The moment that spawned this article was when I asked myself how hard can it be to make sure two types have the same size at compile time? Well… it’s complicated. In here, we’ll do a deep dive into the limits of compile time metaprogamming in today’s (and tonight’s) Rust.

Motivation

In my last article I wrote a function to perform an in place mapping from a Vec<T> to a Vec<U> where the important precondition was that T and U have the same size and alignment. The function looked something like this:

fn map_in_place<T,U,F>(v: Vec<T>, mut f: F) -> Vec<U> 
    where F: FnMut(T) -> U {
    assert_eq!(std::alloc::Layout::<T>::new(), 
        std::alloc::Layout::<U>::new());
    // loads of unsafe code here
}

I won’t go into the unsafe code here because that was the topic of the aforementioned article. The thing that bugged me was that assert_eq! in there. Not the fact that it was in there at all, but that the panic would only occur at runtime ¹. As a die-hard metaprogramming fan, it felt weird to check a condition at runtime that we can clearly check at compile time. It would be great if we can stick this condition into the function signature, possibly using traits to make it blatantly obvious that we want the types T and U to have the same size and alignment.

Today’s Problem: Same Size for Two Types

For this post, let’s consider a slightly simplified problem and just check that the two types T and U have the same size and not bother with the alignment. This is just to keep the examples concise, because once we figure out how to check for size, adding an alignment requirement is trivial. So what we want is this:

fn do_somehting<T,U>(t: T, u: U) 
    // where: T and U have same size 
{
    // do something
}

// this compiles
do_something(1u8,2u8);
// this also compiles
do_somehting(1f32,2i32);
// this must not compile
do_something(1u8,2u32);

Our goal is to make the compiler accept the code only if our two types T and U have the same size and emit an error message otherwise. The compiler knows the size of a (sized) type at compiletime. That’s why it should be simple enough to find a solution that enforces identical size at compile time,right? Right?

Using Associated Constants

My first intuition was that associated constants would help us elegantly enforce trait bounds to restrict our do_something function arguments to types with same size.

There are many ways to skin this cat and my ideas were definitely influenced by how metaprogramming in C++ uses associated types and compile time constants to give us metafunctions. I married this with a more Rusty idea of trait bounds and I came up with the following, pretty straightforward, code:

pub trait SameSizeAs<T> {
    const VALUE: bool;
}

impl<T,U> SameSizeAs<T> for U {
    const VALUE: bool = 
        std::mem::size_of::<T>() == std::mem::size_of::<U>();
}

So what we do is implement a trait SameSizeAs<T> for every type U, which indicates whether T and U have the same size via an associated constant. That’s not too bad. We can use the trait like so:

pub fn do_something<T,U>(t: T, u: U) 
where U: SameSizeAs<T,VALUE=true> {
    // do something
}

I find this pretty elegant and concise and it turns out the error messages are very readable if we try to call the function with two types of different size. There’s just one problem with this: it does not compile on stable Rust. Current stable Rust (1.72 at the time of writing) does not allow us to compare associated constants for equality; we need the feature associated_const_equality to compile it. I found that a bit disappointing because I liked the simplicity of the solution and I would like this to work on stable Rust.

For completeness let me link to another known way of using compile time booleans in where clauses via a clever combination of Const Generics and Traits. However, it requires the unstable feature generic_const_exprs. I won’t go into detail here but we will see this feature pop up in a different context.

Using Associated Types

So the problem with the solution above was that we cannot yet compare associated constants for equality in trait bounds. But we definitely can compare types for equality in trait bounds and so that is the core of the next approach I took. Translate the boolean values into types and do the equality comparison on the types rather than the values. So that’s what I tried next.

Coming from C++, I know that metaprogramming with types can get a bit hairy at times. However, I was pretty confident that I could find a solution. Because after all I was only trying to make the compiler enforce something that it already knows!

Now, since I didn’t want to use actual boolean values at compile time I had to translate the idea of booleans into types:

struct TrueType;
struct FalseType;

trait BoolType {}
impl BoolType for TrueType {}
impl BoolType for FalseType {}

Strictly speaking, the whole BoolType trait is not necessary but I feel it makes the downstream code easier to read. Now we can define a trait that tells us whether a type T that implements it has the same size as another type U:

pub trait SameSizeAs<U> {
    type Value : BoolType;
}

You can see why I like the BoolType trait here: it mirrors the syntax we would use to define the type of a struct field or an associated constant. Compare this implementation with the way we did it above. Finally, we can add a nice where clause into our function definition:

pub fn do_something<T,U>(t: T, u: U) 
where T: SameSizeAs<U,Value=TrueType> {
    // do something
}

This reads very similar to the constant based code above but now it is fine to write Value=TrueType in the where clause in stable Rust. The reason is that we are testing for equality of an associated type and not a compile time constant value.

Finally, there is just one thing missing and that is to write a blanket implementation for SameSizeAs that serves our purpose. We need to have some way to go from a compile time known condition (a const bool) to a type. Since Rust 1.51 we have Const Generics to help us make this transition. That’s the only way I saw how to do that. In C++ we would use a templated struct with boolean template parameters and associated typed. In Rust we can to a similar thing when we bring traits into the mix:

pub struct Condition<const B: bool>;

pub trait TruthType {
    type ValueType : BoolType;
}

impl TruthType for Condition<true> {
    type ValueType = TrueType;
}

impl TruthType for Condition<false> {
    type ValueType = FalseType;
}

We can use the struct and the trait together to go from a compile time known condition to a type. Unfortunately, as of the time of writing we cannot simply use it as Condition<Cond>::ValueType but we have to use the fully qualified type so that the compiler can understand the associated type, even if it is actually unambiguous. That means we must use it as <Condition<Cond> as TruthType>::ValueType which is a bit cumbersome but does the trick ²:

impl<T,U> SameSizeAs<U> for T 
where Condition<{core::mem::size_of::<T>() 
        == core::mem::size_of::<U>()}>: TruthType {
    type Value = <Condition<{core::mem::size_of::<T>() 
        == core::mem::size_of::<U>()}> as TruthType>::ValueType;
}

We have now created a metafunction that transforms a compile time known boolean into a type. We can use it to find out whether two given types are of the same size. That’s great and all, but we again have to use an unstable feature for that. This where the feature generic_const_exprs pops up again. We need this to use generic parameters T and U as part of the Const Generic parameter for Condition. It’s a bit unfortunate since the whole exercise was to go from a compile time boolean to a type and it seems to me we need an unstable feature to accomplish that in our particular case. I would be happy to be proven wrong here.

Be that as it may, we can now use our type and trait to restrict the generic types passed to our do_something function:

pub fn do_something<T,U>(t: T, u: U) 
where T: SameSizeAs<U,Value=TrueType> {
    // do something 
}

Now the compiler will let us invoke do_something with types of the same size and will give an error otherwise. I find it hard to compare which unsafe feature has a better chance of making it to stable soon, but it is worth noting that as of now, generic_const_exprs is still described as “highly experimental” in the associated tracking issue and that the compiler issues a dedicated warning when it is used.

Rethinking and Making it Work on Stable

There is another way to go about the whole problem, which does not involve traits. For a while stable Rust has offered the possibility of panicking in const evaluated contexts. A panic in const context will produce a compile error, though I can’t find the exact Rust version that stabilized it. Framing the problem like this makes it conceptually similar to a static_assert in C++, though it is not quite as straightforward.

What we need to do to invoke a const panic is to force the compiler to constant evaluate the panic. What we do is:

const ASSERTION : () = assert!(Cond,"condition was not satisfied");

Here, Cond needs to be a compile time known boolean. This code produces a compile error if and only if Cond evaluates to false. So now we might just try to replace the runtime assertion in our function by a compile time assertion like so:

fn do_something<T,U>(t: T, u: U) {
    const ASSERTION : () = assert!(core::mem::size_of::<T>()
                            ==core::mem::size_of::<U>(),
                           "T and U must have the same size");
    // do something
}

However, this does not compile because the compiler points to T and U with the error message use of generic parameter from outer function. What does that mean? The way it was explained to me is that const items exist as if they were global, even if they were defined inside a function. That is why we cannot access the generic parameters of the function in the const item ASSERTION. But there is a way around it. Let’s make ASSERTION an associated constant of a struct:

struct SameSize<T, U> {
    phantom: std::marker::PhantomData<(T, U)>,
}

impl<T,U> SameSize<T,U> {
    const ASSERTION: () = assert!(std::mem::size_of::<T>() 
                           == std::mem::size_of::<U>(),
                          "types do not have the same size");
}

Now what we have to do is force the creation of that constant inside the function. But we can’t just use another const item to do that because that would, again, not allow us to access the types T and U for the reasons stated above. However, we can do it in a context that is not const evaluated and whose only purpose is to force the monomorphization of the compile time assertion we are interested in.

pub fn do_something<T,U>(t: T, u: U) {
    _ = SameSize::<T,U>::ASSERTION;
    // do something
}

Now when we try to invoke do_something with types of different sizes the compiler will print an error message. This one finally works on stable Rust, which is pretty satisfying. However, while it is nice that this does work at compile time, there is no indication in the function signature that we require T and U to be of the same size. We must relegate this fact to the documentation.

Providing Fallback Implementations

The stated goal of this article was to enforce that T and U have the same size at compile time and we have achieved that in different ways, one of which works on stable. But what if we did not want to issue a compile error in case T and U have different sizes but rather provide a fallback implementation? Let’s go very briefly through the presented solutions starting with the last one:

I see no way of using compile time assertions for branching in code generation because their only purpose is to emit a compile error. So that one is out, I think. The case is different when using associated types in traits, because in principle we could write two incarnations of do_something: one where T: SameSizeAs<U,Value=TrueType> and one where T: SameSizeAs<U,Value=FalseType>. However, currently the trait solver in Rust does not recognize these two things as disjoint cases so that one won’t work yet. There’s some clever ways around those limitations, but I am not sure they’ll work for this case. You can read all about it –shameless plug incoming– in my article on mutually exclusive traits in Rust. Lastly, using associated constants: again, we could in theory write two implementations, but as of now the cases U: SameSizeAs<T,VALUE=true> and U: SameSizeAs<T,VALUE=false> are not recognized as disjoint. However, it is stated as a future goal in the associated tracking issue.

If you are aware of specialization you’ll recognize that this would offer another way of providing a fallback implementation. It does not work quite like the solutions outlined above but it can be used to achieve something to that effect. Specialization is a big complex of features that is, as of the time of writing, unsound and even a minimal subset is still unstable.

Fallback Implementations without Specialization

This section is an idea that was posted by reddit user u/Dragon-Hatcher here that has to be one of the most brilliant applications of the KISS principle I have seen. Let’s just do the obvious thing and stick an if inside the function like so:

fn do_stuff<T,U>(t: T, u: U) {
    use std::mem::size_of;
    if size_of::<T>() == size_of::<U>() {
        // do one thing
    } else {
        // do another thing
    }
}

What happens here is that the compiler will evaluate the condition at compile time and just optimize out the branch that is not taken. Don’t believe me? Try it on godbolt. The one thing I don’t know is that the compiler will always evaluate a const fn at compile time when it can ³, but here it clearly works.

Final Thoughts

First of all, I’m happy to hear all the things I got wrong in this article because this is indeed a complex topic. Secondly, I would be interested in other ways to solve this problem that I missed here, especially ones that work on stable.

While this writeup has been fun, it has demonstrated to me that interacting with types in nontrivial ways during metaprogramming in Rust is hard, especially in the context of conditional compilation. Furthermore, the trait system still has some rough edges, where stuff that intuitively should work does not ⁴. That’s a compliment to Rust because it is surprising to run into these problems in such a well designed language. I’m also not trying to say that the current trait system is badly implemented because when it works (which is almost all of the time) it works amazingly, but this exercise would have been a oneliner in Modern C++™ ⁵.

Other Solutions

I’ll collect some other solutions that people send me here:

• I already added a section on fallbacks without specialization above. (thanks again u/Dragon-Hatcher)
• Another nightly solution that works entirely inside a where clause. (thanks u/matthieum, thanks u/Dragon-Hatcher)

Endnotes

const rust traits unstable

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