Auto merge of #4384 - ivanbakel:unused_mut, r=alexcrichton

Fixed some variables being unnecessarily mutable

### Changes

Some variables are marked `mut` when they don't need to be. This PR changes those variables to no longer be `mut`.

### Context

PR on https://github.com/rust-lang/rust/pull/43582

tl:dr; There's a bug with the mutability checker that sometimes marks mutable ref variables as being used when they're not.