error[E0308]: mismatched types
  --> src/format.rs:51:5
   |
51 |   ) -> Option<String> {
   |        -------------- expected `std::option::Option<std::string::String>` because of return type
52 | /     for ann in annotations {
53 | |         match (ann.range.0, ann.range.1) {
54 | |             (None, None) => continue,
55 | |             (Some(start), Some(end)) if start > end_index || end < start_index => continue,
...  |
71 | |         }
72 | |     }
   | |_____^ expected enum `std::option::Option`, found ()
   |
