Assignment to something that cannot change

Built from the repository at commit a8614dc.

Only a variable, declared with let mut, a mut field, or an element of an array held in one of them can be assigned. A let binding, a parameter, a binding made by for, if let or while let, a field without mut and a constant keep the value they were given. This holds for = and for compound assignments such as +=.

Why

Sisu marks everything that can change with mut (ADR 0011), so a reader knows that a binding or field without it keeps its value. A parameter keeps the value it was passed, so assigning to one never changes the caller's value.

Broken

fn main() {
    let total = 0
    total = 5
    print(total)
}

What sisu says

error[E0001]: cannot assign to `total`
 --> E0001.sisu:3:5
  |
2 |     let total = 0
  |         ----- declared with `let` here
3 |     total = 5
  |     ^^^^^^^^^ cannot assign twice
  |
help: declare it with `let mut`
  |
2 |     let mut total = 0
  |         +++

sisu: 1 error
sisu: for more on a code, run `sisu explain <code>`: E0001

Fixed

fn main() {
    let mut total = 0
    total = 5
    print(total)
}

Related

• W0002: a let mut that is never reassigned.