Rust's Unsafe Code Guidelines Reference
This document is produced by the UCG WG to provide a "guide" for writing unsafe code that "recommends" what kinds of things unsafe code can and cannot do, and that documents which guarantees unsafe code may rely on. It is largely a work-in-progress right now.
Unless we state otherwise, the information in the guide is mostly a "recommendation" and still subject to change. The main goal for the WG in 2019 is to RFC parts of the guide into the language specification, and move those parts into the Rust Language Reference proper.