Error code E0531

An unknown tuple struct/variant has been used.

Erroneous code example:

#![allow(unused)]
fn main() {
let Type(x) = Type(12); // error!
match Bar(12) {
    Bar(x) => {} // error!
    _ => {}
}
}

In most cases, it's either a forgotten import or a typo. However, let's look at how you can have such a type:

#![allow(unused)]
fn main() {
struct Type(u32); // this is a tuple struct

enum Foo {
    Bar(u32), // this is a tuple variant
}

use Foo::*; // To use Foo's variant directly, we need to import them in
            // the scope.
}

Either way, it should work fine with our previous code:

#![allow(unused)]
fn main() {
struct Type(u32);

enum Foo {
    Bar(u32),
}
use Foo::*;

let Type(x) = Type(12); // ok!
match Type(12) {
    Type(x) => {} // ok!
    _ => {}
}
}