E0784 - Error codes index
Description: Book listing all Rust error codes

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# Error code E0784

A union expression does not have exactly one field.

Erroneous code example:

    #![allow(unused)]
    fn main() {
    union Bird {
        pigeon: u8,
        turtledove: u16,
    }

    let bird = Bird {}; // error
    let bird = Bird { pigeon: 0, turtledove: 1 }; // error
    }

The key property of unions is that all fields of a union share common storage. As a result, writes to one field of a union can overwrite its other fields, and size of a union is determined by the size of its largest field.

You can find more information about the union types in the Rust reference.

Working example:

    #![allow(unused)]
    fn main() {
    union Bird {
        pigeon: u8,
        turtledove: u16,
    }

    let bird = Bird { pigeon: 0 }; // OK
    }