error mutate-immutable

What it means#

A let binding is immutable; only var bindings can be assigned to or mutated through var fn methods and &var borrows (§4.1, §5.1). The same applies to parameters, which are immutable by default.

Example#

fn main() {
  let count = 0
  count += 1      // <- mutate-immutable
}

mutate-immutable

How to fix#

fn main() {
  var count = 0
  count += 1
}

Declare the binding with var, or compute a new value with a new let (shadowing is allowed).

See also#

• not-assignable
• borrow-conflict