You are trying to use an identifier that is either undefined or not a struct.
Erroneous code example:
```compile_fail,E0422
fn main () {
let x = Foo { x: 1, y: 2 };
}
```
In this case, `Foo` is undefined, so it inherently isn't anything, and
definitely not a struct.
```compile_fail
fn main () {
let foo = 1;
let x = foo { x: 1, y: 2 };
}
```
In this case, `foo` is defined, but is not a struct, so Rust can't use it as
one.