rust/transmutes.md

% Transmutes

Get out of our way type system! We're going to reinterpret these bits or die trying! Even though this book is all about doing things that are unsafe, I really can't emphasize that you should deeply think about finding Another Way than the operations covered in this section. This is really, truly, the most horribly unsafe thing you can do in Rust. The railguards here are dental floss.

mem::transmute<T, U> takes a value of type T and reinterprets it to have type U. The only restriction is that the T and U are verified to have the same size. The ways to cause Undefined Behaviour with this are mind boggling.

• First and foremost, creating an instance of any type with an invalid state is going to cause arbitrary chaos that can't really be predicted.
• Transmute has an overloaded return type. If you do not specify the return type it may produce a surprising type to satisfy inference.
• Making a primitive with an invalid value is UB
• Transmuting between non-repr(C) types is UB
• Transmuting an & to &mut is UB
• Transmuting to a reference without an explicitly provided lifetime produces an unbound lifetime

mem::transmute_copy<T, U> somehow manages to be even more wildly unsafe than this. It copies size_of<U> bytes out of an &T and interprets them as a U. The size check that mem::transmute has is gone (as it may be valid to copy out a prefix), though it is Undefined Behaviour for U to be larger than T.

Also of course you can get most of the functionality of these functions using pointer casts.