r[type.union]
r[type.union.intro]
A union type is a nominal, heterogeneous C-like union, denoted by the name of
a union item.
r[type.union.access] 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.
r[type.union.safety]
Since transmutes can cause unexpected or undefined behaviour, unsafe is
required to read from a union field.
r[type.union.constraint] Union field types are also restricted to a subset of types which ensures that they never need dropping. See the item documentation for further details.
r[type.union.layout]
The memory layout of a union is undefined by default (in particular, fields do
not have to be at offset 0), but the #[repr(...)] attribute can be used to
fix a layout.