Error code E0422

An identifier that is neither defined nor a struct was used.

Erroneous code example:

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.

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.