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40 lines
1.8 KiB
40 lines
1.8 KiB
# Transmutes
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Get out of our way type system! We're going to reinterpret these bits or die
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trying! Even though this book is all about doing things that are unsafe, I
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really can't emphasize that you should deeply think about finding Another Way
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than the operations covered in this section. This is really, truly, the most
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horribly unsafe thing you can do in Rust. The guardrails here are dental floss.
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[`mem::transmute<T, U>`][transmute] takes a value of type `T` and reinterprets
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it to have type `U`. The only restriction is that the `T` and `U` are verified
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to have the same size. The ways to cause Undefined Behavior with this are mind
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boggling.
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* First and foremost, creating an instance of *any* type with an invalid state
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is going to cause arbitrary chaos that can't really be predicted.
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* Transmute has an overloaded return type. If you do not specify the return type
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it may produce a surprising type to satisfy inference.
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* Making a primitive with an invalid value is UB
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* Transmuting between non-repr(C) types is UB
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* Transmuting an & to &mut is UB
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* Transmuting an & to &mut is *always* UB
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* No you can't do it
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* No you're not special
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* Transmuting to a reference without an explicitly provided lifetime
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produces an [unbounded lifetime]
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[`mem::transmute_copy<T, U>`][transmute_copy] somehow manages to be *even more*
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wildly unsafe than this. It copies `size_of<U>` bytes out of an `&T` and
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interprets them as a `U`. The size check that `mem::transmute` has is gone (as
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it may be valid to copy out a prefix), though it is Undefined Behavior for `U`
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to be larger than `T`.
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Also of course you can get most of the functionality of these functions using
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pointer casts.
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[unbounded lifetime]: unbounded-lifetimes.html
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[transmute]: ../std/mem/fn.transmute.html
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[transmute_copy]: ../std/mem/fn.transmute_copy.html
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