Attempted to access a non-existent field in a struct.

Erroneous code example:

```compile_fail,E0609
struct StructWithFields {
    x: u32,
}

let s = StructWithFields { x: 0 };
println!("{}", s.foo); // error: no field `foo` on type `StructWithFields`
```

To fix this error, check that you didn't misspell the field's name or that the
field actually exists. Example:

```
struct StructWithFields {
    x: u32,
}

let s = StructWithFields { x: 0 };
println!("{}", s.x); // ok!
```