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.