basic chapter for representation of unions

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reference/src/representation/unions.md

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+# Representation of unions
+
+**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.
+
+[#13]: https://github.com/rust-rfcs/unsafe-code-guidelines/issues/13
+
+The only degree of freedom the compiler has when computing the layout of a union
+like
+
+```rust
+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.
+
+### C-compatible layout ("repr C")
+
+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.
+
+[6.5.8.5]: http://port70.net/~nsz/c/c11/n1570.html#6.5.8p5
+[6.7.2.1.16]: http://port70.net/~nsz/c/c11/n1570.html#6.7.2.1p16
+
+### Default layout ("repr rust")
+
+**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.