mirror of https://github.com/rust-lang/nomicon
You can not select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
36 lines
1.7 KiB
36 lines
1.7 KiB
% 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 an & to &mut is *always* UB
|
|
* No you can't do it
|
|
* No you're not special
|
|
* Transmuting to a reference without an explicitly provided lifetime
|
|
produces an [unbounded 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.
|
|
|
|
|
|
[unbounded lifetime]: unbounded-lifetimes.html
|