fix: use byte length instead of char count

Author: 0xrusowsky

crates/fmt/src/state/mod.rs

@@ -599,8 +599,13 @@ impl<'sess> State<'sess, '_> {
             // Doc comments preserve leading whitespaces (right after the prefix).
             self.word(prefix);
             let content = &line[prefix.len()..];
-            let (leading_ws, rest) =
-                content.split_at(content.chars().take_while(|&c| c.is_whitespace()).count());
+            let (leading_ws, rest) = content.split_at(
+                content
+                    .char_indices()
+                    .take_while(|(_, c)| c.is_whitespace())
+                    .last()
+                    .map_or(0, |(idx, c)| idx + c.len_utf8()),
+            );
             if !leading_ws.is_empty() {
                 self.word(leading_ws.to_owned());
             }