Union types - The Rust Reference

# Union types

A union type is a nominal, heterogeneous C-like union, denoted by the name of a `union` item.

A union contains the value of any one of its fields. Since the accessing the wrong field can cause unexpected or undefined behaviour, `unsafe` is required to read from a union field or to write to a field that doesn't implement `Copy`.

The memory layout of a `union` is undefined by default, but the `#[repr(...)]` attribute can be used to fix a layout.