+++ /dev/null
----
-title: "Why even unused data needs to be valid"
-categories: rust
-forum: https://internals.rust-lang.org/t/why-even-unused-data-needs-to-be-valid/12734
----
-
-The Rust compiler has a few assumptions that it makes about the behavior of all code.
-Violations of those assumptions are referred to as [Undefined Behavior][ub].
-Since Rust is a safe-by-default language, programmers usually do not have to worry about those rules (the compiler and libraries ensure that safe code always satisfies the assumptions),
-but authors of `unsafe` code are themselves responsible for upholding these requirements.
-
-Those assumptions are [listed in the Rust reference](https://doc.rust-lang.org/reference/behavior-considered-undefined.html).
-The one that seems to be most surprising to many people is the clause which says that Rust code may not *produce* "[...] an invalid value, even in private fields and locals".
-The reference goes on to explain that "*producing* a value happens any time a value is assigned to or read from a place, passed to a function/primitive operation or returned from a function/primitive operation".
-In other words, even just *constructing*, for example, an invalid `bool`, is Undefined Behavior---no matter whether that `bool` is ever actually "used" by the program.
-The purpose of this post is to explain why that rule is so strict.
-
-[ub]: https://rust-lang.github.io/unsafe-code-guidelines/glossary.html#undefined-behavior
-
-<!-- MORE -->
-
-First of all, let me clarify what is meant by "used" here, as that term is used to mean very different things.
-The following code "uses" `b`:
-
-{% highlight rust %}
-fn example(b: bool) -> i32 {
- if b { 42 } else { 23 }
-}
-{% endhighlight %}
-
-I hope it is not very surprising that calling `example` on, e.g., `3` transmuted to `bool` is Undefined Behavior (UB).
-When compiling `if`, the compiler assumes that `0` and `1` are the only possible values; there is no saying what could go wrong when that assumption is violated.
-(This is a compiler-understood *validity invariant* that is fixed in the language specification, which is very different from a user-defined *safety invariant*.
-See [this earlier post]({% post_url 2018-08-22-two-kinds-of-invariants %}) for more details on that distinction.)
-
-What is less obvious is why calling `example` on `3` is UB even when there is no such `if` being executed.
-To understand why that is important, let us consider the following example:
-
-{% highlight rust %}
-fn example(b: bool, num: u32) -> i32 {
- let mut acc = 0;
- for _i in 0..num {
- acc += if b { 42 } else { 23 };
- }
- acc
-}
-{% endhighlight %}
-
-Now assume we were working in a slightly different version of Rust, where transmuting `3` to a `bool` is fine as long as you do not "use" the `bool`.
-That would mean that calling `example(transmute(3u8), 0)` is actually allowed, because in that case the loop never gets executed, so we never "use" `b`.
-
-However, this is a problem for a very important transformation called [loop-invariant code motion](https://en.wikipedia.org/wiki/Loop-invariant_code_motion).
-That transformation can be used to turn our `example` function into the following:
-
-{% highlight rust %}
-fn example(b: bool, num: u32) -> i32 {
- let mut acc = 0;
- let incr = if b { 42 } else { 23 }
- for _i in 0..num {
- acc += incr;
- }
- acc
-}
-{% endhighlight %}
-
-The increment `if b { 42 } else { 23 }`, now called `incr`, is "invariant" during the execution of the loop, and thus computing the increment can be moved out.
-Why is this a good transformation?
-Instead of determining the increment each time around the loop, we do that just once, thus saving a lot of conditional jumps that the CPU is unhappy about.
-This also enables further transformations down the road, e.g. the compiler could notice that this is just `num*incr`.
-
-However, in our hypothetical Rust where "unused" values may be invalid, this important optimization is actually incorrect!
-To see why, consider again calling `example(transmute(3u8), 0)`.
-Before the optimization, that call was fine.
-After the optimization, that call is UB because we are doing `if b` where `b` is `3`.
-
-This is because loop-invariant code motion makes dead code live when the loop is not actually executed.
-To fix this, we could require the compiler to prove that the loop runs at least once, but in general that will be hard (and in `example` it is impossible, since `num` can indeed be `0`).
-So the alternative that Rust uses is to require that even "unused" data satisfies some basic validity.
-That makes `example(transmute(3u8), 0)` UB in *both* versions of `example`, and as such the optimization is correct for this input (and indeed for all possible inputs).
-
-Now, one might argue that `b` in the example here was not truly "unused", it was just "used in dead code".
-So instead of saying that `b` must be *always* valid, we could somehow try to make the use of `b` in dead code affect whether or not the program is UB.
-However, that is a huge can of worms.
-Right now, we have the fundamental principle that *dead code cannot affect program behavior*.
-This principle is crucial for tools like [Miri](https://github.com/rust-lang/miri/): since Miri is an interpreter, it never even sees dead code.
-I would argue that being able to have tools like Miri is hugely important, and it is worth having a semantics that enables such tools to exist.
-But this means our hands are pretty much tied: since we cannot take into account of `b` is "used in dead code", we simply have to require `b` to always be valid, no matter whether and where it is "used" or not.
-To support inlining and outlining, we also do not want the function boundary to be relevant, which ultimately leads us to the rule that Rust requires today: whenever data of a given type is *produced* anywhere, the data needs to be valid for that type.
-
-I hope this post was helpful in explaining why Undefined Behavior in Rust is defined the way it is.
-As usual, if you have any comments or questions, let me know in the [forums](https://internals.rust-lang.org/t/why-even-unused-data-needs-to-be-valid/12734).
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