fix: avoid char-boundary panic in NBReader::try_read

Author: SAY-5

src/reader.rs

@@ -203,11 +203,9 @@ impl NBReader {
     pub fn try_read(&mut self) -> Option<char> {
         // discard eventual errors, EOF will be handled in read_until correctly
         let _ = self.read_into_buffer();
-        if !self.buffer.is_empty() {
-            self.buffer.drain(..1).last()
-        } else {
-            None
-        }
+        let first = self.buffer.chars().next()?;
+        self.buffer.drain(..first.len_utf8());
+        Some(first)
     }
 }
 
@@ -458,18 +456,20 @@ mod tests {
         assert_eq!(None, r.try_read());
     }
 
-    // Multi-byte UTF-8 characters in the buffer cause `drain(..1)` to panic at
-    // the char boundary. Marked `should_panic` to capture the current behavior;
-    // the follow-up fix commit removes the attribute and asserts correct decode.
     #[test]
-    #[should_panic = "is_char_boundary"]
     fn test_try_read_multibyte() {
         let f = io::Cursor::new("\u{c3}");
         let mut r = NBReader::new(f, Options::default());
         // pump bytes from the reader thread into the buffer
+        let mut chars = String::new();
         for _ in 0..10 {
-            let _ = r.try_read();
+            if let Some(c) = r.try_read() {
+                chars.push(c);
+            }
             thread::sleep(time::Duration::from_millis(5));
         }
+        // Each raw byte is cast to a char before reaching the buffer, so the two
+        // UTF-8 bytes of "\u{c3}" surface as separate Latin-1 chars.
+        assert_eq!(chars, "\u{c3}\u{83}");
     }
 }