refactor: needless_pass_by_ref_mut (#1137)

<https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut>

Cargo.toml

@@ -88,6 +88,7 @@ map_err_ignore = "warn"
 missing_const_for_fn = "warn"
 mixed_read_write_in_expression = "warn"
 mod_module_files = "warn"
+needless_pass_by_ref_mut = "warn"
 needless_raw_strings = "warn"
 redundant_type_annotations = "warn"
 rest_pat_in_fully_bound_structs = "warn"

examples/user_input.rs

@@ -86,7 +86,7 @@ impl App {
     ///
     /// Since each character in a string can be contain multiple bytes, it's necessary to calculate
     /// the byte index based on the index of the character.
-    fn byte_index(&mut self) -> usize {
+    fn byte_index(&self) -> usize {
         self.input
             .char_indices()
             .map(|(i, _)| i)