Note: this error code is no longer emitted by the compiler.

Closures cannot mutate immutable captured variables.

Erroneous code example:

let x = 3; // error: closure cannot assign to immutable local variable `x`
let mut c = || { x += 1 };

Make the variable binding mutable:

let mut x = 3; // ok!
let mut c = || { x += 1 };