I believe this is a regression in rust2018 nightly (it wasn't triggering in earlier rust2018 builds).

Playground

#![deny(warnings)] #[allow(unreachable_code)] pub fn sum_nan() { // return; let mut v = 0; assert_eq!(v, 0); v = 1; assert_eq!(v, 1); } fn main() {}

works correctly, but uncommenting the return; (play) errors with:

error: variable does not need to be mutable --> src/main.rs:7:9 | 7 | let mut v = 0; | ----^ | | | help: remove this `mut` 

This only happens with Rust2018 on nightly (Rust2015 works fine).