa79245dc
--- /dev/null
+++ b/intl/components/src/ScopedICUObject.h
@@ -0,0 +1,40 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+#ifndef intl_components_ScopedICUObject_h
+#define intl_components_ScopedICUObject_h
+
+/*
+ * A simple RAII class to assure ICU objects are automatically deallocated at
+ * scope end.  Unfortunately, ICU's C++ API is uniformly unstable, so we can't
+ * use its smart pointers for this.
+ */
+
+namespace mozilla::intl {
+
+template <typename T, void(Delete)(T*)>
+class ScopedICUObject {
+  T* ptr_;
+
+ public:
+  explicit ScopedICUObject(T* ptr) : ptr_(ptr) {}
+
+  ~ScopedICUObject() {
+    if (ptr_) {
+      Delete(ptr_);
+    }
+  }
+
+  // In cases where an object should be deleted on abnormal exits,
+  // but returned to the caller if everything goes well, call forget()
+  // to transfer the object just before returning.
+  T* forget() {
+    T* tmp = ptr_;
+    ptr_ = nullptr;
+    return tmp;
+  }
+};
+
+}  // namespace mozilla::intl
+
+#endif /* intl_components_ScopedICUObject_h */
diff --git a/intl/components/src/String.cpp b/intl/components/src/String.cpp
new file mode 100644
index 0000000000..de24ab804f
--- /dev/null
+++ b/intl/components/src/String.cpp
@@ -0,0 +1,13 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "mozilla/intl/String.h"
+
+namespace mozilla::intl {
+
+Span<const char> String::GetUnicodeVersion() {
+  return MakeStringSpan(U_UNICODE_VERSION);
+}
+
+}  // namespace mozilla::intl
diff --git a/intl/components/src/String.h b/intl/components/src/String.h
new file mode 100644
index 0000000000..c44c6b3443
--- /dev/null
+++ b/intl/components/src/String.h
@@ -0,0 +1,256 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef intl_components_String_h_
+#define intl_components_String_h_
+
+#include "mozilla/Assertions.h"
+#include "mozilla/Casting.h"
+#include "mozilla/intl/ICU4CGlue.h"
+#include "mozilla/intl/ICUError.h"
+#include "mozilla/PodOperations.h"
+#include "mozilla/Span.h"
+#include "mozilla/Try.h"
+
+#include "unicode/uchar.h"
+#include "unicode/unorm2.h"
+#include "unicode/ustring.h"
+#include "unicode/utext.h"
+#include "unicode/utypes.h"
+
+namespace mozilla::intl {
+
+/**
+ * This component is a Mozilla-focused API for working with strings in
+ * internationalization code.
+ */
+class String final {
+ public:
+  String() = delete;
+
+  /**
+   * Return the locale-sensitive lower case string of the input.
+   */
+  template <typename B>
+  static Result<Ok, ICUError> ToLocaleLowerCase(const char* aLocale,
+                                                Span<const char16_t> aString,
+                                                B& aBuffer) {
+    if (!aBuffer.reserve(aString.size())) {
+      return Err(ICUError::OutOfMemory);
+    }
+    return FillBufferWithICUCall(
+        aBuffer, [&](UChar* target, int32_t length, UErrorCode* status) {
+          return u_strToLower(target, length, aString.data(), aString.size(),
+                              aLocale, status);
+        });
+  }
+
+  /**
+   * Return the locale-sensitive upper case string of the input.
+   */
+  template <typename B>
+  static Result<Ok, ICUError> ToLocaleUpperCase(const char* aLocale,
+                                                Span<const char16_t> aString,
+                                                B& aBuffer) {
+    if (!aBuffer.reserve(aString.size())) {
+      return Err(ICUError::OutOfMemory);
+    }
+    return FillBufferWithICUCall(
+        aBuffer, [&](UChar* target, int32_t length, UErrorCode* status) {
+          return u_strToUpper(target, length, aString.data(), aString.size(),
+                              aLocale, status);
+        });
+  }
+
+  /**
+   * Normalization form constants to describe which normalization algorithm
+   * should be performed.
+   *
+   * Also see:
+   * - Unicode Standard, §2.12 Equivalent Sequences
+   * - Unicode Standard, §3.11 Normalization Forms
+   * - https://unicode.org/reports/tr15/
+   */
+  enum class NormalizationForm {
+    /**
+     * Normalization Form C
+     */
+    NFC,
+
+    /**
+     * Normalization Form D
+     */
+    NFD,
+
+    /**
+     * Normalization Form KC
+     */
+    NFKC,
+
+    /**
+     * Normalization Form KD
+     */
+    NFKD,
+  };
+
+  enum class AlreadyNormalized : bool { No, Yes };
+
+  /**
+   * Normalize the input string according to requested normalization form.
+   *
+   * Returns `AlreadyNormalized::Yes` when the string is already in normalized
+   * form. The output buffer is unchanged in this case. Otherwise returns
+   * `AlreadyNormalized::No` and places the normalized string into the output
+   * buffer.
+   */
+  template <typename B>
+  static Result<AlreadyNormalized, ICUError> Normalize(
+      NormalizationForm aForm, Span<const char16_t> aString, B& aBuffer) {
+    // The unorm2_getXXXInstance() methods return a shared instance which must
+    // not be deleted.
+    UErrorCode status = U_ZERO_ERROR;
+    const UNormalizer2* normalizer;
+    switch (aForm) {
+      case NormalizationForm::NFC:
+        normalizer = unorm2_getNFCInstance(&status);
+        break;
+      case NormalizationForm::NFD:
+        normalizer = unorm2_getNFDInstance(&status);
+        break;
+      case NormalizationForm::NFKC:
+        normalizer = unorm2_getNFKCInstance(&status);
+        break;
+      case NormalizationForm::NFKD:
+        normalizer = unorm2_getNFKDInstance(&status);
+        break;
+    }
+    if (U_FAILURE(status)) {
+      return Err(ToICUError(status));
+    }
+
+    int32_t spanLengthInt = unorm2_spanQuickCheckYes(normalizer, aString.data(),
+                                                     aString.size(), &status);
+    if (U_FAILURE(status)) {
+      return Err(ToICUError(status));
+    }
+
+    size_t spanLength = AssertedCast<size_t>(spanLengthInt);
+    MOZ_ASSERT(spanLength <= aString.size());
+
+    // Return if the input string is already normalized.
+    if (spanLength == aString.size()) {
+      return AlreadyNormalized::Yes;
+    }
+
+    if (!aBuffer.reserve(aString.size())) {
+      return Err(ICUError::OutOfMemory);
+    }
+
+    // Copy the already normalized prefix.
+    if (spanLength > 0) {
+      PodCopy(aBuffer.data(), aString.data(), spanLength);
+
+      aBuffer.written(spanLength);
+    }
+
+    MOZ_TRY(FillBufferWithICUCall(
+        aBuffer, [&](UChar* target, int32_t length, UErrorCode* status) {
+          Span<const char16_t> remaining = aString.From(spanLength);
+          return unorm2_normalizeSecondAndAppend(normalizer, target, spanLength,
+                                                 length, remaining.data(),
+                                                 remaining.size(), status);
+        }));
+
+    return AlreadyNormalized::No;
+  }
+
+  /**
+   * Return true if the code point has the binary property "Cased".
+   */
+  static bool IsCased(char32_t codePoint) {
+    return u_hasBinaryProperty(static_cast<UChar32>(codePoint), UCHAR_CASED);
+  }
+
+  /**
+   * Return true if the code point has the binary property "Case_Ignorable".
+   */
+  static bool IsCaseIgnorable(char32_t codePoint) {
+    return u_hasBinaryProperty(static_cast<UChar32>(codePoint),
+                               UCHAR_CASE_IGNORABLE);
+  }
+
+  /**
+   * Return the NFC pairwise composition of the two input characters, if any;
+   * returns 0 (which we know is not a composed char!) if none exists.
+   */
+  static char32_t ComposePairNFC(char32_t a, char32_t b) {
+    // unorm2_getNFCInstance returns a static instance that does not have to be
+    // released here. If it fails, we just return 0 (no composition) always.
+    static UErrorCode status = U_ZERO_ERROR;
+    static const UNormalizer2* normalizer = unorm2_getNFCInstance(&status);
+    if (U_FAILURE(status)) {
+      return 0;
+    }
+    UChar32 ch = unorm2_composePair(normalizer, static_cast<UChar32>(a),
+                                    static_cast<UChar32>(b));
+    return ch < 0 ? 0 : static_cast<char32_t>(ch);
+  }
+
+  /**
+   * Put the "raw" (single-level) canonical decomposition of the input char, if
+   * any, into the provided buffer. Canonical decomps are never more than two
+   * chars in length (although full normalization may result in longer output
+   * due to recursion).
+   * Returns the length of the decomposition (0 if none, else 1 or 2).
+   */
+  static int DecomposeRawNFD(char32_t ab, char32_t decomp[2]) {
+    // unorm2_getNFCInstance returns a static instance that does not have to be
+    // released here. If it fails, we just return 0 (no decomposition) always.
+    // Although we are using it to query for a decomposition, the mode of the
+    // Normalizer2 is irrelevant here, so we may as well use the same singleton
+    // instance as ComposePairNFC.
+    static UErrorCode status = U_ZERO_ERROR;
+    static const UNormalizer2* normalizer = unorm2_getNFCInstance(&status);
+    if (U_FAILURE(status)) {
+      return 0;
+    }
+
+    // Canonical decompositions are never more than two Unicode characters,
+    // or a maximum of 4 utf-16 code units.
+    const unsigned MAX_DECOMP_LENGTH = 4;
+    UErrorCode error = U_ZERO_ERROR;
+    UChar decompUtf16[MAX_DECOMP_LENGTH];
+    int32_t len =
+        unorm2_getRawDecomposition(normalizer, static_cast<UChar32>(ab),
+                                   decompUtf16, MAX_DECOMP_LENGTH, &error);
+    if (U_FAILURE(error) || len < 0) {
+      return 0;
+    }
+    UText text = UTEXT_INITIALIZER;
+    utext_openUChars(&text, decompUtf16, len, &error);
+    MOZ_ASSERT(U_SUCCESS(error));
+    UChar32 ch = UTEXT_NEXT32(&text);
+    len = 0;
+    if (ch != U_SENTINEL) {
+      decomp[0] = static_cast<char32_t>(ch);
+      ++len;
+      ch = UTEXT_NEXT32(&text);
+      if (ch != U_SENTINEL) {
+        decomp[1] = static_cast<char32_t>(ch);
+        ++len;
+      }
+    }
+    utext_close(&text);
+    return len;
+  }
+
+  /**
+   * Return the Unicode version, for example "13.0".
+   */
+  static Span<const char> GetUnicodeVersion();
+};
+
+}  // namespace mozilla::intl
+
+#endif
diff --git a/intl/components/src/TimeZone.cpp b/intl/components/src/TimeZone.cpp
new file mode 100644
index 0000000000..7a069ef0c5
--- /dev/null
+++ b/intl/components/src/TimeZone.cpp
@@ -0,0 +1,423 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "mozilla/intl/TimeZone.h"
+
+#include "mozilla/Vector.h"
+
+#include <algorithm>
+#include <string_view>
+
+#include "unicode/uenum.h"
+#if MOZ_INTL_USE_ICU_CPP_TIMEZONE
+#  include "unicode/basictz.h"
+#endif
+
+namespace mozilla::intl {
+
+/* static */
+Result<UniquePtr<TimeZone>, ICUError> TimeZone::TryCreate(
+    Maybe<Span<const char16_t>> aTimeZoneOverride) {
+  const UChar* zoneID = nullptr;
+  int32_t zoneIDLen = 0;
+  if (aTimeZoneOverride) {
+    zoneIDLen = static_cast<int32_t>(aTimeZoneOverride->Length());
+    zoneID = aTimeZoneOverride->Elements();
+  }
+
+#if MOZ_INTL_USE_ICU_CPP_TIMEZONE
+  UniquePtr<icu::TimeZone> tz;
+  if (zoneID) {
+    tz.reset(
+        icu::TimeZone::createTimeZone(icu::UnicodeString(zoneID, zoneIDLen)));
+  } else {
+    tz.reset(icu::TimeZone::createDefault());
+  }
+  MOZ_ASSERT(tz);
+
+  if (*tz == icu::TimeZone::getUnknown()) {
+    return Err(ICUError::InternalError);
+  }
+
+  return MakeUnique<TimeZone>(std::move(tz));
+#else
+  // An empty string is used for the root locale. This is regarded as the base
+  // locale of all locales, and is used as the language/country neutral locale
+  // for locale sensitive operations.
+  const char* rootLocale = "";
+
+  UErrorCode status = U_ZERO_ERROR;
+  UCalendar* calendar =
+      ucal_open(zoneID, zoneIDLen, rootLocale, UCAL_DEFAULT, &status);
+
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+
+  // https://tc39.es/ecma262/#sec-time-values-and-time-range
+  //
+  // A time value supports a slightly smaller range of -8,640,000,000,000,000 to
+  // 8,640,000,000,000,000 milliseconds.
+  constexpr double StartOfTime = -8.64e15;
+
+  // Ensure all computations are performed in the proleptic Gregorian calendar.
+  ucal_setGregorianChange(calendar, StartOfTime, &status);
+
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+
+  return MakeUnique<TimeZone>(calendar);
+#endif
+}
+
+Result<int32_t, ICUError> TimeZone::GetRawOffsetMs() {
+#if MOZ_INTL_USE_ICU_CPP_TIMEZONE
+  return mTimeZone->getRawOffset();
+#else
+  // Reset the time in case the calendar has been modified.
+  UErrorCode status = U_ZERO_ERROR;
+  ucal_setMillis(mCalendar, ucal_getNow(), &status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+
+  int32_t offset = ucal_get(mCalendar, UCAL_ZONE_OFFSET, &status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+
+  return offset;
+#endif
+}
+
+Result<int32_t, ICUError> TimeZone::GetDSTOffsetMs(int64_t aUTCMilliseconds) {
+  UDate date = UDate(aUTCMilliseconds);
+
+#if MOZ_INTL_USE_ICU_CPP_TIMEZONE
+  constexpr bool dateIsLocalTime = false;
+  int32_t rawOffset, dstOffset;
+  UErrorCode status = U_ZERO_ERROR;
+
+  mTimeZone->getOffset(date, dateIsLocalTime, rawOffset, dstOffset, status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+
+  return dstOffset;
+#else
+  UErrorCode status = U_ZERO_ERROR;
+  ucal_setMillis(mCalendar, date, &status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+
+  int32_t dstOffset = ucal_get(mCalendar, UCAL_DST_OFFSET, &status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+
+  return dstOffset;
+#endif
+}
+
+Result<int32_t, ICUError> TimeZone::GetOffsetMs(int64_t aUTCMilliseconds) {
+  UDate date = UDate(aUTCMilliseconds);
+
+#if MOZ_INTL_USE_ICU_CPP_TIMEZONE
+  constexpr bool dateIsLocalTime = false;
+  int32_t rawOffset, dstOffset;
+  UErrorCode status = U_ZERO_ERROR;
+
+  mTimeZone->getOffset(date, dateIsLocalTime, rawOffset, dstOffset, status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+
+  return rawOffset + dstOffset;
+#else
+  UErrorCode status = U_ZERO_ERROR;
+  ucal_setMillis(mCalendar, date, &status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+
+  int32_t rawOffset = ucal_get(mCalendar, UCAL_ZONE_OFFSET, &status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+
+  int32_t dstOffset = ucal_get(mCalendar, UCAL_DST_OFFSET, &status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+
+  return rawOffset + dstOffset;
+#endif
+}
+
+Result<int32_t, ICUError> TimeZone::GetUTCOffsetMs(int64_t aLocalMilliseconds) {
+  // https://tc39.es/ecma262/#sec-local-time-zone-adjustment
+  //
+  // LocalTZA ( t, isUTC )
+  //
+  // When t_local represents local time repeating multiple times at a negative
+  // time zone transition (e.g. when the daylight saving time ends or the time
+  // zone offset is decreased due to a time zone rule change) or skipped local
+  // time at a positive time zone transitions (e.g. when the daylight saving
+  // time starts or the time zone offset is increased due to a time zone rule
+  // change), t_local must be interpreted using the time zone offset before the
+  // transition.
+  constexpr LocalOption skippedTime = LocalOption::Former;
+  constexpr LocalOption repeatedTime = LocalOption::Former;
+
+  return GetUTCOffsetMs(aLocalMilliseconds, skippedTime, repeatedTime);
+}
+
+static UTimeZoneLocalOption ToUTimeZoneLocalOption(
+    TimeZone::LocalOption aOption) {
+  switch (aOption) {
+    case TimeZone::LocalOption::Former:
+      return UTimeZoneLocalOption::UCAL_TZ_LOCAL_FORMER;
+    case TimeZone::LocalOption::Latter:
+      return UTimeZoneLocalOption::UCAL_TZ_LOCAL_LATTER;
+  }
+  MOZ_CRASH("Unexpected TimeZone::LocalOption");
+}
+
+Result<int32_t, ICUError> TimeZone::GetUTCOffsetMs(int64_t aLocalMilliseconds,
+                                                   LocalOption aSkippedTime,
+                                                   LocalOption aRepeatedTime) {
+  UDate date = UDate(aLocalMilliseconds);
+  UTimeZoneLocalOption skippedTime = ToUTimeZoneLocalOption(aSkippedTime);
+  UTimeZoneLocalOption repeatedTime = ToUTimeZoneLocalOption(aRepeatedTime);
+
+#if MOZ_INTL_USE_ICU_CPP_TIMEZONE
+  int32_t rawOffset, dstOffset;
+  UErrorCode status = U_ZERO_ERROR;
+
+  // All ICU TimeZone classes derive from BasicTimeZone, so we can safely
+  // perform the static_cast.
+  // Once <https://unicode-org.atlassian.net/browse/ICU-13705> is fixed we
+  // can remove this extra cast.
+  auto* basicTz = static_cast<icu::BasicTimeZone*>(mTimeZone.get());
+  basicTz->getOffsetFromLocal(date, skippedTime, repeatedTime, rawOffset,
+                              dstOffset, status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+
+  return rawOffset + dstOffset;
+#else
+  UErrorCode status = U_ZERO_ERROR;
+  ucal_setMillis(mCalendar, date, &status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+
+  int32_t rawOffset, dstOffset;
+  ucal_getTimeZoneOffsetFromLocal(mCalendar, skippedTime, repeatedTime,
+                                  &rawOffset, &dstOffset, &status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+
+  return rawOffset + dstOffset;
+#endif
+}
+
+Result<Maybe<int64_t>, ICUError> TimeZone::GetPreviousTransition(
+    int64_t aUTCMilliseconds) {
+  UDate date = UDate(aUTCMilliseconds);
+
+#if MOZ_INTL_USE_ICU_CPP_TIMEZONE
+  // All ICU TimeZone classes derive from BasicTimeZone, so we can safely
+  // perform the static_cast.
+  auto* basicTz = static_cast<icu::BasicTimeZone*>(mTimeZone.get());
+
+  constexpr bool inclusive = false;
+  icu::TimeZoneTransition transition;
+  if (!basicTz->getPreviousTransition(date, inclusive, transition)) {
+    return Maybe<int64_t>();
+  }
+  return Some(int64_t(transition.getTime()));
+#else
+  UDate transition = 0;
+  UErrorCode status = U_ZERO_ERROR;
+  bool found = ucal_getTimeZoneTransitionDate(
+      mCalendar, UCAL_TZ_TRANSITION_PREVIOUS, &transition, &status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+  if (!found) {
+    return Maybe<int64_t>();
+  }
+  return Some(int64_t(transition));
+#endif
+}
+
+Result<Maybe<int64_t>, ICUError> TimeZone::GetNextTransition(
+    int64_t aUTCMilliseconds) {
+  UDate date = UDate(aUTCMilliseconds);
+
+#if MOZ_INTL_USE_ICU_CPP_TIMEZONE
+  // All ICU TimeZone classes derive from BasicTimeZone, so we can safely
+  // perform the static_cast.
+  auto* basicTz = static_cast<icu::BasicTimeZone*>(mTimeZone.get());
+
+  constexpr bool inclusive = false;
+  icu::TimeZoneTransition transition;
+  if (!basicTz->getNextTransition(date, inclusive, transition)) {
+    return Maybe<int64_t>();
+  }
+  return Some(int64_t(transition.getTime()));
+#else
+  UDate transition = 0;
+  UErrorCode status = U_ZERO_ERROR;
+  bool found = ucal_getTimeZoneTransitionDate(
+      mCalendar, UCAL_TZ_TRANSITION_NEXT, &transition, &status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+  if (!found) {
+    return Maybe<int64_t>();
+  }
+  return Some(int64_t(transition));
+#endif
+}
+
+using TimeZoneIdentifierVector =
+    Vector<char16_t, TimeZone::TimeZoneIdentifierLength>;
+
+#if !MOZ_INTL_USE_ICU_CPP_TIMEZONE
+static bool IsUnknownTimeZone(const TimeZoneIdentifierVector& timeZone) {
+  constexpr std::string_view unknownTimeZone = UCAL_UNKNOWN_ZONE_ID;
+
+  return timeZone.length() == unknownTimeZone.length() &&
+         std::equal(timeZone.begin(), timeZone.end(), unknownTimeZone.begin(),
+                    unknownTimeZone.end());
+}
+
+static ICUResult SetDefaultTimeZone(TimeZoneIdentifierVector& timeZone) {
+  // The string mustn't already be null-terminated.
+  MOZ_ASSERT_IF(!timeZone.empty(), timeZone.end()[-1] != '\0');
+
+  // The time zone identifier must be a null-terminated string.
+  if (!timeZone.append('\0')) {
+    return Err(ICUError::OutOfMemory);
+  }
+
+  UErrorCode status = U_ZERO_ERROR;
+  ucal_setDefaultTimeZone(timeZone.begin(), &status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+
+  return Ok{};
+}
+#endif
+
+Result<bool, ICUError> TimeZone::SetDefaultTimeZone(
+    Span<const char> aTimeZone) {
+#if MOZ_INTL_USE_ICU_CPP_TIMEZONE
+  icu::UnicodeString tzid(aTimeZone.data(), aTimeZone.size(), US_INV);
+  if (tzid.isBogus()) {
+    return Err(ICUError::OutOfMemory);
+  }
+
+  UniquePtr<icu::TimeZone> newTimeZone(icu::TimeZone::createTimeZone(tzid));
+  MOZ_ASSERT(newTimeZone);
+
+  if (*newTimeZone != icu::TimeZone::getUnknown()) {
+    // adoptDefault() takes ownership of the time zone.
+    icu::TimeZone::adoptDefault(newTimeZone.release());
+    return true;
+  }
+#else
+  TimeZoneIdentifierVector tzid;
+  if (!tzid.append(aTimeZone.data(), aTimeZone.size())) {
+    return Err(ICUError::OutOfMemory);
+  }
+
+  // Retrieve the current default time zone in case we need to restore it.
+  TimeZoneIdentifierVector defaultTimeZone;
+  MOZ_TRY(FillBufferWithICUCall(defaultTimeZone, ucal_getDefaultTimeZone));
+
+  // Try to set the new time zone.
+  MOZ_TRY(mozilla::intl::SetDefaultTimeZone(tzid));
+
+  // Check if the time zone was actually applied.
+  TimeZoneIdentifierVector newTimeZone;
+  MOZ_TRY(FillBufferWithICUCall(newTimeZone, ucal_getDefaultTimeZone));
+
+  // Return if the new time zone was successfully applied.
+  if (!IsUnknownTimeZone(newTimeZone)) {
+    return true;
+  }
+
+  // Otherwise restore the original time zone.
+  MOZ_TRY(mozilla::intl::SetDefaultTimeZone(defaultTimeZone));
+#endif
+
+  return false;
+}
+
+ICUResult TimeZone::SetDefaultTimeZoneFromHostTimeZone() {
+#if MOZ_INTL_USE_ICU_CPP_TIMEZONE
+  if (icu::TimeZone* defaultZone = icu::TimeZone::detectHostTimeZone()) {
+    icu::TimeZone::adoptDefault(defaultZone);
+  }
+#else
+  TimeZoneIdentifierVector hostTimeZone;
+  MOZ_TRY(FillBufferWithICUCall(hostTimeZone, ucal_getHostTimeZone));
+
+  MOZ_TRY(mozilla::intl::SetDefaultTimeZone(hostTimeZone));
+#endif
+
+  return Ok{};
+}
+
+Result<Span<const char>, ICUError> TimeZone::GetTZDataVersion() {
+  UErrorCode status = U_ZERO_ERROR;
+  const char* tzdataVersion = ucal_getTZDataVersion(&status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+  return MakeStringSpan(tzdataVersion);
+}
+
+Result<SpanEnumeration<char>, ICUError> TimeZone::GetAvailableTimeZones(
+    const char* aRegion) {
+  // Get the time zones that are commonly used in the given region. Uses the
+  // UCAL_ZONE_TYPE_ANY filter so we have more fine-grained control over the
+  // returned time zones and don't omit time zones which are considered links in
+  // ICU, but are treated as proper zones in IANA.
+  UErrorCode status = U_ZERO_ERROR;
+  UEnumeration* enumeration = ucal_openTimeZoneIDEnumeration(
+      UCAL_ZONE_TYPE_ANY, aRegion, nullptr, &status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+
+  return SpanEnumeration<char>(enumeration);
+}
+
+Result<SpanEnumeration<char>, ICUError> TimeZone::GetAvailableTimeZones() {
+  UErrorCode status = U_ZERO_ERROR;
+  UEnumeration* enumeration = ucal_openTimeZones(&status);
+  if (U_FAILURE(status)) {
+    return Err(ToICUError(status));
+  }
+
+  return SpanEnumeration<char>(enumeration);
+}
+
+#if !MOZ_INTL_USE_ICU_CPP_TIMEZONE
+TimeZone::~TimeZone() {
+  MOZ_ASSERT(mCalendar);
+  ucal_close(mCalendar);
+}
+#endif
+
+}  // namespace mozilla::intl
diff --git a/intl/components/src/TimeZone.h b/intl/components/src/TimeZone.h
new file mode 100644
index 0000000000..9d0423ef13
--- /dev/null
+++ b/intl/components/src/TimeZone.h
@@ -0,0 +1,292 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+#ifndef intl_components_TimeZone_h_
+#define intl_components_TimeZone_h_
+
+// ICU doesn't provide a separate C API for time zone functions, but instead
+// requires to use UCalendar. This adds a measurable overhead when compared to
+// using ICU's C++ TimeZone API, therefore we prefer to use the C++ API when
+// possible. Due to the lack of a stable ABI in C++, it's only possible to use
+// the C++ API when we use our in-tree ICU copy.
+#if !MOZ_SYSTEM_ICU
+#  define MOZ_INTL_USE_ICU_CPP_TIMEZONE 1
+#else
+#  define MOZ_INTL_USE_ICU_CPP_TIMEZONE 0
+#endif
+
+#include <stdint.h>
+#include <utility>
+
+#include "unicode/ucal.h"
+#include "unicode/utypes.h"
+#if MOZ_INTL_USE_ICU_CPP_TIMEZONE
+#  include "unicode/locid.h"
+#  include "unicode/timezone.h"
+#  include "unicode/unistr.h"
+#endif
+
+#include "mozilla/Assertions.h"
+#include "mozilla/Casting.h"
+#include "mozilla/intl/ICU4CGlue.h"
+#include "mozilla/intl/ICUError.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/Result.h"
+#include "mozilla/Span.h"
+#include "mozilla/UniquePtr.h"
+
+namespace mozilla::intl {
+
+/**
+ * This component is a Mozilla-focused API for working with time zones in
+ * internationalization code. It is used in coordination with other operations
+ * such as datetime formatting.
+ */
+class TimeZone final {
+ public:
+#if MOZ_INTL_USE_ICU_CPP_TIMEZONE
+  explicit TimeZone(UniquePtr<icu::TimeZone> aTimeZone)
+      : mTimeZone(std::move(aTimeZone)) {
+    MOZ_ASSERT(mTimeZone);
+  }
+#else
+  explicit TimeZone(UCalendar* aCalendar) : mCalendar(aCalendar) {
+    MOZ_ASSERT(mCalendar);
+  }
+#endif
+
+  // Do not allow copy as this class owns the ICU resource. Move is not
+  // currently implemented, but a custom move operator could be created if
+  // needed.
+  TimeZone(const TimeZone&) = delete;
+  TimeZone& operator=(const TimeZone&) = delete;
+
+#if MOZ_INTL_USE_ICU_CPP_TIMEZONE
+  ~TimeZone() = default;
+#else
+  ~TimeZone();
+#endif
+
+  /**
+   * Create a TimeZone.
+   */
+  static Result<UniquePtr<TimeZone>, ICUError> TryCreate(
+      Maybe<Span<const char16_t>> aTimeZoneOverride = Nothing{});
+
+  /**
+   * A number indicating the raw offset from GMT in milliseconds.
+   */
+  Result<int32_t, ICUError> GetRawOffsetMs();
+
+  /**
+   * Return the daylight saving offset in milliseconds at the given UTC time.
+   */
+  Result<int32_t, ICUError> GetDSTOffsetMs(int64_t aUTCMilliseconds);
+
+  /**
+   * Return the local offset in milliseconds at the given UTC time.
+   */
+  Result<int32_t, ICUError> GetOffsetMs(int64_t aUTCMilliseconds);
+
+  /**
+   * Return the UTC offset in milliseconds at the given local time.
+   */
+  Result<int32_t, ICUError> GetUTCOffsetMs(int64_t aLocalMilliseconds);
+
+  enum class LocalOption {
+    /**
+     * The input is interpreted as local time before a time zone transition.
+     */
+    Former,
+
+    /**
+     * The input is interpreted as local time after a time zone transition.
+     */
+    Latter,
+  };
+
+  /**
+   * Return the UTC offset in milliseconds at the given local time.
+   *
+   * `aSkippedTime` and `aRepeatedTime` select how to interpret skipped and
+   * repeated local times.
+   */
+  Result<int32_t, ICUError> GetUTCOffsetMs(int64_t aLocalMilliseconds,
+                                           LocalOption aSkippedTime,
+                                           LocalOption aRepeatedTime);
+
+  /**
+   * Return the previous time zone transition before the given UTC time. If no
+   * transition was found, return Nothing.
+   */
+  Result<Maybe<int64_t>, ICUError> GetPreviousTransition(
+      int64_t aUTCMilliseconds);
+
+  /**
+   * Return the next time zone transition after the given UTC time. If no
+   * transition was found, return Nothing.
+   */
+  Result<Maybe<int64_t>, ICUError> GetNextTransition(int64_t aUTCMilliseconds);
+
+  enum class DaylightSavings : bool { No, Yes };
+
+  /**
+   * Return the display name for this time zone.
+   */
+  template <typename B>
+  ICUResult GetDisplayName(const char* aLocale,
+                           DaylightSavings aDaylightSavings, B& aBuffer) {
+#if MOZ_INTL_USE_ICU_CPP_TIMEZONE
+    icu::UnicodeString displayName;
+    mTimeZone->getDisplayName(static_cast<bool>(aDaylightSavings),
+                              icu::TimeZone::LONG, icu::Locale(aLocale),
+                              displayName);
+    return FillBuffer(displayName, aBuffer);
+#else
+    return FillBufferWithICUCall(
+        aBuffer, [&](UChar* target, int32_t length, UErrorCode* status) {
+          UCalendarDisplayNameType type =
+              static_cast<bool>(aDaylightSavings) ? UCAL_DST : UCAL_STANDARD;
+          return ucal_getTimeZoneDisplayName(mCalendar, type, aLocale, target,
+                                             length, status);
+        });
+#endif
+  }
+
+  /**
+   * Return the identifier for this time zone.
+   */
+  template <typename B>
+  ICUResult GetId(B& aBuffer) {
+#if MOZ_INTL_USE_ICU_CPP_TIMEZONE
+    icu::UnicodeString id;
+    mTimeZone->getID(id);
+    return FillBuffer(id, aBuffer);
+#else
+    return FillBufferWithICUCall(
+        aBuffer, [&](UChar* target, int32_t length, UErrorCode* status) {
+          return ucal_getTimeZoneID(mCalendar, target, length, status);
+        });
+#endif
+  }
+
+  /**
+   * Fill the buffer with the system's default IANA time zone identifier, e.g.
+   * "America/Chicago".
+   */
+  template <typename B>
+  static ICUResult GetDefaultTimeZone(B& aBuffer) {
+    return FillBufferWithICUCall(aBuffer, ucal_getDefaultTimeZone);
+  }
+
+  /**
+   * Fill the buffer with the host system's default IANA time zone identifier,
+   * e.g. "America/Chicago".
+   *
+   * NOTE: This function is not thread-safe.
+   */
+  template <typename B>
+  static ICUResult GetHostTimeZone(B& aBuffer) {
+    return FillBufferWithICUCall(aBuffer, ucal_getHostTimeZone);
+  }
+
+  /**
+   * Set the default time zone.
+   */
+  static Result<bool, ICUError> SetDefaultTimeZone(Span<const char> aTimeZone);
+
+  /**
+   * Set the default time zone using the host system's time zone.
+   *
+   * NOTE: This function is not thread-safe.
+   */
+  static ICUResult SetDefaultTimeZoneFromHostTimeZone();
+
+  /**
+   * Return the tzdata version.
+   *
+   * The tzdata version is a string of the form "<year><release>", e.g. "2021a".
+   */
+  static Result<Span<const char>, ICUError> GetTZDataVersion();
+
+  /**
+   * Constant for the typical maximal length of a time zone identifier.
+   *
+   * At the time of this writing 32 characters fits every supported time zone:
+   *
+   * Intl.supportedValuesOf("timeZone")
+   *     .reduce((acc, v) => Math.max(acc, v.length), 0)
+   */
+  static constexpr size_t TimeZoneIdentifierLength = 32;
+
+  /**
+   * Returns the canonical system time zone ID or the normalized custom time
+   * zone ID for the given time zone ID.
+   */
+  template <typename B>
+  static ICUResult GetCanonicalTimeZoneID(Span<const char16_t> inputTimeZone,
+                                          B& aBuffer) {
+    static_assert(std::is_same_v<typename B::CharType, char16_t>,
+                  "Currently only UTF-16 buffers are supported.");
+
+    if (aBuffer.capacity() == 0) {
+      // ucal_getCanonicalTimeZoneID differs from other API calls and fails when
+      // passed a nullptr or 0 length result. Reserve some space initially so
+      // that a real pointer will be used in the API.
+      if (!aBuffer.reserve(TimeZoneIdentifierLength)) {
+        return Err(ICUError::OutOfMemory);
+      }
+    }
+
+    return FillBufferWithICUCall(
+        aBuffer,
+        [&inputTimeZone](UChar* target, int32_t length, UErrorCode* status) {
+          return ucal_getCanonicalTimeZoneID(
+              inputTimeZone.Elements(),
+              static_cast<int32_t>(inputTimeZone.Length()), target, length,
+              /* isSystemID */ nullptr, status);
+        });
+  }
+
+  /**
+   * Return an enumeration over all time zones commonly used in the given
+   * region.
+   */
+  static Result<SpanEnumeration<char>, ICUError> GetAvailableTimeZones(
+      const char* aRegion);
+
+  /**
+   * Return an enumeration over all available time zones.
+   */
+  static Result<SpanEnumeration<char>, ICUError> GetAvailableTimeZones();
+
+ private:
+#if MOZ_INTL_USE_ICU_CPP_TIMEZONE
+  template <typename B>
+  static ICUResult FillBuffer(const icu::UnicodeString& aString, B& aBuffer) {
+    int32_t length = aString.length();
+    if (!aBuffer.reserve(AssertedCast<size_t>(length))) {
+      return Err(ICUError::OutOfMemory);
+    }
+
+    UErrorCode status = U_ZERO_ERROR;
+    int32_t written = aString.extract(aBuffer.data(), length, status);
+    if (!ICUSuccessForStringSpan(status)) {
+      return Err(ToICUError(status));
+    }
+    MOZ_ASSERT(written == length);
+
+    aBuffer.written(written);
+
+    return Ok{};
+  }
+
+  UniquePtr<icu::TimeZone> mTimeZone = nullptr;
+#else
+  UCalendar* mCalendar = nullptr;
+#endif
+};
+
+}  // namespace mozilla::intl
+
+#endif
diff --git a/intl/components/src/UnicodeProperties.h b/intl/components/src/UnicodeProperties.h
new file mode 100644
index 0000000000..7fd64e099e
--- /dev/null
+++ b/intl/components/src/UnicodeProperties.h
@@ -0,0 +1,310 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+#ifndef intl_components_UnicodeProperties_h_
+#define intl_components_UnicodeProperties_h_
+
+#include "mozilla/intl/BidiClass.h"
+#include "mozilla/intl/GeneralCategory.h"
+#include "mozilla/intl/ICU4CGlue.h"
+#include "mozilla/intl/UnicodeScriptCodes.h"
+#include "mozilla/Vector.h"
+
+#include "unicode/uchar.h"
+#include "unicode/uscript.h"
+
+namespace mozilla::intl {
+
+/**
+ * This component is a Mozilla-focused API for working with text properties.
+ */
+class UnicodeProperties final {
+ public:
+  /**
+   * Return the BidiClass for the character.
+   */
+  static inline BidiClass GetBidiClass(uint32_t aCh) {
+    return BidiClass(u_charDirection(aCh));
+  }
+
+  /**
+   * Maps the specified character to a "mirror-image" character.
+   */
+  static inline uint32_t CharMirror(uint32_t aCh) { return u_charMirror(aCh); }
+
+  /**
+   * Return the general category value for the code point.
+   */
+  static inline GeneralCategory CharType(uint32_t aCh) {
+    return GeneralCategory(u_charType(aCh));
+  }
+
+  /**
+   * Determine whether the code point has the Bidi_Mirrored property.
+   */
+  static inline bool IsMirrored(uint32_t aCh) { return u_isMirrored(aCh); }
+
+  /**
+   * Returns the combining class of the code point as specified in
+   * UnicodeData.txt.
+   */
+  static inline uint8_t GetCombiningClass(uint32_t aCh) {
+    return u_getCombiningClass(aCh);
+  }
+
+  enum class IntProperty {
+    BidiPairedBracketType,
+    EastAsianWidth,
+    HangulSyllableType,
+    LineBreak,
+    NumericType,
+  };
+
+  /**
+   * Get the property value for an enumerated or integer Unicode property for a
+   * code point.
+   */
+  static inline int32_t GetIntPropertyValue(uint32_t aCh, IntProperty aProp) {
+    UProperty prop;
+    switch (aProp) {
+      case IntProperty::BidiPairedBracketType:
+        prop = UCHAR_BIDI_PAIRED_BRACKET_TYPE;
+        break;
+      case IntProperty::EastAsianWidth:
+        prop = UCHAR_EAST_ASIAN_WIDTH;
+        break;
+      case IntProperty::HangulSyllableType:
+        prop = UCHAR_HANGUL_SYLLABLE_TYPE;
+        break;
+      case IntProperty::LineBreak:
+        prop = UCHAR_LINE_BREAK;
+        break;
+      case IntProperty::NumericType:
+        prop = UCHAR_NUMERIC_TYPE;
+        break;
+    }
+    return u_getIntPropertyValue(aCh, prop);
+  }
+
+  /**
+   * Get the numeric value for a Unicode code point as defined in the
+   * Unicode Character Database if the input is decimal or a digit,
+   * otherwise, returns -1.
+   */
+  static inline int8_t GetNumericValue(uint32_t aCh) {
+    UNumericType type =
+        UNumericType(GetIntPropertyValue(aCh, IntProperty::NumericType));
+    return type == U_NT_DECIMAL || type == U_NT_DIGIT
+               ? int8_t(u_getNumericValue(aCh))
+               : -1;
+  }
+
+  /**
+   * Maps the specified character to its paired bracket character.
+   */
+  static inline uint32_t GetBidiPairedBracket(uint32_t aCh) {
+    return u_getBidiPairedBracket(aCh);
+  }
+
+  /**
+   * The given character is mapped to its uppercase equivalent according to
+   * UnicodeData.txt; if the character has no uppercase equivalent, the
+   * character itself is returned.
+   */
+  static inline uint32_t ToUpper(uint32_t aCh) { return u_toupper(aCh); }
+
+  /**
+   * The given character is mapped to its lowercase equivalent according to
+   * UnicodeData.txt; if the character has no lowercase equivalent, the
+   * character itself is returned.
+   */
+  static inline uint32_t ToLower(uint32_t aCh) { return u_tolower(aCh); }
+
+  /**
+   * Check if a code point has the Lowercase Unicode property.
+   */
+  static inline bool IsLowercase(uint32_t aCh) { return u_isULowercase(aCh); }
+
+  /**
+   * The given character is mapped to its titlecase equivalent according to
+   * UnicodeData.txt; if the character has no titlecase equivalent, the
+   * character itself is returned.
+   */
+  static inline uint32_t ToTitle(uint32_t aCh) { return u_totitle(aCh); }
+
+  /**
+   * The given character is mapped to its case folding equivalent according to
+   * UnicodeData.txt and CaseFolding.txt;
+   * if the character has no case folding equivalent, the character
+   * itself is returned.
+   */
+  static inline uint32_t FoldCase(uint32_t aCh) {
+    return u_foldCase(aCh, U_FOLD_CASE_DEFAULT);
+  }
+
+  enum class BinaryProperty {
+    DefaultIgnorableCodePoint,
+    Emoji,
+    EmojiPresentation,
+  };
+
+  /**
+   * Check a binary Unicode property for a code point.
+   */
+  static inline bool HasBinaryProperty(uint32_t aCh, BinaryProperty aProp) {
+    UProperty prop;
+    switch (aProp) {
+      case BinaryProperty::DefaultIgnorableCodePoint:
+        prop = UCHAR_DEFAULT_IGNORABLE_CODE_POINT;
+        break;
+      case BinaryProperty::Emoji:
+        prop = UCHAR_EMOJI;
+        break;
+      case BinaryProperty::EmojiPresentation:
+        prop = UCHAR_EMOJI_PRESENTATION;
+        break;
+    }
+    return u_hasBinaryProperty(aCh, prop);
+  }
+
+  /**
+   * Check if the width of aCh is full width, half width or wide
+   * excluding emoji.
+   */
+  static inline bool IsEastAsianWidthFHWexcludingEmoji(uint32_t aCh) {
+    switch (GetIntPropertyValue(aCh, IntProperty::EastAsianWidth)) {
+      case U_EA_FULLWIDTH:
+      case U_EA_HALFWIDTH:
+        return true;
+      case U_EA_WIDE:
+        return HasBinaryProperty(aCh, BinaryProperty::Emoji) ? false : true;
+      case U_EA_AMBIGUOUS:
+      case U_EA_NARROW:
+      case U_EA_NEUTRAL:
+        return false;
+    }
+    return false;
+  }
+
+  /**
+   * Check if the width of aCh is ambiguous, full width, or wide.
+   */
+  static inline bool IsEastAsianWidthAFW(uint32_t aCh) {
+    switch (GetIntPropertyValue(aCh, IntProperty::EastAsianWidth)) {
+      case U_EA_AMBIGUOUS:
+      case U_EA_FULLWIDTH:
+      case U_EA_WIDE:
+        return true;
+      case U_EA_HALFWIDTH:
+      case U_EA_NARROW:
+      case U_EA_NEUTRAL:
+        return false;
+    }
+    return false;
+  }
+
+  /**
+   * Check if the width of aCh is full width, or wide.
+   */
+  static inline bool IsEastAsianWidthFW(uint32_t aCh) {
+    switch (GetIntPropertyValue(aCh, IntProperty::EastAsianWidth)) {
+      case U_EA_FULLWIDTH:
+      case U_EA_WIDE:
+        return true;
+      case U_EA_AMBIGUOUS:
+      case U_EA_HALFWIDTH:
+      case U_EA_NARROW:
+      case U_EA_NEUTRAL:
+        return false;
+    }
+    return false;
+  }
+
+  /**
+   * Check if the CharType of aCh is math or other symbol.
+   */
+  static inline bool IsMathOrMusicSymbol(uint32_t aCh) {
+    // Keep this function in sync with is_math_symbol in base_chars.py.
+    return CharType(aCh) == GeneralCategory::Math_Symbol ||
+           CharType(aCh) == GeneralCategory::Other_Symbol;
+  }
+
+  static inline Script GetScriptCode(uint32_t aCh) {
+    // We can safely ignore the error code here because uscript_getScript
+    // returns USCRIPT_INVALID_CODE in the event of an error.
+    UErrorCode err = U_ZERO_ERROR;
+    return Script(uscript_getScript(aCh, &err));
+  }
+
+  static inline bool HasScript(uint32_t aCh, Script aScript) {
+    return uscript_hasScript(aCh, UScriptCode(aScript));
+  }
+
+  static inline const char* GetScriptShortName(Script aScript) {
+    return uscript_getShortName(UScriptCode(aScript));
+  }
+
+  static inline int32_t GetMaxNumberOfScripts() {
+    return u_getIntPropertyMaxValue(UCHAR_SCRIPT);
+  }
+
+  // The code point which has the most script extensions is 0x0965, which has 21
+  // script extensions, so choose the vector size as 32 to prevent heap
+  // allocation.
+  static constexpr size_t kMaxScripts = 32;
+
+  using ScriptExtensionVector = Vector<Script, kMaxScripts>;
+
+  /**
+   * Get the script extensions for the given code point, and write the script
+   * extensions to aExtensions vector. If the code point has script extensions,
+   * the script code (Script::COMMON or Script::INHERITED) will be excluded.
+   *
+   * If the code point doesn't have any script extension, then its script code
+   * will be written to aExtensions vector.
+   *
+   * If the code point is invalid, Script::UNKNOWN will be written to
+   * aExtensions vector.
+   *
+   * Note: aExtensions will be cleared after calling this method regardless of
+   * failure.
+   *
+   * See [1] for the script code of the code point, [2] for the script
+   * extensions.
+   *
+   * https://www.unicode.org/Public/UNIDATA/Scripts.txt
+   * https://www.unicode.org/Public/UNIDATA/ScriptExtensions.txt
+   */
+  static ICUResult GetExtensions(char32_t aCodePoint,
+                                 ScriptExtensionVector& aExtensions) {
+    // Clear the vector first.
+    aExtensions.clear();
+
+    // We cannot pass aExtensions to uscript_getScriptExtension as USCriptCode
+    // takes 4 bytes, so create a local UScriptCode array to get the extensions.
+    UScriptCode ext[kMaxScripts];
+    UErrorCode status = U_ZERO_ERROR;
+    int32_t len = uscript_getScriptExtensions(static_cast<UChar32>(aCodePoint),
+                                              ext, kMaxScripts, &status);
+    if (U_FAILURE(status)) {
+      // kMaxScripts should be large enough to hold the maximun number of script
+      // extensions.
+      MOZ_DIAGNOSTIC_ASSERT(status != U_BUFFER_OVERFLOW_ERROR);
+      return Err(ToICUError(status));
+    }
+
+    if (!aExtensions.reserve(len)) {
+      return Err(ICUError::OutOfMemory);
+    }
+
+    for (int32_t i = 0; i < len; i++) {
+      aExtensions.infallibleAppend(Script(ext[i]));
+    }
+
+    return Ok();
+  }
+};
+
+}  // namespace mozilla::intl
+
+#endif
diff --git a/intl/components/src/UnicodeScriptCodes.h b/intl/components/src/UnicodeScriptCodes.h
new file mode 100644
index 0000000000..b5d6f490ee
--- /dev/null
+++ b/intl/components/src/UnicodeScriptCodes.h
@@ -0,0 +1,261 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/*
+ * Derived from the Unicode Character Database by genUnicodePropertyData.pl
+ *
+ * For Unicode terms of use, see http://www.unicode.org/terms_of_use.html
+ */
+
+/*
+ * Created on Tue Oct 25 06:53:25 2022 from UCD data files with version info:
+ *
+
+# Unicode Character Database
+# Date: 2022-09-02
+# © 2022 Unicode®, Inc.
+# Unicode and the Unicode Logo are registered trademarks of Unicode, Inc. in the U.S. and other countries.
+# For terms of use, see https://www.unicode.org/terms_of_use.html
+#
+# For documentation, see the following:
+# NamesList.html
+# UAX #38, "Unicode Han Database (Unihan)"
+# UAX #44, "Unicode Character Database"
+# UTS #51, "Unicode Emoji"
+#
+# The UAXes and UTS #51 can be accessed at https://www.unicode.org/versions/Unicode15.0.0/
+
+This directory contains the final data files
+for the Unicode Character Database, for Version 15.0.0 of the Unicode Standard.
+
+# IdentifierStatus.txt
+# Date: 2022-08-26, 16:49:09 GMT
+
+#
+# Unihan_Variants.txt
+# Date: 2022-08-01 16:36:07 GMT [JHJ]
+
+# VerticalOrientation-17.txt
+# Date: 2016-10-20, 07:00:00 GMT [EM, KI, LI]
+
+ *
+ * * * * * This file contains MACHINE-GENERATED DATA, do not edit! * * * * *
+ */
+
+#ifndef intl_components_UnicodeScriptCodes_h_
+#define intl_components_UnicodeScriptCodes_h_
+
+namespace mozilla::intl {
+enum class Script : int16_t {
+  COMMON = 0,
+  INHERITED = 1,
+  ARABIC = 2,
+  ARMENIAN = 3,
+  BENGALI = 4,
+  BOPOMOFO = 5,
+  CHEROKEE = 6,
+  COPTIC = 7,
+  CYRILLIC = 8,
+  DESERET = 9,
+  DEVANAGARI = 10,
+  ETHIOPIC = 11,
+  GEORGIAN = 12,
+  GOTHIC = 13,
+  GREEK = 14,
+  GUJARATI = 15,
+  GURMUKHI = 16,
+  HAN = 17,
+  HANGUL = 18,
+  HEBREW = 19,
+  HIRAGANA = 20,
+  KANNADA = 21,
+  KATAKANA = 22,
+  KHMER = 23,
+  LAO = 24,
+  LATIN = 25,
+  MALAYALAM = 26,
+  MONGOLIAN = 27,
+  MYANMAR = 28,
+  OGHAM = 29,
+  OLD_ITALIC = 30,
+  ORIYA = 31,
+  RUNIC = 32,
+  SINHALA = 33,
+  SYRIAC = 34,
+  TAMIL = 35,
+  TELUGU = 36,
+  THAANA = 37,
+  THAI = 38,
+  TIBETAN = 39,
+  CANADIAN_ABORIGINAL = 40,
+  YI = 41,
+  TAGALOG = 42,
+  HANUNOO = 43,
+  BUHID = 44,
+  TAGBANWA = 45,
+  BRAILLE = 46,
+  CYPRIOT = 47,
+  LIMBU = 48,
+  LINEAR_B = 49,
+  OSMANYA = 50,
+  SHAVIAN = 51,
+  TAI_LE = 52,
+  UGARITIC = 53,
+  KATAKANA_OR_HIRAGANA = 54,
+  BUGINESE = 55,
+  GLAGOLITIC = 56,
+  KHAROSHTHI = 57,
+  SYLOTI_NAGRI = 58,
+  NEW_TAI_LUE = 59,
+  TIFINAGH = 60,
+  OLD_PERSIAN = 61,
+  BALINESE = 62,
+  BATAK = 63,
+  BLISSYMBOLS = 64,
+  BRAHMI = 65,
+  CHAM = 66,
+  CIRTH = 67,
+  OLD_CHURCH_SLAVONIC_CYRILLIC = 68,
+  DEMOTIC_EGYPTIAN = 69,
+  HIERATIC_EGYPTIAN = 70,
+  EGYPTIAN_HIEROGLYPHS = 71,
+  KHUTSURI = 72,
+  SIMPLIFIED_HAN = 73,
+  TRADITIONAL_HAN = 74,
+  PAHAWH_HMONG = 75,
+  OLD_HUNGARIAN = 76,
+  HARAPPAN_INDUS = 77,
+  JAVANESE = 78,
+  KAYAH_LI = 79,
+  LATIN_FRAKTUR = 80,
+  LATIN_GAELIC = 81,
+  LEPCHA = 82,
+  LINEAR_A = 83,
+  MANDAIC = 84,
+  MAYAN_HIEROGLYPHS = 85,
+  MEROITIC_HIEROGLYPHS = 86,
+  NKO = 87,
+  OLD_TURKIC = 88,
+  OLD_PERMIC = 89,
+  PHAGS_PA = 90,
+  PHOENICIAN = 91,
+  MIAO = 92,
+  RONGORONGO = 93,
+  SARATI = 94,
+  ESTRANGELO_SYRIAC = 95,
+  WESTERN_SYRIAC = 96,
+  EASTERN_SYRIAC = 97,
+  TENGWAR = 98,
+  VAI = 99,
+  VISIBLE_SPEECH = 100,
+  CUNEIFORM = 101,
+  UNWRITTEN_LANGUAGES = 102,
+  UNKNOWN = 103,
+  CARIAN = 104,
+  JAPANESE = 105,
+  TAI_THAM = 106,
+  LYCIAN = 107,
+  LYDIAN = 108,
+  OL_CHIKI = 109,
+  REJANG = 110,
+  SAURASHTRA = 111,
+  SIGNWRITING = 112,
+  SUNDANESE = 113,
+  MOON = 114,
+  MEETEI_MAYEK = 115,
+  IMPERIAL_ARAMAIC = 116,
+  AVESTAN = 117,
+  CHAKMA = 118,
+  KOREAN = 119,
+  KAITHI = 120,
+  MANICHAEAN = 121,
+  INSCRIPTIONAL_PAHLAVI = 122,
+  PSALTER_PAHLAVI = 123,
+  BOOK_PAHLAVI = 124,
+  INSCRIPTIONAL_PARTHIAN = 125,
+  SAMARITAN = 126,
+  TAI_VIET = 127,
+  MATHEMATICAL_NOTATION = 128,
+  SYMBOLS = 129,
+  BAMUM = 130,
+  LISU = 131,
+  NAKHI_GEBA = 132,
+  OLD_SOUTH_ARABIAN = 133,
+  BASSA_VAH = 134,
+  DUPLOYAN = 135,
+  ELBASAN = 136,
+  GRANTHA = 137,
+  KPELLE = 138,
+  LOMA = 139,
+  MENDE_KIKAKUI = 140,
+  MEROITIC_CURSIVE = 141,
+  OLD_NORTH_ARABIAN = 142,
+  NABATAEAN = 143,
+  PALMYRENE = 144,
+  KHUDAWADI = 145,
+  WARANG_CITI = 146,
+  AFAKA = 147,
+  JURCHEN = 148,
+  MRO = 149,
+  NUSHU = 150,
+  SHARADA = 151,
+  SORA_SOMPENG = 152,
+  TAKRI = 153,
+  TANGUT = 154,
+  WOLEAI = 155,
+  ANATOLIAN_HIEROGLYPHS = 156,
+  KHOJKI = 157,
+  TIRHUTA = 158,
+  CAUCASIAN_ALBANIAN = 159,
+  MAHAJANI = 160,
+  AHOM = 161,
+  HATRAN = 162,
+  MODI = 163,
+  MULTANI = 164,
+  PAU_CIN_HAU = 165,
+  SIDDHAM = 166,
+  ADLAM = 167,
+  BHAIKSUKI = 168,
+  MARCHEN = 169,
+  NEWA = 170,
+  OSAGE = 171,
+  HAN_WITH_BOPOMOFO = 172,
+  JAMO = 173,
+  SYMBOLS_EMOJI = 174,
+  MASARAM_GONDI = 175,
+  SOYOMBO = 176,
+  ZANABAZAR_SQUARE = 177,
+  DOGRA = 178,
+  GUNJALA_GONDI = 179,
+  MAKASAR = 180,
+  MEDEFAIDRIN = 181,
+  HANIFI_ROHINGYA = 182,
+  SOGDIAN = 183,
+  OLD_SOGDIAN = 184,
+  ELYMAIC = 185,
+  NYIAKENG_PUACHUE_HMONG = 186,
+  NANDINAGARI = 187,
+  WANCHO = 188,
+  CHORASMIAN = 189,
+  DIVES_AKURU = 190,
+  KHITAN_SMALL_SCRIPT = 191,
+  YEZIDI = 192,
+  CYPRO_MINOAN = 193,
+  OLD_UYGHUR = 194,
+  TANGSA = 195,
+  TOTO = 196,
+  VITHKUQI = 197,
+  KAWI = 198,
+  NAG_MUNDARI = 199,
+
+  NUM_SCRIPT_CODES = 200,
+
+  INVALID = -1
+};
+} // namespace mozilla::intl
+
+#endif
+/*
+ * * * * * This file contains MACHINE-GENERATED DATA, do not edit! * * * * *
+ */