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 }