Bug 1719735 - Add a UTF-16 span overload for FillBuffer; r=platform-i18n-reviewers,dminor

Differential Revision: https://phabricator.services.mozilla.com/D130798

intl/components/src/ICU4CGlue.h

@@ -290,8 +290,9 @@ static ICUResult FillBufferWithICUCall(AutoTArray<CharType, N>& array,
 #endif
 
 /**
- * ICU4C works with UTF-16 strings, but consumers of mozilla::intl may require
- * UTF-8 strings.
+ * Fill a UTF-8 or a UTF-16 buffer with a UTF-16 span. ICU4C mostly uses UTF-16
+ * internally, but different consumers may have different situations with their
+ * buffers.
  */
 template <typename Buffer>
 [[nodiscard]] bool FillBuffer(Span<const char16_t> utf16Span,
@@ -331,25 +332,74 @@ template <typename Buffer>
   return true;
 }
 
+/**
+ * Fill a UTF-8 or a UTF-16 buffer with a UTF-8 span. ICU4C mostly uses UTF-16
+ * internally, but different consumers may have different situations with their
+ * buffers.
+ */
+template <typename Buffer>
+[[nodiscard]] bool FillBuffer(Span<const char> utf8Span, Buffer& targetBuffer) {
+  static_assert(std::is_same_v<typename Buffer::CharType, char> ||
+                std::is_same_v<typename Buffer::CharType, unsigned char> ||
+                std::is_same_v<typename Buffer::CharType, char16_t>);
+
+  if constexpr (std::is_same_v<typename Buffer::CharType, char> ||
+                std::is_same_v<typename Buffer::CharType, unsigned char>) {
+    size_t amount = utf8Span.Length();
+    if (!targetBuffer.reserve(amount)) {
+      return false;
+    }
+    for (size_t i = 0; i < amount; i++) {
+      targetBuffer.data()[i] =
+          // Static cast in case of a mismatch between `unsigned char` and
+          // `char`
+          static_cast<typename Buffer::CharType>(utf8Span[i]);
+    }
+    targetBuffer.written(amount);
+  }
+  if constexpr (std::is_same_v<typename Buffer::CharType, char16_t>) {
+    if (!targetBuffer.reserve(utf8Span.Length() + 1)) {
+      return false;
+    }
+
+    size_t amount = ConvertUtf8toUtf16(
+        utf8Span, Span(targetBuffer.data(), targetBuffer.capacity()));
+
+    targetBuffer.written(amount);
+  }
+
+  return true;
+}
+
 /**
  * It is convenient for callers to be able to pass in UTF-8 strings to the API.
  * This function can be used to convert that to a stack-allocated UTF-16
- * mozilla::Vector that can then be passed into ICU calls.
+ * mozilla::Vector that can then be passed into ICU calls. The string will be
+ * null terminated.
  */
 template <size_t StackSize>
 [[nodiscard]] static bool FillUTF16Vector(
     Span<const char> utf8Span,
     mozilla::Vector<char16_t, StackSize>& utf16TargetVec) {
   // Per ConvertUtf8toUtf16: The length of aDest must be at least one greater
-  // than the length of aSource
+  // than the length of aSource. This additional length will be used for null
+  // termination.
   if (!utf16TargetVec.reserve(utf8Span.Length() + 1)) {
     return false;
   }
+
   // ConvertUtf8toUtf16 fills the buffer with the data, but the length of the
-  // vector is unchanged. The call to resizeUninitialized notifies the vector of
-  // how much was written.
-  return utf16TargetVec.resizeUninitialized(ConvertUtf8toUtf16(
-      utf8Span, Span(utf16TargetVec.begin(), utf16TargetVec.capacity())));
+  // vector is unchanged.
+  size_t length = ConvertUtf8toUtf16(
+      utf8Span, Span(utf16TargetVec.begin(), utf16TargetVec.capacity()));
+
+  // Assert that the last element is free for writing a null terminator.
+  MOZ_ASSERT(length < utf16TargetVec.capacity());
+  utf16TargetVec.begin()[length] = '\0';
+
+  // The call to resizeUninitialized notifies the vector of how much was written
+  // exclusive of the null terminated character.
+  return utf16TargetVec.resizeUninitialized(length);
 }
 
 /**