E1015

The mutability is never used. It is a common mistake to declare an array to be mutable when it is not necessary. Setting the value of an element in an array does not require the array to be mutable. For example, a[0] = 0 does not require a to be mutable, but a = [0, 1, 2] does.

Erroneous example

fn main {
  let mut x = [1, 2, 3] // Warning: The mutability of 'x' is never used.
  x[0] = 0
  println(x)
}

Suggestion

Remove the mut keyword from the variable declaration.

fn main {
  let x = [1, 2, 3]
  x[0] = 0
  println(x)
}