diff --git a/src/de/mod.rs b/src/de/mod.rs
index 3c6c1c7..335995f 100644
--- a/src/de/mod.rs
+++ b/src/de/mod.rs
@@ -458,6 +458,7 @@ static UTF8_CHAR_WIDTH: [u8; 256] = [
 4,4,4,4,4,0,0,0,0,0,0,0,0,0,0,0, // 0xFF
 ];
 
+// This function is a copy of core::str::utf8_char_width
 fn utf8_char_width(b: u8) -> usize {
     UTF8_CHAR_WIDTH[b as usize] as usize
 }