How about something more generic like  ```  pub fn sub_span(self, begin: usize, end: usize) -> Span  ```  and, optionally,  ```  pub fn length(&self) -> usize  ```  This would then also solve my use case of pointing an error to part of a template string like the compiler can do  ```    |  6 |     format!("{.}", 1);    |               ^ expected `}` in format string  ```