A non-mutable value was assigned a value.
Erroneous code example:
```compile_fail,E0594
struct SolarSystem {
earth: i32,
}
let ss = SolarSystem { earth: 3 };
ss.earth = 2; // error!
```
To fix this error, declare `ss` as mutable by using the `mut` keyword:
```
struct SolarSystem {
earth: i32,
}
let mut ss = SolarSystem { earth: 3 }; // declaring `ss` as mutable
ss.earth = 2; // ok!
```