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A union expression does not have exactly one field.
Erroneous code example:
```compile_fail,E0784
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:
```
union Bird {
pigeon: u8,
turtledove: u16,
}
let bird = Bird { pigeon: 0 }; // OK
```
[Rust reference]: https://doc.rust-lang.org/reference/items/unions.html
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