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diff --git a/intl/components/gtest/TestBuffer.h b/intl/components/gtest/TestBuffer.h
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+++ b/intl/components/gtest/TestBuffer.h
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+/* 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