warning: unused variable: `n`
  --> /w/foo.rs:21:13
   |
21 | fn calc(mut n: u32, mut numbers: Vec<u32>) -> u32 {
   |             ^ help: if this is intentional, prefix it with an underscore: `_n`
   |
   = note: `#[warn(unused_variables)]` on by default

warning: variable does not need to be mutable
  --> /w/foo.rs:21:9
   |
21 | fn calc(mut n: u32, mut numbers: Vec<u32>) -> u32 {
   |         ----^
   |         |
   |         help: remove this `mut`
   |
   = note: `#[warn(unused_mut)]` on by default

warning: 2 warnings emitted

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