For now, this reference is a best-effort document. We strive for validity and completeness, but are not yet there. In the future, the docs and lang teams will work together to figure out how best to do this. Until then, this is a best-effort attempt. If you find something wrong or missing, file an issue or send in a pull request.

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.