This change removes an unused 'mut' qualifier of the 'source' variable
in src/input.rs.

> warning: variable does not need to be mutable
>   --> src/input.rs:52:13
>    |
> 52 |         let mut source = &mut self.source;
>    |             ----^^^^^^
>    |             |
>    |             help: remove this `mut`
>    |
>    = note: #[warn(unused_mut)] on by default