Count a pending U+0000 in the UTF-16 max length queries

src/utf_16.rs

@@ -29,8 +29,13 @@ impl Utf16Decoder {
     }
 
     pub fn additional_from_state(&self) -> usize {
+        // A pending BMP code unit is stored in `lead_surrogate` and may be zero.
         1 + if self.lead_byte.is_some() { 1 } else { 0 }
-            + if self.lead_surrogate == 0 { 0 } else { 2 }
+            + if self.lead_surrogate == 0 && !self.pending_bmp {
+                0
+            } else {
+                2
+            }
     }
 
     pub fn max_utf16_buffer_length(&self, byte_length: usize) -> Option<usize> {
@@ -444,6 +449,34 @@ mod tests {
         }
     }
 
+    #[test]
+    fn test_utf_16le_pending_nul_max_length() {
+        // An unpaired high surrogate followed by U+0000 leaves U+0000 pending.
+        // The max length queries must count it like any other pending code unit.
+        let mut decoder = UTF_16LE.new_decoder_without_bom_handling();
+        let mut output = [0u8; 16];
+        let (result, _, _) =
+            decoder.decode_to_utf8_without_replacement(&[0x00, 0xD8], &mut output[..], false);
+        assert_eq!(result, DecoderResult::InputEmpty);
+        let (result, read, _) = decoder.decode_to_utf8_without_replacement(
+            &[0x00, 0x00, 0x00, 0x4E, 0x00, 0x4E],
+            &mut output[..],
+            false,
+        );
+        assert_eq!(result, DecoderResult::Malformed(2, 2));
+        assert_eq!(read, 2);
+        let rest = [0x00, 0x4E, 0x00, 0x4E];
+        let max = decoder
+            .max_utf8_buffer_length_without_replacement(rest.len())
+            .unwrap();
+        let mut output = vec![0u8; max];
+        let (result, read, written) =
+            decoder.decode_to_utf8_without_replacement(&rest, &mut output[..], true);
+        assert_eq!(result, DecoderResult::InputEmpty);
+        assert_eq!(read, 4);
+        assert_eq!(&output[..written], "\u{0000}\u{4E00}\u{4E00}".as_bytes());
+    }
+
     #[test]
     fn test_utf_16be_decode_near_end() {
         let mut output = [0u8; 4];