Union类型

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

A union access transmutes 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.

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