"UB" vs "Undefined Behavior" (#349)

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Arthur Milchior 3 years ago committed by GitHub
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@ -135,7 +135,7 @@ In principle, Rust can do some interesting analyses and optimizations based
on this fact. For instance, `Result<T, Void>` is represented as just `T`, on this fact. For instance, `Result<T, Void>` is represented as just `T`,
because the `Err` case doesn't actually exist (strictly speaking, this is only because the `Err` case doesn't actually exist (strictly speaking, this is only
an optimization that is not guaranteed, so for example transmuting one into the an optimization that is not guaranteed, so for example transmuting one into the
other is still UB). other is still Undefined Behavior).
The following *could* also compile: The following *could* also compile:
@ -165,7 +165,7 @@ construct.
`*const ()` (or equivalent) works reasonably well for `void*`, and can be made `*const ()` (or equivalent) works reasonably well for `void*`, and can be made
into a reference without any safety problems. It still doesn't prevent you from into a reference without any safety problems. It still doesn't prevent you from
trying to read or write values, but at least it compiles to a no-op instead trying to read or write values, but at least it compiles to a no-op instead
of UB. of Undefined Behavior.
## Extern Types ## Extern Types

@ -41,7 +41,7 @@ do, but we'll do anyway.
Safe Rust is the *true* Rust programming language. If all you do is write Safe Safe Rust is the *true* Rust programming language. If all you do is write Safe
Rust, you will never have to worry about type-safety or memory-safety. You will Rust, you will never have to worry about type-safety or memory-safety. You will
never endure a dangling pointer, a use-after-free, or any other kind of never endure a dangling pointer, a use-after-free, or any other kind of
Undefined Behavior. Undefined Behavior (a.k.a. UB).
The standard library also gives you enough utilities out of the box that you'll The standard library also gives you enough utilities out of the box that you'll
be able to write high-performance applications and libraries in pure idiomatic be able to write high-performance applications and libraries in pure idiomatic

@ -19,11 +19,11 @@ boggling.
* Transmute has an overloaded return type. If you do not specify the return type * Transmute has an overloaded return type. If you do not specify the return type
it may produce a surprising type to satisfy inference. it may produce a surprising type to satisfy inference.
* Transmuting an `&` to `&mut` is UB. While certain usages may *appear* safe, * Transmuting an `&` to `&mut` is Undefined Behavior. While certain usages may
note that the Rust optimizer is free to assume that a shared reference won't *appear* safe, note that the Rust optimizer is free to assume that a shared
change through its lifetime and thus such transmutation will run afoul of those reference won't change through its lifetime and thus such transmutation will
assumptions. So: run afoul of those assumptions. So:
* Transmuting an `&` to `&mut` is *always* UB. * Transmuting an `&` to `&mut` is *always* Undefined Behavior.
* No you can't do it. * No you can't do it.
* No you're not special. * No you're not special.
@ -32,8 +32,8 @@ boggling.
* When transmuting between different compound types, you have to make sure they * When transmuting between different compound types, you have to make sure they
are laid out the same way! If layouts differ, the wrong fields are going to are laid out the same way! If layouts differ, the wrong fields are going to
get filled with the wrong data, which will make you unhappy and can also be UB get filled with the wrong data, which will make you unhappy and can also be
(see above). Undefined Behavior (see above).
So how do you know if the layouts are the same? For `repr(C)` types and So how do you know if the layouts are the same? For `repr(C)` types and
`repr(transparent)` types, layout is precisely defined. But for your `repr(transparent)` types, layout is precisely defined. But for your

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