error[E0308]: mismatched types
  --> src/factoid.rs:90:40
   |
90 |     let &mut bs: Iterator<Item=char> = my_base.chars().peekable();
   |                                        ^^^^^^^^^^^^^^^^^^^^^^^^^^ expected trait std::iter::Iterator, found struct `std::iter::Peekable`
   |
   = note: expected type `dyn std::iter::Iterator<Item=char>`
              found type `std::iter::Peekable<std::str::Chars<'_>>`