Disclaimer: This chapter represents the consensus from issue #13. The statements in here are not (yet) "guaranteed" not to change until an RFC ratifies them.
The only degree of freedom the compiler has when computing the layout of a union like
union U { f1: T1, f2: T2 }is to determine the offset of the fields. The layout of these fields themselves
is already entirely determined by their types, and since we intend to allow
creating references to fields (&u.f1), unions do not have any wiggle-room
there.
For unions tagged #[repr(C)], the compiler will apply the C layout scheme. Per
sections 6.5.8.5 and 6.7.2.1.16 of the C11 specification, this means that
the offset of every field is 0. Unsafe code can case a pointer to the union to
a field type to obtain a pointer to any field, and vice versa.
The default layout of unions is not specified. As of this writing, we want to keep the option of using non-zero offsets open for the future; whether this is useful depends on what exactly the compiler-assumed invariants about union contents are.