Adding upstream version 115.7.0esr.upstream/115.7.0esrupstream

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

23 files changed, 4835 insertions, 0 deletions

diff --git a/intl/components/gtest/TestBidi.cpp b/intl/components/gtest/TestBidi.cpp
new file mode 100644
index 0000000000..f928f890a6
--- /dev/null
+++ b/intl/components/gtest/TestBidi.cpp
@@ -0,0 +1,314 @@
+/* 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 "gtest/gtest.h"
+
+#include "mozilla/intl/Bidi.h"
+#include "mozilla/Span.h"
+namespace mozilla::intl {
+
+struct VisualRun {
+  Span<const char16_t> string;
+  BidiDirection direction;
+};
+
+/**
+ * An iterator for visual runs in a paragraph. See Bug 1736597 for integrating
+ * this into the public API.
+ */
+class MOZ_STACK_CLASS VisualRunIter {
+ public:
+  VisualRunIter(Bidi& aBidi, Span<const char16_t> aParagraph,
+                BidiEmbeddingLevel aLevel)
+      : mBidi(aBidi), mParagraph(aParagraph) {
+    // Crash in case of errors by calling unwrap. If this were a real API, this
+    // would be a TryCreate call.
+    mBidi.SetParagraph(aParagraph, aLevel).unwrap();
+    mRunCount = mBidi.CountRuns().unwrap();
+  }
+
+  Maybe<VisualRun> Next() {
+    if (mRunIndex >= mRunCount) {
+      return Nothing();
+    }
+
+    int32_t stringIndex = -1;
+    int32_t stringLength = -1;
+
+    BidiDirection direction =
+        mBidi.GetVisualRun(mRunIndex, &stringIndex, &stringLength);
+
+    Span<const char16_t> string(mParagraph.Elements() + stringIndex,
+                                stringLength);
+    mRunIndex++;
+    return Some(VisualRun{string, direction});
+  }
+
+ private:
+  Bidi& mBidi;
+  Span<const char16_t> mParagraph = Span<const char16_t>();
+  int32_t mRunIndex = 0;
+  int32_t mRunCount = 0;
+};
+
+struct LogicalRun {
+  Span<const char16_t> string;
+  BidiEmbeddingLevel embeddingLevel;
+};
+
+/**
+ * An iterator for logical runs in a paragraph. See Bug 1736597 for integrating
+ * this into the public API.
+ */
+class MOZ_STACK_CLASS LogicalRunIter {
+ public:
+  LogicalRunIter(Bidi& aBidi, Span<const char16_t> aParagraph,
+                 BidiEmbeddingLevel aLevel)
+      : mBidi(aBidi), mParagraph(aParagraph) {
+    // Crash in case of errors by calling unwrap. If this were a real API, this
+    // would be a TryCreate call.
+    mBidi.SetParagraph(aParagraph, aLevel).unwrap();
+    mBidi.CountRuns().unwrap();
+  }
+
+  Maybe<LogicalRun> Next() {
+    if (mRunIndex >= static_cast<int32_t>(mParagraph.Length())) {
+      return Nothing();
+    }
+
+    int32_t logicalLimit;
+
+    BidiEmbeddingLevel embeddingLevel;
+    mBidi.GetLogicalRun(mRunIndex, &logicalLimit, &embeddingLevel);
+
+    Span<const char16_t> string(mParagraph.Elements() + mRunIndex,
+                                logicalLimit - mRunIndex);
+
+    mRunIndex = logicalLimit;
+    return Some(LogicalRun{string, embeddingLevel});
+  }
+
+ private:
+  Bidi& mBidi;
+  Span<const char16_t> mParagraph = Span<const char16_t>();
+  int32_t mRunIndex = 0;
+};
+
+TEST(IntlBidi, SimpleLTR)
+{
+  Bidi bidi{};
+  LogicalRunIter logicalRunIter(bidi, MakeStringSpan(u"this is a paragraph"),
+                                BidiEmbeddingLevel::DefaultLTR());
+  ASSERT_EQ(bidi.GetParagraphEmbeddingLevel(), 0);
+  ASSERT_EQ(bidi.GetParagraphDirection(), Bidi::ParagraphDirection::LTR);
+
+  {
+    auto logicalRun = logicalRunIter.Next();
+    ASSERT_TRUE(logicalRun.isSome());
+    ASSERT_EQ(logicalRun->string, MakeStringSpan(u"this is a paragraph"));
+    ASSERT_EQ(logicalRun->embeddingLevel, 0);
+    ASSERT_EQ(logicalRun->embeddingLevel.Direction(), BidiDirection::LTR);
+  }
+
+  {
+    auto logicalRun = logicalRunIter.Next();
+    ASSERT_TRUE(logicalRun.isNothing());
+  }
+}
+
+TEST(IntlBidi, SimpleRTL)
+{
+  Bidi bidi{};
+  LogicalRunIter logicalRunIter(bidi, MakeStringSpan(u"فايرفوكس رائع"),
+                                BidiEmbeddingLevel::DefaultLTR());
+  ASSERT_EQ(bidi.GetParagraphEmbeddingLevel(), 1);
+  ASSERT_EQ(bidi.GetParagraphDirection(), Bidi::ParagraphDirection::RTL);
+
+  {
+    auto logicalRun = logicalRunIter.Next();
+    ASSERT_TRUE(logicalRun.isSome());
+    ASSERT_EQ(logicalRun->string, MakeStringSpan(u"فايرفوكس رائع"));
+    ASSERT_EQ(logicalRun->embeddingLevel.Direction(), BidiDirection::RTL);
+    ASSERT_EQ(logicalRun->embeddingLevel, 1);
+  }
+
+  {
+    auto logicalRun = logicalRunIter.Next();
+    ASSERT_TRUE(logicalRun.isNothing());
+  }
+}
+
+TEST(IntlBidi, MultiLevel)
+{
+  Bidi bidi{};
+  LogicalRunIter logicalRunIter(
+      bidi, MakeStringSpan(u"Firefox is awesome: رائع Firefox"),
+      BidiEmbeddingLevel::DefaultLTR());
+  ASSERT_EQ(bidi.GetParagraphEmbeddingLevel(), 0);
+  ASSERT_EQ(bidi.GetParagraphDirection(), Bidi::ParagraphDirection::Mixed);
+
+  {
+    auto logicalRun = logicalRunIter.Next();
+    ASSERT_TRUE(logicalRun.isSome());
+    ASSERT_EQ(logicalRun->string, MakeStringSpan(u"Firefox is awesome: "));
+    ASSERT_EQ(logicalRun->embeddingLevel, 0);
+  }
+  {
+    auto logicalRun = logicalRunIter.Next();
+    ASSERT_TRUE(logicalRun.isSome());
+    ASSERT_EQ(logicalRun->string, MakeStringSpan(u"رائع"));
+    ASSERT_EQ(logicalRun->embeddingLevel, 1);
+  }
+  {
+    auto logicalRun = logicalRunIter.Next();
+    ASSERT_TRUE(logicalRun.isSome());
+    ASSERT_EQ(logicalRun->string, MakeStringSpan(u" Firefox"));
+    ASSERT_EQ(logicalRun->embeddingLevel, 0);
+  }
+  {
+    auto logicalRun = logicalRunIter.Next();
+    ASSERT_TRUE(logicalRun.isNothing());
+  }
+}
+
+TEST(IntlBidi, RtlOverride)
+{
+  Bidi bidi{};
+  // Set the paragraph using the RTL embedding mark U+202B, and the LTR
+  // embedding mark U+202A to increase the embedding level. This mark switches
+  // the weakly directional character "_". This demonstrates that embedding
+  // levels can be computed.
+  LogicalRunIter logicalRunIter(
+      bidi, MakeStringSpan(u"ltr\u202b___رائع___\u202a___ltr__"),
+      BidiEmbeddingLevel::DefaultLTR());
+  ASSERT_EQ(bidi.GetParagraphEmbeddingLevel(), 0);
+  ASSERT_EQ(bidi.GetParagraphDirection(), Bidi::ParagraphDirection::Mixed);
+
+  {
+    auto logicalRun = logicalRunIter.Next();
+    ASSERT_TRUE(logicalRun.isSome());
+    ASSERT_EQ(logicalRun->string, MakeStringSpan(u"ltr"));
+    ASSERT_EQ(logicalRun->embeddingLevel, 0);
+    ASSERT_EQ(logicalRun->embeddingLevel.Direction(), BidiDirection::LTR);
+  }
+  {
+    auto logicalRun = logicalRunIter.Next();
+    ASSERT_TRUE(logicalRun.isSome());
+    ASSERT_EQ(logicalRun->string, MakeStringSpan(u"\u202b___رائع___"));
+    ASSERT_EQ(logicalRun->embeddingLevel, 1);
+    ASSERT_EQ(logicalRun->embeddingLevel.Direction(), BidiDirection::RTL);
+  }
+  {
+    auto logicalRun = logicalRunIter.Next();
+    ASSERT_TRUE(logicalRun.isSome());
+    ASSERT_EQ(logicalRun->string, MakeStringSpan(u"\u202a___ltr__"));
+    ASSERT_EQ(logicalRun->embeddingLevel, 2);
+    ASSERT_EQ(logicalRun->embeddingLevel.Direction(), BidiDirection::LTR);
+  }
+  {
+    auto logicalRun = logicalRunIter.Next();
+    ASSERT_TRUE(logicalRun.isNothing());
+  }
+}
+
+TEST(IntlBidi, VisualRuns)
+{
+  Bidi bidi{};
+
+  VisualRunIter visualRunIter(
+      bidi,
+      MakeStringSpan(
+          u"first visual run التشغيل البصري الثاني third visual run"),
+      BidiEmbeddingLevel::DefaultLTR());
+  {
+    Maybe<VisualRun> run = visualRunIter.Next();
+    ASSERT_TRUE(run.isSome());
+    ASSERT_EQ(run->string, MakeStringSpan(u"first visual run "));
+    ASSERT_EQ(run->direction, BidiDirection::LTR);
+  }
+  {
+    Maybe<VisualRun> run = visualRunIter.Next();
+    ASSERT_TRUE(run.isSome());
+    ASSERT_EQ(run->string, MakeStringSpan(u"التشغيل البصري الثاني"));
+    ASSERT_EQ(run->direction, BidiDirection::RTL);
+  }
+  {
+    Maybe<VisualRun> run = visualRunIter.Next();
+    ASSERT_TRUE(run.isSome());
+    ASSERT_EQ(run->string, MakeStringSpan(u" third visual run"));
+    ASSERT_EQ(run->direction, BidiDirection::LTR);
+  }
+  {
+    Maybe<VisualRun> run = visualRunIter.Next();
+    ASSERT_TRUE(run.isNothing());
+  }
+}
+
+TEST(IntlBidi, VisualRunsWithEmbeds)
+{
+  // Compare this test to the logical order test.
+  Bidi bidi{};
+  VisualRunIter visualRunIter(
+      bidi, MakeStringSpan(u"ltr\u202b___رائع___\u202a___ltr___"),
+      BidiEmbeddingLevel::DefaultLTR());
+  {
+    Maybe<VisualRun> run = visualRunIter.Next();
+    ASSERT_TRUE(run.isSome());
+    ASSERT_EQ(run->string, MakeStringSpan(u"ltr"));
+    ASSERT_EQ(run->direction, BidiDirection::LTR);
+  }
+  {
+    Maybe<VisualRun> run = visualRunIter.Next();
+    ASSERT_TRUE(run.isSome());
+    ASSERT_EQ(run->string, MakeStringSpan(u"\u202a___ltr___"));
+    ASSERT_EQ(run->direction, BidiDirection::LTR);
+  }
+  {
+    Maybe<VisualRun> run = visualRunIter.Next();
+    ASSERT_TRUE(run.isSome());
+    ASSERT_EQ(run->string, MakeStringSpan(u"\u202b___رائع___"));
+    ASSERT_EQ(run->direction, BidiDirection::RTL);
+  }
+  {
+    Maybe<VisualRun> run = visualRunIter.Next();
+    ASSERT_TRUE(run.isNothing());
+  }
+}
+
+// The full Bidi class can be found in [1].
+//
+// [1]: https://www.unicode.org/Public/UNIDATA/extracted/DerivedBidiClass.txt
+TEST(IntlBidi, GetBaseDirection)
+{
+  // Return Neutral as default if empty string is provided.
+  ASSERT_EQ(Bidi::GetBaseDirection(nullptr), Bidi::BaseDirection::Neutral);
+
+  // White space(WS) is classified as Neutral.
+  ASSERT_EQ(Bidi::GetBaseDirection(MakeStringSpan(u" ")),
+            Bidi::BaseDirection::Neutral);
+
+  // 000A and 000D are paragraph separators(BS), which are also classified as
+  // Neutral.
+  ASSERT_EQ(Bidi::GetBaseDirection(MakeStringSpan(u"\u000A")),
+            Bidi::BaseDirection::Neutral);
+  ASSERT_EQ(Bidi::GetBaseDirection(MakeStringSpan(u"\u000D")),
+            Bidi::BaseDirection::Neutral);
+
+  // 0620..063f are Arabic letters, which is of type AL.
+  ASSERT_EQ(Bidi::GetBaseDirection(MakeStringSpan(u"\u0620\u0621\u0622")),
+            Bidi::BaseDirection::RTL);
+  ASSERT_EQ(Bidi::GetBaseDirection(MakeStringSpan(u" \u0620\u0621\u0622")),
+            Bidi::BaseDirection::RTL);
+  ASSERT_EQ(Bidi::GetBaseDirection(MakeStringSpan(u"\u0620\u0621\u0622ABC")),
+            Bidi::BaseDirection::RTL);
+
+  // First strong character is of English letters.
+  ASSERT_EQ(Bidi::GetBaseDirection(MakeStringSpan(u"ABC")),
+            Bidi::BaseDirection::LTR);
+  ASSERT_EQ(Bidi::GetBaseDirection(MakeStringSpan(u" ABC")),
+            Bidi::BaseDirection::LTR);
+  ASSERT_EQ(Bidi::GetBaseDirection(MakeStringSpan(u"ABC\u0620")),
+            Bidi::BaseDirection::LTR);
+}
+
+}  // namespace mozilla::intl
diff --git a/intl/components/gtest/TestBuffer.h b/intl/components/gtest/TestBuffer.h
new file mode 100644
index 0000000000..69412ba521
--- /dev/null
+++ b/intl/components/gtest/TestBuffer.h
@@ -0,0 +1,149 @@
+/* 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_gtest_TestBuffer_h_
+#define intl_components_gtest_TestBuffer_h_
+
+#include <string_view>
+#include "mozilla/DebugOnly.h"
+#include "mozilla/Utf8.h"
+#include "mozilla/Vector.h"
+
+namespace mozilla::intl {
+
+/**
+ * A test buffer for interfacing with unified intl classes.
+ * Closely resembles the FormatBuffer class, but without
+ * JavaScript-specific implementation details.
+ */
+template <typename C, size_t inlineCapacity = 0>
+class TestBuffer {
+ public:
+  using CharType = C;
+
+  // Only allow moves, and not copies, as this class owns the mozilla::Vector.
+  TestBuffer(TestBuffer&& other) noexcept = default;
+  TestBuffer& operator=(TestBuffer&& other) noexcept = default;
+
+  explicit TestBuffer(const size_t aSize = 0) { reserve(aSize); }
+
+  /**
+   * Ensures the buffer has enough space to accommodate |aSize| elemtns.
+   */
+  bool reserve(const size_t aSize) { return mBuffer.reserve(aSize); }
+
+  /**
+   * Returns the raw data inside the buffer.
+   */
+  CharType* data() { return mBuffer.begin(); }
+
+  /**
+   * Returns the count of elements in written to the buffer.
+   */
+  size_t length() const { return mBuffer.length(); }
+
+  /**
+   * Returns the buffer's overall capacity.
+   */
+  size_t capacity() const { return mBuffer.capacity(); }
+
+  /**
+   * Resizes the buffer to the given amount of written elements.
+   * This is necessary because the buffer gets written to across
+   * FFI boundaries, so this needs to happen in a separate step.
+   */
+  void written(size_t aAmount) {
+    MOZ_ASSERT(aAmount <= mBuffer.capacity());
+    mozilla::DebugOnly<bool> result = mBuffer.resizeUninitialized(aAmount);
+    MOZ_ASSERT(result);
+  }
+
+  /**
+   * Get a string view into the buffer, which is useful for test assertions.
+   */
+  std::basic_string_view<CharType> get_string_view() {
+    return std::basic_string_view<CharType>(data(), length());
+  }
+
+  /**
+   * Clear the buffer, allowing it to be re-used.
+   */
+  void clear() { mBuffer.clear(); }
+
+  /**
+   * A utility function to convert UTF-16 strings to UTF-8 strings so that they
+   * can be logged to stderr.
+   */
+  static std::string toUtf8(mozilla::Span<const char16_t> input) {
+    size_t buff_len = input.Length() * 3;
+    std::string result(buff_len, ' ');
+    result.reserve(buff_len);
+    size_t result_len =
+        ConvertUtf16toUtf8(input, mozilla::Span(result.data(), buff_len));
+    result.resize(result_len);
+    return result;
+  }
+
+  /**
+   * String buffers, especially UTF-16, do not assert nicely, and are difficult
+   * to debug. This function is verbose in that it prints the buffer contents
+   * and expected contents to stderr when they do not match.
+   *
+   * Usage:
+   *   ASSERT_TRUE(buffer.assertStringView(u"9/23/2002, 8:07:30 PM"));
+   *
+   * Here is what gtests output:
+   *
+   *   Expected equality of these values:
+   *   buffer.get_string_view()
+   *     Which is: { '0' (48, 0x30), '9' (57, 0x39), '/' (47, 0x2F), ... }
+   *   "9/23/2002, 8:07:30 PM"
+   *     Which is: 0x11600afb9
+   *
+   * Here is what this method outputs:
+   *
+   *   The buffer did not match:
+   *     Buffer:
+   *      u"9/23/2002, 8:07:30 PM"
+   *     Expected:
+   *      u"09/23/2002, 08:07:30 PM"
+   */
+  [[nodiscard]] bool verboseMatches(const CharType* aExpected) {
+    std::basic_string_view<CharType> actualSV(data(), length());
+    std::basic_string_view<CharType> expectedSV(aExpected);
+
+    if (actualSV.compare(expectedSV) == 0) {
+      return true;
+    }
+
+    static_assert(std::is_same_v<CharType, char> ||
+                  std::is_same_v<CharType, char16_t>);
+
+    std::string actual;
+    std::string expected;
+    const char* startQuote;
+
+    if constexpr (std::is_same_v<CharType, char>) {
+      actual = std::string(actualSV);
+      expected = std::string(expectedSV);
+      startQuote = "\"";
+    }
+    if constexpr (std::is_same_v<CharType, char16_t>) {
+      actual = toUtf8(actualSV);
+      expected = toUtf8(expectedSV);
+      startQuote = "u\"";
+    }
+
+    fprintf(stderr, "The buffer did not match:\n");
+    fprintf(stderr, "  Buffer:\n    %s%s\"\n", startQuote, actual.c_str());
+    fprintf(stderr, "  Expected:\n    %s%s\"\n", startQuote, expected.c_str());
+
+    return false;
+  }
+
+  Vector<C, inlineCapacity> mBuffer{};
+};
+
+}  // namespace mozilla::intl
+
+#endif
diff --git a/intl/components/gtest/TestCalendar.cpp b/intl/components/gtest/TestCalendar.cpp
new file mode 100644
index 0000000000..a40fea0027
--- /dev/null
+++ b/intl/components/gtest/TestCalendar.cpp
@@ -0,0 +1,156 @@
+/* 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 "gtest/gtest.h"
+
+#include "mozilla/intl/Calendar.h"
+#include "mozilla/intl/DateTimeFormat.h"
+#include "mozilla/Span.h"
+#include "TestBuffer.h"
+
+namespace mozilla::intl {
+
+// Firefox 1.0 release date.
+const double CALENDAR_DATE = 1032800850000.0;
+
+TEST(IntlCalendar, GetLegacyKeywordValuesForLocale)
+{
+  bool hasGregorian = false;
+  bool hasIslamic = false;
+  auto gregorian = MakeStringSpan("gregorian");
+  auto islamic = MakeStringSpan("islamic");
+  auto keywords = Calendar::GetLegacyKeywordValuesForLocale("en-US").unwrap();
+  for (auto name : keywords) {
+    // Check a few keywords, as this list may not be stable between ICU updates.
+    if (name.unwrap() == gregorian) {
+      hasGregorian = true;
+    }
+    if (name.unwrap() == islamic) {
+      hasIslamic = true;
+    }
+  }
+  ASSERT_TRUE(hasGregorian);
+  ASSERT_TRUE(hasIslamic);
+}
+
+TEST(IntlCalendar, GetBcp47KeywordValuesForLocale)
+{
+  bool hasGregory = false;
+  bool hasIslamic = false;
+  auto gregory = MakeStringSpan("gregory");
+  auto islamic = MakeStringSpan("islamic");
+  auto keywords = Calendar::GetBcp47KeywordValuesForLocale("en-US").unwrap();
+  for (auto name : keywords) {
+    // Check a few keywords, as this list may not be stable between ICU updates.
+    if (name.unwrap() == gregory) {
+      hasGregory = true;
+    }
+    if (name.unwrap() == islamic) {
+      hasIslamic = true;
+    }
+  }
+  ASSERT_TRUE(hasGregory);
+  ASSERT_TRUE(hasIslamic);
+}
+
+TEST(IntlCalendar, GetBcp47KeywordValuesForLocaleCommonlyUsed)
+{
+  bool hasGregory = false;
+  bool hasIslamic = false;
+  auto gregory = MakeStringSpan("gregory");
+  auto islamic = MakeStringSpan("islamic");
+  auto keywords = Calendar::GetBcp47KeywordValuesForLocale(
+                      "en-US", Calendar::CommonlyUsed::Yes)
+                      .unwrap();
+  for (auto name : keywords) {
+    // Check a few keywords, as this list may not be stable between ICU updates.
+    if (name.unwrap() == gregory) {
+      hasGregory = true;
+    }
+    if (name.unwrap() == islamic) {
+      hasIslamic = true;
+    }
+  }
+  ASSERT_TRUE(hasGregory);
+  ASSERT_FALSE(hasIslamic);
+}
+
+TEST(IntlCalendar, GetBcp47Type)
+{
+  auto calendar =
+      Calendar::TryCreate("en-US", Some(MakeStringSpan(u"GMT+3"))).unwrap();
+  ASSERT_STREQ(calendar->GetBcp47Type().unwrap().data(), "gregory");
+}
+
+TEST(IntlCalendar, SetTimeInMs)
+{
+  auto calendar =
+      Calendar::TryCreate("en-US", Some(MakeStringSpan(u"GMT+3"))).unwrap();
+
+  // There is no way to verify the results. Unwrap the results to ensure it
+  // doesn't fail, but don't check the values.
+  calendar->SetTimeInMs(CALENDAR_DATE).unwrap();
+}
+
+TEST(IntlCalendar, CloneFrom)
+{
+  DateTimeFormat::StyleBag style;
+  style.date = Some(DateTimeFormat::Style::Medium);
+  style.time = Some(DateTimeFormat::Style::Medium);
+  auto dtFormat = DateTimeFormat::TryCreateFromStyle(
+                      MakeStringSpan("en-US"), style,
+                      // It's ok to pass nullptr here, as it will cause format
+                      // operations to fail, but this test is only checking
+                      // calendar cloning.
+                      nullptr, Some(MakeStringSpan(u"America/Chicago")))
+                      .unwrap();
+
+  dtFormat->CloneCalendar(CALENDAR_DATE).unwrap();
+}
+
+TEST(IntlCalendar, GetWeekend)
+{
+  auto calendar_en_US = Calendar::TryCreate("en-US").unwrap();
+  auto weekend_en_US = calendar_en_US->GetWeekend().unwrap();
+  ASSERT_EQ(weekend_en_US, EnumSet({Weekday::Saturday, Weekday::Sunday}));
+
+  auto calendar_de_DE = Calendar::TryCreate("de-DE").unwrap();
+  auto weekend_de_DE = calendar_de_DE->GetWeekend().unwrap();
+  ASSERT_EQ(weekend_de_DE, EnumSet({Weekday::Saturday, Weekday::Sunday}));
+
+  auto calendar_ar_EG = Calendar::TryCreate("ar-EG").unwrap();
+  auto weekend_ar_EG = calendar_ar_EG->GetWeekend().unwrap();
+  ASSERT_EQ(weekend_ar_EG, EnumSet({Weekday::Friday, Weekday::Saturday}));
+}
+
+TEST(IntlCalendar, GetFirstDayOfWeek)
+{
+  auto calendar_en_US = Calendar::TryCreate("en-US").unwrap();
+  auto firstDayOfWeek_en_US = calendar_en_US->GetFirstDayOfWeek();
+  ASSERT_EQ(firstDayOfWeek_en_US, Weekday::Sunday);
+
+  auto calendar_de_DE = Calendar::TryCreate("de-DE").unwrap();
+  auto firstDayOfWeek_de_DE = calendar_de_DE->GetFirstDayOfWeek();
+  ASSERT_EQ(firstDayOfWeek_de_DE, Weekday::Monday);
+
+  auto calendar_ar_EG = Calendar::TryCreate("ar-EG").unwrap();
+  auto firstDayOfWeek_ar_EG = calendar_ar_EG->GetFirstDayOfWeek();
+  ASSERT_EQ(firstDayOfWeek_ar_EG, Weekday::Saturday);
+}
+
+TEST(IntlCalendar, GetMinimalDaysInFirstWeek)
+{
+  auto calendar_en_US = Calendar::TryCreate("en-US").unwrap();
+  auto minimalDays_en_US = calendar_en_US->GetMinimalDaysInFirstWeek();
+  ASSERT_EQ(minimalDays_en_US, 1);
+
+  auto calendar_de_DE = Calendar::TryCreate("de-DE").unwrap();
+  auto minimalDays_de_DE = calendar_de_DE->GetMinimalDaysInFirstWeek();
+  ASSERT_EQ(minimalDays_de_DE, 4);
+
+  auto calendar_ar_EG = Calendar::TryCreate("ar-EG").unwrap();
+  auto minimalDays_ar_EG = calendar_ar_EG->GetMinimalDaysInFirstWeek();
+  ASSERT_EQ(minimalDays_ar_EG, 1);
+}
+
+}  // namespace mozilla::intl
diff --git a/intl/components/gtest/TestCollator.cpp b/intl/components/gtest/TestCollator.cpp
new file mode 100644
index 0000000000..d19eeff089
--- /dev/null
+++ b/intl/components/gtest/TestCollator.cpp
@@ -0,0 +1,347 @@
+/* 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 "gtest/gtest.h"
+
+#include <string.h>
+#include <string_view>
+#include "mozilla/intl/Collator.h"
+#include "mozilla/Span.h"
+#include "TestBuffer.h"
+
+namespace mozilla::intl {
+
+TEST(IntlCollator, SetAttributesInternal)
+{
+  // Run through each settings to make sure MOZ_ASSERT is not triggered for
+  // misconfigured attributes.
+  auto result = Collator::TryCreate("en-US");
+  ASSERT_TRUE(result.isOk());
+  auto collator = result.unwrap();
+
+  collator->SetStrength(Collator::Strength::Primary);
+  collator->SetStrength(Collator::Strength::Secondary);
+  collator->SetStrength(Collator::Strength::Tertiary);
+  collator->SetStrength(Collator::Strength::Quaternary);
+  collator->SetStrength(Collator::Strength::Identical);
+  collator->SetStrength(Collator::Strength::Default);
+
+  collator->SetAlternateHandling(Collator::AlternateHandling::NonIgnorable)
+      .unwrap();
+  collator->SetAlternateHandling(Collator::AlternateHandling::Shifted).unwrap();
+  collator->SetAlternateHandling(Collator::AlternateHandling::Default).unwrap();
+
+  collator->SetCaseFirst(Collator::CaseFirst::False).unwrap();
+  collator->SetCaseFirst(Collator::CaseFirst::Upper).unwrap();
+  collator->SetCaseFirst(Collator::CaseFirst::Lower).unwrap();
+
+  collator->SetCaseLevel(Collator::Feature::On).unwrap();
+  collator->SetCaseLevel(Collator::Feature::Off).unwrap();
+  collator->SetCaseLevel(Collator::Feature::Default).unwrap();
+
+  collator->SetNumericCollation(Collator::Feature::On).unwrap();
+  collator->SetNumericCollation(Collator::Feature::Off).unwrap();
+  collator->SetNumericCollation(Collator::Feature::Default).unwrap();
+
+  collator->SetNormalizationMode(Collator::Feature::On).unwrap();
+  collator->SetNormalizationMode(Collator::Feature::Off).unwrap();
+  collator->SetNormalizationMode(Collator::Feature::Default).unwrap();
+}
+
+TEST(IntlCollator, GetSortKey)
+{
+  // Do some light sort key comparisons to ensure everything is wired up
+  // correctly. This is not doing extensive correctness testing.
+  auto result = Collator::TryCreate("en-US");
+  ASSERT_TRUE(result.isOk());
+  auto collator = result.unwrap();
+  TestBuffer<uint8_t> bufferA;
+  TestBuffer<uint8_t> bufferB;
+
+  auto compareSortKeys = [&](const char16_t* a, const char16_t* b) {
+    collator->GetSortKey(MakeStringSpan(a), bufferA).unwrap();
+    collator->GetSortKey(MakeStringSpan(b), bufferB).unwrap();
+    return strcmp(reinterpret_cast<const char*>(bufferA.data()),
+                  reinterpret_cast<const char*>(bufferB.data()));
+  };
+
+  ASSERT_TRUE(compareSortKeys(u"aaa", u"bbb") < 0);
+  ASSERT_TRUE(compareSortKeys(u"bbb", u"aaa") > 0);
+  ASSERT_TRUE(compareSortKeys(u"aaa", u"aaa") == 0);
+  ASSERT_TRUE(compareSortKeys(u"👍", u"👎") < 0);
+}
+
+TEST(IntlCollator, CompareStrings)
+{
+  // Do some light string comparisons to ensure everything is wired up
+  // correctly. This is not doing extensive correctness testing.
+  auto result = Collator::TryCreate("en-US");
+  ASSERT_TRUE(result.isOk());
+  auto collator = result.unwrap();
+  TestBuffer<uint8_t> bufferA;
+  TestBuffer<uint8_t> bufferB;
+
+  ASSERT_EQ(collator->CompareStrings(u"aaa", u"bbb"), -1);
+  ASSERT_EQ(collator->CompareStrings(u"bbb", u"aaa"), 1);
+  ASSERT_EQ(collator->CompareStrings(u"aaa", u"aaa"), 0);
+  ASSERT_EQ(collator->CompareStrings(u"👍", u"👎"), -1);
+}
+
+TEST(IntlCollator, SetOptionsSensitivity)
+{
+  // Test the ECMA 402 sensitivity behavior per:
+  // https://tc39.es/ecma402/#sec-collator-comparestrings
+  auto result = Collator::TryCreate("en-US");
+  ASSERT_TRUE(result.isOk());
+  auto collator = result.unwrap();
+
+  TestBuffer<uint8_t> bufferA;
+  TestBuffer<uint8_t> bufferB;
+  ICUResult optResult = Ok();
+  Collator::Options options{};
+
+  options.sensitivity = Collator::Sensitivity::Base;
+  optResult = collator->SetOptions(options);
+  ASSERT_TRUE(optResult.isOk());
+  ASSERT_EQ(collator->CompareStrings(u"a", u"b"), -1);
+  ASSERT_EQ(collator->CompareStrings(u"a", u"á"), 0);
+  ASSERT_EQ(collator->CompareStrings(u"a", u"A"), 0);
+
+  options.sensitivity = Collator::Sensitivity::Accent;
+  optResult = collator->SetOptions(options);
+  ASSERT_TRUE(optResult.isOk());
+  ASSERT_EQ(collator->CompareStrings(u"a", u"b"), -1);
+  ASSERT_EQ(collator->CompareStrings(u"a", u"á"), -1);
+  ASSERT_EQ(collator->CompareStrings(u"a", u"A"), 0);
+
+  options.sensitivity = Collator::Sensitivity::Case;
+  optResult = collator->SetOptions(options);
+  ASSERT_TRUE(optResult.isOk());
+  ASSERT_EQ(collator->CompareStrings(u"a", u"b"), -1);
+  ASSERT_EQ(collator->CompareStrings(u"a", u"á"), 0);
+  ASSERT_EQ(collator->CompareStrings(u"a", u"A"), -1);
+
+  options.sensitivity = Collator::Sensitivity::Variant;
+  optResult = collator->SetOptions(options);
+  ASSERT_TRUE(optResult.isOk());
+  ASSERT_EQ(collator->CompareStrings(u"a", u"b"), -1);
+  ASSERT_EQ(collator->CompareStrings(u"a", u"á"), -1);
+  ASSERT_EQ(collator->CompareStrings(u"a", u"A"), -1);
+}
+
+TEST(IntlCollator, LocaleSensitiveCollations)
+{
+  UniquePtr<Collator> collator = nullptr;
+  TestBuffer<uint8_t> bufferA;
+  TestBuffer<uint8_t> bufferB;
+
+  auto changeLocale = [&](const char* locale) {
+    auto result = Collator::TryCreate(locale);
+    ASSERT_TRUE(result.isOk());
+    collator = result.unwrap();
+
+    Collator::Options options{};
+    options.sensitivity = Collator::Sensitivity::Base;
+    auto optResult = collator->SetOptions(options);
+    ASSERT_TRUE(optResult.isOk());
+  };
+
+  // Swedish treats "Ö" as a separate character, which sorts after "Z".
+  changeLocale("en-US");
+  ASSERT_EQ(collator->CompareStrings(u"Österreich", u"Västervik"), -1);
+  changeLocale("sv-SE");
+  ASSERT_EQ(collator->CompareStrings(u"Österreich", u"Västervik"), 1);
+
+  // Country names in their respective scripts.
+  auto china = MakeStringSpan(u"中国");
+  auto japan = MakeStringSpan(u"日本");
+  auto korea = MakeStringSpan(u"한국");
+
+  changeLocale("en-US");
+  ASSERT_EQ(collator->CompareStrings(china, japan), -1);
+  ASSERT_EQ(collator->CompareStrings(china, korea), 1);
+  changeLocale("zh");
+  ASSERT_EQ(collator->CompareStrings(china, japan), 1);
+  ASSERT_EQ(collator->CompareStrings(china, korea), -1);
+  changeLocale("ja");
+  ASSERT_EQ(collator->CompareStrings(china, japan), -1);
+  ASSERT_EQ(collator->CompareStrings(china, korea), -1);
+  changeLocale("ko");
+  ASSERT_EQ(collator->CompareStrings(china, japan), 1);
+  ASSERT_EQ(collator->CompareStrings(china, korea), -1);
+}
+
+TEST(IntlCollator, IgnorePunctuation)
+{
+  TestBuffer<uint8_t> bufferA;
+  TestBuffer<uint8_t> bufferB;
+
+  auto result = Collator::TryCreate("en-US");
+  ASSERT_TRUE(result.isOk());
+  auto collator = result.unwrap();
+  Collator::Options options{};
+  options.ignorePunctuation = true;
+
+  auto optResult = collator->SetOptions(options);
+  ASSERT_TRUE(optResult.isOk());
+
+  ASSERT_EQ(collator->CompareStrings(u"aa", u".bb"), -1);
+
+  options.ignorePunctuation = false;
+  optResult = collator->SetOptions(options);
+  ASSERT_TRUE(optResult.isOk());
+
+  ASSERT_EQ(collator->CompareStrings(u"aa", u".bb"), 1);
+}
+
+TEST(IntlCollator, GetBcp47KeywordValuesForLocale)
+{
+  auto extsResult = Collator::GetBcp47KeywordValuesForLocale("de");
+  ASSERT_TRUE(extsResult.isOk());
+  auto extensions = extsResult.unwrap();
+
+  // Since this list is dependent on ICU, and may change between upgrades, only
+  // test a subset of the keywords.
+  auto standard = MakeStringSpan("standard");
+  auto search = MakeStringSpan("search");
+  auto phonebk = MakeStringSpan("phonebk");      // Valid BCP 47.
+  auto phonebook = MakeStringSpan("phonebook");  // Not valid BCP 47.
+  bool hasStandard = false;
+  bool hasSearch = false;
+  bool hasPhonebk = false;
+  bool hasPhonebook = false;
+
+  for (auto extensionResult : extensions) {
+    ASSERT_TRUE(extensionResult.isOk());
+    auto extension = extensionResult.unwrap();
+    hasStandard |= extension == standard;
+    hasSearch |= extension == search;
+    hasPhonebk |= extension == phonebk;
+    hasPhonebook |= extension == phonebook;
+  }
+
+  ASSERT_TRUE(hasStandard);
+  ASSERT_TRUE(hasSearch);
+  ASSERT_TRUE(hasPhonebk);
+
+  ASSERT_FALSE(hasPhonebook);  // Not valid BCP 47.
+}
+
+TEST(IntlCollator, GetBcp47KeywordValuesForLocaleCommonlyUsed)
+{
+  auto extsResult = Collator::GetBcp47KeywordValuesForLocale(
+      "fr", Collator::CommonlyUsed::Yes);
+  ASSERT_TRUE(extsResult.isOk());
+  auto extensions = extsResult.unwrap();
+
+  // Since this list is dependent on ICU, and may change between upgrades, only
+  // test a subset of the keywords.
+  auto standard = MakeStringSpan("standard");
+  auto search = MakeStringSpan("search");
+  auto phonebk = MakeStringSpan("phonebk");      // Valid BCP 47.
+  auto phonebook = MakeStringSpan("phonebook");  // Not valid BCP 47.
+  bool hasStandard = false;
+  bool hasSearch = false;
+  bool hasPhonebk = false;
+  bool hasPhonebook = false;
+
+  for (auto extensionResult : extensions) {
+    ASSERT_TRUE(extensionResult.isOk());
+    auto extension = extensionResult.unwrap();
+    hasStandard |= extension == standard;
+    hasSearch |= extension == search;
+    hasPhonebk |= extension == phonebk;
+    hasPhonebook |= extension == phonebook;
+  }
+
+  ASSERT_TRUE(hasStandard);
+  ASSERT_TRUE(hasSearch);
+
+  ASSERT_FALSE(hasPhonebk);    // Not commonly used in French.
+  ASSERT_FALSE(hasPhonebook);  // Not valid BCP 47.
+}
+
+TEST(IntlCollator, GetBcp47KeywordValues)
+{
+  auto extsResult = Collator::GetBcp47KeywordValues();
+  ASSERT_TRUE(extsResult.isOk());
+  auto extensions = extsResult.unwrap();
+
+  // Since this list is dependent on ICU, and may change between upgrades, only
+  // test a subset of the keywords.
+  auto standard = MakeStringSpan("standard");
+  auto search = MakeStringSpan("search");
+  auto phonebk = MakeStringSpan("phonebk");      // Valid BCP 47.
+  auto phonebook = MakeStringSpan("phonebook");  // Not valid BCP 47.
+  bool hasStandard = false;
+  bool hasSearch = false;
+  bool hasPhonebk = false;
+  bool hasPhonebook = false;
+
+  for (auto extensionResult : extensions) {
+    ASSERT_TRUE(extensionResult.isOk());
+    auto extension = extensionResult.unwrap();
+    hasStandard |= extension == standard;
+    hasSearch |= extension == search;
+    hasPhonebk |= extension == phonebk;
+    hasPhonebook |= extension == phonebook;
+  }
+
+  ASSERT_TRUE(hasStandard);
+  ASSERT_TRUE(hasSearch);
+  ASSERT_TRUE(hasPhonebk);
+
+  ASSERT_FALSE(hasPhonebook);  // Not valid BCP 47.
+}
+
+TEST(IntlCollator, GetAvailableLocales)
+{
+  using namespace std::literals;
+
+  int32_t english = 0;
+  int32_t german = 0;
+  int32_t chinese = 0;
+
+  // Since this list is dependent on ICU, and may change between upgrades, only
+  // test a subset of the available locales.
+  for (const char* locale : Collator::GetAvailableLocales()) {
+    if (locale == "en"sv) {
+      english++;
+    } else if (locale == "de"sv) {
+      german++;
+    } else if (locale == "zh"sv) {
+      chinese++;
+    }
+  }
+
+  // Each locale should be found exactly once.
+  ASSERT_EQ(english, 1);
+  ASSERT_EQ(german, 1);
+  ASSERT_EQ(chinese, 1);
+}
+
+TEST(IntlCollator, GetCaseFirst)
+{
+  auto result = Collator::TryCreate("en-US");
+  ASSERT_TRUE(result.isOk());
+  auto collator = result.unwrap();
+
+  auto caseFirst = collator->GetCaseFirst();
+  ASSERT_TRUE(caseFirst.isOk());
+  ASSERT_EQ(caseFirst.unwrap(), Collator::CaseFirst::False);
+
+  for (auto kf : {Collator::CaseFirst::Upper, Collator::CaseFirst::Lower,
+                  Collator::CaseFirst::False}) {
+    Collator::Options options{};
+    options.caseFirst = kf;
+
+    auto optResult = collator->SetOptions(options);
+    ASSERT_TRUE(optResult.isOk());
+
+    auto caseFirst = collator->GetCaseFirst();
+    ASSERT_TRUE(caseFirst.isOk());
+    ASSERT_EQ(caseFirst.unwrap(), kf);
+  }
+}
+
+}  // namespace mozilla::intl
diff --git a/intl/components/gtest/TestCurrency.cpp b/intl/components/gtest/TestCurrency.cpp
new file mode 100644
index 0000000000..c0afcfb326
--- /dev/null
+++ b/intl/components/gtest/TestCurrency.cpp
@@ -0,0 +1,34 @@
+/* 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 "gtest/gtest.h"
+
+#include "mozilla/intl/Currency.h"
+#include "mozilla/Span.h"
+
+namespace mozilla::intl {
+
+TEST(IntlCurrency, GetISOCurrencies)
+{
+  bool hasUSDollar = false;
+  bool hasEuro = false;
+  constexpr auto usdollar = MakeStringSpan("USD");
+  constexpr auto euro = MakeStringSpan("EUR");
+
+  auto currencies = Currency::GetISOCurrencies().unwrap();
+  for (auto currency : currencies) {
+    // Check a few currencies, as the list may not be stable between ICU
+    // updates.
+    if (currency.unwrap() == usdollar) {
+      hasUSDollar = true;
+    }
+    if (currency.unwrap() == euro) {
+      hasEuro = true;
+    }
+  }
+
+  ASSERT_TRUE(hasUSDollar);
+  ASSERT_TRUE(hasEuro);
+}
+
+}  // namespace mozilla::intl
diff --git a/intl/components/gtest/TestDateIntervalFormat.cpp b/intl/components/gtest/TestDateIntervalFormat.cpp
new file mode 100644
index 0000000000..cc4d4b9552
--- /dev/null
+++ b/intl/components/gtest/TestDateIntervalFormat.cpp
@@ -0,0 +1,200 @@
+/* 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 "gtest/gtest.h"
+
+#include "mozilla/intl/DateIntervalFormat.h"
+#include "mozilla/intl/DateTimeFormat.h"
+#include "mozilla/intl/DateTimePart.h"
+#include "mozilla/Span.h"
+
+#include "unicode/uformattedvalue.h"
+
+#include "TestBuffer.h"
+
+namespace mozilla::intl {
+
+const double DATE201901030000GMT = 1546473600000.0;
+const double DATE201901050000GMT = 1546646400000.0;
+
+TEST(IntlDateIntervalFormat, TryFormatDateTime)
+{
+  UniquePtr<DateIntervalFormat> dif =
+      DateIntervalFormat::TryCreate(MakeStringSpan("en-US"),
+                                    MakeStringSpan(u"MMddHHmm"),
+                                    MakeStringSpan(u"GMT"))
+          .unwrap();
+
+  AutoFormattedDateInterval formatted;
+
+  // Pass two same Date time, 'equal' should be true.
+  bool equal;
+  auto result = dif->TryFormatDateTime(DATE201901030000GMT, DATE201901030000GMT,
+                                       formatted, &equal);
+  ASSERT_TRUE(result.isOk());
+  ASSERT_TRUE(equal);
+
+  auto spanResult = formatted.ToSpan();
+  ASSERT_TRUE(spanResult.isOk());
+
+  ASSERT_EQ(spanResult.unwrap(), MakeStringSpan(u"01/03, 00:00"));
+
+  result = dif->TryFormatDateTime(DATE201901030000GMT, DATE201901050000GMT,
+                                  formatted, &equal);
+  ASSERT_TRUE(result.isOk());
+  ASSERT_FALSE(equal);
+
+  spanResult = formatted.ToSpan();
+  ASSERT_TRUE(spanResult.isOk());
+  ASSERT_EQ(spanResult.unwrap(),
+            MakeStringSpan(u"01/03, 00:00 – 01/05, 00:00"));
+}
+
+TEST(IntlDateIntervalFormat, TryFormatCalendar)
+{
+  auto dateTimePatternGenerator =
+      DateTimePatternGenerator::TryCreate("en").unwrap();
+
+  DateTimeFormat::ComponentsBag components;
+  components.year = Some(DateTimeFormat::Numeric::Numeric);
+  components.month = Some(DateTimeFormat::Month::TwoDigit);
+  components.day = Some(DateTimeFormat::Numeric::TwoDigit);
+  components.hour = Some(DateTimeFormat::Numeric::TwoDigit);
+  components.minute = Some(DateTimeFormat::Numeric::TwoDigit);
+  components.hour12 = Some(false);
+
+  UniquePtr<DateTimeFormat> dtFormat =
+      DateTimeFormat::TryCreateFromComponents(
+          MakeStringSpan("en-US"), components, dateTimePatternGenerator.get(),
+          Some(MakeStringSpan(u"GMT")))
+          .unwrap();
+
+  UniquePtr<DateIntervalFormat> dif =
+      DateIntervalFormat::TryCreate(MakeStringSpan("en-US"),
+                                    MakeStringSpan(u"MMddHHmm"),
+                                    MakeStringSpan(u"GMT"))
+          .unwrap();
+
+  AutoFormattedDateInterval formatted;
+
+  // Two Calendar objects with the same date time.
+  auto sameCal = dtFormat->CloneCalendar(DATE201901030000GMT);
+  ASSERT_TRUE(sameCal.isOk());
+
+  auto cal = sameCal.unwrap();
+  bool equal;
+  auto result = dif->TryFormatCalendar(*cal, *cal, formatted, &equal);
+  ASSERT_TRUE(result.isOk());
+  ASSERT_TRUE(equal);
+
+  auto spanResult = formatted.ToSpan();
+  ASSERT_TRUE(spanResult.isOk());
+  ASSERT_EQ(spanResult.unwrap(), MakeStringSpan(u"01/03, 00:00"));
+
+  auto startCal = dtFormat->CloneCalendar(DATE201901030000GMT);
+  ASSERT_TRUE(startCal.isOk());
+  auto endCal = dtFormat->CloneCalendar(DATE201901050000GMT);
+  ASSERT_TRUE(endCal.isOk());
+
+  result = dif->TryFormatCalendar(*startCal.unwrap(), *endCal.unwrap(),
+                                  formatted, &equal);
+  ASSERT_TRUE(result.isOk());
+  ASSERT_FALSE(equal);
+
+  spanResult = formatted.ToSpan();
+  ASSERT_TRUE(spanResult.isOk());
+  ASSERT_EQ(spanResult.unwrap(),
+            MakeStringSpan(u"01/03, 00:00 – 01/05, 00:00"));
+}
+
+TEST(IntlDateIntervalFormat, TryFormattedToParts)
+{
+  UniquePtr<DateIntervalFormat> dif =
+      DateIntervalFormat::TryCreate(MakeStringSpan("en-US"),
+                                    MakeStringSpan(u"MMddHHmm"),
+                                    MakeStringSpan(u"GMT"))
+          .unwrap();
+
+  AutoFormattedDateInterval formatted;
+  bool equal;
+  auto result = dif->TryFormatDateTime(DATE201901030000GMT, DATE201901050000GMT,
+                                       formatted, &equal);
+  ASSERT_TRUE(result.isOk());
+  ASSERT_FALSE(equal);
+
+  Span<const char16_t> formattedSpan = formatted.ToSpan().unwrap();
+  ASSERT_EQ(formattedSpan, MakeStringSpan(u"01/03, 00:00 – 01/05, 00:00"));
+
+  mozilla::intl::DateTimePartVector parts;
+  result = dif->TryFormattedToParts(formatted, parts);
+  ASSERT_TRUE(result.isOk());
+
+  auto getSubSpan = [formattedSpan, &parts](size_t index) {
+    size_t start = index == 0 ? 0 : parts[index - 1].mEndIndex;
+    size_t end = parts[index].mEndIndex;
+    return formattedSpan.FromTo(start, end);
+  };
+
+  ASSERT_EQ(parts[0].mType, DateTimePartType::Month);
+  ASSERT_EQ(getSubSpan(0), MakeStringSpan(u"01"));
+  ASSERT_EQ(parts[0].mSource, DateTimePartSource::StartRange);
+
+  ASSERT_EQ(parts[1].mType, DateTimePartType::Literal);
+  ASSERT_EQ(getSubSpan(1), MakeStringSpan(u"/"));
+  ASSERT_EQ(parts[1].mSource, DateTimePartSource::StartRange);
+
+  ASSERT_EQ(parts[2].mType, DateTimePartType::Day);
+  ASSERT_EQ(getSubSpan(2), MakeStringSpan(u"03"));
+  ASSERT_EQ(parts[2].mSource, DateTimePartSource::StartRange);
+
+  ASSERT_EQ(parts[3].mType, DateTimePartType::Literal);
+  ASSERT_EQ(getSubSpan(3), MakeStringSpan(u", "));
+  ASSERT_EQ(parts[3].mSource, DateTimePartSource::StartRange);
+
+  ASSERT_EQ(parts[4].mType, DateTimePartType::Hour);
+  ASSERT_EQ(getSubSpan(4), MakeStringSpan(u"00"));
+  ASSERT_EQ(parts[4].mSource, DateTimePartSource::StartRange);
+
+  ASSERT_EQ(parts[5].mType, DateTimePartType::Literal);
+  ASSERT_EQ(getSubSpan(5), MakeStringSpan(u":"));
+  ASSERT_EQ(parts[5].mSource, DateTimePartSource::StartRange);
+
+  ASSERT_EQ(parts[6].mType, DateTimePartType::Minute);
+  ASSERT_EQ(getSubSpan(6), MakeStringSpan(u"00"));
+  ASSERT_EQ(parts[6].mSource, DateTimePartSource::StartRange);
+
+  ASSERT_EQ(parts[7].mType, DateTimePartType::Literal);
+  ASSERT_EQ(getSubSpan(7), MakeStringSpan(u" – "));
+  ASSERT_EQ(parts[7].mSource, DateTimePartSource::Shared);
+
+  ASSERT_EQ(parts[8].mType, DateTimePartType::Month);
+  ASSERT_EQ(getSubSpan(8), MakeStringSpan(u"01"));
+  ASSERT_EQ(parts[8].mSource, DateTimePartSource::EndRange);
+
+  ASSERT_EQ(parts[9].mType, DateTimePartType::Literal);
+  ASSERT_EQ(getSubSpan(9), MakeStringSpan(u"/"));
+  ASSERT_EQ(parts[9].mSource, DateTimePartSource::EndRange);
+
+  ASSERT_EQ(parts[10].mType, DateTimePartType::Day);
+  ASSERT_EQ(getSubSpan(10), MakeStringSpan(u"05"));
+  ASSERT_EQ(parts[10].mSource, DateTimePartSource::EndRange);
+
+  ASSERT_EQ(parts[11].mType, DateTimePartType::Literal);
+  ASSERT_EQ(getSubSpan(11), MakeStringSpan(u", "));
+  ASSERT_EQ(parts[11].mSource, DateTimePartSource::EndRange);
+
+  ASSERT_EQ(parts[12].mType, DateTimePartType::Hour);
+  ASSERT_EQ(getSubSpan(12), MakeStringSpan(u"00"));
+  ASSERT_EQ(parts[12].mSource, DateTimePartSource::EndRange);
+
+  ASSERT_EQ(parts[13].mType, DateTimePartType::Literal);
+  ASSERT_EQ(getSubSpan(13), MakeStringSpan(u":"));
+  ASSERT_EQ(parts[13].mSource, DateTimePartSource::EndRange);
+
+  ASSERT_EQ(parts[14].mType, DateTimePartType::Minute);
+  ASSERT_EQ(getSubSpan(14), MakeStringSpan(u"00"));
+  ASSERT_EQ(parts[14].mSource, DateTimePartSource::EndRange);
+
+  ASSERT_EQ(parts.length(), 15u);
+}
+}  // namespace mozilla::intl
diff --git a/intl/components/gtest/TestDateTimeFormat.cpp b/intl/components/gtest/TestDateTimeFormat.cpp
new file mode 100644
index 0000000000..057e26ea13
--- /dev/null
+++ b/intl/components/gtest/TestDateTimeFormat.cpp
@@ -0,0 +1,621 @@
+/* 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 "gtest/gtest.h"
+
+#include "mozilla/intl/Calendar.h"
+#include "mozilla/intl/DateTimeFormat.h"
+#include "mozilla/intl/DateTimePart.h"
+#include "mozilla/intl/DateTimePatternGenerator.h"
+#include "mozilla/Span.h"
+#include "TestBuffer.h"
+
+#include <string_view>
+
+namespace mozilla::intl {
+
+// Firefox 1.0 release date.
+const double DATE = 1032800850000.0;
+
+static UniquePtr<DateTimeFormat> testStyle(
+    const char* aLocale, DateTimeFormat::StyleBag& aStyleBag) {
+  // Always specify a time zone in the tests, otherwise it will use the system
+  // time zone which can vary between test runs.
+  auto timeZone = Some(MakeStringSpan(u"GMT+3"));
+  auto gen = DateTimePatternGenerator::TryCreate("en").unwrap();
+  return DateTimeFormat::TryCreateFromStyle(MakeStringSpan(aLocale), aStyleBag,
+                                            gen.get(), timeZone)
+      .unwrap();
+}
+
+TEST(IntlDateTimeFormat, Style_enUS_utf8)
+{
+  DateTimeFormat::StyleBag style;
+  style.date = Some(DateTimeFormat::Style::Medium);
+  style.time = Some(DateTimeFormat::Style::Medium);
+
+  auto dtFormat = testStyle("en-US", style);
+  TestBuffer<char> buffer;
+  dtFormat->TryFormat(DATE, buffer).unwrap();
+
+  ASSERT_TRUE(buffer.verboseMatches("Sep 23, 2002, 8:07:30 PM"));
+}
+
+TEST(IntlDateTimeFormat, Style_enUS_utf16)
+{
+  DateTimeFormat::StyleBag style;
+  style.date = Some(DateTimeFormat::Style::Medium);
+  style.time = Some(DateTimeFormat::Style::Medium);
+
+  auto dtFormat = testStyle("en-US", style);
+  TestBuffer<char16_t> buffer;
+  dtFormat->TryFormat(DATE, buffer).unwrap();
+
+  ASSERT_TRUE(buffer.verboseMatches(u"Sep 23, 2002, 8:07:30 PM"));
+}
+
+TEST(IntlDateTimeFormat, Style_ar_utf8)
+{
+  DateTimeFormat::StyleBag style;
+  style.time = Some(DateTimeFormat::Style::Medium);
+
+  auto dtFormat = testStyle("ar", style);
+  TestBuffer<char> buffer;
+  dtFormat->TryFormat(DATE, buffer).unwrap();
+
+  ASSERT_TRUE(buffer.verboseMatches("٨:٠٧:٣٠ م"));
+}
+
+TEST(IntlDateTimeFormat, Style_ar_utf16)
+{
+  DateTimeFormat::StyleBag style;
+  style.time = Some(DateTimeFormat::Style::Medium);
+
+  auto dtFormat = testStyle("ar", style);
+  TestBuffer<char16_t> buffer;
+  dtFormat->TryFormat(DATE, buffer).unwrap();
+
+  ASSERT_TRUE(buffer.verboseMatches(u"٨:٠٧:٣٠ م"));
+}
+
+TEST(IntlDateTimeFormat, Style_enUS_fallback_to_default_styles)
+{
+  DateTimeFormat::StyleBag style;
+
+  auto dtFormat = testStyle("en-US", style);
+  TestBuffer<char> buffer;
+  dtFormat->TryFormat(DATE, buffer).unwrap();
+
+  ASSERT_TRUE(buffer.verboseMatches("Sep 23, 2002, 8:07:30 PM"));
+}
+
+TEST(IntlDateTimeFormat, Time_zone_IANA_identifier)
+{
+  auto gen = DateTimePatternGenerator::TryCreate("en").unwrap();
+
+  DateTimeFormat::StyleBag style;
+  style.date = Some(DateTimeFormat::Style::Medium);
+  style.time = Some(DateTimeFormat::Style::Medium);
+
+  auto dtFormat = DateTimeFormat::TryCreateFromStyle(
+                      MakeStringSpan("en-US"), style, gen.get(),
+                      Some(MakeStringSpan(u"America/Chicago")))
+                      .unwrap();
+  TestBuffer<char> buffer;
+  dtFormat->TryFormat(DATE, buffer).unwrap();
+  ASSERT_TRUE(buffer.verboseMatches("Sep 23, 2002, 12:07:30 PM"));
+}
+
+TEST(IntlDateTimeFormat, GetAllowedHourCycles)
+{
+  auto allowed_en_US = DateTimeFormat::GetAllowedHourCycles(
+                           MakeStringSpan("en"), Some(MakeStringSpan("US")))
+                           .unwrap();
+
+  ASSERT_TRUE(allowed_en_US.length() == 2);
+  ASSERT_EQ(allowed_en_US[0], DateTimeFormat::HourCycle::H12);
+  ASSERT_EQ(allowed_en_US[1], DateTimeFormat::HourCycle::H23);
+
+  auto allowed_de =
+      DateTimeFormat::GetAllowedHourCycles(MakeStringSpan("de"), Nothing())
+          .unwrap();
+
+  ASSERT_TRUE(allowed_de.length() == 2);
+  ASSERT_EQ(allowed_de[0], DateTimeFormat::HourCycle::H23);
+  ASSERT_EQ(allowed_de[1], DateTimeFormat::HourCycle::H12);
+}
+
+TEST(IntlDateTimePatternGenerator, GetBestPattern)
+{
+  auto gen = DateTimePatternGenerator::TryCreate("en").unwrap();
+  TestBuffer<char16_t> buffer;
+
+  gen->GetBestPattern(MakeStringSpan(u"yMd"), buffer).unwrap();
+  ASSERT_TRUE(buffer.verboseMatches(u"M/d/y"));
+}
+
+TEST(IntlDateTimePatternGenerator, GetSkeleton)
+{
+  auto gen = DateTimePatternGenerator::TryCreate("en").unwrap();
+  TestBuffer<char16_t> buffer;
+
+  DateTimePatternGenerator::GetSkeleton(MakeStringSpan(u"M/d/y"), buffer)
+      .unwrap();
+  ASSERT_TRUE(buffer.verboseMatches(u"yMd"));
+}
+
+TEST(IntlDateTimePatternGenerator, GetPlaceholderPattern)
+{
+  auto gen = DateTimePatternGenerator::TryCreate("en").unwrap();
+  auto span = gen->GetPlaceholderPattern();
+  // The default date-time pattern for 'en' locale is u"{1}, {0}".
+  ASSERT_EQ(span, MakeStringSpan(u"{1}, {0}"));
+}
+
+// A utility function to help test the DateTimeFormat::ComponentsBag.
+[[nodiscard]] bool FormatComponents(
+    TestBuffer<char16_t>& aBuffer, DateTimeFormat::ComponentsBag& aComponents,
+    Span<const char> aLocale = MakeStringSpan("en-US")) {
+  UniquePtr<DateTimePatternGenerator> gen = nullptr;
+  auto dateTimePatternGenerator =
+      DateTimePatternGenerator::TryCreate(aLocale.data()).unwrap();
+
+  auto dtFormat = DateTimeFormat::TryCreateFromComponents(
+      aLocale, aComponents, dateTimePatternGenerator.get(),
+      Some(MakeStringSpan(u"GMT+3")));
+  if (dtFormat.isErr()) {
+    fprintf(stderr, "Could not create a DateTimeFormat\n");
+    return false;
+  }
+
+  auto result = dtFormat.unwrap()->TryFormat(DATE, aBuffer);
+  if (result.isErr()) {
+    fprintf(stderr, "Could not format a DateTimeFormat\n");
+    return false;
+  }
+
+  return true;
+}
+
+TEST(IntlDateTimeFormat, Components)
+{
+  DateTimeFormat::ComponentsBag components{};
+
+  components.year = Some(DateTimeFormat::Numeric::Numeric);
+  components.month = Some(DateTimeFormat::Month::Numeric);
+  components.day = Some(DateTimeFormat::Numeric::Numeric);
+
+  components.hour = Some(DateTimeFormat::Numeric::Numeric);
+  components.minute = Some(DateTimeFormat::Numeric::TwoDigit);
+  components.second = Some(DateTimeFormat::Numeric::TwoDigit);
+
+  TestBuffer<char16_t> buffer;
+  ASSERT_TRUE(FormatComponents(buffer, components));
+  ASSERT_TRUE(buffer.verboseMatches(u"9/23/2002, 8:07:30 PM"));
+}
+
+TEST(IntlDateTimeFormat, Components_es_ES)
+{
+  DateTimeFormat::ComponentsBag components{};
+
+  components.year = Some(DateTimeFormat::Numeric::Numeric);
+  components.month = Some(DateTimeFormat::Month::Numeric);
+  components.day = Some(DateTimeFormat::Numeric::Numeric);
+
+  components.hour = Some(DateTimeFormat::Numeric::Numeric);
+  components.minute = Some(DateTimeFormat::Numeric::TwoDigit);
+  components.second = Some(DateTimeFormat::Numeric::TwoDigit);
+
+  TestBuffer<char16_t> buffer;
+  ASSERT_TRUE(FormatComponents(buffer, components, MakeStringSpan("es-ES")));
+  ASSERT_TRUE(buffer.verboseMatches(u"23/9/2002, 20:07:30"));
+}
+
+TEST(IntlDateTimeFormat, ComponentsAll)
+{
+  // Use most all of the components.
+  DateTimeFormat::ComponentsBag components{};
+
+  components.era = Some(DateTimeFormat::Text::Short);
+
+  components.year = Some(DateTimeFormat::Numeric::Numeric);
+  components.month = Some(DateTimeFormat::Month::Numeric);
+  components.day = Some(DateTimeFormat::Numeric::Numeric);
+
+  components.weekday = Some(DateTimeFormat::Text::Short);
+
+  components.hour = Some(DateTimeFormat::Numeric::Numeric);
+  components.minute = Some(DateTimeFormat::Numeric::TwoDigit);
+  components.second = Some(DateTimeFormat::Numeric::TwoDigit);
+
+  components.timeZoneName = Some(DateTimeFormat::TimeZoneName::Short);
+  components.hourCycle = Some(DateTimeFormat::HourCycle::H24);
+  components.fractionalSecondDigits = Some(3);
+
+  TestBuffer<char16_t> buffer;
+  ASSERT_TRUE(FormatComponents(buffer, components));
+  ASSERT_TRUE(buffer.verboseMatches(u"Mon, 9 23, 2002 AD, 20:07:30.000 GMT+3"));
+}
+
+TEST(IntlDateTimeFormat, ComponentsHour12Default)
+{
+  // Assert the behavior of the default "en-US" 12 hour time with day period.
+  DateTimeFormat::ComponentsBag components{};
+  components.hour = Some(DateTimeFormat::Numeric::Numeric);
+  components.minute = Some(DateTimeFormat::Numeric::Numeric);
+
+  TestBuffer<char16_t> buffer;
+  ASSERT_TRUE(FormatComponents(buffer, components));
+  ASSERT_TRUE(buffer.verboseMatches(u"8:07 PM"));
+}
+
+TEST(IntlDateTimeFormat, ComponentsHour24)
+{
+  // Test the behavior of using 24 hour time to override the default of
+  // hour 12 with a day period.
+  DateTimeFormat::ComponentsBag components{};
+  components.hour = Some(DateTimeFormat::Numeric::Numeric);
+  components.minute = Some(DateTimeFormat::Numeric::Numeric);
+  components.hour12 = Some(false);
+
+  TestBuffer<char16_t> buffer;
+  ASSERT_TRUE(FormatComponents(buffer, components));
+  ASSERT_TRUE(buffer.verboseMatches(u"20:07"));
+}
+
+TEST(IntlDateTimeFormat, ComponentsHour12DayPeriod)
+{
+  // Test the behavior of specifying a specific day period.
+  DateTimeFormat::ComponentsBag components{};
+
+  components.hour = Some(DateTimeFormat::Numeric::Numeric);
+  components.minute = Some(DateTimeFormat::Numeric::Numeric);
+  components.dayPeriod = Some(DateTimeFormat::Text::Long);
+
+  TestBuffer<char16_t> buffer;
+  ASSERT_TRUE(FormatComponents(buffer, components));
+  ASSERT_TRUE(buffer.verboseMatches(u"8:07 in the evening"));
+}
+
+const char* ToString(uint8_t b) { return "uint8_t"; }
+const char* ToString(bool b) { return b ? "true" : "false"; }
+
+template <typename T>
+const char* ToString(Maybe<T> option) {
+  if (option) {
+    if constexpr (std::is_same_v<T, bool> || std::is_same_v<T, uint8_t>) {
+      return ToString(*option);
+    } else {
+      return DateTimeFormat::ToString(*option);
+    }
+  }
+  return "Nothing";
+}
+
+template <typename T>
+[[nodiscard]] bool VerboseEquals(T expected, T actual, const char* msg) {
+  if (expected != actual) {
+    fprintf(stderr, "%s\n  Actual: %s\nExpected: %s\n", msg, ToString(actual),
+            ToString(expected));
+    return false;
+  }
+  return true;
+}
+
+// A testing utility for getting nice errors when ComponentsBags don't match.
+[[nodiscard]] bool VerboseEquals(DateTimeFormat::ComponentsBag& expected,
+                                 DateTimeFormat::ComponentsBag& actual) {
+  // clang-format off
+  return
+      VerboseEquals(expected.era, actual.era, "Components do not match: bag.era") &&
+      VerboseEquals(expected.year, actual.year, "Components do not match: bag.year") &&
+      VerboseEquals(expected.month, actual.month, "Components do not match: bag.month") &&
+      VerboseEquals(expected.day, actual.day, "Components do not match: bag.day") &&
+      VerboseEquals(expected.weekday, actual.weekday, "Components do not match: bag.weekday") &&
+      VerboseEquals(expected.hour, actual.hour, "Components do not match: bag.hour") &&
+      VerboseEquals(expected.minute, actual.minute, "Components do not match: bag.minute") &&
+      VerboseEquals(expected.second, actual.second, "Components do not match: bag.second") &&
+      VerboseEquals(expected.timeZoneName, actual.timeZoneName, "Components do not match: bag.timeZoneName") &&
+      VerboseEquals(expected.hour12, actual.hour12, "Components do not match: bag.hour12") &&
+      VerboseEquals(expected.hourCycle, actual.hourCycle, "Components do not match: bag.hourCycle") &&
+      VerboseEquals(expected.dayPeriod, actual.dayPeriod, "Components do not match: bag.dayPeriod") &&
+      VerboseEquals(expected.fractionalSecondDigits, actual.fractionalSecondDigits, "Components do not match: bag.fractionalSecondDigits");
+  // clang-format on
+}
+
+// A utility function to help test the DateTimeFormat::ComponentsBag.
+[[nodiscard]] bool ResolveComponentsBag(
+    DateTimeFormat::ComponentsBag& aComponentsIn,
+    DateTimeFormat::ComponentsBag* aComponentsOut,
+    Span<const char> aLocale = MakeStringSpan("en-US")) {
+  UniquePtr<DateTimePatternGenerator> gen = nullptr;
+  auto dateTimePatternGenerator =
+      DateTimePatternGenerator::TryCreate("en").unwrap();
+  auto dtFormat = DateTimeFormat::TryCreateFromComponents(
+      aLocale, aComponentsIn, dateTimePatternGenerator.get(),
+      Some(MakeStringSpan(u"GMT+3")));
+  if (dtFormat.isErr()) {
+    fprintf(stderr, "Could not create a DateTimeFormat\n");
+    return false;
+  }
+
+  auto result = dtFormat.unwrap()->ResolveComponents();
+  if (result.isErr()) {
+    fprintf(stderr, "Could not resolve the components\n");
+    return false;
+  }
+
+  *aComponentsOut = result.unwrap();
+  return true;
+}
+
+TEST(IntlDateTimeFormat, ResolvedComponentsDate)
+{
+  DateTimeFormat::ComponentsBag input{};
+  {
+    input.year = Some(DateTimeFormat::Numeric::Numeric);
+    input.month = Some(DateTimeFormat::Month::Numeric);
+    input.day = Some(DateTimeFormat::Numeric::Numeric);
+  }
+
+  DateTimeFormat::ComponentsBag expected = input;
+
+  DateTimeFormat::ComponentsBag resolved{};
+  ASSERT_TRUE(ResolveComponentsBag(input, &resolved));
+  ASSERT_TRUE(VerboseEquals(expected, resolved));
+}
+
+TEST(IntlDateTimeFormat, ResolvedComponentsAll)
+{
+  DateTimeFormat::ComponentsBag input{};
+  {
+    input.era = Some(DateTimeFormat::Text::Short);
+
+    input.year = Some(DateTimeFormat::Numeric::Numeric);
+    input.month = Some(DateTimeFormat::Month::Numeric);
+    input.day = Some(DateTimeFormat::Numeric::Numeric);
+
+    input.weekday = Some(DateTimeFormat::Text::Short);
+
+    input.hour = Some(DateTimeFormat::Numeric::Numeric);
+    input.minute = Some(DateTimeFormat::Numeric::TwoDigit);
+    input.second = Some(DateTimeFormat::Numeric::TwoDigit);
+
+    input.timeZoneName = Some(DateTimeFormat::TimeZoneName::Short);
+    input.hourCycle = Some(DateTimeFormat::HourCycle::H24);
+    input.fractionalSecondDigits = Some(3);
+  }
+
+  DateTimeFormat::ComponentsBag expected = input;
+  {
+    expected.hour = Some(DateTimeFormat::Numeric::TwoDigit);
+    expected.hourCycle = Some(DateTimeFormat::HourCycle::H24);
+    expected.hour12 = Some(false);
+  }
+
+  DateTimeFormat::ComponentsBag resolved{};
+  ASSERT_TRUE(ResolveComponentsBag(input, &resolved));
+  ASSERT_TRUE(VerboseEquals(expected, resolved));
+}
+
+TEST(IntlDateTimeFormat, ResolvedComponentsHourDayPeriod)
+{
+  DateTimeFormat::ComponentsBag input{};
+  {
+    input.hour = Some(DateTimeFormat::Numeric::Numeric);
+    input.minute = Some(DateTimeFormat::Numeric::Numeric);
+  }
+
+  DateTimeFormat::ComponentsBag expected = input;
+  {
+    expected.minute = Some(DateTimeFormat::Numeric::TwoDigit);
+    expected.hourCycle = Some(DateTimeFormat::HourCycle::H12);
+    expected.hour12 = Some(true);
+  }
+
+  DateTimeFormat::ComponentsBag resolved{};
+  ASSERT_TRUE(ResolveComponentsBag(input, &resolved));
+  ASSERT_TRUE(VerboseEquals(expected, resolved));
+}
+
+TEST(IntlDateTimeFormat, ResolvedComponentsHour12)
+{
+  DateTimeFormat::ComponentsBag input{};
+  {
+    input.hour = Some(DateTimeFormat::Numeric::Numeric);
+    input.minute = Some(DateTimeFormat::Numeric::Numeric);
+    input.hour12 = Some(false);
+  }
+
+  DateTimeFormat::ComponentsBag expected = input;
+  {
+    expected.hour = Some(DateTimeFormat::Numeric::TwoDigit);
+    expected.minute = Some(DateTimeFormat::Numeric::TwoDigit);
+    expected.hourCycle = Some(DateTimeFormat::HourCycle::H23);
+    expected.hour12 = Some(false);
+  }
+
+  DateTimeFormat::ComponentsBag resolved{};
+  ASSERT_TRUE(ResolveComponentsBag(input, &resolved));
+  ASSERT_TRUE(VerboseEquals(expected, resolved));
+}
+
+TEST(IntlDateTimeFormat, GetOriginalSkeleton)
+{
+  // Demonstrate that the original skeleton and the resolved skeleton can
+  // differ.
+  DateTimeFormat::ComponentsBag components{};
+  components.month = Some(DateTimeFormat::Month::Narrow);
+  components.day = Some(DateTimeFormat::Numeric::TwoDigit);
+
+  const char* locale = "zh-Hans-CN";
+  auto dateTimePatternGenerator =
+      DateTimePatternGenerator::TryCreate(locale).unwrap();
+
+  auto result = DateTimeFormat::TryCreateFromComponents(
+      MakeStringSpan(locale), components, dateTimePatternGenerator.get(),
+      Some(MakeStringSpan(u"GMT+3")));
+  ASSERT_TRUE(result.isOk());
+  auto dtFormat = result.unwrap();
+
+  TestBuffer<char16_t> originalSkeleton;
+  auto originalSkeletonResult = dtFormat->GetOriginalSkeleton(originalSkeleton);
+  ASSERT_TRUE(originalSkeletonResult.isOk());
+  ASSERT_TRUE(originalSkeleton.verboseMatches(u"MMMMMdd"));
+
+  TestBuffer<char16_t> pattern;
+  auto patternResult = dtFormat->GetPattern(pattern);
+  ASSERT_TRUE(patternResult.isOk());
+  ASSERT_TRUE(pattern.verboseMatches(u"M月dd日"));
+
+  TestBuffer<char16_t> resolvedSkeleton;
+  auto resolvedSkeletonResult = DateTimePatternGenerator::GetSkeleton(
+      Span(pattern.data(), pattern.length()), resolvedSkeleton);
+
+  ASSERT_TRUE(resolvedSkeletonResult.isOk());
+  ASSERT_TRUE(resolvedSkeleton.verboseMatches(u"Mdd"));
+}
+
+TEST(IntlDateTimeFormat, GetAvailableLocales)
+{
+  using namespace std::literals;
+
+  int32_t english = 0;
+  int32_t german = 0;
+  int32_t chinese = 0;
+
+  // Since this list is dependent on ICU, and may change between upgrades, only
+  // test a subset of the available locales.
+  for (const char* locale : DateTimeFormat::GetAvailableLocales()) {
+    if (locale == "en"sv) {
+      english++;
+    } else if (locale == "de"sv) {
+      german++;
+    } else if (locale == "zh"sv) {
+      chinese++;
+    }
+  }
+
+  // Each locale should be found exactly once.
+  ASSERT_EQ(english, 1);
+  ASSERT_EQ(german, 1);
+  ASSERT_EQ(chinese, 1);
+}
+
+TEST(IntlDateTimeFormat, TryFormatToParts)
+{
+  auto dateTimePatternGenerator =
+      DateTimePatternGenerator::TryCreate("en").unwrap();
+
+  DateTimeFormat::ComponentsBag components;
+  components.year = Some(DateTimeFormat::Numeric::Numeric);
+  components.month = Some(DateTimeFormat::Month::TwoDigit);
+  components.day = Some(DateTimeFormat::Numeric::TwoDigit);
+  components.hour = Some(DateTimeFormat::Numeric::TwoDigit);
+  components.minute = Some(DateTimeFormat::Numeric::TwoDigit);
+  components.hour12 = Some(false);
+
+  UniquePtr<DateTimeFormat> dtFormat =
+      DateTimeFormat::TryCreateFromComponents(
+          MakeStringSpan("en-US"), components, dateTimePatternGenerator.get(),
+          Some(MakeStringSpan(u"GMT")))
+          .unwrap();
+
+  TestBuffer<char16_t> buffer;
+  mozilla::intl::DateTimePartVector parts;
+  auto result = dtFormat->TryFormatToParts(DATE, buffer, parts);
+  ASSERT_TRUE(result.isOk());
+
+  std::u16string_view strView = buffer.get_string_view();
+  ASSERT_EQ(strView, u"09/23/2002, 17:07");
+
+  auto getSubStringView = [strView, &parts](size_t index) {
+    size_t pos = index == 0 ? 0 : parts[index - 1].mEndIndex;
+    size_t count = parts[index].mEndIndex - pos;
+    return strView.substr(pos, count);
+  };
+
+  ASSERT_EQ(parts[0].mType, DateTimePartType::Month);
+  ASSERT_EQ(getSubStringView(0), u"09");
+
+  ASSERT_EQ(parts[1].mType, DateTimePartType::Literal);
+  ASSERT_EQ(getSubStringView(1), u"/");
+
+  ASSERT_EQ(parts[2].mType, DateTimePartType::Day);
+  ASSERT_EQ(getSubStringView(2), u"23");
+
+  ASSERT_EQ(parts[3].mType, DateTimePartType::Literal);
+  ASSERT_EQ(getSubStringView(3), u"/");
+
+  ASSERT_EQ(parts[4].mType, DateTimePartType::Year);
+  ASSERT_EQ(getSubStringView(4), u"2002");
+
+  ASSERT_EQ(parts[5].mType, DateTimePartType::Literal);
+  ASSERT_EQ(getSubStringView(5), u", ");
+
+  ASSERT_EQ(parts[6].mType, DateTimePartType::Hour);
+  ASSERT_EQ(getSubStringView(6), u"17");
+
+  ASSERT_EQ(parts[7].mType, DateTimePartType::Literal);
+  ASSERT_EQ(getSubStringView(7), u":");
+
+  ASSERT_EQ(parts[8].mType, DateTimePartType::Minute);
+  ASSERT_EQ(getSubStringView(8), u"07");
+
+  ASSERT_EQ(parts.length(), 9u);
+}
+
+TEST(IntlDateTimeFormat, SetStartTimeIfGregorian)
+{
+  DateTimeFormat::StyleBag style{};
+  style.date = Some(DateTimeFormat::Style::Long);
+
+  auto timeZone = Some(MakeStringSpan(u"UTC"));
+
+  // Beginning of ECMAScript time.
+  constexpr double StartOfTime = -8.64e15;
+
+  // Gregorian change date defaults to October 15, 1582 in ICU. Test with a date
+  // before the default change date, in this case January 1, 1582.
+  constexpr double FirstJanuary1582 = -12244089600000.0;
+
+  // One year expressed in milliseconds.
+  constexpr double oneYear = (365 * 24 * 60 * 60) * 1000.0;
+
+  // Test with and without explicit calendar. The start time of the calendar can
+  // only be adjusted for the Gregorian and the ISO-8601 calendar.
+  for (const char* locale : {
+           "en-US",
+           "en-US-u-ca-gregory",
+           "en-US-u-ca-iso8601",
+       }) {
+    auto gen = DateTimePatternGenerator::TryCreate(locale).unwrap();
+
+    auto dtFormat = DateTimeFormat::TryCreateFromStyle(
+                        MakeStringSpan(locale), style, gen.get(), timeZone)
+                        .unwrap();
+
+    TestBuffer<char> buffer;
+
+    // Before the default Gregorian change date, so interpreted in the Julian
+    // calendar, which is December 22, 1581.
+    dtFormat->TryFormat(FirstJanuary1582, buffer).unwrap();
+    ASSERT_TRUE(buffer.verboseMatches("December 22, 1581"));
+
+    // After default Gregorian change date, so January 1, 1583.
+    dtFormat->TryFormat(FirstJanuary1582 + oneYear, buffer).unwrap();
+    ASSERT_TRUE(buffer.verboseMatches("January 1, 1583"));
+
+    // Adjust the start time to use a proleptic Gregorian calendar.
+    dtFormat->SetStartTimeIfGregorian(StartOfTime);
+
+    // Now interpreted in proleptic Gregorian calendar at January 1, 1582.
+    dtFormat->TryFormat(FirstJanuary1582, buffer).unwrap();
+    ASSERT_TRUE(buffer.verboseMatches("January 1, 1582"));
+
+    // Still January 1, 1583.
+    dtFormat->TryFormat(FirstJanuary1582 + oneYear, buffer).unwrap();
+    ASSERT_TRUE(buffer.verboseMatches("January 1, 1583"));
+  }
+}
+}  // namespace mozilla::intl
diff --git a/intl/components/gtest/TestDisplayNames.cpp b/intl/components/gtest/TestDisplayNames.cpp
new file mode 100644
index 0000000000..36980f484b
--- /dev/null
+++ b/intl/components/gtest/TestDisplayNames.cpp
@@ -0,0 +1,635 @@
+/* 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 "gtest/gtest.h"
+
+#include "TestBuffer.h"
+#include "mozilla/intl/DisplayNames.h"
+
+namespace mozilla::intl {
+
+TEST(IntlDisplayNames, Script)
+{
+  TestBuffer<char16_t> buffer;
+
+  DisplayNames::Options options{};
+  options.style = DisplayNames::Style::Long;
+
+  {
+    auto result = DisplayNames::TryCreate("en-US", options);
+    ASSERT_TRUE(result.isOk());
+    auto displayNames = result.unwrap();
+
+    ASSERT_TRUE(displayNames->GetScript(buffer, MakeStringSpan("Hans")).isOk());
+    ASSERT_TRUE(buffer.verboseMatches(u"Simplified Han"));
+
+    buffer.clear();
+    {
+      // The code is too long here.
+      auto err =
+          displayNames->GetScript(buffer, MakeStringSpan("ThisIsTooLong"));
+      ASSERT_TRUE(err.isErr());
+      ASSERT_EQ(err.unwrapErr(), DisplayNamesError::InvalidOption);
+      ASSERT_TRUE(buffer.verboseMatches(u""));
+    }
+
+    // Test fallbacking for unknown scripts.
+
+    buffer.clear();
+    ASSERT_TRUE(displayNames
+                    ->GetScript(buffer, MakeStringSpan("Fake"),
+                                DisplayNames::Fallback::None)
+                    .isOk());
+    ASSERT_TRUE(buffer.verboseMatches(u""));
+
+    buffer.clear();
+    ASSERT_TRUE(displayNames
+                    ->GetScript(buffer, MakeStringSpan("Fake"),
+                                DisplayNames::Fallback::Code)
+                    .isOk());
+    ASSERT_TRUE(buffer.verboseMatches(u"Fake"));
+  }
+
+  {
+    auto result = DisplayNames::TryCreate("es-ES", options);
+    ASSERT_TRUE(result.isOk());
+    auto displayNames = result.unwrap();
+
+    buffer.clear();
+    ASSERT_TRUE(displayNames->GetScript(buffer, MakeStringSpan("Hans")).isOk());
+    ASSERT_TRUE(buffer.verboseMatches(u"han simplificado"));
+  }
+
+  options.style = DisplayNames::Style::Short;
+  {
+    auto result = DisplayNames::TryCreate("en-US", options);
+    ASSERT_TRUE(result.isOk());
+    auto displayNames = result.unwrap();
+
+    buffer.clear();
+    ASSERT_TRUE(displayNames->GetScript(buffer, MakeStringSpan("Hans")).isOk());
+    ASSERT_TRUE(buffer.verboseMatches(u"Simplified"));
+
+    // Test fallbacking for unknown scripts.
+    buffer.clear();
+    ASSERT_TRUE(displayNames
+                    ->GetScript(buffer, MakeStringSpan("Fake"),
+                                DisplayNames::Fallback::None)
+                    .isOk());
+    ASSERT_TRUE(buffer.verboseMatches(u""));
+
+    buffer.clear();
+    ASSERT_TRUE(displayNames
+                    ->GetScript(buffer, MakeStringSpan("Fake"),
+                                DisplayNames::Fallback::Code)
+                    .isOk());
+    ASSERT_TRUE(buffer.verboseMatches(u"Fake"));
+  }
+
+  {
+    auto result = DisplayNames::TryCreate("es-ES", options);
+    ASSERT_TRUE(result.isOk());
+    auto displayNames = result.unwrap();
+
+    buffer.clear();
+    ASSERT_TRUE(displayNames->GetScript(buffer, MakeStringSpan("Hans")).isOk());
+    ASSERT_TRUE(buffer.verboseMatches(u"simplificado"));
+  }
+}
+
+TEST(IntlDisplayNames, Language)
+{
+  TestBuffer<char16_t> buffer;
+  DisplayNames::Options options{};
+
+  {
+    auto result = DisplayNames::TryCreate("en-US", options);
+    ASSERT_TRUE(result.isOk());
+    auto displayNames = result.unwrap();
+
+    ASSERT_TRUE(
+        displayNames->GetLanguage(buffer, MakeStringSpan("es-ES")).isOk());
+    ASSERT_TRUE(buffer.verboseMatches(u"Spanish (Spain)"));
+
+    buffer.clear();
+    ASSERT_TRUE(
+        displayNames->GetLanguage(buffer, MakeStringSpan("zh-Hant")).isOk());
+    ASSERT_TRUE(buffer.verboseMatches(u"Chinese (Traditional)"));
+
+    // The undefined locale returns an empty string.
+    buffer.clear();
+    ASSERT_TRUE(
+        displayNames->GetLanguage(buffer, MakeStringSpan("und")).isOk());
+    ASSERT_TRUE(buffer.get_string_view().empty());
+
+    // Invalid locales are an error.
+    buffer.clear();
+    ASSERT_EQ(
+        displayNames->GetLanguage(buffer, MakeStringSpan("asdf")).unwrapErr(),
+        DisplayNamesError::InvalidOption);
+    ASSERT_TRUE(buffer.get_string_view().empty());
+
+    // Unknown locales return an empty string.
+    buffer.clear();
+    ASSERT_TRUE(displayNames
+                    ->GetLanguage(buffer, MakeStringSpan("zz"),
+                                  DisplayNames::Fallback::None)
+                    .isOk());
+    ASSERT_TRUE(buffer.verboseMatches(u""));
+
+    // Unknown locales can fallback to the language code.
+    buffer.clear();
+    ASSERT_TRUE(displayNames
+                    ->GetLanguage(buffer, MakeStringSpan("zz-US"),
+                                  DisplayNames::Fallback::Code)
+                    .isOk());
+    ASSERT_TRUE(buffer.verboseMatches(u"zz-US"));
+
+    // Unknown locales with a unicode extension error. Is this correct?
+    buffer.clear();
+    ASSERT_TRUE(displayNames
+                    ->GetLanguage(buffer, MakeStringSpan("zz-US-u-ca-chinese"),
+                                  DisplayNames::Fallback::Code)
+                    .isErr());
+    ASSERT_TRUE(buffer.verboseMatches(u""));
+  }
+  {
+    auto result = DisplayNames::TryCreate("es-ES", options);
+    ASSERT_TRUE(result.isOk());
+    auto displayNames = result.unwrap();
+
+    buffer.clear();
+    ASSERT_TRUE(
+        displayNames->GetLanguage(buffer, MakeStringSpan("es-ES")).isOk());
+    ASSERT_TRUE(buffer.verboseMatches(u"español (España)"));
+
+    buffer.clear();
+    ASSERT_TRUE(
+        displayNames->GetLanguage(buffer, MakeStringSpan("zh-Hant")).isOk());
+    ASSERT_TRUE(buffer.verboseMatches(u"chino (tradicional)"));
+  }
+}
+
+TEST(IntlDisplayNames, Region)
+{
+  TestBuffer<char16_t> buffer;
+
+  DisplayNames::Options options{};
+  options.style = DisplayNames::Style::Long;
+
+  {
+    auto result = DisplayNames::TryCreate("en-US", options);
+    ASSERT_TRUE(result.isOk());
+    auto displayNames = result.unwrap();
+
+    ASSERT_TRUE(displayNames->GetRegion(buffer, MakeStringSpan("US")).isOk());
+    ASSERT_TRUE(buffer.verboseMatches(u"United States"));
+
+    ASSERT_TRUE(displayNames->GetRegion(buffer, MakeStringSpan("ES")).isOk());
+    ASSERT_TRUE(buffer.verboseMatches(u"Spain"));
+
+    ASSERT_TRUE(displayNames
+                    ->GetRegion(buffer, MakeStringSpan("ZX"),
+                                DisplayNames::Fallback::None)
+                    .isOk());
+    ASSERT_TRUE(buffer.verboseMatches(u""));
+
+    ASSERT_TRUE(displayNames
+                    ->GetRegion(buffer, MakeStringSpan("ZX"),
+                                DisplayNames::Fallback::Code)
+                    .isOk());
+    ASSERT_TRUE(buffer.verboseMatches(u"ZX"));
+  }
+  {
+    auto result = DisplayNames::TryCreate("es-ES", options);
+    ASSERT_TRUE(result.isOk());
+    auto displayNames = result.unwrap();
+
+    ASSERT_TRUE(displayNames->GetRegion(buffer, MakeStringSpan("US")).isOk());
+    ASSERT_TRUE(buffer.verboseMatches(u"Estados Unidos"));
+
+    ASSERT_TRUE(displayNames->GetRegion(buffer, MakeStringSpan("ES")).isOk());
+    ASSERT_TRUE(buffer.verboseMatches(u"España"));
+  }
+}
+
+TEST(IntlDisplayNames, Currency)
+{
+  TestBuffer<char16_t> buffer;
+
+  DisplayNames::Options options{};
+  options.style = DisplayNames::Style::Long;
+
+  auto result = DisplayNames::TryCreate("en-US", options);
+  ASSERT_TRUE(result.isOk());
+  auto displayNames = result.unwrap();
+  ASSERT_TRUE(displayNames->GetCurrency(buffer, MakeStringSpan("EUR")).isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"Euro"));
+
+  ASSERT_TRUE(displayNames->GetCurrency(buffer, MakeStringSpan("USD")).isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"US Dollar"));
+
+  ASSERT_TRUE(displayNames
+                  ->GetCurrency(buffer, MakeStringSpan("moz"),
+                                DisplayNames::Fallback::None)
+                  .isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u""));
+
+  ASSERT_TRUE(displayNames
+                  ->GetCurrency(buffer, MakeStringSpan("moz"),
+                                DisplayNames::Fallback::Code)
+                  .isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"MOZ"));
+
+  // Invalid options.
+  {
+    // Code with fewer than 3 characters.
+    auto err = displayNames->GetCurrency(buffer, MakeStringSpan("US"));
+    ASSERT_TRUE(err.isErr());
+    ASSERT_EQ(err.unwrapErr(), DisplayNamesError::InvalidOption);
+  }
+  {
+    // Code with more than 3 characters.
+    auto err = displayNames->GetCurrency(buffer, MakeStringSpan("USDDDDDDD"));
+    ASSERT_TRUE(err.isErr());
+    ASSERT_EQ(err.unwrapErr(), DisplayNamesError::InvalidOption);
+  }
+  {
+    // Code with non-ascii alpha letters/
+    auto err = displayNames->GetCurrency(buffer, MakeStringSpan("US1"));
+    ASSERT_TRUE(err.isErr());
+    ASSERT_EQ(err.unwrapErr(), DisplayNamesError::InvalidOption);
+  }
+}
+
+TEST(IntlDisplayNames, Calendar)
+{
+  TestBuffer<char16_t> buffer;
+
+  DisplayNames::Options options{};
+  auto result = DisplayNames::TryCreate("en-US", options);
+  ASSERT_TRUE(result.isOk());
+  auto displayNames = result.unwrap();
+
+  ASSERT_TRUE(
+      displayNames->GetCalendar(buffer, MakeStringSpan("buddhist")).isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"Buddhist Calendar"));
+
+  ASSERT_TRUE(
+      displayNames->GetCalendar(buffer, MakeStringSpan("gregory")).isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"Gregorian Calendar"));
+
+  ASSERT_TRUE(
+      displayNames->GetCalendar(buffer, MakeStringSpan("GREGORY")).isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"Gregorian Calendar"));
+
+  {
+    // Code with non-ascii alpha letters.
+    auto err = displayNames->GetCalendar(buffer, MakeStringSpan("🥸 not ascii"));
+    ASSERT_TRUE(err.isErr());
+    ASSERT_EQ(err.unwrapErr(), DisplayNamesError::InvalidOption);
+  }
+  {
+    // Empty string.
+    auto err = displayNames->GetCalendar(buffer, MakeStringSpan(""));
+    ASSERT_TRUE(err.isErr());
+    ASSERT_EQ(err.unwrapErr(), DisplayNamesError::InvalidOption);
+  }
+  {
+    // Non-valid ascii.
+    auto err = displayNames->GetCalendar(
+        buffer, MakeStringSpan("ascii-but_not(valid)1234"));
+    ASSERT_TRUE(err.isErr());
+    ASSERT_EQ(err.unwrapErr(), DisplayNamesError::InvalidOption);
+  }
+
+  // Test fallbacking.
+
+  ASSERT_TRUE(displayNames
+                  ->GetCalendar(buffer, MakeStringSpan("moz"),
+                                DisplayNames::Fallback::None)
+                  .isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u""));
+
+  ASSERT_TRUE(displayNames
+                  ->GetCalendar(buffer, MakeStringSpan("moz"),
+                                DisplayNames::Fallback::Code)
+                  .isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"moz"));
+
+  ASSERT_TRUE(displayNames
+                  ->GetCalendar(buffer, MakeStringSpan("MOZ"),
+                                DisplayNames::Fallback::Code)
+                  .isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"moz"));
+}
+
+TEST(IntlDisplayNames, Weekday)
+{
+  TestBuffer<char16_t> buffer;
+
+  DisplayNames::Options options{};
+  Span<const char> calendar{};
+  auto result = DisplayNames::TryCreate("en-US", options);
+  ASSERT_TRUE(result.isOk());
+  auto displayNames = result.unwrap();
+
+  ASSERT_TRUE(
+      displayNames->GetWeekday(buffer, Weekday::Monday, calendar).isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"Monday"));
+  ASSERT_TRUE(
+      displayNames->GetWeekday(buffer, Weekday::Tuesday, calendar).isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"Tuesday"));
+  ASSERT_TRUE(
+      displayNames->GetWeekday(buffer, Weekday::Wednesday, calendar).isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"Wednesday"));
+  ASSERT_TRUE(
+      displayNames->GetWeekday(buffer, Weekday::Thursday, calendar).isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"Thursday"));
+  ASSERT_TRUE(
+      displayNames->GetWeekday(buffer, Weekday::Friday, calendar).isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"Friday"));
+  ASSERT_TRUE(
+      displayNames->GetWeekday(buffer, Weekday::Saturday, calendar).isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"Saturday"));
+  ASSERT_TRUE(
+      displayNames->GetWeekday(buffer, Weekday::Sunday, calendar).isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"Sunday"));
+}
+
+TEST(IntlDisplayNames, WeekdaySpanish)
+{
+  TestBuffer<char16_t> buffer;
+
+  DisplayNames::Options options{};
+  Span<const char> calendar{};
+  auto result = DisplayNames::TryCreate("es-ES", options);
+  ASSERT_TRUE(result.isOk());
+  auto displayNames = result.unwrap();
+
+  ASSERT_TRUE(
+      displayNames->GetWeekday(buffer, Weekday::Monday, calendar).isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"lunes"));
+  ASSERT_TRUE(
+      displayNames->GetWeekday(buffer, Weekday::Tuesday, calendar).isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"martes"));
+  ASSERT_TRUE(
+      displayNames->GetWeekday(buffer, Weekday::Wednesday, calendar).isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"miércoles"));
+  ASSERT_TRUE(
+      displayNames->GetWeekday(buffer, Weekday::Thursday, calendar).isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"jueves"));
+  ASSERT_TRUE(
+      displayNames->GetWeekday(buffer, Weekday::Friday, calendar).isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"viernes"));
+  ASSERT_TRUE(
+      displayNames->GetWeekday(buffer, Weekday::Saturday, calendar).isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"sábado"));
+  ASSERT_TRUE(
+      displayNames->GetWeekday(buffer, Weekday::Sunday, calendar).isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"domingo"));
+}
+
+TEST(IntlDisplayNames, Month)
+{
+  TestBuffer<char16_t> buffer;
+
+  DisplayNames::Options options{};
+  Span<const char> calendar{};
+  auto result = DisplayNames::TryCreate("en-US", options);
+  ASSERT_TRUE(result.isOk());
+  auto displayNames = result.unwrap();
+
+  ASSERT_TRUE(displayNames->GetMonth(buffer, Month::January, calendar).isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"January"));
+  ASSERT_TRUE(displayNames->GetMonth(buffer, Month::February, calendar).isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"February"));
+  ASSERT_TRUE(displayNames->GetMonth(buffer, Month::March, calendar).isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"March"));
+  ASSERT_TRUE(displayNames->GetMonth(buffer, Month::April, calendar).isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"April"));
+  ASSERT_TRUE(displayNames->GetMonth(buffer, Month::May, calendar).isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"May"));
+  ASSERT_TRUE(displayNames->GetMonth(buffer, Month::June, calendar).isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"June"));
+  ASSERT_TRUE(displayNames->GetMonth(buffer, Month::July, calendar).isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"July"));
+  ASSERT_TRUE(displayNames->GetMonth(buffer, Month::August, calendar).isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"August"));
+  ASSERT_TRUE(
+      displayNames->GetMonth(buffer, Month::September, calendar).isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"September"));
+  ASSERT_TRUE(displayNames->GetMonth(buffer, Month::October, calendar).isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"October"));
+  ASSERT_TRUE(displayNames->GetMonth(buffer, Month::November, calendar).isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"November"));
+  ASSERT_TRUE(displayNames->GetMonth(buffer, Month::December, calendar).isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"December"));
+  ASSERT_TRUE(
+      displayNames->GetMonth(buffer, Month::Undecimber, calendar).isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u""));
+}
+
+TEST(IntlDisplayNames, MonthHebrew)
+{
+  TestBuffer<char16_t> buffer;
+
+  DisplayNames::Options options{};
+  Span<const char> calendar{};
+  auto result = DisplayNames::TryCreate("en-u-ca-hebrew", options);
+  ASSERT_TRUE(result.isOk());
+  auto displayNames = result.unwrap();
+
+  ASSERT_TRUE(displayNames->GetMonth(buffer, Month::January, calendar).isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"Tishri"));
+  ASSERT_TRUE(displayNames->GetMonth(buffer, Month::February, calendar).isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"Heshvan"));
+  ASSERT_TRUE(displayNames->GetMonth(buffer, Month::March, calendar).isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"Kislev"));
+  ASSERT_TRUE(displayNames->GetMonth(buffer, Month::April, calendar).isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"Tevet"));
+  ASSERT_TRUE(displayNames->GetMonth(buffer, Month::May, calendar).isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"Shevat"));
+  ASSERT_TRUE(displayNames->GetMonth(buffer, Month::June, calendar).isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"Adar I"));
+  ASSERT_TRUE(displayNames->GetMonth(buffer, Month::July, calendar).isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"Adar"));
+  ASSERT_TRUE(displayNames->GetMonth(buffer, Month::August, calendar).isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"Nisan"));
+  ASSERT_TRUE(
+      displayNames->GetMonth(buffer, Month::September, calendar).isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"Iyar"));
+  ASSERT_TRUE(displayNames->GetMonth(buffer, Month::October, calendar).isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"Sivan"));
+  ASSERT_TRUE(displayNames->GetMonth(buffer, Month::November, calendar).isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"Tamuz"));
+  ASSERT_TRUE(displayNames->GetMonth(buffer, Month::December, calendar).isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"Av"));
+  ASSERT_TRUE(
+      displayNames->GetMonth(buffer, Month::Undecimber, calendar).isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"Elul"));
+}
+
+TEST(IntlDisplayNames, MonthCalendarOption)
+{
+  TestBuffer<char16_t> buffer;
+
+  {
+    // No calendar.
+    DisplayNames::Options options{};
+    Span<const char> calendar{};
+    auto result = DisplayNames::TryCreate("en", options);
+    ASSERT_TRUE(result.isOk());
+    auto displayNames = result.unwrap();
+
+    ASSERT_TRUE(
+        displayNames->GetMonth(buffer, Month::January, calendar).isOk());
+    ASSERT_TRUE(buffer.verboseMatches(u"January"));
+  }
+  {
+    // Switch to hebrew.
+    DisplayNames::Options options{};
+    Span<const char> calendar = MakeStringSpan("hebrew");
+    auto result = DisplayNames::TryCreate("en", options);
+    ASSERT_TRUE(result.isOk());
+    auto displayNames = result.unwrap();
+
+    ASSERT_TRUE(
+        displayNames->GetMonth(buffer, Month::January, calendar).isOk());
+    ASSERT_TRUE(buffer.verboseMatches(u"Tishri"));
+  }
+  {
+    // Conflicting tags.
+    DisplayNames::Options options{};
+    Span<const char> calendar = MakeStringSpan("hebrew");
+    auto result = DisplayNames::TryCreate("en-u-ca-gregory", options);
+    ASSERT_TRUE(result.isOk());
+    auto displayNames = result.unwrap();
+
+    ASSERT_TRUE(
+        displayNames->GetMonth(buffer, Month::January, calendar).isOk());
+    ASSERT_TRUE(buffer.verboseMatches(u"Tishri"));
+  }
+  {
+    // Conflicting tags.
+    DisplayNames::Options options{};
+    Span<const char> calendar = MakeStringSpan("gregory");
+    auto result = DisplayNames::TryCreate("en-u-ca-hebrew", options);
+    ASSERT_TRUE(result.isOk());
+    auto displayNames = result.unwrap();
+
+    ASSERT_TRUE(
+        displayNames->GetMonth(buffer, Month::January, calendar).isOk());
+    ASSERT_TRUE(buffer.verboseMatches(u"January"));
+  }
+}
+
+TEST(IntlDisplayNames, Quarter)
+{
+  TestBuffer<char16_t> buffer;
+
+  DisplayNames::Options options{};
+  Span<const char> calendar{};
+  auto result = DisplayNames::TryCreate("en-US", options);
+  ASSERT_TRUE(result.isOk());
+  auto displayNames = result.unwrap();
+
+  ASSERT_TRUE(displayNames->GetQuarter(buffer, Quarter::Q1, calendar).isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"1st quarter"));
+  ASSERT_TRUE(displayNames->GetQuarter(buffer, Quarter::Q2, calendar).isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"2nd quarter"));
+  ASSERT_TRUE(displayNames->GetQuarter(buffer, Quarter::Q3, calendar).isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"3rd quarter"));
+  ASSERT_TRUE(displayNames->GetQuarter(buffer, Quarter::Q4, calendar).isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"4th quarter"));
+}
+
+TEST(IntlDisplayNames, DayPeriod_en_US)
+{
+  TestBuffer<char16_t> buffer;
+
+  DisplayNames::Options options{};
+  Span<const char> calendar{};
+  auto result = DisplayNames::TryCreate("en-US", options);
+  ASSERT_TRUE(result.isOk());
+  auto displayNames = result.unwrap();
+
+  ASSERT_TRUE(
+      displayNames->GetDayPeriod(buffer, DayPeriod::AM, calendar).isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"AM"));
+  ASSERT_TRUE(
+      displayNames->GetDayPeriod(buffer, DayPeriod::PM, calendar).isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"PM"));
+}
+
+TEST(IntlDisplayNames, DayPeriod_ar)
+{
+  TestBuffer<char16_t> buffer;
+  DisplayNames::Options options{};
+  Span<const char> calendar{};
+  auto result = DisplayNames::TryCreate("ar", options);
+  ASSERT_TRUE(result.isOk());
+  auto displayNames = result.unwrap();
+
+  ASSERT_TRUE(
+      displayNames->GetDayPeriod(buffer, DayPeriod::AM, calendar).isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"ص"));
+  ASSERT_TRUE(
+      displayNames->GetDayPeriod(buffer, DayPeriod::PM, calendar).isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"م"));
+}
+
+TEST(IntlDisplayNames, DateTimeField)
+{
+  TestBuffer<char16_t> buffer;
+
+  DisplayNames::Options options{};
+  Span<const char> locale = MakeStringSpan("en-US");
+  auto result = DisplayNames::TryCreate(locale.data(), options);
+  ASSERT_TRUE(result.isOk());
+  auto displayNames = result.unwrap();
+  auto gen = DateTimePatternGenerator::TryCreate(locale.data()).unwrap();
+
+  ASSERT_TRUE(
+      displayNames->GetDateTimeField(buffer, DateTimeField::Year, *gen).isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"year"));
+  ASSERT_TRUE(
+      displayNames->GetDateTimeField(buffer, DateTimeField::Quarter, *gen)
+          .isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"quarter"));
+  ASSERT_TRUE(displayNames->GetDateTimeField(buffer, DateTimeField::Month, *gen)
+                  .isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"month"));
+  ASSERT_TRUE(
+      displayNames->GetDateTimeField(buffer, DateTimeField::WeekOfYear, *gen)
+          .isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"week"));
+  ASSERT_TRUE(
+      displayNames->GetDateTimeField(buffer, DateTimeField::Weekday, *gen)
+          .isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"day of the week"));
+  ASSERT_TRUE(
+      displayNames->GetDateTimeField(buffer, DateTimeField::Day, *gen).isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"day"));
+  ASSERT_TRUE(
+      displayNames->GetDateTimeField(buffer, DateTimeField::DayPeriod, *gen)
+          .isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"AM/PM"));
+  ASSERT_TRUE(
+      displayNames->GetDateTimeField(buffer, DateTimeField::Hour, *gen).isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"hour"));
+  ASSERT_TRUE(
+      displayNames->GetDateTimeField(buffer, DateTimeField::Minute, *gen)
+          .isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"minute"));
+  ASSERT_TRUE(
+      displayNames->GetDateTimeField(buffer, DateTimeField::Second, *gen)
+          .isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"second"));
+  ASSERT_TRUE(
+      displayNames->GetDateTimeField(buffer, DateTimeField::TimeZoneName, *gen)
+          .isOk());
+  ASSERT_TRUE(buffer.verboseMatches(u"time zone"));
+}
+
+}  // namespace mozilla::intl
diff --git a/intl/components/gtest/TestIDNA.cpp b/intl/components/gtest/TestIDNA.cpp
new file mode 100644
index 0000000000..e1c7a501bc
--- /dev/null
+++ b/intl/components/gtest/TestIDNA.cpp
@@ -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/. */
+#include "gtest/gtest.h"
+
+#include "mozilla/intl/IDNA.h"
+#include "mozilla/Span.h"
+#include "TestBuffer.h"
+
+namespace mozilla::intl {
+
+TEST(IntlIDNA, LabelToUnicodeBasic)
+{
+  auto createResult = IDNA::TryCreate(IDNA::ProcessingType::NonTransitional);
+  ASSERT_TRUE(createResult.isOk());
+  auto idna = createResult.unwrap();
+
+  // 'A' and 'ª' are mapped to 'a'.
+  TestBuffer<char16_t> buf16;
+  auto convertResult = idna->LabelToUnicode(MakeStringSpan(u"Aa\u00aa"), buf16);
+  ASSERT_TRUE(convertResult.isOk());
+  intl::IDNA::Info info = convertResult.unwrap();
+  ASSERT_TRUE(!info.HasErrors());
+  ASSERT_EQ(buf16.get_string_view(), u"aaa");
+
+  buf16.clear();
+  // For nontransitional processing 'ß' is still mapped to 'ß'.
+  convertResult = idna->LabelToUnicode(MakeStringSpan(u"Faß"), buf16);
+  ASSERT_TRUE(convertResult.isOk());
+  info = convertResult.unwrap();
+  ASSERT_TRUE(!info.HasErrors());
+  ASSERT_EQ(buf16.get_string_view(), u"faß");
+}
+
+TEST(IntlIDNA, LabelToUnicodeBasicTransitional)
+{
+  auto createResult = IDNA::TryCreate(IDNA::ProcessingType::Transitional);
+  ASSERT_TRUE(createResult.isOk());
+  auto idna = createResult.unwrap();
+
+  TestBuffer<char16_t> buf16;
+  // For transitional processing 'ß' will be mapped to 'ss'.
+  auto convertResult = idna->LabelToUnicode(MakeStringSpan(u"Faß"), buf16);
+  ASSERT_TRUE(convertResult.isOk());
+  intl::IDNA::Info info = convertResult.unwrap();
+  ASSERT_TRUE(!info.HasErrors());
+  ASSERT_EQ(buf16.get_string_view(), u"fass");
+}
+
+TEST(IntlIDNA, LabelToUnicodeHasErrors)
+{
+  auto createResult = IDNA::TryCreate(IDNA::ProcessingType::NonTransitional);
+  ASSERT_TRUE(createResult.isOk());
+  auto idna = createResult.unwrap();
+  TestBuffer<char16_t> buf16;
+  // \u0378 is a reserved charactor, conversion should be disallowed.
+  auto convertResult = idna->LabelToUnicode(MakeStringSpan(u"\u0378"), buf16);
+  ASSERT_TRUE(convertResult.isOk());
+  intl::IDNA::Info info = convertResult.unwrap();
+  ASSERT_TRUE(info.HasErrors());
+
+  buf16.clear();
+  // FULL STOP '.' is not allowed.
+  convertResult = idna->LabelToUnicode(MakeStringSpan(u"a.b"), buf16);
+  ASSERT_TRUE(convertResult.isOk());
+  info = convertResult.unwrap();
+  ASSERT_TRUE(info.HasErrors());
+}
+
+TEST(IntlIDNA, LabelToUnicodeHasInvalidPunycode)
+{
+  auto createResult = IDNA::TryCreate(IDNA::ProcessingType::NonTransitional);
+  ASSERT_TRUE(createResult.isOk());
+  auto idna = createResult.unwrap();
+  TestBuffer<char16_t> buf16;
+  auto convertResult =
+      idna->LabelToUnicode(MakeStringSpan(u"xn--a-ecp.ru"), buf16);
+  ASSERT_TRUE(convertResult.isOk());
+  intl::IDNA::Info info = convertResult.unwrap();
+  ASSERT_TRUE(info.HasInvalidPunycode());
+
+  buf16.clear();
+  convertResult = idna->LabelToUnicode(MakeStringSpan(u"xn--0.pt"), buf16);
+  ASSERT_TRUE(convertResult.isOk());
+  info = convertResult.unwrap();
+  ASSERT_TRUE(info.HasInvalidPunycode());
+}
+
+TEST(IntlIDNA, LabelToUnicodeHasInvalidHyphen)
+{
+  auto createResult = IDNA::TryCreate(IDNA::ProcessingType::NonTransitional);
+  ASSERT_TRUE(createResult.isOk());
+  auto idna = createResult.unwrap();
+  TestBuffer<char16_t> buf16;
+
+  // Leading hyphen.
+  auto convertResult = idna->LabelToUnicode(MakeStringSpan(u"-a"), buf16);
+  ASSERT_TRUE(convertResult.isOk());
+  intl::IDNA::Info info = convertResult.unwrap();
+  ASSERT_TRUE(info.HasErrors());
+  ASSERT_TRUE(info.HasInvalidHyphen());
+
+  buf16.clear();
+  // Trailing hyphen.
+  convertResult = idna->LabelToUnicode(MakeStringSpan(u"a-"), buf16);
+  ASSERT_TRUE(convertResult.isOk());
+  info = convertResult.unwrap();
+  ASSERT_TRUE(info.HasInvalidHyphen());
+
+  buf16.clear();
+  // Contains hyphens in both 3rd and 4th positions.
+  convertResult = idna->LabelToUnicode(MakeStringSpan(u"ab--c"), buf16);
+  ASSERT_TRUE(convertResult.isOk());
+  info = convertResult.unwrap();
+  ASSERT_TRUE(info.HasInvalidHyphen());
+}
+
+}  // namespace mozilla::intl
diff --git a/intl/components/gtest/TestListFormat.cpp b/intl/components/gtest/TestListFormat.cpp
new file mode 100644
index 0000000000..85759d4985
--- /dev/null
+++ b/intl/components/gtest/TestListFormat.cpp
@@ -0,0 +1,162 @@
+/* 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 "gtest/gtest.h"
+
+#include "mozilla/intl/ListFormat.h"
+#include "mozilla/Span.h"
+#include "TestBuffer.h"
+
+namespace mozilla::intl {
+
+// Test ListFormat.format with default options.
+TEST(IntlListFormat, FormatDefault)
+{
+  ListFormat::Options options;
+  UniquePtr<ListFormat> lf =
+      ListFormat::TryCreate(MakeStringSpan("en-US"), options).unwrap();
+  ListFormat::StringList list;
+  MOZ_RELEASE_ASSERT(list.append(MakeStringSpan(u"Alice")));
+  MOZ_RELEASE_ASSERT(list.append(MakeStringSpan(u"Bob")));
+  MOZ_RELEASE_ASSERT(list.append(MakeStringSpan(u"Charlie")));
+  TestBuffer<char16_t> buf16;
+  ASSERT_TRUE(lf->Format(list, buf16).isOk());
+  ASSERT_EQ(buf16.get_string_view(), u"Alice, Bob, and Charlie");
+
+  UniquePtr<ListFormat> lfDe =
+      ListFormat::TryCreate(MakeStringSpan("de"), options).unwrap();
+  ASSERT_TRUE(lfDe->Format(list, buf16).isOk());
+  ASSERT_EQ(buf16.get_string_view(), u"Alice, Bob und Charlie");
+}
+
+// Test ListFormat.format with Type::Conjunction and other styles.
+TEST(IntlListFormat, FormatConjunction)
+{
+  ListFormat::Options options{ListFormat::Type::Conjunction,
+                              ListFormat::Style::Narrow};
+  UniquePtr<ListFormat> lf =
+      ListFormat::TryCreate(MakeStringSpan("en-US"), options).unwrap();
+  ListFormat::StringList list;
+  MOZ_RELEASE_ASSERT(list.append(MakeStringSpan(u"Alice")));
+  MOZ_RELEASE_ASSERT(list.append(MakeStringSpan(u"Bob")));
+  MOZ_RELEASE_ASSERT(list.append(MakeStringSpan(u"Charlie")));
+  TestBuffer<char16_t> buf16;
+  ASSERT_TRUE(lf->Format(list, buf16).isOk());
+  ASSERT_EQ(buf16.get_string_view(), u"Alice, Bob, Charlie");
+
+  ListFormat::Options optionsSh{ListFormat::Type::Conjunction,
+                                ListFormat::Style::Short};
+  UniquePtr<ListFormat> lfSh =
+      ListFormat::TryCreate(MakeStringSpan("en-US"), optionsSh).unwrap();
+  ASSERT_TRUE(lfSh->Format(list, buf16).isOk());
+  ASSERT_EQ(buf16.get_string_view(), u"Alice, Bob, & Charlie");
+}
+
+// Test ListFormat.format with Type::Disjunction.
+TEST(IntlListFormat, FormatDisjunction)
+{
+  // When Type is Disjunction, the results will be the same regardless of the
+  // style for most locales, so simply test with Style::Long.
+  ListFormat::Options options{ListFormat::Type::Disjunction,
+                              ListFormat::Style::Long};
+  UniquePtr<ListFormat> lf =
+      ListFormat::TryCreate(MakeStringSpan("en-US"), options).unwrap();
+  ListFormat::StringList list;
+  MOZ_RELEASE_ASSERT(list.append(MakeStringSpan(u"Alice")));
+  MOZ_RELEASE_ASSERT(list.append(MakeStringSpan(u"Bob")));
+  MOZ_RELEASE_ASSERT(list.append(MakeStringSpan(u"Charlie")));
+  TestBuffer<char16_t> buf16;
+  ASSERT_TRUE(lf->Format(list, buf16).isOk());
+  ASSERT_EQ(buf16.get_string_view(), u"Alice, Bob, or Charlie");
+}
+
+// Test ListFormat.format with Type::Unit.
+TEST(IntlListFormat, FormatUnit)
+{
+  ListFormat::Options options{ListFormat::Type::Unit, ListFormat::Style::Long};
+  // For locale "en", Style::Long and Style::Short have the same result, so just
+  // test Style::Long here.
+  UniquePtr<ListFormat> lf =
+      ListFormat::TryCreate(MakeStringSpan("en-US"), options).unwrap();
+  ListFormat::StringList list;
+  MOZ_RELEASE_ASSERT(list.append(MakeStringSpan(u"Alice")));
+  MOZ_RELEASE_ASSERT(list.append(MakeStringSpan(u"Bob")));
+  MOZ_RELEASE_ASSERT(list.append(MakeStringSpan(u"Charlie")));
+  TestBuffer<char16_t> buf16;
+  ASSERT_TRUE(lf->Format(list, buf16).isOk());
+  ASSERT_EQ(buf16.get_string_view(), u"Alice, Bob, Charlie");
+
+  ListFormat::Options optionsNa{ListFormat::Type::Unit,
+                                ListFormat::Style::Narrow};
+  UniquePtr<ListFormat> lfNa =
+      ListFormat::TryCreate(MakeStringSpan("en-US"), optionsNa).unwrap();
+  ASSERT_TRUE(lfNa->Format(list, buf16).isOk());
+  ASSERT_EQ(buf16.get_string_view(), u"Alice Bob Charlie");
+}
+
+// Pass a long list (list.length() > DEFAULT_LIST_LENGTH) and check the result
+// is still correct. (result.length > INITIAL_CHAR_BUFFER_SIZE)
+TEST(IntlListFormat, FormatBufferLength)
+{
+  ListFormat::Options options;
+  UniquePtr<ListFormat> lf =
+      ListFormat::TryCreate(MakeStringSpan("en-US"), options).unwrap();
+  ListFormat::StringList list;
+  MOZ_RELEASE_ASSERT(list.append(MakeStringSpan(u"Alice")));
+  MOZ_RELEASE_ASSERT(list.append(MakeStringSpan(u"Bob")));
+  MOZ_RELEASE_ASSERT(list.append(MakeStringSpan(u"Charlie")));
+  MOZ_RELEASE_ASSERT(list.append(MakeStringSpan(u"David")));
+  MOZ_RELEASE_ASSERT(list.append(MakeStringSpan(u"Eve")));
+  MOZ_RELEASE_ASSERT(list.append(MakeStringSpan(u"Frank")));
+  MOZ_RELEASE_ASSERT(list.append(MakeStringSpan(u"Grace")));
+  MOZ_RELEASE_ASSERT(list.append(MakeStringSpan(u"Heidi")));
+  MOZ_RELEASE_ASSERT(list.append(MakeStringSpan(u"Ivan")));
+  TestBuffer<char16_t> buf16;
+  ASSERT_TRUE(lf->Format(list, buf16).isOk());
+  ASSERT_EQ(buf16.get_string_view(),
+            u"Alice, Bob, Charlie, David, Eve, Frank, Grace, Heidi, and Ivan");
+}
+
+TEST(IntlListFormat, FormatToParts)
+{
+  ListFormat::Options options;
+  UniquePtr<ListFormat> lf =
+      ListFormat::TryCreate(MakeStringSpan("en-US"), options).unwrap();
+  ListFormat::StringList list;
+  MOZ_RELEASE_ASSERT(list.append(MakeStringSpan(u"Alice")));
+  MOZ_RELEASE_ASSERT(list.append(MakeStringSpan(u"Bob")));
+  MOZ_RELEASE_ASSERT(list.append(MakeStringSpan(u"Charlie")));
+
+  TestBuffer<char16_t> buf16;
+  mozilla::intl::ListFormat::PartVector parts;
+  ASSERT_TRUE(lf->FormatToParts(list, buf16, parts).isOk());
+
+  std::u16string_view strView = buf16.get_string_view();
+  ASSERT_EQ(strView, u"Alice, Bob, and Charlie");
+
+  // 3 elements, and 2 literals.
+  ASSERT_EQ((parts.length()), (5u));
+
+  auto getSubStringView = [strView, &parts](size_t index) {
+    size_t pos = index == 0 ? 0 : parts[index - 1].second;
+    size_t count = parts[index].second - pos;
+    return strView.substr(pos, count);
+  };
+
+  ASSERT_EQ(parts[0].first, ListFormat::PartType::Element);
+  ASSERT_EQ(getSubStringView(0), u"Alice");
+
+  ASSERT_EQ(parts[1].first, ListFormat::PartType::Literal);
+  ASSERT_EQ(getSubStringView(1), u", ");
+
+  ASSERT_EQ(parts[2].first, ListFormat::PartType::Element);
+  ASSERT_EQ(getSubStringView(2), u"Bob");
+
+  ASSERT_EQ(parts[3].first, ListFormat::PartType::Literal);
+  ASSERT_EQ(getSubStringView(3), u", and ");
+
+  ASSERT_EQ(parts[4].first, ListFormat::PartType::Element);
+  ASSERT_EQ(getSubStringView(4), u"Charlie");
+}
+
+}  // namespace mozilla::intl
diff --git a/intl/components/gtest/TestLocale.cpp b/intl/components/gtest/TestLocale.cpp
new file mode 100644
index 0000000000..e4cc6a093b
--- /dev/null
+++ b/intl/components/gtest/TestLocale.cpp
@@ -0,0 +1,152 @@
+/* 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 "gtest/gtest.h"
+
+#include "mozilla/intl/Locale.h"
+#include "mozilla/Span.h"
+
+#include "TestBuffer.h"
+
+namespace mozilla::intl {
+
+TEST(IntlLocale, LocaleSettersAndGetters)
+{
+  Locale locale;
+  locale.SetLanguage("fr");
+  locale.SetRegion("CA");
+  locale.SetScript("Latn");
+  ASSERT_TRUE(
+      locale.SetUnicodeExtension(MakeStringSpan("u-ca-gregory")).isOk());
+  ASSERT_TRUE(locale.Language().EqualTo("fr"));
+  ASSERT_TRUE(locale.Region().EqualTo("CA"));
+  ASSERT_TRUE(locale.Script().EqualTo("Latn"));
+  ASSERT_EQ(locale.GetUnicodeExtension().value(),
+            MakeStringSpan("u-ca-gregory"));
+
+  TestBuffer<char> buffer;
+  ASSERT_TRUE(locale.ToString(buffer).isOk());
+  ASSERT_TRUE(buffer.verboseMatches("fr-Latn-CA-u-ca-gregory"));
+
+  // No setters for variants or other extensions...
+  Locale locale2;
+  ASSERT_TRUE(LocaleParser::TryParse(
+                  MakeStringSpan("fr-CA-fonipa-t-es-AR-h0-hybrid"), locale2)
+                  .isOk());
+  ASSERT_EQ(locale2.Variants()[0], MakeStringSpan("fonipa"));
+  ASSERT_EQ(locale2.Extensions()[0], MakeStringSpan("t-es-AR-h0-hybrid"));
+  locale2.ClearVariants();
+  ASSERT_EQ(locale2.Variants().length(), 0UL);
+}
+
+TEST(IntlLocale, LocaleMove)
+{
+  Locale locale;
+  ASSERT_TRUE(
+      LocaleParser::TryParse(
+          MakeStringSpan(
+              "fr-Latn-CA-fonipa-u-ca-gregory-t-es-AR-h0-hybrid-x-private"),
+          locale)
+          .isOk());
+
+  ASSERT_TRUE(locale.Language().EqualTo("fr"));
+  ASSERT_TRUE(locale.Script().EqualTo("Latn"));
+  ASSERT_TRUE(locale.Region().EqualTo("CA"));
+  ASSERT_EQ(locale.Variants()[0], MakeStringSpan("fonipa"));
+  ASSERT_EQ(locale.Extensions()[0], MakeStringSpan("u-ca-gregory"));
+  ASSERT_EQ(locale.Extensions()[1], MakeStringSpan("t-es-AR-h0-hybrid"));
+  ASSERT_EQ(locale.GetUnicodeExtension().value(),
+            MakeStringSpan("u-ca-gregory"));
+  ASSERT_EQ(locale.PrivateUse().value(), MakeStringSpan("x-private"));
+
+  Locale locale2 = std::move(locale);
+
+  ASSERT_TRUE(locale2.Language().EqualTo("fr"));
+  ASSERT_TRUE(locale2.Script().EqualTo("Latn"));
+  ASSERT_TRUE(locale2.Region().EqualTo("CA"));
+  ASSERT_EQ(locale2.Variants()[0], MakeStringSpan("fonipa"));
+  ASSERT_EQ(locale2.Extensions()[0], MakeStringSpan("u-ca-gregory"));
+  ASSERT_EQ(locale2.Extensions()[1], MakeStringSpan("t-es-AR-h0-hybrid"));
+  ASSERT_EQ(locale2.GetUnicodeExtension().value(),
+            MakeStringSpan("u-ca-gregory"));
+  ASSERT_EQ(locale2.PrivateUse().value(), MakeStringSpan("x-private"));
+}
+
+TEST(IntlLocale, LocaleParser)
+{
+  const char* tags[] = {
+      "en-US",       "en-GB",       "es-AR",      "it",         "zh-Hans-CN",
+      "de-AT",       "pl",          "fr-FR",      "de-AT",      "sr-Cyrl-SR",
+      "nb-NO",       "fr-FR",       "mk",         "uk",         "und-PL",
+      "und-Latn-AM", "ug-Cyrl",     "sr-ME",      "mn-Mong",    "lif-Limb",
+      "gan",         "zh-Hant",     "yue-Hans",   "unr",        "unr-Deva",
+      "und-Thai-CN", "ug-Cyrl",     "en-Latn-DE", "pl-FR",      "de-CH",
+      "tuq",         "sr-ME",       "ng",         "klx",        "kk-Arab",
+      "en-Cyrl",     "und-Cyrl-UK", "und-Arab",   "und-Arab-FO"};
+
+  for (const auto* tag : tags) {
+    Locale locale;
+    ASSERT_TRUE(LocaleParser::TryParse(MakeStringSpan(tag), locale).isOk());
+  }
+}
+
+TEST(IntlLocale, LikelySubtags)
+{
+  Locale locale;
+  ASSERT_TRUE(LocaleParser::TryParse(MakeStringSpan("zh"), locale).isOk());
+  ASSERT_TRUE(locale.AddLikelySubtags().isOk());
+  TestBuffer<char> buffer;
+  ASSERT_TRUE(locale.ToString(buffer).isOk());
+  ASSERT_TRUE(buffer.verboseMatches("zh-Hans-CN"));
+  ASSERT_TRUE(locale.RemoveLikelySubtags().isOk());
+  buffer.clear();
+  ASSERT_TRUE(locale.ToString(buffer).isOk());
+  ASSERT_TRUE(buffer.verboseMatches("zh"));
+}
+
+TEST(IntlLocale, Canonicalize)
+{
+  Locale locale;
+  ASSERT_TRUE(
+      LocaleParser::TryParse(MakeStringSpan("nob-bokmal"), locale).isOk());
+  ASSERT_TRUE(locale.Canonicalize().isOk());
+  TestBuffer<char> buffer;
+  ASSERT_TRUE(locale.ToString(buffer).isOk());
+  ASSERT_TRUE(buffer.verboseMatches("nb"));
+}
+
+// These tests are dependent on the machine that this test is being run on.
+TEST(IntlLocale, SystemDependentTests)
+{
+  // e.g. "en_US"
+  const char* locale = Locale::GetDefaultLocale();
+  ASSERT_TRUE(locale != nullptr);
+}
+
+TEST(IntlLocale, GetAvailableLocales)
+{
+  using namespace std::literals;
+
+  int32_t english = 0;
+  int32_t german = 0;
+  int32_t chinese = 0;
+
+  // Since this list is dependent on ICU, and may change between upgrades, only
+  // test a subset of the available locales.
+  for (const char* locale : Locale::GetAvailableLocales()) {
+    if (locale == "en"sv) {
+      english++;
+    } else if (locale == "de"sv) {
+      german++;
+    } else if (locale == "zh"sv) {
+      chinese++;
+    }
+  }
+
+  // Each locale should be found exactly once.
+  ASSERT_EQ(english, 1);
+  ASSERT_EQ(german, 1);
+  ASSERT_EQ(chinese, 1);
+}
+
+}  // namespace mozilla::intl
diff --git a/intl/components/gtest/TestLocaleCanonicalizer.cpp b/intl/components/gtest/TestLocaleCanonicalizer.cpp
new file mode 100644
index 0000000000..de1d7d1353
--- /dev/null
+++ b/intl/components/gtest/TestLocaleCanonicalizer.cpp
@@ -0,0 +1,60 @@
+/* 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 "gtest/gtest.h"
+
+#include "mozilla/intl/LocaleCanonicalizer.h"
+#include "mozilla/Span.h"
+
+namespace mozilla::intl {
+
+static void CheckLocaleResult(LocaleCanonicalizer::Vector& ascii,
+                              const char* before, const char* after) {
+  auto result = LocaleCanonicalizer::CanonicalizeICULevel1(before, ascii);
+  ASSERT_TRUE(result.isOk());
+  ASSERT_EQ(Span(const_cast<const char*>(ascii.begin()), ascii.length()),
+            MakeStringSpan(after));
+}
+
+/**
+ * Asserts the behavior of canonicalization as defined in:
+ * http://userguide.icu-project.org/locale#TOC-Canonicalization
+ */
+TEST(IntlLocaleCanonicalizer, CanonicalizeICULevel1)
+{
+  LocaleCanonicalizer::Vector ascii{};
+
+  // Canonicalizes en-US
+  CheckLocaleResult(ascii, "en-US", "en_US");
+  // Canonicalizes POSIX
+  CheckLocaleResult(ascii, "en-US-posix", "en_US_POSIX");
+  // und gets changed to an empty string
+  CheckLocaleResult(ascii, "und", "");
+  // retains incorrect locales
+  CheckLocaleResult(ascii, "asdf", "asdf");
+  // makes text uppercase
+  CheckLocaleResult(ascii, "es-es", "es_ES");
+  // Converts 3 letter country codes to 2 letter.
+  CheckLocaleResult(ascii, "en-USA", "en_US");
+  // Does not perform level 2 canonicalization where the result would be
+  // fr_FR@currency=EUR
+  CheckLocaleResult(ascii, "fr-fr@EURO", "fr_FR_EURO");
+  // Removes the .utf8 ends
+  CheckLocaleResult(ascii, "ar-MA.utf8", "ar_MA");
+
+  // Allows valid ascii inputs
+  CheckLocaleResult(
+      ascii,
+      "abcdefghijlkmnopqrstuvwxyzABCDEFGHIJLKMNOPQRSTUVWXYZ-_.0123456789",
+      "abcdefghijlkmnopqrstuvwxyzabcdefghijlkmnopqrstuvwxyz__");
+  CheckLocaleResult(ascii, "exotic ascii:
", "exotic ascii:
");
+
+  // Does not accept non-ascii inputs.
+  ASSERT_EQ(LocaleCanonicalizer::CanonicalizeICULevel1("👍", ascii).unwrapErr(),
+            ICUError::InternalError);
+  ASSERT_EQ(
+      LocaleCanonicalizer::CanonicalizeICULevel1("ᏣᎳᎩ", ascii).unwrapErr(),
+      ICUError::InternalError);
+}
+
+}  // namespace mozilla::intl
diff --git a/intl/components/gtest/TestMeasureUnit.cpp b/intl/components/gtest/TestMeasureUnit.cpp
new file mode 100644
index 0000000000..377ad9dc02
--- /dev/null
+++ b/intl/components/gtest/TestMeasureUnit.cpp
@@ -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/. */
+#include "gtest/gtest.h"
+
+#include "mozilla/intl/MeasureUnit.h"
+#include "mozilla/Span.h"
+
+namespace mozilla::intl {
+
+TEST(IntlMeasureUnit, GetAvailable)
+{
+  auto units = MeasureUnit::GetAvailable();
+  ASSERT_TRUE(units.isOk());
+
+  // Test a subset of the installed measurement units.
+  auto gigabyte = MakeStringSpan("gigabyte");
+  auto liter = MakeStringSpan("liter");
+  auto meter = MakeStringSpan("meter");
+  auto meters = MakeStringSpan("meters");  // Plural "meters" is invalid.
+
+  bool hasGigabyte = false;
+  bool hasLiter = false;
+  bool hasMeter = false;
+  bool hasMeters = false;
+
+  for (auto unit : units.unwrap()) {
+    ASSERT_TRUE(unit.isOk());
+    auto span = unit.unwrap();
+
+    hasGigabyte |= span == gigabyte;
+    hasLiter |= span == liter;
+    hasMeter |= span == meter;
+    hasMeters |= span == meters;
+  }
+
+  ASSERT_TRUE(hasGigabyte);
+  ASSERT_TRUE(hasLiter);
+  ASSERT_TRUE(hasMeter);
+  ASSERT_FALSE(hasMeters);
+}
+
+}  // namespace mozilla::intl
diff --git a/intl/components/gtest/TestNumberFormat.cpp b/intl/components/gtest/TestNumberFormat.cpp
new file mode 100644
index 0000000000..e13ca7fc36
--- /dev/null
+++ b/intl/components/gtest/TestNumberFormat.cpp
@@ -0,0 +1,236 @@
+/* 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 "gtest/gtest.h"
+
+#include "mozilla/intl/NumberFormat.h"
+#include "TestBuffer.h"
+
+#include <string_view>
+
+namespace mozilla {
+namespace intl {
+
+TEST(IntlNumberFormat, Basic)
+{
+  NumberFormatOptions options;
+  UniquePtr<NumberFormat> nf =
+      NumberFormat::TryCreate("en-US", options).unwrap();
+  TestBuffer<char> buf8;
+  ASSERT_TRUE(nf->format(1234.56, buf8).isOk());
+  ASSERT_EQ(buf8.get_string_view(), "1,234.56");
+  TestBuffer<char16_t> buf16;
+  ASSERT_TRUE(nf->format(1234.56, buf16).isOk());
+  ASSERT_EQ(buf16.get_string_view(), u"1,234.56");
+  const char16_t* res16 = nf->format(1234.56).unwrap().data();
+  ASSERT_TRUE(res16 != nullptr);
+  ASSERT_EQ(std::u16string_view(res16), u"1,234.56");
+
+  UniquePtr<NumberFormat> nfAr =
+      NumberFormat::TryCreate("ar", options).unwrap();
+  ASSERT_TRUE(nfAr->format(1234.56, buf8).isOk());
+  ASSERT_EQ(buf8.get_string_view(), "١٬٢٣٤٫٥٦");
+  ASSERT_TRUE(nfAr->format(1234.56, buf16).isOk());
+  ASSERT_EQ(buf16.get_string_view(), u"١٬٢٣٤٫٥٦");
+  res16 = nfAr->format(1234.56).unwrap().data();
+  ASSERT_TRUE(res16 != nullptr);
+  ASSERT_EQ(std::u16string_view(res16), u"١٬٢٣٤٫٥٦");
+}
+
+TEST(IntlNumberFormat, Numbers)
+{
+  NumberFormatOptions options;
+  UniquePtr<NumberFormat> nf =
+      NumberFormat::TryCreate("es-ES", options).unwrap();
+  TestBuffer<char> buf8;
+  ASSERT_TRUE(nf->format(123456.789, buf8).isOk());
+  ASSERT_EQ(buf8.get_string_view(), "123.456,789");
+  TestBuffer<char16_t> buf16;
+  ASSERT_TRUE(nf->format(123456.789, buf16).isOk());
+  ASSERT_EQ(buf16.get_string_view(), u"123.456,789");
+
+  const char16_t* res = nf->format(123456.789).unwrap().data();
+  ASSERT_TRUE(res != nullptr);
+  ASSERT_EQ(std::u16string_view(res), u"123.456,789");
+}
+
+TEST(IntlNumberFormat, SignificantDigits)
+{
+  NumberFormatOptions options;
+  options.mSignificantDigits = Some(std::make_pair(3, 5));
+  UniquePtr<NumberFormat> nf =
+      NumberFormat::TryCreate("es-ES", options).unwrap();
+  TestBuffer<char> buf8;
+  ASSERT_TRUE(nf->format(123456.789, buf8).isOk());
+  ASSERT_EQ(buf8.get_string_view(), "123.460");
+  ASSERT_TRUE(nf->format(0.7, buf8).isOk());
+  ASSERT_EQ(buf8.get_string_view(), "0,700");
+}
+
+TEST(IntlNumberFormat, Currency)
+{
+  NumberFormatOptions options;
+  options.mCurrency =
+      Some(std::make_pair("MXN", NumberFormatOptions::CurrencyDisplay::Symbol));
+  UniquePtr<NumberFormat> nf =
+      NumberFormat::TryCreate("es-MX", options).unwrap();
+  TestBuffer<char> buf8;
+  ASSERT_TRUE(nf->format(123456.789, buf8).isOk());
+  ASSERT_EQ(buf8.get_string_view(), "$123,456.79");
+  TestBuffer<char16_t> buf16;
+  ASSERT_TRUE(nf->format(123456.789, buf16).isOk());
+  ASSERT_EQ(buf16.get_string_view(), u"$123,456.79");
+  const char16_t* res = nf->format(123456.789).unwrap().data();
+  ASSERT_TRUE(res != nullptr);
+  ASSERT_EQ(std::u16string_view(res), u"$123,456.79");
+}
+
+TEST(IntlNumberFormat, Unit)
+{
+  NumberFormatOptions options;
+  options.mUnit = Some(std::make_pair("meter-per-second",
+                                      NumberFormatOptions::UnitDisplay::Long));
+  UniquePtr<NumberFormat> nf =
+      NumberFormat::TryCreate("es-MX", options).unwrap();
+  TestBuffer<char> buf8;
+  ASSERT_TRUE(nf->format(12.34, buf8).isOk());
+  ASSERT_EQ(buf8.get_string_view(), "12.34 metros por segundo");
+  TestBuffer<char16_t> buf16;
+  ASSERT_TRUE(nf->format(12.34, buf16).isOk());
+  ASSERT_EQ(buf16.get_string_view(), u"12.34 metros por segundo");
+  const char16_t* res = nf->format(12.34).unwrap().data();
+  ASSERT_TRUE(res != nullptr);
+  ASSERT_EQ(std::u16string_view(res), u"12.34 metros por segundo");
+
+  // Create a string view into a longer string and make sure everything works
+  // correctly.
+  const char* unit = "meter-per-second-with-some-trailing-garbage";
+  options.mUnit = Some(std::make_pair(std::string_view(unit, 5),
+                                      NumberFormatOptions::UnitDisplay::Long));
+  UniquePtr<NumberFormat> nf2 =
+      NumberFormat::TryCreate("es-MX", options).unwrap();
+  res = nf2->format(12.34).unwrap().data();
+  ASSERT_TRUE(res != nullptr);
+  ASSERT_EQ(std::u16string_view(res), u"12.34 metros");
+
+  options.mUnit = Some(std::make_pair(std::string_view(unit, 16),
+                                      NumberFormatOptions::UnitDisplay::Long));
+  UniquePtr<NumberFormat> nf3 =
+      NumberFormat::TryCreate("es-MX", options).unwrap();
+  res = nf3->format(12.34).unwrap().data();
+  ASSERT_TRUE(res != nullptr);
+  ASSERT_EQ(std::u16string_view(res), u"12.34 metros por segundo");
+}
+
+TEST(IntlNumberFormat, RoundingMode)
+{
+  NumberFormatOptions options;
+  options.mFractionDigits = Some(std::make_pair(0, 2));
+  options.mStripTrailingZero = true;
+  options.mRoundingIncrement = 5;
+  options.mRoundingMode = NumberFormatOptions::RoundingMode::Ceil;
+
+  UniquePtr<NumberFormat> nf = NumberFormat::TryCreate("en", options).unwrap();
+
+  const char16_t* res16 = nf->format(1.92).unwrap().data();
+  ASSERT_TRUE(res16 != nullptr);
+  ASSERT_EQ(std::u16string_view(res16), u"1.95");
+
+  res16 = nf->format(1.96).unwrap().data();
+  ASSERT_TRUE(res16 != nullptr);
+  ASSERT_EQ(std::u16string_view(res16), u"2");
+}
+
+TEST(IntlNumberFormat, Grouping)
+{
+  NumberFormatOptions options;
+  options.mGrouping = NumberFormatOptions::Grouping::Min2;
+
+  UniquePtr<NumberFormat> nf = NumberFormat::TryCreate("en", options).unwrap();
+
+  const char16_t* res16 = nf->format(1'000.0).unwrap().data();
+  ASSERT_TRUE(res16 != nullptr);
+  ASSERT_EQ(std::u16string_view(res16), u"1000");
+
+  res16 = nf->format(10'000.0).unwrap().data();
+  ASSERT_TRUE(res16 != nullptr);
+  ASSERT_EQ(std::u16string_view(res16), u"10,000");
+}
+
+TEST(IntlNumberFormat, RoundingPriority)
+{
+  NumberFormatOptions options;
+  options.mFractionDigits = Some(std::make_pair(2, 2));
+  options.mSignificantDigits = Some(std::make_pair(1, 2));
+  options.mRoundingPriority =
+      NumberFormatOptions::RoundingPriority::LessPrecision;
+
+  UniquePtr<NumberFormat> nf1 = NumberFormat::TryCreate("en", options).unwrap();
+
+  const char16_t* res16 = nf1->format(4.321).unwrap().data();
+  ASSERT_TRUE(res16 != nullptr);
+  ASSERT_EQ(std::u16string_view(res16), u"4.3");
+
+  options.mRoundingPriority =
+      NumberFormatOptions::RoundingPriority::MorePrecision;
+
+  UniquePtr<NumberFormat> nf2 = NumberFormat::TryCreate("en", options).unwrap();
+
+  res16 = nf2->format(4.321).unwrap().data();
+  ASSERT_TRUE(res16 != nullptr);
+  ASSERT_EQ(std::u16string_view(res16), u"4.32");
+}
+
+TEST(IntlNumberFormat, FormatToParts)
+{
+  NumberFormatOptions options;
+  UniquePtr<NumberFormat> nf =
+      NumberFormat::TryCreate("es-ES", options).unwrap();
+  NumberPartVector parts;
+  const char16_t* res = nf->formatToParts(123456.789, parts).unwrap().data();
+  ASSERT_TRUE(res != nullptr);
+  ASSERT_EQ(std::u16string_view(res), u"123.456,789");
+  ASSERT_EQ(parts.length(), 5U);
+
+  // NumberFormat only ever produces number parts with NumberPartSource::Shared.
+
+  ASSERT_EQ(parts[0],
+            (NumberPart{NumberPartType::Integer, NumberPartSource::Shared, 3}));
+  ASSERT_EQ(parts[1],
+            (NumberPart{NumberPartType::Group, NumberPartSource::Shared, 4}));
+  ASSERT_EQ(parts[2],
+            (NumberPart{NumberPartType::Integer, NumberPartSource::Shared, 7}));
+  ASSERT_EQ(parts[3],
+            (NumberPart{NumberPartType::Decimal, NumberPartSource::Shared, 8}));
+  ASSERT_EQ(parts[4], (NumberPart{NumberPartType::Fraction,
+                                  NumberPartSource::Shared, 11}));
+}
+
+TEST(IntlNumberFormat, GetAvailableLocales)
+{
+  using namespace std::literals;
+
+  int32_t english = 0;
+  int32_t german = 0;
+  int32_t chinese = 0;
+
+  // Since this list is dependent on ICU, and may change between upgrades, only
+  // test a subset of the available locales.
+  for (const char* locale : NumberFormat::GetAvailableLocales()) {
+    if (locale == "en"sv) {
+      english++;
+    } else if (locale == "de"sv) {
+      german++;
+    } else if (locale == "zh"sv) {
+      chinese++;
+    }
+  }
+
+  // Each locale should be found exactly once.
+  ASSERT_EQ(english, 1);
+  ASSERT_EQ(german, 1);
+  ASSERT_EQ(chinese, 1);
+}
+
+}  // namespace intl
+}  // namespace mozilla
diff --git a/intl/components/gtest/TestNumberParser.cpp b/intl/components/gtest/TestNumberParser.cpp
new file mode 100644
index 0000000000..3995295913
--- /dev/null
+++ b/intl/components/gtest/TestNumberParser.cpp
@@ -0,0 +1,34 @@
+/* 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 "gtest/gtest.h"
+
+#include "mozilla/intl/NumberParser.h"
+
+namespace mozilla::intl {
+
+TEST(IntlNumberParser, Basic)
+{
+  // Try with an English locale
+  UniquePtr<NumberParser> np = NumberParser::TryCreate("en-US", true).unwrap();
+  auto result = np->ParseDouble(MakeStringSpan(u"1,234.56"));
+  ASSERT_TRUE(result.isOk());
+  ASSERT_EQ(result.unwrap().first, 1234.56);
+  ASSERT_EQ(result.unwrap().second, 8);
+
+  // Disable grouping, parsing will stop at the first comma
+  np = NumberParser::TryCreate("en-US", false).unwrap();
+  result = np->ParseDouble(MakeStringSpan(u"1,234.56"));
+  ASSERT_TRUE(result.isOk());
+  ASSERT_EQ(result.unwrap().first, 1);
+  ASSERT_EQ(result.unwrap().second, 1);
+
+  // Try with a Spanish locale
+  np = NumberParser::TryCreate("es-CR", true).unwrap();
+  result = np->ParseDouble(MakeStringSpan(u"1234,56"));
+  ASSERT_TRUE(result.isOk());
+  ASSERT_EQ(result.unwrap().first, 1234.56);
+  ASSERT_EQ(result.unwrap().second, 7);
+}
+
+}  // namespace mozilla::intl
diff --git a/intl/components/gtest/TestNumberRangeFormat.cpp b/intl/components/gtest/TestNumberRangeFormat.cpp
new file mode 100644
index 0000000000..f26212e3d5
--- /dev/null
+++ b/intl/components/gtest/TestNumberRangeFormat.cpp
@@ -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/. */
+#include "gtest/gtest.h"
+
+#include <string_view>
+
+#include "mozilla/intl/NumberRangeFormat.h"
+#include "./TestBuffer.h"
+
+namespace mozilla {
+namespace intl {
+
+using namespace std::literals;
+
+TEST(IntlNumberRangeFormat, Basic)
+{
+  NumberRangeFormatOptions options;
+  UniquePtr<NumberRangeFormat> nf =
+      NumberRangeFormat::TryCreate("en-US", options).unwrap();
+
+  const char16_t* res16 = nf->format(1234.56, 1234.56).unwrap().data();
+  ASSERT_TRUE(res16 != nullptr);
+  ASSERT_EQ(res16, u"1,234.56"sv);
+
+  options.mRangeIdentityFallback = NumberRangeFormatOptions::Approximately;
+  nf = std::move(NumberRangeFormat::TryCreate("en-US", options).unwrap());
+
+  res16 = nf->format("1234.56", "1234.56").unwrap().data();
+  ASSERT_TRUE(res16 != nullptr);
+  ASSERT_EQ(res16, u"~1,234.56"sv);
+
+  res16 = nf->format("1234.56", "2999.89").unwrap().data();
+  ASSERT_TRUE(res16 != nullptr);
+  ASSERT_EQ(res16, u"1,234.56–2,999.89"sv);
+
+  nf = std::move(NumberRangeFormat::TryCreate("ar", options).unwrap());
+
+  res16 = nfAr->format(1234.56, 1234.56).unwrap().data();
+  ASSERT_TRUE(res16 != nullptr);
+  ASSERT_EQ(res16, u"~١٬٢٣٤٫٥٦"sv);
+
+  res16 = nfAr->format(1234.56, 2999.89).unwrap().data();
+  ASSERT_TRUE(res16 != nullptr);
+  ASSERT_EQ(res16, u"١٬٢٣٤٫٥٦–٢٬٩٩٩٫٨٩"sv);
+}
+
+TEST(IntlNumberRangeFormat, Currency)
+{
+  NumberRangeFormatOptions options;
+  options.mCurrency = Some(
+      std::make_pair("MXN", NumberRangeFormatOptions::CurrencyDisplay::Symbol));
+  UniquePtr<NumberRangeFormat> nf =
+      NumberRangeFormat::TryCreate("es-MX", options).unwrap();
+
+  const char16_t* res = nf->format(123456.789, 299999.89).unwrap().data();
+  ASSERT_TRUE(res != nullptr);
+  ASSERT_EQ(std::u16string_view(res), u"$123,456.79 - $299,999.89"sv);
+
+  options.mCurrency = Some(
+      std::make_pair("EUR", NumberRangeFormatOptions::CurrencyDisplay::Symbol));
+  nf = std::move(NumberRangeFormat::TryCreate("fr", options).unwrap());
+
+  res = nf->format(123456.789, 299999.89).unwrap().data();
+  ASSERT_TRUE(res != nullptr);
+  ASSERT_EQ(std::u16string_view(res), u"123 456,79–299 999,89 €"sv);
+}
+
+TEST(IntlNumberRangeFormat, Unit)
+{
+  NumberRangeFormatOptions options;
+  options.mUnit = Some(std::make_pair(
+      "meter-per-second", NumberRangeFormatOptions::UnitDisplay::Long));
+  UniquePtr<NumberRangeFormat> nf =
+      NumberRangeFormat::TryCreate("es-MX", options).unwrap();
+
+  const char16_t* res = nf->format(12.34, 56.78).unwrap().data();
+  ASSERT_TRUE(res != nullptr);
+  ASSERT_EQ(std::u16string_view(res), u"12.34-56.78 metros por segundo");
+}
+
+TEST(IntlNumberRangeFormat, FormatToParts)
+{
+  NumberRangeFormatOptions options;
+  UniquePtr<NumberRangeFormat> nf =
+      NumberRangeFormat::TryCreate("es-ES", options).unwrap();
+  NumberPartVector parts;
+  const char16_t* res =
+      nf->formatToParts(123456.789, 299999.89, parts).unwrap().data();
+  ASSERT_TRUE(res != nullptr);
+  ASSERT_EQ(std::u16string_view(res), u"123.456,789-299.999,89"sv);
+  ASSERT_EQ(parts.length(), 11U);
+  ASSERT_EQ(parts[0],
+            (NumberPart{NumberPartType::Integer, NumberPartSource::Start, 3}));
+  ASSERT_EQ(parts[1],
+            (NumberPart{NumberPartType::Group, NumberPartSource::Start, 4}));
+  ASSERT_EQ(parts[2],
+            (NumberPart{NumberPartType::Integer, NumberPartSource::Start, 7}));
+  ASSERT_EQ(parts[3],
+            (NumberPart{NumberPartType::Decimal, NumberPartSource::Start, 8}));
+  ASSERT_EQ(parts[4], (NumberPart{NumberPartType::Fraction,
+                                  NumberPartSource::Start, 11}));
+  ASSERT_EQ(parts[5], (NumberPart{NumberPartType::Fraction,
+                                  NumberPartSource::Shared, 12}));
+  ASSERT_EQ(parts[6],
+            (NumberPart{NumberPartType::Integer, NumberPartSource::End, 15}));
+  ASSERT_EQ(parts[7],
+            (NumberPart{NumberPartType::Group, NumberPartSource::End, 16}));
+  ASSERT_EQ(parts[8],
+            (NumberPart{NumberPartType::Integer, NumberPartSource::End, 19}));
+  ASSERT_EQ(parts[9],
+            (NumberPart{NumberPartType::Decimal, NumberPartSource::End, 20}));
+  ASSERT_EQ(parts[10],
+            (NumberPart{NumberPartType::Fraction, NumberPartSource::End, 23}));
+}
+
+}  // namespace intl
+}  // namespace mozilla
diff --git a/intl/components/gtest/TestNumberingSystem.cpp b/intl/components/gtest/TestNumberingSystem.cpp
new file mode 100644
index 0000000000..c6f15b6559
--- /dev/null
+++ b/intl/components/gtest/TestNumberingSystem.cpp
@@ -0,0 +1,23 @@
+/* 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 "gtest/gtest.h"
+
+#include "mozilla/intl/NumberingSystem.h"
+#include "mozilla/Span.h"
+
+namespace mozilla::intl {
+
+TEST(IntlNumberingSystem, GetName)
+{
+  auto numbers_en = NumberingSystem::TryCreate("en").unwrap();
+  ASSERT_EQ(numbers_en->GetName().unwrap(), MakeStringSpan("latn"));
+
+  auto numbers_ar = NumberingSystem::TryCreate("ar").unwrap();
+  ASSERT_EQ(numbers_ar->GetName().unwrap(), MakeStringSpan("arab"));
+
+  auto numbers_ff_Adlm = NumberingSystem::TryCreate("ff-Adlm").unwrap();
+  ASSERT_EQ(numbers_ff_Adlm->GetName().unwrap(), MakeStringSpan("adlm"));
+}
+
+}  // namespace mozilla::intl
diff --git a/intl/components/gtest/TestPluralRules.cpp b/intl/components/gtest/TestPluralRules.cpp
new file mode 100644
index 0000000000..d02b1ff5fa
--- /dev/null
+++ b/intl/components/gtest/TestPluralRules.cpp
@@ -0,0 +1,671 @@
+/* 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 "gtest/gtest.h"
+#include "mozilla/Vector.h"
+#include "mozilla/intl/PluralRules.h"
+
+#define TEST_SELECT(actual, expected)     \
+  do {                                    \
+    ASSERT_TRUE(actual.isOk());           \
+    ASSERT_EQ(actual.unwrap(), expected); \
+  } while (false)
+
+namespace mozilla {
+namespace intl {
+
+TEST(IntlPluralRules, CategoriesEnCardinal)
+{
+  PluralRulesOptions defaultOptions;
+
+  auto prResult = PluralRules::TryCreate("en", defaultOptions);
+  ASSERT_TRUE(prResult.isOk());
+  auto pr = prResult.unwrap();
+
+  auto catResult = pr->Categories();
+  ASSERT_TRUE(catResult.isOk());
+  auto categories = catResult.unwrap();
+
+  ASSERT_EQ(categories.size(), 2u);
+  ASSERT_TRUE(categories.contains(PluralRules::Keyword::One));
+  ASSERT_TRUE(categories.contains(PluralRules::Keyword::Other));
+}
+
+TEST(IntlPluralRules, CategoriesEnOrdinal)
+{
+  PluralRulesOptions options;
+  options.mPluralType = PluralRules::Type::Ordinal;
+
+  auto prResult = PluralRules::TryCreate("en", options);
+  ASSERT_TRUE(prResult.isOk());
+  auto pr = prResult.unwrap();
+
+  auto catResult = pr->Categories();
+  ASSERT_TRUE(catResult.isOk());
+  auto categories = catResult.unwrap();
+
+  ASSERT_EQ(categories.size(), 4u);
+  ASSERT_TRUE(categories.contains(PluralRules::Keyword::Few));
+  ASSERT_TRUE(categories.contains(PluralRules::Keyword::One));
+  ASSERT_TRUE(categories.contains(PluralRules::Keyword::Other));
+  ASSERT_TRUE(categories.contains(PluralRules::Keyword::Two));
+}
+
+TEST(IntlPluralRules, CategoriesCyCardinal)
+{
+  PluralRulesOptions defaultOptions;
+
+  auto prResult = PluralRules::TryCreate("cy", defaultOptions);
+  ASSERT_TRUE(prResult.isOk());
+  auto pr = prResult.unwrap();
+
+  auto catResult = pr->Categories();
+  ASSERT_TRUE(catResult.isOk());
+  auto categories = catResult.unwrap();
+
+  ASSERT_EQ(categories.size(), 6u);
+  ASSERT_TRUE(categories.contains(PluralRules::Keyword::Few));
+  ASSERT_TRUE(categories.contains(PluralRules::Keyword::One));
+  ASSERT_TRUE(categories.contains(PluralRules::Keyword::Other));
+  ASSERT_TRUE(categories.contains(PluralRules::Keyword::Many));
+  ASSERT_TRUE(categories.contains(PluralRules::Keyword::Two));
+  ASSERT_TRUE(categories.contains(PluralRules::Keyword::Zero));
+}
+
+TEST(IntlPluralRules, CategoriesCyOrdinal)
+{
+  PluralRulesOptions options;
+  options.mPluralType = PluralRules::Type::Ordinal;
+
+  auto prResult = PluralRules::TryCreate("cy", options);
+  ASSERT_TRUE(prResult.isOk());
+  auto pr = prResult.unwrap();
+
+  auto catResult = pr->Categories();
+  ASSERT_TRUE(catResult.isOk());
+  auto categories = catResult.unwrap();
+
+  ASSERT_EQ(categories.size(), 6u);
+  ASSERT_TRUE(categories.contains(PluralRules::Keyword::Few));
+  ASSERT_TRUE(categories.contains(PluralRules::Keyword::One));
+  ASSERT_TRUE(categories.contains(PluralRules::Keyword::Other));
+  ASSERT_TRUE(categories.contains(PluralRules::Keyword::Many));
+  ASSERT_TRUE(categories.contains(PluralRules::Keyword::Two));
+  ASSERT_TRUE(categories.contains(PluralRules::Keyword::Zero));
+}
+
+TEST(IntlPluralRules, CategoriesBrCardinal)
+{
+  PluralRulesOptions defaultOptions;
+
+  auto prResult = PluralRules::TryCreate("br", defaultOptions);
+  ASSERT_TRUE(prResult.isOk());
+  auto pr = prResult.unwrap();
+
+  auto catResult = pr->Categories();
+  ASSERT_TRUE(catResult.isOk());
+  auto categories = catResult.unwrap();
+
+  ASSERT_EQ(categories.size(), 5u);
+  ASSERT_TRUE(categories.contains(PluralRules::Keyword::Few));
+  ASSERT_TRUE(categories.contains(PluralRules::Keyword::One));
+  ASSERT_TRUE(categories.contains(PluralRules::Keyword::Other));
+  ASSERT_TRUE(categories.contains(PluralRules::Keyword::Many));
+  ASSERT_TRUE(categories.contains(PluralRules::Keyword::Two));
+}
+
+TEST(IntlPluralRules, CategoriesBrOrdinal)
+{
+  PluralRulesOptions options;
+  options.mPluralType = PluralRules::Type::Ordinal;
+
+  auto prResult = PluralRules::TryCreate("br", options);
+  ASSERT_TRUE(prResult.isOk());
+  auto pr = prResult.unwrap();
+
+  auto catResult = pr->Categories();
+  ASSERT_TRUE(catResult.isOk());
+  auto categories = catResult.unwrap();
+
+  ASSERT_EQ(categories.size(), 1u);
+  ASSERT_TRUE(categories.contains(PluralRules::Keyword::Other));
+}
+
+TEST(IntlPluralRules, CategoriesHsbCardinal)
+{
+  PluralRulesOptions defaultOptions;
+
+  auto prResult = PluralRules::TryCreate("hsb", defaultOptions);
+  ASSERT_TRUE(prResult.isOk());
+  auto pr = prResult.unwrap();
+
+  auto catResult = pr->Categories();
+  ASSERT_TRUE(catResult.isOk());
+  auto categories = catResult.unwrap();
+
+  ASSERT_EQ(categories.size(), 4u);
+  ASSERT_TRUE(categories.contains(PluralRules::Keyword::Few));
+  ASSERT_TRUE(categories.contains(PluralRules::Keyword::One));
+  ASSERT_TRUE(categories.contains(PluralRules::Keyword::Other));
+  ASSERT_TRUE(categories.contains(PluralRules::Keyword::Two));
+}
+
+TEST(IntlPluralRules, CategoriesHsbOrdinal)
+{
+  PluralRulesOptions options;
+  options.mPluralType = PluralRules::Type::Ordinal;
+
+  auto prResult = PluralRules::TryCreate("hsb", options);
+  ASSERT_TRUE(prResult.isOk());
+  auto pr = prResult.unwrap();
+
+  auto catResult = pr->Categories();
+  ASSERT_TRUE(catResult.isOk());
+  auto categories = catResult.unwrap();
+
+  ASSERT_EQ(categories.size(), 1u);
+  ASSERT_TRUE(categories.contains(PluralRules::Keyword::Other));
+}
+
+// PluralRules should define the sort order of the keywords.
+// ICU returns these keywords in alphabetical order, so our implementation
+// should do the same.
+//
+// https://github.com/tc39/ecma402/issues/578
+TEST(IntlPluralRules, CategoriesSortOrder)
+{
+  PluralRulesOptions defaultOptions;
+
+  auto prResult = PluralRules::TryCreate("cy", defaultOptions);
+  ASSERT_TRUE(prResult.isOk());
+  auto pr = prResult.unwrap();
+
+  PluralRules::Keyword expected[] = {
+      PluralRules::Keyword::Few, PluralRules::Keyword::Many,
+      PluralRules::Keyword::One, PluralRules::Keyword::Other,
+      PluralRules::Keyword::Two, PluralRules::Keyword::Zero,
+  };
+
+  // Categories() returns an EnumSet so we want to ensure it still iterates
+  // over elements in the expected sorted order.
+  size_t index = 0;
+
+  auto catResult = pr->Categories();
+  ASSERT_TRUE(catResult.isOk());
+  auto categories = catResult.unwrap();
+
+  for (const PluralRules::Keyword keyword : categories) {
+    ASSERT_EQ(keyword, expected[index++]);
+  }
+}
+
+// en Cardinal Plural Rules
+//   one: i = 1 and v = 0 @integer 1
+// other: @integer 0, 2~16, 100, 1000, 10000, 100000, 1000000, …
+//        @decimal 0.0~1.5, 10.0, 100.0, 1000.0, 10000.0, 100000.0, 1000000.0, …
+TEST(IntlPluralRules, SelectEnCardinal)
+{
+  PluralRulesOptions defaultOptions;
+
+  auto prResult = PluralRules::TryCreate("en", defaultOptions);
+  ASSERT_TRUE(prResult.isOk());
+  auto pr = prResult.unwrap();
+
+  TEST_SELECT(pr->Select(0.00), PluralRules::Keyword::Other);
+  TEST_SELECT(pr->Select(1.00), PluralRules::Keyword::One);
+  TEST_SELECT(pr->Select(1.01), PluralRules::Keyword::Other);
+  TEST_SELECT(pr->Select(0.99), PluralRules::Keyword::Other);
+}
+
+// en Ordinal Plural Rules
+//   one: n % 10 = 1 and n % 100 != 11
+//        @integer 1, 21, 31, 41, 51, 61, 71, 81, 101, 1001, …,
+//   two: n % 10 = 2 and n % 100 != 12
+//        @integer 2, 22, 32, 42, 52, 62, 72, 82, 102, 1002, …,
+//   few: n % 10 = 3 and n % 100 != 13
+//        @integer 3, 23, 33, 43, 53, 63, 73, 83, 103, 1003, …,
+// other: @integer 0, 4~18, 100, 1000, 10000, 100000, 1000000, …
+TEST(IntlPluralRules, SelectEnOrdinal)
+{
+  PluralRulesOptions options;
+  options.mPluralType = PluralRules::Type::Ordinal;
+
+  auto prResult = PluralRules::TryCreate("en", options);
+  ASSERT_TRUE(prResult.isOk());
+  auto pr = prResult.unwrap();
+
+  TEST_SELECT(pr->Select(01.00), PluralRules::Keyword::One);
+  TEST_SELECT(pr->Select(21.00), PluralRules::Keyword::One);
+  TEST_SELECT(pr->Select(31.00), PluralRules::Keyword::One);
+  TEST_SELECT(pr->Select(41.00), PluralRules::Keyword::One);
+
+  TEST_SELECT(pr->Select(02.00), PluralRules::Keyword::Two);
+  TEST_SELECT(pr->Select(22.00), PluralRules::Keyword::Two);
+  TEST_SELECT(pr->Select(32.00), PluralRules::Keyword::Two);
+  TEST_SELECT(pr->Select(42.00), PluralRules::Keyword::Two);
+
+  TEST_SELECT(pr->Select(03.00), PluralRules::Keyword::Few);
+  TEST_SELECT(pr->Select(23.00), PluralRules::Keyword::Few);
+  TEST_SELECT(pr->Select(33.00), PluralRules::Keyword::Few);
+  TEST_SELECT(pr->Select(43.00), PluralRules::Keyword::Few);
+
+  TEST_SELECT(pr->Select(00.00), PluralRules::Keyword::Other);
+  TEST_SELECT(pr->Select(11.00), PluralRules::Keyword::Other);
+  TEST_SELECT(pr->Select(12.00), PluralRules::Keyword::Other);
+  TEST_SELECT(pr->Select(13.00), PluralRules::Keyword::Other);
+}
+
+// cy Cardinal Plural Rules
+//  zero: n = 0 @integer 0 @decimal 0.0, 0.00, 0.000, 0.0000,
+//   one: n = 1 @integer 1 @decimal 1.0, 1.00, 1.000, 1.0000,
+//   two: n = 2 @integer 2 @decimal 2.0, 2.00, 2.000, 2.0000,
+//   few: n = 3 @integer 3 @decimal 3.0, 3.00, 3.000, 3.0000,
+//  many: n = 6 @integer 6 @decimal 6.0, 6.00, 6.000, 6.0000,
+// other: @integer 4, 5, 7~20, 100, 1000, 10000, 100000, 1000000, …
+//        @decimal 0.1~0.9, 1.1~1.7, 10.0, 100.0, 1000.0, 10000.0, 100000.0, …
+TEST(IntlPluralRules, SelectCyCardinal)
+{
+  PluralRulesOptions defaultOptions;
+
+  auto prResult = PluralRules::TryCreate("cy", defaultOptions);
+  ASSERT_TRUE(prResult.isOk());
+  auto pr = prResult.unwrap();
+
+  TEST_SELECT(pr->Select(0.00), PluralRules::Keyword::Zero);
+  TEST_SELECT(pr->Select(1.00), PluralRules::Keyword::One);
+  TEST_SELECT(pr->Select(2.00), PluralRules::Keyword::Two);
+  TEST_SELECT(pr->Select(3.00), PluralRules::Keyword::Few);
+  TEST_SELECT(pr->Select(4.00), PluralRules::Keyword::Other);
+  TEST_SELECT(pr->Select(5.00), PluralRules::Keyword::Other);
+  TEST_SELECT(pr->Select(6.00), PluralRules::Keyword::Many);
+  TEST_SELECT(pr->Select(7.00), PluralRules::Keyword::Other);
+}
+
+// cy Ordinal Plural Rules
+//  zero: n = 0,7,8,9 @integer 0, 7~9,
+//   one: n = 1 @integer 1,
+//   two: n = 2 @integer 2,
+//   few: n = 3,4 @integer 3, 4,
+//  many: n = 5,6 @integer 5, 6,
+// other: @integer 10~25, 100, 1000, 10000, 100000, 1000000, …
+TEST(IntlPluralRules, SelectCyOrdinal)
+{
+  PluralRulesOptions options;
+  options.mPluralType = PluralRules::Type::Ordinal;
+
+  auto prResult = PluralRules::TryCreate("cy", options);
+  ASSERT_TRUE(prResult.isOk());
+  auto pr = prResult.unwrap();
+
+  TEST_SELECT(pr->Select(0.00), PluralRules::Keyword::Zero);
+  TEST_SELECT(pr->Select(7.00), PluralRules::Keyword::Zero);
+  TEST_SELECT(pr->Select(8.00), PluralRules::Keyword::Zero);
+  TEST_SELECT(pr->Select(9.00), PluralRules::Keyword::Zero);
+
+  TEST_SELECT(pr->Select(1.00), PluralRules::Keyword::One);
+
+  TEST_SELECT(pr->Select(2.00), PluralRules::Keyword::Two);
+
+  TEST_SELECT(pr->Select(3.00), PluralRules::Keyword::Few);
+  TEST_SELECT(pr->Select(4.00), PluralRules::Keyword::Few);
+
+  TEST_SELECT(pr->Select(5.00), PluralRules::Keyword::Many);
+  TEST_SELECT(pr->Select(6.00), PluralRules::Keyword::Many);
+
+  TEST_SELECT(pr->Select(10.00), PluralRules::Keyword::Other);
+  TEST_SELECT(pr->Select(11.00), PluralRules::Keyword::Other);
+}
+
+// br Cardinal Plural Rules
+//   one: n % 10 = 1 and n % 100 != 11,71,91
+//        @integer 1, 21, 31, 41, 51, 61, 81, 101, 1001, …
+//        @decimal 1.0, 21.0, 31.0, 41.0, 51.0, 61.0, 81.0, 101.0, 1001.0, …
+//   two: n % 10 = 2 and n % 100 != 12,72,92
+//        @integer 2, 22, 32, 42, 52, 62, 82, 102, 1002, …
+//        @decimal 2.0, 22.0, 32.0, 42.0, 52.0, 62.0, 82.0, 102.0, 1002.0, …
+//   few: n % 10 = 3..4,9 and n % 100 != 10..19,70..79,90..99
+//        @integer 3, 4, 9, 23, 24, 29, 33, 34, 39, 43, 44, 49, 103, 1003, …
+//        @decimal 3.0, 4.0, 9.0, 23.0, 24.0, 29.0, 33.0, 34.0, 103.0, 1003.0, …
+//  many: n != 0 and n % 1000000 = 0
+//        @integer 1000000, …
+//        @decimal 1000000.0, 1000000.00, 1000000.000, 1000000.0000, …
+// other: @integer 0, 5~8, 10~20, 100, 1000, 10000, 100000, …
+//        @decimal 0.0~0.9, 1.1~1.6, 10.0, 100.0, 1000.0, 10000.0, 100000.0, …
+TEST(IntlPluralRules, SelectBrCardinal)
+{
+  PluralRulesOptions defaultOptions;
+
+  auto prResult = PluralRules::TryCreate("br", defaultOptions);
+  ASSERT_TRUE(prResult.isOk());
+  auto pr = prResult.unwrap();
+
+  TEST_SELECT(pr->Select(00.00), PluralRules::Keyword::Other);
+
+  TEST_SELECT(pr->Select(01.00), PluralRules::Keyword::One);
+  TEST_SELECT(pr->Select(11.00), PluralRules::Keyword::Other);
+  TEST_SELECT(pr->Select(21.00), PluralRules::Keyword::One);
+  TEST_SELECT(pr->Select(31.00), PluralRules::Keyword::One);
+
+  TEST_SELECT(pr->Select(02.00), PluralRules::Keyword::Two);
+  TEST_SELECT(pr->Select(12.00), PluralRules::Keyword::Other);
+  TEST_SELECT(pr->Select(22.00), PluralRules::Keyword::Two);
+  TEST_SELECT(pr->Select(32.00), PluralRules::Keyword::Two);
+
+  TEST_SELECT(pr->Select(03.00), PluralRules::Keyword::Few);
+  TEST_SELECT(pr->Select(04.00), PluralRules::Keyword::Few);
+  TEST_SELECT(pr->Select(09.00), PluralRules::Keyword::Few);
+  TEST_SELECT(pr->Select(23.00), PluralRules::Keyword::Few);
+
+  TEST_SELECT(pr->Select(1000000), PluralRules::Keyword::Many);
+
+  TEST_SELECT(pr->Select(999999), PluralRules::Keyword::Other);
+  TEST_SELECT(pr->Select(1000005), PluralRules::Keyword::Other);
+}
+
+// br Ordinal Plural Rules
+// br has no rules for Ordinal, so everything is PluralRules::Keyword::Other.
+TEST(IntlPluralRules, SelectBrOrdinal)
+{
+  PluralRulesOptions options;
+  options.mPluralType = PluralRules::Type::Ordinal;
+
+  auto prResult = PluralRules::TryCreate("br", options);
+  ASSERT_TRUE(prResult.isOk());
+  auto pr = prResult.unwrap();
+
+  TEST_SELECT(pr->Select(00.00), PluralRules::Keyword::Other);
+
+  TEST_SELECT(pr->Select(01.00), PluralRules::Keyword::Other);
+  TEST_SELECT(pr->Select(11.00), PluralRules::Keyword::Other);
+  TEST_SELECT(pr->Select(21.00), PluralRules::Keyword::Other);
+  TEST_SELECT(pr->Select(31.00), PluralRules::Keyword::Other);
+
+  TEST_SELECT(pr->Select(02.00), PluralRules::Keyword::Other);
+  TEST_SELECT(pr->Select(12.00), PluralRules::Keyword::Other);
+  TEST_SELECT(pr->Select(22.00), PluralRules::Keyword::Other);
+  TEST_SELECT(pr->Select(32.00), PluralRules::Keyword::Other);
+
+  TEST_SELECT(pr->Select(03.00), PluralRules::Keyword::Other);
+  TEST_SELECT(pr->Select(04.00), PluralRules::Keyword::Other);
+  TEST_SELECT(pr->Select(09.00), PluralRules::Keyword::Other);
+  TEST_SELECT(pr->Select(23.00), PluralRules::Keyword::Other);
+
+  TEST_SELECT(pr->Select(1000000), PluralRules::Keyword::Other);
+
+  TEST_SELECT(pr->Select(999999), PluralRules::Keyword::Other);
+  TEST_SELECT(pr->Select(1000005), PluralRules::Keyword::Other);
+}
+
+// hsb Cardinal Plural Rules
+//   one: v = 0 and i % 100 = 1 or f % 100 = 1
+//        @integer 1, 101, 201, 301, 401, 501, 601, 701, 1001, …
+//        @decimal 0.1, 1.1, 2.1, 3.1, 4.1, 5.1, 6.1, 7.1, 10.1, 100.1, 1000.1,
+//        …,
+//   two: v = 0 and i % 100 = 2 or f % 100 = 2
+//        @integer 2, 102, 202, 302, 402, 502, 602, 702, 1002, …
+//        @decimal 0.2, 1.2, 2.2, 3.2, 4.2, 5.2, 6.2, 7.2, 10.2, 100.2, 1000.2,
+//        …,
+//   few: v = 0 and i % 100 = 3..4 or f % 100 = 3..4
+//        @integer 3, 4, 103, 104, 203, 204, 303, 304, 403, 404, 503, 504, 603,
+//        604, 703, 704, 1003, …
+//        @decimal 0.3,
+//        0.4, 1.3, 1.4, 2.3, 2.4, 3.3, 3.4, 4.3, 4.4, 5.3, 5.4, 6.3, 6.4, 7.3, 7.4,
+//        10.3, 100.3, 1000.3, …,
+// other: @integer 0, 5~19, 100, 1000, 10000, 100000, 1000000, …
+//        @decimal 0.0, 0.5~1.0, 1.5~2.0, 2.5~2.7, 10.0, 100.0, 1000.0, 10000.0,
+//        100000.0, 1000000.0, …
+TEST(IntlPluralRules, SelectHsbCardinal)
+{
+  PluralRulesOptions defaultOptions;
+
+  auto prResult = PluralRules::TryCreate("hsb", defaultOptions);
+  ASSERT_TRUE(prResult.isOk());
+  auto pr = prResult.unwrap();
+
+  TEST_SELECT(pr->Select(1.00), PluralRules::Keyword::One);
+  TEST_SELECT(pr->Select(101.00), PluralRules::Keyword::One);
+
+  TEST_SELECT(pr->Select(2.00), PluralRules::Keyword::Two);
+  TEST_SELECT(pr->Select(102.00), PluralRules::Keyword::Two);
+
+  TEST_SELECT(pr->Select(3.00), PluralRules::Keyword::Few);
+  TEST_SELECT(pr->Select(4.00), PluralRules::Keyword::Few);
+  TEST_SELECT(pr->Select(103.00), PluralRules::Keyword::Few);
+
+  TEST_SELECT(pr->Select(0.00), PluralRules::Keyword::Other);
+  TEST_SELECT(pr->Select(5.00), PluralRules::Keyword::Other);
+  TEST_SELECT(pr->Select(19.00), PluralRules::Keyword::Other);
+  TEST_SELECT(pr->Select(100.00), PluralRules::Keyword::Other);
+}
+
+// hsb Ordinal Plural Rules
+// other: @integer 0~15, 100, 1000, 10000, 100000, 1000000, …
+TEST(IntlPluralRules, SelectHsbOrdinal)
+{
+  PluralRulesOptions options;
+  options.mPluralType = PluralRules::Type::Ordinal;
+
+  auto prResult = PluralRules::TryCreate("hsb", options);
+  ASSERT_TRUE(prResult.isOk());
+  auto pr = prResult.unwrap();
+
+  TEST_SELECT(pr->Select(00.00), PluralRules::Keyword::Other);
+
+  TEST_SELECT(pr->Select(01.00), PluralRules::Keyword::Other);
+  TEST_SELECT(pr->Select(11.00), PluralRules::Keyword::Other);
+  TEST_SELECT(pr->Select(21.00), PluralRules::Keyword::Other);
+  TEST_SELECT(pr->Select(31.00), PluralRules::Keyword::Other);
+
+  TEST_SELECT(pr->Select(02.00), PluralRules::Keyword::Other);
+  TEST_SELECT(pr->Select(12.00), PluralRules::Keyword::Other);
+  TEST_SELECT(pr->Select(22.00), PluralRules::Keyword::Other);
+  TEST_SELECT(pr->Select(32.00), PluralRules::Keyword::Other);
+
+  TEST_SELECT(pr->Select(03.00), PluralRules::Keyword::Other);
+  TEST_SELECT(pr->Select(04.00), PluralRules::Keyword::Other);
+  TEST_SELECT(pr->Select(09.00), PluralRules::Keyword::Other);
+  TEST_SELECT(pr->Select(23.00), PluralRules::Keyword::Other);
+
+  TEST_SELECT(pr->Select(1000000), PluralRules::Keyword::Other);
+
+  TEST_SELECT(pr->Select(999999), PluralRules::Keyword::Other);
+  TEST_SELECT(pr->Select(1000005), PluralRules::Keyword::Other);
+}
+
+TEST(IntlPluralRules, DefaultFractionDigits)
+{
+  PluralRulesOptions defaultOptions;
+
+  auto prResult = PluralRules::TryCreate("en", defaultOptions);
+  ASSERT_TRUE(prResult.isOk());
+  auto pr = prResult.unwrap();
+
+  TEST_SELECT(pr->Select(1.000), PluralRules::Keyword::One);
+
+  TEST_SELECT(pr->Select(1.100), PluralRules::Keyword::Other);
+  TEST_SELECT(pr->Select(1.010), PluralRules::Keyword::Other);
+  TEST_SELECT(pr->Select(1.001), PluralRules::Keyword::Other);
+
+  TEST_SELECT(pr->Select(0.900), PluralRules::Keyword::Other);
+  TEST_SELECT(pr->Select(0.990), PluralRules::Keyword::Other);
+  TEST_SELECT(pr->Select(0.999), PluralRules::Keyword::Other);
+}
+
+TEST(IntlPluralRules, MaxFractionDigitsZero)
+{
+  PluralRulesOptions options;
+  options.mFractionDigits = Some(std::pair<uint32_t, uint32_t>(0, 0));
+
+  auto prResult = PluralRules::TryCreate("en", options);
+  ASSERT_TRUE(prResult.isOk());
+  auto pr = prResult.unwrap();
+
+  TEST_SELECT(pr->Select(1.000), PluralRules::Keyword::One);
+  TEST_SELECT(pr->Select(1.100), PluralRules::Keyword::One);
+  TEST_SELECT(pr->Select(1.010), PluralRules::Keyword::One);
+  TEST_SELECT(pr->Select(1.001), PluralRules::Keyword::One);
+  TEST_SELECT(pr->Select(0.900), PluralRules::Keyword::One);
+  TEST_SELECT(pr->Select(0.990), PluralRules::Keyword::One);
+  TEST_SELECT(pr->Select(0.999), PluralRules::Keyword::One);
+}
+
+TEST(IntlPluralRules, MaxFractionDigitsOne)
+{
+  PluralRulesOptions options;
+  options.mFractionDigits = Some(std::pair<uint32_t, uint32_t>(0, 1));
+
+  auto prResult = PluralRules::TryCreate("en", options);
+  ASSERT_TRUE(prResult.isOk());
+  auto pr = prResult.unwrap();
+
+  TEST_SELECT(pr->Select(1.000), PluralRules::Keyword::One);
+  TEST_SELECT(pr->Select(1.010), PluralRules::Keyword::One);
+  TEST_SELECT(pr->Select(1.001), PluralRules::Keyword::One);
+  TEST_SELECT(pr->Select(0.990), PluralRules::Keyword::One);
+  TEST_SELECT(pr->Select(0.999), PluralRules::Keyword::One);
+
+  TEST_SELECT(pr->Select(1.100), PluralRules::Keyword::Other);
+  TEST_SELECT(pr->Select(0.900), PluralRules::Keyword::Other);
+}
+
+TEST(IntlPluralRules, MaxSignificantDigitsOne)
+{
+  PluralRulesOptions options;
+  options.mSignificantDigits = Some(std::pair<uint32_t, uint32_t>(1, 1));
+
+  auto prResult = PluralRules::TryCreate("en", options);
+  ASSERT_TRUE(prResult.isOk());
+  auto pr = prResult.unwrap();
+
+  TEST_SELECT(pr->Select(1.000), PluralRules::Keyword::One);
+  TEST_SELECT(pr->Select(1.100), PluralRules::Keyword::One);
+  TEST_SELECT(pr->Select(1.010), PluralRules::Keyword::One);
+  TEST_SELECT(pr->Select(1.001), PluralRules::Keyword::One);
+  TEST_SELECT(pr->Select(0.990), PluralRules::Keyword::One);
+  TEST_SELECT(pr->Select(0.999), PluralRules::Keyword::One);
+
+  TEST_SELECT(pr->Select(0.900), PluralRules::Keyword::Other);
+}
+
+TEST(IntlPluralRules, MaxFractionDigitsTwo)
+{
+  PluralRulesOptions options;
+  options.mFractionDigits = Some(std::pair<uint32_t, uint32_t>(0, 2));
+
+  auto prResult = PluralRules::TryCreate("en", options);
+  ASSERT_TRUE(prResult.isOk());
+  auto pr = prResult.unwrap();
+
+  TEST_SELECT(pr->Select(1.000), PluralRules::Keyword::One);
+  TEST_SELECT(pr->Select(1.001), PluralRules::Keyword::One);
+  TEST_SELECT(pr->Select(0.999), PluralRules::Keyword::One);
+
+  TEST_SELECT(pr->Select(1.100), PluralRules::Keyword::Other);
+  TEST_SELECT(pr->Select(1.010), PluralRules::Keyword::Other);
+  TEST_SELECT(pr->Select(0.900), PluralRules::Keyword::Other);
+  TEST_SELECT(pr->Select(0.990), PluralRules::Keyword::Other);
+}
+
+TEST(IntlPluralRules, MaxSignificantDigitsTwo)
+{
+  PluralRulesOptions options;
+  options.mSignificantDigits = Some(std::pair<uint32_t, uint32_t>(1, 2));
+
+  auto prResult = PluralRules::TryCreate("en", options);
+  ASSERT_TRUE(prResult.isOk());
+  auto pr = prResult.unwrap();
+
+  TEST_SELECT(pr->Select(1.000), PluralRules::Keyword::One);
+  TEST_SELECT(pr->Select(1.010), PluralRules::Keyword::One);
+  TEST_SELECT(pr->Select(1.001), PluralRules::Keyword::One);
+  TEST_SELECT(pr->Select(0.999), PluralRules::Keyword::One);
+
+  TEST_SELECT(pr->Select(1.100), PluralRules::Keyword::Other);
+  TEST_SELECT(pr->Select(0.900), PluralRules::Keyword::Other);
+  TEST_SELECT(pr->Select(0.990), PluralRules::Keyword::Other);
+}
+
+// en Plural Range Rules
+//   other: one other
+//   other: other one
+//   other: other other
+TEST(IntlPluralRules, SelectRangeEn)
+{
+  for (auto type : {PluralRules::Type::Cardinal, PluralRules::Type::Ordinal}) {
+    PluralRulesOptions options;
+    options.mPluralType = type;
+    auto prResult = PluralRules::TryCreate("en", options);
+    ASSERT_TRUE(prResult.isOk());
+    auto pr = prResult.unwrap();
+
+    TEST_SELECT(pr->SelectRange(0, 0), PluralRules::Keyword::Other);
+    TEST_SELECT(pr->SelectRange(0, 1), PluralRules::Keyword::Other);
+    TEST_SELECT(pr->SelectRange(0, 2), PluralRules::Keyword::Other);
+    TEST_SELECT(pr->SelectRange(1, 1), PluralRules::Keyword::Other);
+    TEST_SELECT(pr->SelectRange(1, 2), PluralRules::Keyword::Other);
+    TEST_SELECT(pr->SelectRange(1, 10), PluralRules::Keyword::Other);
+  }
+}
+
+// fr Cardinal Plural Rules
+//   one: i = 0,1
+//        @integer 0, 1
+//        @decimal 0.0~1.5
+//   many: e = 0 and i != 0 and i % 1000000 = 0 and v = 0 or e != 0..5
+//         @integer 1000000, 1c6, 2c6, 3c6, 4c6, 5c6, 6c6, …
+//         @decimal 1.0000001c6, 1.1c6, 2.0000001c6, 2.1c6, 3.0000001c6, 3.1c6,
+//         …
+//   other: @integer 2~17, 100, 1000, 10000, 100000, 1c3, 2c3, 3c3, 4c3, 5c3,
+//   6c3, …
+//          @decimal 2.0~3.5, 10.0, 100.0, 1000.0, 10000.0, 100000.0,
+//          1000000.0, 1.0001c3, 1.1c3, 2.0001c3, 2.1c3, 3.0001c3, 3.1c3, …
+//
+// fr Plural Range Rules
+//   one: one one
+//   other: one other
+//   other: other other
+TEST(IntlPluralRules, SelectRangeFrCardinal)
+{
+  PluralRulesOptions options;
+  options.mPluralType = PluralRules::Type::Cardinal;
+  auto prResult = PluralRules::TryCreate("fr", options);
+  ASSERT_TRUE(prResult.isOk());
+  auto pr = prResult.unwrap();
+
+  TEST_SELECT(pr->SelectRange(0, 0), PluralRules::Keyword::One);
+  TEST_SELECT(pr->SelectRange(0, 1), PluralRules::Keyword::One);
+  TEST_SELECT(pr->SelectRange(0, 2), PluralRules::Keyword::Other);
+  TEST_SELECT(pr->SelectRange(1, 1), PluralRules::Keyword::One);
+  TEST_SELECT(pr->SelectRange(1, 2), PluralRules::Keyword::Other);
+  TEST_SELECT(pr->SelectRange(1, 10), PluralRules::Keyword::Other);
+  TEST_SELECT(pr->SelectRange(1, 1000000), PluralRules::Keyword::Other);
+}
+
+// fr Ordinal Plural Rules
+//   one: n = 1 @integer 1
+//   other: @integer 0, 2~16, 100, 1000, 10000, 100000, 1000000, …
+//
+// fr Plural Range Rules
+//   one: one one
+//   other: one other
+//   other: other other
+TEST(IntlPluralRules, SelectRangeFrOrdinal)
+{
+  PluralRulesOptions options;
+  options.mPluralType = PluralRules::Type::Ordinal;
+  auto prResult = PluralRules::TryCreate("fr", options);
+  ASSERT_TRUE(prResult.isOk());
+  auto pr = prResult.unwrap();
+
+  TEST_SELECT(pr->SelectRange(0, 0), PluralRules::Keyword::Other);
+  TEST_SELECT(pr->SelectRange(0, 1), PluralRules::Keyword::Other);
+  TEST_SELECT(pr->SelectRange(0, 2), PluralRules::Keyword::Other);
+  TEST_SELECT(pr->SelectRange(1, 1), PluralRules::Keyword::One);
+  TEST_SELECT(pr->SelectRange(1, 2), PluralRules::Keyword::Other);
+  TEST_SELECT(pr->SelectRange(1, 10), PluralRules::Keyword::Other);
+  TEST_SELECT(pr->SelectRange(1, 1000000), PluralRules::Keyword::Other);
+}
+
+}  // namespace intl
+}  // namespace mozilla
diff --git a/intl/components/gtest/TestRelativeTimeFormat.cpp b/intl/components/gtest/TestRelativeTimeFormat.cpp
new file mode 100644
index 0000000000..eced3961d7
--- /dev/null
+++ b/intl/components/gtest/TestRelativeTimeFormat.cpp
@@ -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/. */
+#include "gtest/gtest.h"
+
+#include "mozilla/intl/RelativeTimeFormat.h"
+#include "TestBuffer.h"
+
+namespace mozilla::intl {
+
+TEST(IntlRelativeTimeFormat, Basic)
+{
+  RelativeTimeFormatOptions options = {};
+  Result<UniquePtr<RelativeTimeFormat>, ICUError> res =
+      RelativeTimeFormat::TryCreate("en-US", options);
+  ASSERT_TRUE(res.isOk());
+  UniquePtr<RelativeTimeFormat> rtf = res.unwrap();
+  TestBuffer<char> buf8;
+  ASSERT_TRUE(
+      rtf->format(1.2, RelativeTimeFormat::FormatUnit::Day, buf8).isOk());
+  ASSERT_EQ(buf8.get_string_view(), "in 1.2 days");
+
+  TestBuffer<char16_t> buf16;
+  ASSERT_TRUE(
+      rtf->format(1.2, RelativeTimeFormat::FormatUnit::Day, buf16).isOk());
+  ASSERT_EQ(buf16.get_string_view(), u"in 1.2 days");
+
+  res = RelativeTimeFormat::TryCreate("es-AR", options);
+  ASSERT_TRUE(res.isOk());
+  rtf = res.unwrap();
+  buf8.clear();
+  ASSERT_TRUE(
+      rtf->format(1.2, RelativeTimeFormat::FormatUnit::Day, buf8).isOk());
+  ASSERT_EQ(buf8.get_string_view(), "dentro de 1,2 días");
+
+  buf16.clear();
+  ASSERT_TRUE(
+      rtf->format(1.2, RelativeTimeFormat::FormatUnit::Day, buf16).isOk());
+  ASSERT_EQ(buf16.get_string_view(), u"dentro de 1,2 días");
+
+  res = RelativeTimeFormat::TryCreate("ar", options);
+  ASSERT_TRUE(res.isOk());
+  rtf = res.unwrap();
+  buf8.clear();
+  ASSERT_TRUE(
+      rtf->format(1.2, RelativeTimeFormat::FormatUnit::Day, buf8).isOk());
+  ASSERT_EQ(buf8.get_string_view(), "خلال ١٫٢ يوم");
+
+  buf16.clear();
+  ASSERT_TRUE(
+      rtf->format(1.2, RelativeTimeFormat::FormatUnit::Day, buf16).isOk());
+  ASSERT_EQ(buf16.get_string_view(), u"خلال ١٫٢ يوم");
+}
+
+TEST(IntlRelativeTimeFormat, Options)
+{
+  RelativeTimeFormatOptions options = {
+      RelativeTimeFormatOptions::Style::Short,
+      RelativeTimeFormatOptions::Numeric::Auto};
+  Result<UniquePtr<RelativeTimeFormat>, ICUError> res =
+      RelativeTimeFormat::TryCreate("fr", options);
+  ASSERT_TRUE(res.isOk());
+  UniquePtr<RelativeTimeFormat> rtf = res.unwrap();
+  TestBuffer<char> buf8;
+  ASSERT_TRUE(
+      rtf->format(-3.14, RelativeTimeFormat::FormatUnit::Year, buf8).isOk());
+  ASSERT_EQ(buf8.get_string_view(), "il y a 3,14 a");
+
+  options = {RelativeTimeFormatOptions::Style::Narrow,
+             RelativeTimeFormatOptions::Numeric::Auto};
+  res = RelativeTimeFormat::TryCreate("fr", options);
+  ASSERT_TRUE(res.isOk());
+  rtf = res.unwrap();
+  buf8.clear();
+  ASSERT_TRUE(
+      rtf->format(-3.14, RelativeTimeFormat::FormatUnit::Year, buf8).isOk());
+  ASSERT_EQ(buf8.get_string_view(), "-3,14 a");
+
+  options = {RelativeTimeFormatOptions::Style::Long,
+             RelativeTimeFormatOptions::Numeric::Auto};
+  res = RelativeTimeFormat::TryCreate("fr", options);
+  ASSERT_TRUE(res.isOk());
+  rtf = res.unwrap();
+  buf8.clear();
+  ASSERT_TRUE(
+      rtf->format(-3.14, RelativeTimeFormat::FormatUnit::Year, buf8).isOk());
+  ASSERT_EQ(buf8.get_string_view(), "il y a 3,14 ans");
+
+  options = {RelativeTimeFormatOptions::Style::Long,
+             RelativeTimeFormatOptions::Numeric::Auto};
+  res = RelativeTimeFormat::TryCreate("fr", options);
+  ASSERT_TRUE(res.isOk());
+  rtf = res.unwrap();
+  buf8.clear();
+  ASSERT_TRUE(
+      rtf->format(-1, RelativeTimeFormat::FormatUnit::Year, buf8).isOk());
+  ASSERT_EQ(buf8.get_string_view(), "l’année dernière");
+}
+
+TEST(IntlRelativeTimeFormat, Units)
+{
+  RelativeTimeFormatOptions options = {};
+  Result<UniquePtr<RelativeTimeFormat>, ICUError> res =
+      RelativeTimeFormat::TryCreate("en-US", options);
+  ASSERT_TRUE(res.isOk());
+  UniquePtr<RelativeTimeFormat> rtf = res.unwrap();
+  TestBuffer<char> buf8;
+  ASSERT_TRUE(
+      rtf->format(1.2, RelativeTimeFormat::FormatUnit::Second, buf8).isOk());
+  ASSERT_EQ(buf8.get_string_view(), "in 1.2 seconds");
+  ASSERT_TRUE(
+      rtf->format(1.2, RelativeTimeFormat::FormatUnit::Minute, buf8).isOk());
+  ASSERT_EQ(buf8.get_string_view(), "in 1.2 minutes");
+  ASSERT_TRUE(
+      rtf->format(1.2, RelativeTimeFormat::FormatUnit::Hour, buf8).isOk());
+  ASSERT_EQ(buf8.get_string_view(), "in 1.2 hours");
+  ASSERT_TRUE(
+      rtf->format(1.2, RelativeTimeFormat::FormatUnit::Day, buf8).isOk());
+  ASSERT_EQ(buf8.get_string_view(), "in 1.2 days");
+  ASSERT_TRUE(
+      rtf->format(1.2, RelativeTimeFormat::FormatUnit::Week, buf8).isOk());
+  ASSERT_EQ(buf8.get_string_view(), "in 1.2 weeks");
+  ASSERT_TRUE(
+      rtf->format(1.2, RelativeTimeFormat::FormatUnit::Month, buf8).isOk());
+  ASSERT_EQ(buf8.get_string_view(), "in 1.2 months");
+  ASSERT_TRUE(
+      rtf->format(1.2, RelativeTimeFormat::FormatUnit::Quarter, buf8).isOk());
+  ASSERT_EQ(buf8.get_string_view(), "in 1.2 quarters");
+  ASSERT_TRUE(
+      rtf->format(1.2, RelativeTimeFormat::FormatUnit::Year, buf8).isOk());
+  ASSERT_EQ(buf8.get_string_view(), "in 1.2 years");
+}
+
+TEST(IntlRelativeTimeFormat, FormatToParts)
+{
+  RelativeTimeFormatOptions options = {
+      RelativeTimeFormatOptions::Style::Long,
+      RelativeTimeFormatOptions::Numeric::Auto};
+  Result<UniquePtr<RelativeTimeFormat>, ICUError> res =
+      RelativeTimeFormat::TryCreate("es-AR", options);
+  ASSERT_TRUE(res.isOk());
+  UniquePtr<RelativeTimeFormat> rtf = res.unwrap();
+  NumberPartVector parts;
+  Result<Span<const char16_t>, ICUError> strRes =
+      rtf->formatToParts(-1.2, RelativeTimeFormat::FormatUnit::Year, parts);
+  ASSERT_TRUE(strRes.isOk());
+  ASSERT_EQ(strRes.unwrap(), MakeStringSpan(u"hace 1,2 años"));
+  ASSERT_EQ(parts.length(), 5U);
+  ASSERT_EQ(parts[0],
+            (NumberPart{NumberPartType::Literal, NumberPartSource::Shared, 5}));
+  ASSERT_EQ(parts[1],
+            (NumberPart{NumberPartType::Integer, NumberPartSource::Shared, 6}));
+  ASSERT_EQ(parts[2],
+            (NumberPart{NumberPartType::Decimal, NumberPartSource::Shared, 7}));
+  ASSERT_EQ(parts[3], (NumberPart{NumberPartType::Fraction,
+                                  NumberPartSource::Shared, 8}));
+  ASSERT_EQ(parts[4], (NumberPart{NumberPartType::Literal,
+                                  NumberPartSource::Shared, 13}));
+}
+
+}  // namespace mozilla::intl
diff --git a/intl/components/gtest/TestScript.cpp b/intl/components/gtest/TestScript.cpp
new file mode 100644
index 0000000000..72d8cd1087
--- /dev/null
+++ b/intl/components/gtest/TestScript.cpp
@@ -0,0 +1,62 @@
+/* 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 "gtest/gtest.h"
+
+#include "mozilla/intl/UnicodeProperties.h"
+#include "mozilla/intl/UnicodeScriptCodes.h"
+
+namespace mozilla::intl {
+TEST(IntlScript, GetExtensions)
+{
+  UnicodeProperties::ScriptExtensionVector extensions;
+
+  // 0x0000..0x0040 are Common.
+  for (char32_t ch = 0; ch < 0x0041; ch++) {
+    ASSERT_TRUE(UnicodeProperties::GetExtensions(ch, extensions).isOk());
+    ASSERT_EQ(extensions.length(), 1u);
+    ASSERT_EQ(Script(extensions[0]), Script::COMMON);
+  }
+
+  // 0x0300..0x0341 are Inherited.
+  for (char32_t ch = 0x300; ch < 0x0341; ch++) {
+    ASSERT_TRUE(UnicodeProperties::GetExtensions(ch, extensions).isOk());
+    ASSERT_EQ(extensions.length(), 1u);
+    ASSERT_EQ(Script(extensions[0]), Script::INHERITED);
+  }
+
+  // 0x1cf7's script code is Common, but its script extension is Beng.
+  ASSERT_TRUE(UnicodeProperties::GetExtensions(0x1cf7, extensions).isOk());
+  ASSERT_EQ(extensions.length(), 1u);
+  ASSERT_EQ(Script(extensions[0]), Script::BENGALI);
+
+  // ؿ
+  // https://unicode-table.com/en/063F/
+  // This character doesn't have any script extension, so the script code is
+  // returned.
+  ASSERT_TRUE(UnicodeProperties::GetExtensions(0x063f, extensions).isOk());
+  ASSERT_EQ(extensions.length(), 1u);
+  ASSERT_EQ(Script(extensions[0]), Script::ARABIC);
+
+  // 0xff65 is the unicode character '･', see https://unicode-table.com/en/FF65/
+  // Halfwidth Katakana Middle Dot.
+  ASSERT_TRUE(UnicodeProperties::GetExtensions(0xff65, extensions).isOk());
+
+  // 0xff65 should have the following script extensions:
+  // Bopo Hang Hani Hira Kana Yiii.
+  ASSERT_EQ(extensions.length(), 6u);
+
+  ASSERT_EQ(Script(extensions[0]), Script::BOPOMOFO);
+  ASSERT_EQ(Script(extensions[1]), Script::HAN);
+  ASSERT_EQ(Script(extensions[2]), Script::HANGUL);
+  ASSERT_EQ(Script(extensions[3]), Script::HIRAGANA);
+  ASSERT_EQ(Script(extensions[4]), Script::KATAKANA);
+  ASSERT_EQ(Script(extensions[5]), Script::YI);
+
+  // The max code point is 0x10ffff, so 0x110000 should be invalid.
+  // Script::UNKNOWN should be returned for an invalid code point.
+  ASSERT_TRUE(UnicodeProperties::GetExtensions(0x110000, extensions).isOk());
+  ASSERT_EQ(extensions.length(), 1u);
+  ASSERT_EQ(Script(extensions[0]), Script::UNKNOWN);
+}
+}  // namespace mozilla::intl
diff --git a/intl/components/gtest/TestString.cpp b/intl/components/gtest/TestString.cpp
new file mode 100644
index 0000000000..5b4d24b3b7
--- /dev/null
+++ b/intl/components/gtest/TestString.cpp
@@ -0,0 +1,260 @@
+/* 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 "gtest/gtest.h"
+
+#include "mozilla/intl/String.h"
+#include "mozilla/Span.h"
+#include "mozilla/TextUtils.h"
+
+#include <algorithm>
+
+#include "TestBuffer.h"
+
+namespace mozilla::intl {
+
+static Result<std::u16string_view, ICUError> ToLocaleLowerCase(
+    const char* aLocale, const char16_t* aString,
+    TestBuffer<char16_t>& aBuffer) {
+  aBuffer.clear();
+
+  MOZ_TRY(String::ToLocaleLowerCase(aLocale, MakeStringSpan(aString), aBuffer));
+
+  return aBuffer.get_string_view();
+}
+
+static Result<std::u16string_view, ICUError> ToLocaleUpperCase(
+    const char* aLocale, const char16_t* aString,
+    TestBuffer<char16_t>& aBuffer) {
+  aBuffer.clear();
+
+  MOZ_TRY(String::ToLocaleUpperCase(aLocale, MakeStringSpan(aString), aBuffer));
+
+  return aBuffer.get_string_view();
+}
+
+TEST(IntlString, ToLocaleLowerCase)
+{
+  TestBuffer<char16_t> buf;
+
+  ASSERT_EQ(ToLocaleLowerCase("en", u"test", buf).unwrap(), u"test");
+  ASSERT_EQ(ToLocaleLowerCase("en", u"TEST", buf).unwrap(), u"test");
+
+  // Turkish dotless i.
+  ASSERT_EQ(ToLocaleLowerCase("tr", u"I", buf).unwrap(), u"ı");
+  ASSERT_EQ(ToLocaleLowerCase("tr", u"İ", buf).unwrap(), u"i");
+  ASSERT_EQ(ToLocaleLowerCase("tr", u"I\u0307", buf).unwrap(), u"i");
+}
+
+TEST(IntlString, ToLocaleUpperCase)
+{
+  TestBuffer<char16_t> buf;
+
+  ASSERT_EQ(ToLocaleUpperCase("en", u"test", buf).unwrap(), u"TEST");
+  ASSERT_EQ(ToLocaleUpperCase("en", u"TEST", buf).unwrap(), u"TEST");
+
+  // Turkish dotless i.
+  ASSERT_EQ(ToLocaleUpperCase("tr", u"i", buf).unwrap(), u"İ");
+  ASSERT_EQ(ToLocaleUpperCase("tr", u"ı", buf).unwrap(), u"I");
+
+  // Output can be longer than the input string.
+  ASSERT_EQ(ToLocaleUpperCase("en", u"Größenmaßstäbe", buf).unwrap(),
+            u"GRÖSSENMASSSTÄBE");
+}
+
+TEST(IntlString, NormalizeNFC)
+{
+  using namespace std::literals;
+
+  using NormalizationForm = String::NormalizationForm;
+  using AlreadyNormalized = String::AlreadyNormalized;
+
+  TestBuffer<char16_t> buf;
+
+  auto alreadyNormalized =
+      String::Normalize(NormalizationForm::NFC, u""sv, buf);
+  ASSERT_EQ(alreadyNormalized.unwrap(), AlreadyNormalized::Yes);
+  ASSERT_EQ(buf.get_string_view(), u"");
+
+  alreadyNormalized =
+      String::Normalize(NormalizationForm::NFC, u"abcdef"sv, buf);
+  ASSERT_EQ(alreadyNormalized.unwrap(), AlreadyNormalized::Yes);
+  ASSERT_EQ(buf.get_string_view(), u"");
+
+  alreadyNormalized =
+      String::Normalize(NormalizationForm::NFC, u"a\u0308"sv, buf);
+  ASSERT_EQ(alreadyNormalized.unwrap(), AlreadyNormalized::No);
+  ASSERT_EQ(buf.get_string_view(), u"ä");
+
+  buf.clear();
+
+  alreadyNormalized = String::Normalize(NormalizationForm::NFC, u"½"sv, buf);
+  ASSERT_EQ(alreadyNormalized.unwrap(), AlreadyNormalized::Yes);
+  ASSERT_EQ(buf.get_string_view(), u"");
+}
+
+TEST(IntlString, NormalizeNFD)
+{
+  using namespace std::literals;
+
+  using NormalizationForm = String::NormalizationForm;
+  using AlreadyNormalized = String::AlreadyNormalized;
+
+  TestBuffer<char16_t> buf;
+
+  auto alreadyNormalized =
+      String::Normalize(NormalizationForm::NFD, u""sv, buf);
+  ASSERT_EQ(alreadyNormalized.unwrap(), AlreadyNormalized::Yes);
+  ASSERT_EQ(buf.get_string_view(), u"");
+
+  alreadyNormalized =
+      String::Normalize(NormalizationForm::NFD, u"abcdef"sv, buf);
+  ASSERT_EQ(alreadyNormalized.unwrap(), AlreadyNormalized::Yes);
+  ASSERT_EQ(buf.get_string_view(), u"");
+
+  alreadyNormalized = String::Normalize(NormalizationForm::NFD, u"ä"sv, buf);
+  ASSERT_EQ(alreadyNormalized.unwrap(), AlreadyNormalized::No);
+  ASSERT_EQ(buf.get_string_view(), u"a\u0308");
+
+  buf.clear();
+
+  alreadyNormalized = String::Normalize(NormalizationForm::NFD, u"½"sv, buf);
+  ASSERT_EQ(alreadyNormalized.unwrap(), AlreadyNormalized::Yes);
+  ASSERT_EQ(buf.get_string_view(), u"");
+
+  // Test with inline capacity.
+  TestBuffer<char16_t, 2> buf2;
+
+  alreadyNormalized = String::Normalize(NormalizationForm::NFD, u" ç"sv, buf2);
+  ASSERT_EQ(alreadyNormalized.unwrap(), AlreadyNormalized::No);
+  ASSERT_EQ(buf2.get_string_view(), u" c\u0327");
+}
+
+TEST(IntlString, NormalizeNFKC)
+{
+  using namespace std::literals;
+
+  using NormalizationForm = String::NormalizationForm;
+  using AlreadyNormalized = String::AlreadyNormalized;
+
+  TestBuffer<char16_t> buf;
+
+  auto alreadyNormalized =
+      String::Normalize(NormalizationForm::NFKC, u""sv, buf);
+  ASSERT_EQ(alreadyNormalized.unwrap(), AlreadyNormalized::Yes);
+  ASSERT_EQ(buf.get_string_view(), u"");
+
+  alreadyNormalized =
+      String::Normalize(NormalizationForm::NFKC, u"abcdef"sv, buf);
+  ASSERT_EQ(alreadyNormalized.unwrap(), AlreadyNormalized::Yes);
+  ASSERT_EQ(buf.get_string_view(), u"");
+
+  alreadyNormalized =
+      String::Normalize(NormalizationForm::NFKC, u"a\u0308"sv, buf);
+  ASSERT_EQ(alreadyNormalized.unwrap(), AlreadyNormalized::No);
+  ASSERT_EQ(buf.get_string_view(), u"ä");
+
+  buf.clear();
+
+  alreadyNormalized = String::Normalize(NormalizationForm::NFKC, u"½"sv, buf);
+  ASSERT_EQ(alreadyNormalized.unwrap(), AlreadyNormalized::No);
+  ASSERT_EQ(buf.get_string_view(), u"1⁄2");
+}
+
+TEST(IntlString, NormalizeNFKD)
+{
+  using namespace std::literals;
+
+  using NormalizationForm = String::NormalizationForm;
+  using AlreadyNormalized = String::AlreadyNormalized;
+
+  TestBuffer<char16_t> buf;
+
+  auto alreadyNormalized =
+      String::Normalize(NormalizationForm::NFKD, u""sv, buf);
+  ASSERT_EQ(alreadyNormalized.unwrap(), AlreadyNormalized::Yes);
+  ASSERT_EQ(buf.get_string_view(), u"");
+
+  alreadyNormalized =
+      String::Normalize(NormalizationForm::NFKD, u"abcdef"sv, buf);
+  ASSERT_EQ(alreadyNormalized.unwrap(), AlreadyNormalized::Yes);
+  ASSERT_EQ(buf.get_string_view(), u"");
+
+  alreadyNormalized = String::Normalize(NormalizationForm::NFKD, u"ä"sv, buf);
+  ASSERT_EQ(alreadyNormalized.unwrap(), AlreadyNormalized::No);
+  ASSERT_EQ(buf.get_string_view(), u"a\u0308");
+
+  buf.clear();
+
+  alreadyNormalized = String::Normalize(NormalizationForm::NFKD, u"½"sv, buf);
+  ASSERT_EQ(alreadyNormalized.unwrap(), AlreadyNormalized::No);
+  ASSERT_EQ(buf.get_string_view(), u"1⁄2");
+}
+
+TEST(IntlString, ComposePairNFC)
+{
+  // Pair of base characters do not compose
+  ASSERT_EQ(String::ComposePairNFC(U'a', U'b'), U'\0');
+  // Base letter + accent
+  ASSERT_EQ(String::ComposePairNFC(U'a', U'\u0308'), U'ä');
+  // Accented letter + a further accent
+  ASSERT_EQ(String::ComposePairNFC(U'ä', U'\u0304'), U'ǟ');
+  // Accented letter + a further accent, but doubly-accented form is not
+  // available
+  ASSERT_EQ(String::ComposePairNFC(U'ä', U'\u0301'), U'\0');
+  // These do not compose because although U+0344 has the decomposition <0308,
+  // 0301> (see below), it also has the Full_Composition_Exclusion property.
+  ASSERT_EQ(String::ComposePairNFC(U'\u0308', U'\u0301'), U'\0');
+  // Supplementary-plane letter + accent
+  ASSERT_EQ(String::ComposePairNFC(U'\U00011099', U'\U000110BA'),
+            U'\U0001109A');
+}
+
+TEST(IntlString, DecomposeRawNFD)
+{
+  char32_t buf[2];
+  // Non-decomposable character
+  ASSERT_EQ(String::DecomposeRawNFD(U'a', buf), 0);
+  // Singleton decomposition
+  ASSERT_EQ(String::DecomposeRawNFD(U'\u212A', buf), 1);
+  ASSERT_EQ(buf[0], U'K');
+  // Simple accented letter
+  ASSERT_EQ(String::DecomposeRawNFD(U'ä', buf), 2);
+  ASSERT_EQ(buf[0], U'a');
+  ASSERT_EQ(buf[1], U'\u0308');
+  // Double-accented letter decomposes by only one level
+  ASSERT_EQ(String::DecomposeRawNFD(U'ǟ', buf), 2);
+  ASSERT_EQ(buf[0], U'ä');
+  ASSERT_EQ(buf[1], U'\u0304');
+  // Non-starter can decompose, but will not recompose (see above)
+  ASSERT_EQ(String::DecomposeRawNFD(U'\u0344', buf), 2);
+  ASSERT_EQ(buf[0], U'\u0308');
+  ASSERT_EQ(buf[1], U'\u0301');
+  // Supplementary-plane letter with decomposition
+  ASSERT_EQ(String::DecomposeRawNFD(U'\U0001109A', buf), 2);
+  ASSERT_EQ(buf[0], U'\U00011099');
+  ASSERT_EQ(buf[1], U'\U000110BA');
+}
+
+TEST(IntlString, IsCased)
+{
+  ASSERT_TRUE(String::IsCased(U'a'));
+  ASSERT_FALSE(String::IsCased(U'0'));
+}
+
+TEST(IntlString, IsCaseIgnorable)
+{
+  ASSERT_FALSE(String::IsCaseIgnorable(U'a'));
+  ASSERT_TRUE(String::IsCaseIgnorable(U'.'));
+}
+
+TEST(IntlString, GetUnicodeVersion)
+{
+  auto version = String::GetUnicodeVersion();
+
+  ASSERT_TRUE(std::all_of(version.begin(), version.end(), [](char ch) {
+    return IsAsciiDigit(ch) || ch == '.';
+  }));
+}
+
+}  // namespace mozilla::intl
diff --git a/intl/components/gtest/TestTimeZone.cpp b/intl/components/gtest/TestTimeZone.cpp
new file mode 100644
index 0000000000..dc274150c7
--- /dev/null
+++ b/intl/components/gtest/TestTimeZone.cpp
@@ -0,0 +1,249 @@
+/* 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 "gtest/gtest.h"
+
+#include "mozilla/intl/TimeZone.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/Span.h"
+#include "mozilla/TextUtils.h"
+#include "TestBuffer.h"
+
+#include <algorithm>
+#include <string>
+
+namespace mozilla::intl {
+
+// Firefox 1.0 release date.
+static constexpr int64_t RELEASE_DATE = 1'032'800'850'000;
+
+// Date.UTC(2021, 11-1, 7, 2, 0, 0)
+static constexpr int64_t DST_CHANGE_DATE = 1'636'250'400'000;
+
+// These tests are dependent on the machine that this test is being run on.
+// Unwrap the results to ensure it doesn't fail, but don't check the values.
+TEST(IntlTimeZone, SystemDependentTests)
+{
+  // e.g. "America/Chicago"
+  TestBuffer<char16_t> buffer;
+  TimeZone::GetDefaultTimeZone(buffer).unwrap();
+}
+
+TEST(IntlTimeZone, GetRawOffsetMs)
+{
+  auto timeZone = TimeZone::TryCreate(Some(MakeStringSpan(u"GMT+3"))).unwrap();
+  ASSERT_EQ(timeZone->GetRawOffsetMs().unwrap(), 3 * 60 * 60 * 1000);
+
+  timeZone = TimeZone::TryCreate(Some(MakeStringSpan(u"Etc/GMT+3"))).unwrap();
+  ASSERT_EQ(timeZone->GetRawOffsetMs().unwrap(), -(3 * 60 * 60 * 1000));
+
+  timeZone =
+      TimeZone::TryCreate(Some(MakeStringSpan(u"America/New_York"))).unwrap();
+  ASSERT_EQ(timeZone->GetRawOffsetMs().unwrap(), -(5 * 60 * 60 * 1000));
+}
+
+TEST(IntlTimeZone, GetDSTOffsetMs)
+{
+  auto timeZone = TimeZone::TryCreate(Some(MakeStringSpan(u"GMT+3"))).unwrap();
+  ASSERT_EQ(timeZone->GetDSTOffsetMs(0).unwrap(), 0);
+
+  timeZone = TimeZone::TryCreate(Some(MakeStringSpan(u"Etc/GMT+3"))).unwrap();
+  ASSERT_EQ(timeZone->GetDSTOffsetMs(0).unwrap(), 0);
+
+  timeZone =
+      TimeZone::TryCreate(Some(MakeStringSpan(u"America/New_York"))).unwrap();
+  ASSERT_EQ(timeZone->GetDSTOffsetMs(0).unwrap(), 0);
+  ASSERT_EQ(timeZone->GetDSTOffsetMs(RELEASE_DATE).unwrap(),
+            1 * 60 * 60 * 1000);
+  ASSERT_EQ(timeZone->GetDSTOffsetMs(DST_CHANGE_DATE).unwrap(),
+            1 * 60 * 60 * 1000);
+}
+
+TEST(IntlTimeZone, GetOffsetMs)
+{
+  auto timeZone = TimeZone::TryCreate(Some(MakeStringSpan(u"GMT+3"))).unwrap();
+  ASSERT_EQ(timeZone->GetOffsetMs(0).unwrap(), 3 * 60 * 60 * 1000);
+
+  timeZone = TimeZone::TryCreate(Some(MakeStringSpan(u"Etc/GMT+3"))).unwrap();
+  ASSERT_EQ(timeZone->GetOffsetMs(0).unwrap(), -(3 * 60 * 60 * 1000));
+
+  timeZone =
+      TimeZone::TryCreate(Some(MakeStringSpan(u"America/New_York"))).unwrap();
+  ASSERT_EQ(timeZone->GetOffsetMs(0).unwrap(), -(5 * 60 * 60 * 1000));
+  ASSERT_EQ(timeZone->GetOffsetMs(RELEASE_DATE).unwrap(),
+            -(4 * 60 * 60 * 1000));
+  ASSERT_EQ(timeZone->GetOffsetMs(DST_CHANGE_DATE).unwrap(),
+            -(4 * 60 * 60 * 1000));
+}
+
+TEST(IntlTimeZone, GetUTCOffsetMs)
+{
+  auto timeZone = TimeZone::TryCreate(Some(MakeStringSpan(u"GMT+3"))).unwrap();
+  ASSERT_EQ(timeZone->GetUTCOffsetMs(0).unwrap(), 3 * 60 * 60 * 1000);
+
+  timeZone = TimeZone::TryCreate(Some(MakeStringSpan(u"Etc/GMT+3"))).unwrap();
+  ASSERT_EQ(timeZone->GetUTCOffsetMs(0).unwrap(), -(3 * 60 * 60 * 1000));
+
+  timeZone =
+      TimeZone::TryCreate(Some(MakeStringSpan(u"America/New_York"))).unwrap();
+  ASSERT_EQ(timeZone->GetUTCOffsetMs(0).unwrap(), -(5 * 60 * 60 * 1000));
+  ASSERT_EQ(timeZone->GetUTCOffsetMs(RELEASE_DATE).unwrap(),
+            -(4 * 60 * 60 * 1000));
+  ASSERT_EQ(timeZone->GetUTCOffsetMs(DST_CHANGE_DATE).unwrap(),
+            -(5 * 60 * 60 * 1000));
+}
+
+TEST(IntlTimeZone, GetDisplayName)
+{
+  using DaylightSavings = TimeZone::DaylightSavings;
+
+  TestBuffer<char16_t> buffer;
+
+  auto timeZone = TimeZone::TryCreate(Some(MakeStringSpan(u"GMT+3"))).unwrap();
+
+  buffer.clear();
+  timeZone->GetDisplayName("en", DaylightSavings::No, buffer).unwrap();
+  ASSERT_EQ(buffer.get_string_view(), u"GMT+03:00");
+
+  buffer.clear();
+  timeZone->GetDisplayName("en", DaylightSavings::Yes, buffer).unwrap();
+  ASSERT_EQ(buffer.get_string_view(), u"GMT+03:00");
+
+  timeZone = TimeZone::TryCreate(Some(MakeStringSpan(u"Etc/GMT+3"))).unwrap();
+
+  buffer.clear();
+  timeZone->GetDisplayName("en", DaylightSavings::No, buffer).unwrap();
+  ASSERT_EQ(buffer.get_string_view(), u"GMT-03:00");
+
+  buffer.clear();
+  timeZone->GetDisplayName("en", DaylightSavings::Yes, buffer).unwrap();
+  ASSERT_EQ(buffer.get_string_view(), u"GMT-03:00");
+
+  timeZone =
+      TimeZone::TryCreate(Some(MakeStringSpan(u"America/New_York"))).unwrap();
+
+  buffer.clear();
+  timeZone->GetDisplayName("en", DaylightSavings::No, buffer).unwrap();
+  ASSERT_EQ(buffer.get_string_view(), u"Eastern Standard Time");
+
+  buffer.clear();
+  timeZone->GetDisplayName("en", DaylightSavings::Yes, buffer).unwrap();
+  ASSERT_EQ(buffer.get_string_view(), u"Eastern Daylight Time");
+}
+
+TEST(IntlTimeZone, GetCanonicalTimeZoneID)
+{
+  TestBuffer<char16_t> buffer;
+
+  // Providing a canonical time zone results in the same string at the end.
+  TimeZone::GetCanonicalTimeZoneID(MakeStringSpan(u"America/Chicago"), buffer)
+      .unwrap();
+  ASSERT_EQ(buffer.get_string_view(), u"America/Chicago");
+
+  // Providing an alias will result in the canonical representation.
+  TimeZone::GetCanonicalTimeZoneID(MakeStringSpan(u"Europe/Belfast"), buffer)
+      .unwrap();
+  ASSERT_EQ(buffer.get_string_view(), u"Europe/London");
+
+  // An unknown time zone results in an error.
+  ASSERT_TRUE(TimeZone::GetCanonicalTimeZoneID(
+                  MakeStringSpan(u"Not a time zone"), buffer)
+                  .isErr());
+}
+
+TEST(IntlTimeZone, GetAvailableTimeZones)
+{
+  constexpr auto EuropeBerlin = MakeStringSpan("Europe/Berlin");
+  constexpr auto EuropeBusingen = MakeStringSpan("Europe/Busingen");
+
+  auto timeZones = TimeZone::GetAvailableTimeZones("DE").unwrap();
+
+  bool hasEuropeBerlin = false;
+  bool hasEuropeBusingen = false;
+
+  for (auto timeZone : timeZones) {
+    auto span = timeZone.unwrap();
+    if (span == EuropeBerlin) {
+      ASSERT_FALSE(hasEuropeBerlin);
+      hasEuropeBerlin = true;
+    } else if (span == EuropeBusingen) {
+      ASSERT_FALSE(hasEuropeBusingen);
+      hasEuropeBusingen = true;
+    } else {
+      std::string str(span.data(), span.size());
+      ADD_FAILURE() << "Unexpected time zone: " << str;
+    }
+  }
+
+  ASSERT_TRUE(hasEuropeBerlin);
+  ASSERT_TRUE(hasEuropeBusingen);
+}
+
+TEST(IntlTimeZone, GetAvailableTimeZonesNoRegion)
+{
+  constexpr auto AmericaNewYork = MakeStringSpan("America/New_York");
+  constexpr auto AsiaTokyo = MakeStringSpan("Asia/Tokyo");
+  constexpr auto EuropeParis = MakeStringSpan("Europe/Paris");
+
+  auto timeZones = TimeZone::GetAvailableTimeZones().unwrap();
+
+  bool hasAmericaNewYork = false;
+  bool hasAsiaTokyo = false;
+  bool hasEuropeParis = false;
+
+  for (auto timeZone : timeZones) {
+    auto span = timeZone.unwrap();
+    if (span == AmericaNewYork) {
+      ASSERT_FALSE(hasAmericaNewYork);
+      hasAmericaNewYork = true;
+    } else if (span == AsiaTokyo) {
+      ASSERT_FALSE(hasAsiaTokyo);
+      hasAsiaTokyo = true;
+    } else if (span == EuropeParis) {
+      ASSERT_FALSE(hasEuropeParis);
+      hasEuropeParis = true;
+    }
+  }
+
+  ASSERT_TRUE(hasAmericaNewYork);
+  ASSERT_TRUE(hasAsiaTokyo);
+  ASSERT_TRUE(hasEuropeParis);
+}
+
+TEST(IntlTimeZone, GetTZDataVersion)
+{
+  // From <https://data.iana.org/time-zones/tz-link.html#download>:
+  //
+  // "Since 1996, each version has been a four-digit year followed by lower-case
+  // letter (a through z, then za through zz, then zza through zzz, and so on)."
+  //
+  // More than 26 releases are unlikely or at least never happend. 2009 got
+  // quite close with 21 releases, but that was the first time ever with more
+  // than twenty releases in a single year.
+  //
+  // Should this assertion ever fail, because more than 26 releases were issued,
+  // update it accordingly. And in that case we should be extra cautious that
+  // all time zone functionality in Firefox and in external libraries we're
+  // using can cope with more than 26 tzdata releases.
+  //
+  // Also see <https://mm.icann.org/pipermail/tz/2021-September/030621.html>:
+  //
+  // "For Android having 2021a1 and 2021b would be inconvenient. Because there
+  // are hardcoded places which expect that tzdata version is exactly 5
+  // characters."
+
+  auto version = TimeZone::GetTZDataVersion().unwrap();
+  auto [year, release] = version.SplitAt(4);
+
+  ASSERT_TRUE(std::all_of(year.begin(), year.end(), IsAsciiDigit<char>));
+  ASSERT_TRUE(IsAsciiAlpha(release[0]));
+
+  // ICU issued a non-standard release "2021a1".
+  ASSERT_TRUE(release.Length() == 1 || release.Length() == 2);
+
+  if (release.Length() == 2) {
+    ASSERT_TRUE(IsAsciiDigit(release[1]));
+  }
+}
+
+}  // namespace mozilla::intl
diff --git a/intl/components/gtest/moz.build b/intl/components/gtest/moz.build
new file mode 100644
index 0000000000..c9ccc4cda3
--- /dev/null
+++ b/intl/components/gtest/moz.build
@@ -0,0 +1,30 @@
+# -*- Mode: python; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# 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/.
+
+UNIFIED_SOURCES += [
+    "TestBidi.cpp",
+    "TestCalendar.cpp",
+    "TestCollator.cpp",
+    "TestCurrency.cpp",
+    "TestDateIntervalFormat.cpp",
+    "TestDateTimeFormat.cpp",
+    "TestDisplayNames.cpp",
+    "TestIDNA.cpp",
+    "TestListFormat.cpp",
+    "TestLocale.cpp",
+    "TestLocaleCanonicalizer.cpp",
+    "TestMeasureUnit.cpp",
+    "TestNumberFormat.cpp",
+    "TestNumberingSystem.cpp",
+    "TestNumberParser.cpp",
+    "TestPluralRules.cpp",
+    "TestRelativeTimeFormat.cpp",
+    "TestScript.cpp",
+    "TestString.cpp",
+    "TestTimeZone.cpp",
+]
+
+FINAL_LIBRARY = "xul-gtest"