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e025: Traits Deep Dive, Part III

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Hello, I’m Chris Krycho and this is New Rustacean: a show about the Rust programming language and the people who use it. This is episode 24: Traits Deep Dive, Part III. In the first part of this deep dive series, we talked about how to write and implement traits. In the second part, we talked about using traits as bounds on generic types. In this third and hopefully final part, we’re going to look at using traits directly in argument and return position, including the new impl Trait syntax and the important concept of object safety. If that sounds like a lot to cover to you, it sounds like it to me too, so let’s jump in!

First, though, a quick correction from the last traits deep dive episode: thoughtful commenters on both Reddit and Hacker News correctly pointed out that I was mistaken in my discussion of universal and existential types! I said that in the case of generics with trait bounds, arguments are always “universal” and return values are always “existential” – but this isn’t true with generics – the Iterator::collect method example I discussed on the show is a prime case where the caller is in control of and specifies a specific return type, after all! Thanks for that correction!

Plain traits as arguments and return values

There are several ways you’ve been able to use traits without generics since Rust 1.0. However, they have historically had some gotchas, many of which just got resolved with Rust 1.26 and 1.27, and one of which will be resolved in the future. So let’s dig in!


I’ve spent quite a while thinking about how to explore this particular idea, and had gotten stuck repeatedly. Happily, a couple weeks ago while I was on vacation, a reader of my Exploring 4 Languages series had a question from the most recent post there, on Starting to Model the Domain. In that post, I noted that Rust lets us write out function types as type aliases if we want—though I noted that writing out type aliases for functions isn’t usually something we’d see.

The reader who emailed me asked:

Could you give a small code example of how to use a function type alias like this Rust? It is very hard to find an example of this usage (probably because it is unconventional in Rust).

And as it turns out, this is a perfect setup for the rest of this episode!

One time when it’s convenient to write out an alias for a function – not just for “narrowing” a generic or something like that – is when you have some complex or nested transformation you want to apply to a data structure in the context of a std::iter::Iterator::map invocation. You might want it to be testable, and you might want it to be passed into some other function for the sake of abstraction (maybe there are multiple valid kinds of transformations from input type to output type that you’re interested in).

For example, let’s say we want to write a function which lets us get the distances of a set of points from some other point. Let’s assume some basic machinery:

use std::{

struct Point {
    x: f32,
    y: f32,

impl<'p> Sub for &'p Point {
	type Output = Point;

	fn sub(self, other: &Point) -> Point {
		Point {
			x: self.x - other.x,
			y: self.y - other.y,

Now, if we wanted to get the distances between one of those points and another, we could do that inline, using the Pythagorean theorem:

fn main() {
    let a = Point { x: 1.0, y: 2.0 };
    let b = Point { x: -4.5, y: 12.2 };
    let change = a - b;
    let distance = (change.x.powi(2) + change.y.powi(2)).sqrt();

So far so good, but what if we had a bunch of these points – say, a Vec of them – and wanted to get their distance from some other point? Well, then it makes sense to iterate over them and use the std::iter::Iterator::map method. To do that we need a closure:

fn main() {
	let points = vec![
		Point { x: 1.0, y: 2.0 },
		Point { x: 12.0, y: 4.3 },
		Point { x: -5.4, y: 18.7 },

    let origin = Point::default();
    let distances: Vec<f32> = points
        .map(|&point| {
            let change = point - origin;
            let distance = (change.x.powi(2) + change.y.powi(2)).sqrt();

Now, what if we find this “get the distance between two points” function useful and wanted to extract it? Well, we could do that by extracting a function which takes two points by reference and returns the distance between them:

fn distance_between(a: &Point, b: &Point) -> f32 {
    let change = a - b;
    (change.x.powi(2) + change.y.powi(2)).sqrt();

And this is fine – when we want to use it in the context we laid about above, we’d just have |point| distance_between(&point, &origin) in our map invocation. However, we could actually go one step further here and let ourselves create a function which we can “preload” with the desired comparison point: a function distance_from which takes a Point as its argument and returns a function which takes another Point as its argument and then returns the distance between the two. Then you could just write the invocation like this:

	let origin = Point::default();
	let distances: Vec<f32> = points

That’s much clearer to read (at least to me). Unfortunately, though, writing out the signature for that function has been a pretty complicated thing, historically. For one thing, you had to wrap that up behind a heap allocated pointer, like: Box<Fn(Point) -> f32>, and it’s actually really hard to write this type and get the lifetimes all lining up correctly if you want to use references instead of moving all the Point instances:

fn distance_from<'a, 'b: 'a>(offset: &'b Point) -> Box<Fn(&'a Point) -> f32 + 'a> {
	Box::new(move |point| {
		let change = point - offset;
		(change.x.powi(2) + change.y.powi(2)).sqrt()

Now, to get back to my reader’s question, it would be really nice, since Fn is a trait, if we could write a type alias for this! Something like:

type DistanceFrom<'a> = Fn(&'a Point) -> f32;

Then we could use that in our definition:

fn distance_from<'a, 'b: 'a>(offset: &'b Point) -> DistanceFrom<'a> {
    // ...

Unfortunately, that specific definition isn’t quite valid – remember, we had a box around it before. So the correct definition using a Box would be to wrap that in a box at the call site:

fn distance_from<'a, 'b: 'a>(offset: &'b Point) -> Box<DistanceFrom<'a>> {
    // ...

Now that’s an improvement in a number of ways. But with Rust 1.26, we got impl Trait, so can’t we use that here? It would be really nice to write something like this:

type DistanceFrom<'a> = Fn(&'a Point) -> f32;

and then use impl Trait with it:

fn distance_from<'a, 'b: a>(offset: &'b Point) -> impl DistanceFrom<'a> {
    // ...

Unfortunately, we can’t do this… yet. Type aliases can’t be used for traits types like this. However, I’m definitely not the first one to think it’d be useful to be able to write trait aliases! RFC #1733: Trait aliases will give us just that. The syntax would be:

trait DistanceFrom<'a> = Fn(&'a Point) -> f32 + 'a;

Someday! In the meantime, though, we can still use impl Trait – we just can’t use it with the nice convenient name.

Traits in return position

So… on to a topic we’ve been waiting for for quite a few episodes now. Why would we want to use impl Trait instead of a box here?

Most obviously, it simplifies our (non-aliased) function signature and implementation a bit. Now it reads like this:

fn distance_from<'a, 'b: 'a>(offset: &'b Point) -> impl Fn(&'a Point) -> f32 {
	move |point| {
		let change = point - offset;
		(change.x.powi(2) + change.y.powi(2)).sqrt()

The major change here is that the Box is gone – both in the signature and in the body of the function. Historically, we always had to return any trait object – that is, a piece of data where all we care about is that it implements a specific trait – behind a pointer. Why? Well, because that trait object could be pointing to all sorts of different sizes of things. A two-variant enum and a 40-field struct can implement the same trait – but the Rust compiler requires that every output from a function be the same size.

The only candidate – at least historically – has been a heap-allocated pointer! A Box is always the same size. (A regular reference is, too, but then we’d need a lifetime to tie it to, and… there isn’t one available.) Whatever item we could define in the body of this function actually has to be moved out of it, or it’ll get dropped at the end of the function. So our only option here historically was a heap-allocated pointer.

This had a couple important consequences:

  1. We had to pay the price of that heap allocation.

  2. More importantly, it also requires dynamic dispatch—looking up the specific function to execute at runtime instead of at compile time. One of the main reasons for the monomorphization we talked about earlier is to get rid of this runtime overhead.

Those costs are small, and they’re often trivial—the kinds of things you don’t need to worry about. But they are real costs, and in the contexts where Rust is most useful, it does matter. So those were important limitations.

Traits in bare argument position

Many of those limitations also applied to the other place you might have been tempted to use a trait by itself, as the type of an argument. For example, you might think you could write

fn foo(thing: SomeTrait) {}

But the same basic issue is at play as in the return type context. Rust needs an item with a constant size to be able to do this correctly. So we take a reference to the trait type; using a pointer means we have a type with a known size again:

fn foo(thing: &SomeTrait) {}

We can also accept a heap-allocated pointer here, but in general we don’t actually have to: we can just write &SomeTrait and both regular references and also any type which implements the Borrow trait will work. (For a refresher on Borrow you can go back and listen to e018!)

In any case, we have to put the trait type behind a reference, and as such we have dynamic dispatch with its small runtime cost – though in this case we do not (necessarily) have a heap-allocated pointer backing the trait object.

This is extremely handy any time we don’t have a single specific type we’re going to be returning from or dealing with in a given function. For example: anywhere that we might be returning different specific iterator subtypes (Map and Filter, for example), or where we’re operating over a heterogeneous collection, where the only thing we care about in the collection is that every item in it implements some trait.

impl Trait and dyn Trait

We know enough now to see that at least in principle, the Rust compiler should often be able to figure out exactly what type we’re talking about when we want to return a trait type or take a trait type as an argument. After all, it already does exactly that with generics! And generic types with trait bounds notably do not require a heap allocation or dynamic dispatch. The compiler statically figures out the specific types they’re invoked with and return, and monomorphizes them—makes a version for each. So why can’t it do that for trait types too?

As of Rust 1.26, it can. The new impl Trait feature lets us monomorphize the type here. From our perspective writing the code, the type of a closure – that we can’t name concretely, because it’s anonymous and Rust makes up a name at compile time – is just “something which implements the Fn trait with these argument and return types. But Rust compiles it down to the actual single function with that anonymous type. And now, it has a specific return type size, so we can get rid of the pointer. No more heap allocation, and no more runtime lookup cost. That goes for both argument position and return position.

To return to my correction at the beginning of the episode: this is where universal and existential types come into play! (And I got mixed up with them because of how I split this episode into two parts.) impl Trait in argument position is a universal type, an “any” type specified by the caller; and impl Trait in return position is an existential type, a “some specific” type specific by the callee.

The limitation of impl Trait is all those times we still need dynamic allocation and a heap allocated pointer. For those, we have another slight change landing for the Rust 2018 edition, which just reached stable Rust in 1.27: the dyn Trait syntax. Where before, we were allowed to simply write Box<Trait> or Rc<Trait> or even &Trait, now those invocations should be Box<dyn Trait> or Rc<dyn Trait> or &dyn Trait. The reason is to make explicit that there’s a trait object in play, not just a regular struct or enum type. That’s really important for making sure you have the right mental model in place – and, as I noted in my discussion of the feature in the recent Rust 1.27 news episode, it also makes for a nice symmetry between trait objects with dyn Trait and existential and universal types with impl Trait.

Trait objects and object safety

I’ve used the phrase “trait object” often throughout the episode – and for good reason; this idea is important. Any time we’re talking about dealing with some item as a trait behind a pointer rather than as a generic or concrete type, we’re talking about this idea of “trait objects.” Trait objects let us deal with heterogeneous types dynamically at runtime, but safely, much as interfaces do in tradition object oriented languages.

But there is a very important set of rules which govern “trait objects” and allow us to use them safely in Rust, called “object safety.” This set of rules comes up any time you’re trying to use traits for this kind of abstraction in your programming; and since traits are Rust’s primary tool for abstraction, well… it’s going to come up pretty regularly.

A few minutes ago, I noted that the compiler requires a constant size for return values from functions. The compiler captures this with Sized: a marker trait which tells the compiler that the item in question has a constant size known at compile time. (For a review on marker traits, you can go back and listen to episode 22, on the Send and Sync marker traits).

This marker trait has two rules attached to it for object safety.

  1. The trait itself cannot require that the special Self type be Sized. Instead, trait methods can set that requirement when needed—but only when needed!—with a where clause.
  2. All of a trait’s methods must themselves be object safe. There are also two rules for object safety for trait methods:
    1. They cannot have any type parameters, that is, they cannot be generic.
    2. They must not use the Self type themselves.

These rules come down to the reality that when you’re dealing with a trait object, Rust essentially “throws away” the concrete type or types you’re dealing with – as it has to, since it needs to treat every different concrete implementation behind the trait object the same way. But if you reference Self, Rust needs to be able to get back to the concrete type. These things are clearly at odds! So mostly, if you need a trait to be object safe, just avoid referencing Self.

For further reading on object safety, I’ve linked a few important things in the show notes:


That’s a wrap on our our deep dive on traits! The next main teaching episode will be a look at unsafe and the escape hatches it does and doesn’t allow. Also coming up are a look at functional programming ideas in Rust and a pair of Crates You Should Know episodes focused on Futures and Tokio!

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