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.

Unions have no notion of an "active field". Instead, every union access transmutes parts of the content of the union to the type of the accessed field. Since transmutes 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`. See the item documentation for further details.

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