+---
+title: "Why even unused data needs to be valid"
+categories: rust
+---
+
+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 unsafe 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.
+
+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.
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