Using Rust GATs to improve code and application performance

Rust introduced generic associated types (GATs) to address the language’s limitation in expressing certain kinds of generic patterns.

Before GATs, Rust had associated types that enabled type association with traits. However, these were not directly tied to the generic parameters of the implementing type. GATs enhance this concept by allowing associated types to depend on the generic parameters of a trait.

GATs are useful in several scenarios, including the expression of iterator patterns and futures and asynchronous programming. Here, async libraries can leverage GATs to define traits with associated types that accurately represent these dependencies and provide more precise type information for async operations.

In this article, we will explore some ways we can use GATs in Rust to improve our code and application performance. We will cover:

• Getting started with Rust generic associated types
  • When to use GATs in Rust
• Exploring use cases for GATs in Rust programs
  • Avoiding unnecessary allocations
  • Enabling more efficient code generation
  • Improving the ergonomics of your code

Note that GATs became a stable Rust feature in v1.65. Since GATs were unstable before v1.65, you must add #![feature(generic_associated_types)] to use GATs in earlier Rust versions.

Getting started with Rust generic associated types

Rust traits can have associated types. However, before GATS, these associated types could not have their own generic parameters. GATs lift this restriction, allowing associated types to be generic and have a lifetime or type parameter.

Consider this example Rust trait with an associated type:

trait Producer {
    type Item;
    fn produce(&self) -> Self::Item;
}

The Producer trait has a produce() method that takes a reference to self and returns an item of type Self::Item. The Self::Item syntax refers to the associated Item type defined within the trait.

With GATs, you can extend the trait to allow the associated type to have a generic parameter:

trait Producer {
    type Item<T>;
    fn produce<T>(&self, value: T) -> Self::Item<T>;
}

In this case, the Producer trait declares the produce method that takes a value of type T as a parameter and returns an item of type Self::Item<T>. The syntax Self::Item<T> refers to the associated type Item<T> associated with the implementing type.

GATs form a cornerstone in Rust’s type system, offering a previously unattainable level of expressiveness. They allow you to write more generic and reusable code, reducing boilerplate and enhancing code maintainability.

For instance, GATs enable you to define traits that return types with lifetimes tied to self, which previously wasn’t possible. This capability is crucial for creating iterator traits that yield references to their items, amongst other applications.

When to use GATs in Rust

GATs are handy in scenarios where you need to express complex relationships between types and lifetimes in Rust. They are especially useful when designing APIs that need to return types with lifetimes tied to self or when creating generic data structures.

Consider a scenario where you’re creating a data structure that holds a collection of items and provides an iterator over the items. Without GATs, you’ll have to use Box<dyn Iterator>, which incurs a runtime cost.

With GATs, you can express this naturally like so:

trait Iterable {
    type Item;
    type Iterator<'a>: Iterator<Item = &'a Self::Item> where Self: 'a;

    fn iter<'a>(&'a self) -> Self::Iterator<'a>;
}

Here, the iterable trait has two associated types — Item and Iterator. The Item associated type represents a type of the iterable elements, and the Iterator generic associated type represents the Iterator type that the iter method returns.

Exploring use cases for GATs in Rust programs

There are several reasons and use cases for GATs in your everyday Rust programs, including avoiding unnecessary allocations, enabling more efficient code generation, and improving code ergonomics. Let’s explore each of these in more detail below.

Avoiding unnecessary allocations

One everyday use for GATs is avoiding unnecessary allocations. You can prevent unnecessary allocations by defining a trait for iterators that allocates memory when necessary.

Here’s a trait that defines an iterator that can iterate over the elements of a collection and allocates memory for used elements:

#![feature(generic_associated_types)]

trait LendingIterator {
    type Item<'a>: 'a where Self: 'a;
    fn next<'a>(&'a mut self) -> Option<Self::Item<'a>>;
}

struct MyVec<T>(Vec<T>);

impl<T> LendingIterator for MyVec<T> where T: 'static {
    type Item<'a> = &'a T;

    fn next<'a>(&'a mut self) -> Option<Self::Item<'a>> {
        if self.0.is_empty() {
            None
        } else {
            Some(&self.0[0])
        }
    }
}

fn main() {
    let mut my_vec = MyVec(vec![1, 2, 3, 4, 5]);

    while let Some(item) = my_vec.next() {
        println!("{}", item);
        my_vec.0.remove(0);
    }
}

The Rust program above implements a custom iterator with GATs. The LendingIterator trait mimics the behavior of an iterator.

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The type Item<'a>: 'a where Self: 'a; line defines an associated type for the LendingIterator trait.

Meanwhile, fn next<'a>(&'a mut self) -> Option<Self::Item<'a>> is a signature for the next method required for types that implement the LendingIterator trait.

The MyVec type implements the LendingIterator trait, and the main function creates a MyVec and prints the elements before removing the elements from the vector.

Here’s the output from running the program:

Enabling more efficient code generation

GATs can help with more efficient code generation. You can use GATS to define a trait for generic data structures that then generate code for data structures that your program uses. Let’s explore an example to understand this concept better.

Here’s a trait that defines a generic data structure for representing a tree:

trait Tree {
    type Item<'a>: 'a where Self: 'a;
    type Left<'a>: 'a where Self: 'a;
    type Right<'a>: 'a where Self: 'a;

    fn root(&self) -> &Self::Item<'_>;
    fn left(&self) -> Option<&Self::Left<'_>>;
    fn right(&self) -> Option<&Self::Right<'_>>;
}

The Tree trait represents the type of elements in the tree. The root() method returns a reference to the tree’s root. The left() and right() methods return references to the left and right subtrees of the tree, respectively.

The Tree trait doesn’t generate any code. Instead, it leaves the task to the types that implement the trait to decide the methods to implement depending on the specific tree type being represented.

By using generic associated types, the Tree trait allows for flexibility in defining the types of items and left and right subtrees within a tree data structure. The implementing types of the Tree trait will determine the specific types for Item, Left, and Right.

This approach can help you generate more efficient code for various tree data structures.

Improving the ergonomics of your code

You can use GATs to improve your code ergonomics in multiple ways. You can define a trait for generic builders that eases the creation of complex data structures.

Here’s a trait that defines a generic builder for creating trees:

trait TreeBuilder<T> {
    type Tree<'a>;

    fn root<'a>(self, value: T) -> Self::Tree<'a>;
    fn left<'a>(self, value: T) -> Self::Tree<'a>;
    fn right<'a>(self, value: T) -> Self::Tree<'a>;
    fn build<'a>(self) -> Self::Tree<'a>;
}

You can use the TreeBuilder trait to create complex trees easily. The trait provides methods for creating trees with one root value, along with a left and right subtree. The build method returns a reference to the tree.

Conclusion

In this article, we discussed generic associated types (GATs) in Rust. We explored some of their use cases and how you can use them to improve the overall performance of your Rust applications.

GATs provide a more expressive and powerful way to model relationships between generic and associated types in Rust. They enable more precise type information and help you implement advanced patterns and libraries in areas such as iterator design, asynchronous programming, and type-level computations.

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