Let's say I wrote this critical piece of code:

fn main() { let mut i = 42; }

rustc gives me the following output:

warning: unused variable: `i` --> src/main.rs:2:9 | 2 | let mut i = 42; | ^^^^^ help: consider using `_i` instead | = note: #[warn(unused_variables)] on by default warning: variable does not need to be mutable --> src/main.rs:2:9 | 2 | let mut i = 42; | ----^ | | | help: remove this `mut` | = note: #[warn(unused_mut)] on by default 

Which is correct -- except for one little detail. Which is crucial, if you are not just any developer but actually rustfix.

You see, the mut in mut i is highlighted, but the replacement is just for the binding name, not the mutability. (Why not the mutability? Because the mutability is covered by the next lint we trigger, unused_mut.) And rustfix will rightfully complain if we try to replace parts of the code that we have already replaced.

tl;dr We need to change the span in unused_variables to only cover the i