LoginChanges to `impl Trait` in Rust 2024
The default way impl Trait works in return position is changing in Rust 2024. These changes are meant to simplify impl Trait to better match what people want most of the time. We're also adding a flexible syntax that gives you full control when you need it.
TL;DR
Starting in Rust 2024, we are changing the rules for when a generic parameter can be used in the hidden type of a return-position impl Trait:
- a new default that the hidden types for a return-position
impl Traitcan use any generic parameter in scope, instead of only types (applicable only in Rust 2024); - a syntax to declare explicitly what types may be used (usable in any edition).
The new explicit syntax is called a "use bound": impl Trait + use<'x, T>, for example, would indicate that the hidden type is allowed to use 'x and T (but not any other generic parameters in scope).
Read on for the details!
Background: return-position impl Trait
This blog post concerns return-position impl Trait, such as the following example:
fn process_data(
data: &[Datum]
) -> impl Iterator<Item = ProcessedDatum> {
data
.iter()
.map(|datum| datum.process())
}
The use of -> impl Iterator in return position here means that the function returns "some kind of iterator". The actual type will be determined by the compiler based on the function body. It is called the "hidden type" because callers do not get to know exactly what it is; they have to code against the Iterator trait. However, at code generation time, the compiler will generate code based on the actual precise type, which ensures that callers are fully optimized.
Although callers don't know the exact type, they do need to know that it will continue to borrow the data argument so that they can ensure that the data reference remains valid while iteration occurs. Further, callers must be able to figure this out based solely on the type signature, without looking at the function body.
Rust's current rules are that a return-position impl Trait value can only use a reference if the lifetime of that reference appears in the impl Trait itself. In this example, impl Iterator<Item = ProcessedDatum> does not reference any lifetimes, and therefore capturing data is illegal. You can see this for yourself on the playground.
The error message ("hidden type captures lifetime") you get in this scenario is not the most intuitive, but it does come with a useful suggestion for how to fix it:
help: to declare that
`impl Iterator<Item = ProcessedDatum>`
captures `'_`, you can add an
explicit `'_` lifetime bound
|
5 | ) -> impl Iterator<Item = ProcessedDatum> + '_ {
| ++++
Following a slightly more explicit version of this advice, the function signature becomes:
fn process_data<'d>(
data: &'d [Datum]
) -> impl Iterator<Item = ProcessedDatum> + 'd {
data
.iter()
.map(|datum| datum.process())
}
In this version, the lifetime 'd of the data is explicitly referenced in the impl Trait type, and so it is allowed to be used. This is also a signal to the caller that the borrow for data must last as long as the iterator is in use, which means that it (correctly) flags an error in an example like this (try it on the playground):
let mut data: Vec<Datum> = vec![Datum::default()];
let iter = process_data(&data);
data.push(Datum::default()); // <-- Error!
iter.next();
Usability problems with this design
The rules for what generic parameters can be used in an impl Trait were decided early on based on a limited set of examples. Over time we have noticed a number of problems with them.
not the right default
Surveys of major codebases (both the compiler and crates on crates.io) found that the vast majority of return-position impl trait values need to use lifetimes, so the default behavior of not capturing is not helpful.
not sufficiently flexible
The current rule is that return-position impl trait always allows using type parameters and sometimes allows using lifetime parameters (if they appear in the bounds). As noted above, this default is wrong because most functions actually DO want their return type to be allowed to use lifetime parameters: that at least has a workaround (modulo some details we'll note below). But the default is also wrong because some functions want to explicitly state that they do NOT use type parameters in the return type, and there is no way to override that right now. The original intention was that type alias impl trait would solve this use case, but that would be a very non-ergonomic solution (and stabilizing type alias impl trait is taking longer than anticipated due to other complications).
hard to explain
Because the defaults are wrong, these errors are encountered by users fairly regularly, and yet they are also subtle and hard to explain (as evidenced by this post!). Adding the compiler hint to suggest + '_ helps, but it's not great that users have to follow a hint they don't fully understand.
incorrect suggestion
Adding a + '_ argument to impl Trait may be confusing, but it's not terribly difficult. Unfortunately, it's often the wrong annotation, leading to unnecessary compiler errors -- and the right fix is either complex or sometimes not even possible. Consider an example like this:
fn process<'c, T> {
context: &'c Context,
data: Vec<T>,
) -> impl Iterator<Item = ()> + 'c {
data
.into_iter()
.map(|datum| context.process(datum))
}
Here the process function applies context.process to each of the elements in data (of type T). Because the return value uses context, it is declared as + 'c. Our real goal here is to allow the return type to use 'c; writing + 'c achieves that goal because 'c now appears in the bound listing. However, while writing + 'c is a convenient way to make 'c appear in the bounds, also means that the hidden type must outlive 'c. This requirement is not needed and will in fact lead to a compilation error in this example (try it on the playground).
The reason that this error occurs is a bit subtle. The hidden type is an iterator type based on the result of data.into_iter(), which will include the type T. Because of the + 'c bound, the hidden type must outlive 'c, which in turn means that T must outlive 'c. But T is a generic parameter, so the compiler requires a where-clause like where T: 'c. This where-clause means "it is safe to create a reference with lifetime 'c to the type T". But in fact we don't create any such reference, so the where-clause should not be needed. It is only needed because we used the convenient-but-sometimes-incorrect workaround of adding + 'c to the bounds of our impl Trait.
Just as before, this error is obscure, touching on the more complex aspects of Rust's type system. Unlike before, there is no easy fix! This problem in fact occurred frequently in the compiler, leading to an obscure workaround called the Captures trait. Gross!
We surveyed crates on crates.io and found that the vast majority of cases involving return-position impl trait and generics had bounds that were too strong and which could lead to unnecessary errors (though often they were used in simple ways that didn't trigger an error).
inconsistencies with other parts of Rust
The current design was also introducing inconsistencies with other parts of Rust.
async fn desugaring
Rust defines an async fn as desugaring to a normal fn that returns -> impl Future. You might therefore expect that a function like process:
async fn process(data: &Data) { .. }
...would be (roughly) desugared to:
fn process(
data: &Data
) -> impl Future<Output = ()> {
async move {
..
}
}
In practice, because of the problems with the rules around which lifetimes can be used, this is not the actual desugaring. The actual desugaring is to a special kind of impl Trait that is allowed to use all lifetimes. But that form of impl Trait was not exposed to end-users.
impl trait in traits
As we pursued the design for impl trait in traits (RFC 3425), we encountered a number of challenges related to the capturing of lifetimes. In order to get the symmetries that we wanted to work (e.g., that one can write -> impl Future in a trait and impl with the expected effect), we had to change the rules to allow hidden types to use all generic parameters (type and lifetime) uniformly.
Rust 2024 design
The above problems motivated us to take a new approach in Rust 2024. The approach is a combination of two things:
- a new default that the hidden types for a return-position
impl Traitcan use any generic parameter in scope, instead of only types (applicable only in Rust 2024); - a syntax to declare explicitly what types may be used (usable in any edition).
The new explicit syntax is called a "use bound": impl Trait + use<'x, T>, for example, would indicate that the hidden type is allowed to use 'x and T (but not any other generic parameters in scope).
Lifetimes can now be used by default
In Rust 2024, the default is that the hidden type for a return-position impl Trait values use any generic parameter that is in scope, whether it is a type or a lifetime. This means that the initial example of this blog post will compile just fine in Rust 2024 (try it yourself by setting the Edition in the Playground to 2024):
fn process_data(
data: &[Datum]
) -> impl Iterator<Item = ProcessedDatum> {
data
.iter()
.map(|datum| datum.process())
}
Yay!
Impl Traits can include a use<> bound to specify precisely which generic types and lifetimes they use
As a side-effect of this change, if you move code to Rust 2024 by hand (without cargo fix), you may start getting errors in the callers of functions with an impl Trait return type. This is because those impl Trait types are now assumed to potentially use input lifetimes and not only types. To control this, you can use the new use<> bound syntax that explicitly declares what generic parameters can be used by the hidden type. Our experience porting the compiler suggests that it is very rare to need changes -- most code actually works better with the new default.
The exception to the above is when the function takes in a reference parameter that is only used to read values and doesn't get included in the return value. One such example is the following function indices(): it takes in a slice of type &[T] but the only thing it does is read the length, which is used to create an iterator. The slice itself is not needed in the return value:
fn indices<'s, T>(
slice: &'s [T],
) -> impl Iterator<Item = usize> {
0 .. slice.len()
}
In Rust 2021, this declaration implicitly says that slice is not used in the return type. But in Rust 2024, the default is the opposite. That means that callers like this will stop compiling in Rust 2024, since they now assume that data is borrowed until iteration completes:
fn main() {
let mut data = vec![1, 2, 3];
let i = indices(&data);
data.push(4); // <-- Error!
i.next(); // <-- assumed to access `&data`
}
This may actually be what you want! It means you can modify the definition of indices() later so that it actually does include slice in the result. Put another way, the new default continues the impl Trait tradition of retaining flexibility for the function to change its implementation without breaking callers.
But what if it's not what you want? What if you want to guarantee that indices() will not retain a reference to its argument slice in its return value? You now do that by including a use<> bound in the return type to say explicitly which generic parameters may be included in the return type.
In the case of indices(), the return type actually uses none of the generics, so we would ideally write use<>:
fn indices<'s, T>(
slice: &'s [T],
) -> impl Iterator<Item = usize> + use<> {
// -----
// Return type does not use `'s` or `T`
0 .. slice.len()
}
Implementation limitation. Unfortunately, if you actually try the above example on nightly today, you'll see that it doesn't compile (try it for yourself). That's because use<> bounds have only partially been implemented: currently, they must always include at least the type parameters. This corresponds to the limitations of impl Trait in earlier editions, which always must capture type parameters. In this case, that means we can write the following, which also avoids the compilation error, but is still more conservative than necessary (try it yourself):
fn indices<T>(
slice: &[T],
) -> impl Iterator<Item = usize> + use<T> {
0 .. slice.len()
}
This implementation limitation is only temporary and will hopefully be lifted soon! You can follow the current status at tracking issue #130031.
Alternative: 'static bounds. For the special case of capturing no references at all, it is also possible to use a 'static bound, like so (try it yourself):
fn indices<'s, T>(
slice: &'s [T],
) -> impl Iterator<Item = usize> + 'static {
// -------
// Return type does not capture references.
0 .. slice.len()
}
'static bounds are convenient in this case, particularly given the current implementation limitations around use<> bounds, but use<> bound are more flexible overall, and so we expect them to be used more often. (As an example, the compiler has a variant of indices that returns newtype'd indices I instead of usize values, and it therefore includes a use<I> declaration.)
Conclusion
This example demonstrates the way that editions can help us to remove complexity from Rust. In Rust 2021, the default rules for when lifetime parameters can be used in impl Trait had not aged well. They frequently didn't express what users needed and led to obscure workarounds being required. They led to other inconsistencies, such as between -> impl Future and async fn, or between the semantics of return-position impl Trait in top-level functions and trait functions.
Thanks to editions, we are able to address that without breaking existing code. With the newer rules coming in Rust 2024,
- most code will "just work" in Rust 2024, avoiding confusing errors;
- for the code where annotations are required, we now have a more powerful annotation mechanism that can let you say exactly what you need to say.
Appendix: Relevant links
- Precise capture was proposed in RFC #3617, which left an unresolved question regarding syntax, and its tracking issue was #123432.
- The unresolved syntax question was resolved in issue #125836, which introduced the
+ use<>notation used in this post. - The implementation limitation is tracked in #130031.
