This error occurs when an attempt is made to reassign an immutable variable.
Erroneous code example:
```compile_fail,E0384
fn main() {
let x = 3;
x = 5; // error, reassignment of immutable variable
}
```
By default, variables in Rust are immutable. To fix this error, add the keyword
`mut` after the keyword `let` when declaring the variable. For example:
```
fn main() {
let mut x = 3;
x = 5;
}
```