Variable that is never reassigned

Built from the repository at commit a8614dc.

A binding declared with let mut must be reassigned somewhere in its scope: by =, by a compound assignment such as +=, or by a write to one of its elements, and before a later let of the same name in the same scope shadows it. When it never is, declare it with let.

W0011 warns in place of W0002 when a while condition that nothing in the loop changes reads the binding.

W0014 warns in place of W0002 for a let mut that nothing reads. W0002 still warns for an unread let mut in a function that no live code uses, or when the program has an error.

Why

mut marks what can change (ADR 0011). A let mut that never changes would tell the reader to watch for a change that never comes, so the keyword must always tell the truth.

Broken

fn main() {
    let mut count = 3
    print(count)
}

What sisu says

warning[W0002]: `count` is never reassigned
 --> W0002.sisu:2:13
  |
2 |     let mut count = 3
  |             ^^^^^
  |
help: declare it with `let`
  |
2 -     let mut count = 3
2 +     let count = 3
  |

sisu: 1 warning
sisu: for more on a code, run `sisu explain <code>`: W0002

Fixed

fn main() {
    let count = 3
    print(count)
}

Related

• E0001: an assignment to a binding that cannot change.