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---
tags: draft-rfc
title: Return position `impl Trait` in traits
---
- Feature Name: return_position_impl_trait_in_traits
- Start Date: 2023-04-27
- RFC PR: [rust-lang/rfcs#3193](https://github.com/rust-lang/rfcs/pull/0000)
- Rust Issue: [rust-lang/rust#0000](https://github.com/rust-lang/rust/issues/0000)
- Initiative: [impl trait initiative](https://github.com/rust-lang/impl-trait-initiative)
# Summary
[summary]: #summary
* Permit `impl Trait` in fn return position within traits and trait impls.
* Allow `async fn` in traits and trait impls to be used interchangeably with its equivalent `impl Trait` desugaring.
* Allow trait impls to `#[refine]` an `impl Trait` return type with added bounds or a concrete type.[^refine]
# Motivation
[motivation]: #motivation
The `impl Trait` syntax is currently accepted in a variety of places within the Rust language to mean "some type that implements `Trait`" (for an overview, see the [explainer] from the impl trait initiative). For function arguments, `impl Trait` is [equivalent to a generic parameter][apit] and it is accepted in all kinds of functions (free functions, inherent impls, traits, and trait impls).
In return position, `impl Trait` [corresponds to an opaque type whose value is inferred][rpit]. This is necessary for returning unnameable types, like those created by closures and `async` blocks, and also a convenient way to avoid naming complicated types like nested iterators. In return position, `impl Trait` is currently accepted only in free functions and inherent impls. This RFC extends the support to traits and trait impls.
[explainer]: https://rust-lang.github.io/impl-trait-initiative/explainer.html
[apit]: https://rust-lang.github.io/impl-trait-initiative/explainer/apit.html
[rpit]: https://rust-lang.github.io/impl-trait-initiative/explainer/rpit.html
## Example use case
The use case for `-> impl Trait` in trait functions is similar to its use in other contexts: traits often wish to return "some type" without specifying the exact type. As a simple example that we will use through the RFC, consider the `NewIntoIterator` trait, which is a variant of the existing `IntoIterator` that uses `impl Iterator` as the return type:
```rust
trait NewIntoIterator {
type Item;
fn into_iter(self) -> impl Iterator<Item = Self::Item>;
}
```
# Guide-level explanation
[guide-level-explanation]: #guide-level-explanation
*This section assumes familiarity with the [basic semantics of impl trait in return position][rpit].*
When you use `impl Trait` as the return type for a function within a trait definition or trait impl, the intent is the same: impls that implement this trait return "some type that implements `Trait`", and users of the trait can only rely on that.
<!--However, the desugaring to achieve that effect looks somewhat different than other cases of impl trait in return position. This is because we cannot desugar to a type alias in the surrounding module; we need to desugar to an associated type (effectively, a type alias in the trait).-->
Consider the following trait:
```rust
trait IntoIntIterator {
fn into_int_iter(self) -> impl Iterator<Item = u32>;
}
```
The semantics of this are analogous to introducing a new associated type within the surrounding trait;
```rust
trait IntoIntIterator { // desugared
type IntoIntIter: Iterator<Item = u32>;
fn into_int_iter(self) -> Self::IntoIntIter;
}
```
When using `-> impl Trait`, however, there is no associated type that users can name.
By default, the impl for a trait like `IntoIntIterator` must also use `impl Trait` in return position.
```rust
impl IntoIntIterator for Vec<u32> {
fn into_int_iter(self) -> impl Iterator<Item = u32> {
self.into_iter()
}
}
```
It can, however, give a more specific type with `#[refine]`:[^refine]
```rust
impl IntoIntIterator for Vec<u32> {
#[refine]
fn into_int_iter(self) -> impl Iterator<Item = u32> + ExactSizeIterator {
self.into_iter()
}
// ..or even..
#[refine]
fn into_int_iter(self) -> std::vec::IntoIter<u32> {
self.into_iter()
}
}
```
Users of this impl are then able to rely on the refined return type, as long as the compiler can prove this impl specifically is being used. Conversely, in this example, code that is generic over the trait can only rely on the fact that the return type implements `Iterator<Item = u32>`.
### async fn desugaring
`async fn` always desugars to a regular function returning `-> impl Future`. When used in a trait, the `async fn` syntax can be used interchangeably with the equivalent desugaring in the trait and trait impl:
```rust
trait UsesAsyncFn {
// Equivalent to:
// fn do_something(&self) -> impl Future<Output = ()> + '_;
async fn do_something(&self);
}
// OK!
impl UsesAsyncFn for MyType {
fn do_something(&self) -> impl Future<Output = ()> + '_ {
async {}
}
}
```
```rust
trait UsesDesugaredFn {
// Equivalent to:
// async fn do_something(&self);
fn do_something(&self) -> impl Future<Output = ()> + '_;
}
// Also OK!
impl UsesDesugaredFn for MyType {
async fn do_something(&self) {}
}
```
# Reference-level explanation
[reference-level-explanation]: #reference-level-explanation
## Equivalent desugaring for traits
Each `-> impl Trait` notation appearing in a trait fn return type is effectively desugared to an anonymous associated type. In this RFC, we will use the placeholder name `$` when illustrating desugarings and the like.
As a simple example, consider the following (more complex examples follow):
```rust
trait NewIntoIterator {
type Item;
fn into_iter(self) -> impl Iterator<Item = Self::Item>;
}
// becomes
trait NewIntoIterator {
type Item;
type $: Iterator<Item = Self::Item>;
fn into_iter(self) -> <Self as NewIntoIterator>::$;
}
```
## Equivalent desugaring for trait impls
Each `impl Trait` notation appearing in a trait impl fn return type is desugared to the same anonymous associated type `$` defined in the trait along with a function that returns it. The value of this associated type `$` is an `impl Trait`.
```rust
impl NewIntoIterator for Vec<u32> {
type Item = u32;
fn into_iter(self) -> impl Iterator<Item = Self::Item> {
self.into_iter()
}
}
// becomes
impl NewIntoIterator for Vec<u32> {
type Item = u32;
type $ = impl Iterator<Item = Self::Item>;
fn into_iter(self) -> <Self as NewIntoIterator>::$ {
self.into_iter()
}
}
```
## Generic parameter capture and GATs
Given a trait method with a return type like `-> impl A + ... + Z` and an implementation of that trait, the hidden type for that implementation is allowed to reference:
* Concrete types, constant expressions, and `'static`
* `Self`
* Generics on the impl
* Certain generics on the method
* Explicit type parameters
* Argument-position `impl Trait` types
* Explicit const parameters
* Lifetime parameters that appear anywhere in `A + ... + Z`, including elided lifetimes
We say that a generic parameter is *captured* if it may appear in the hidden type. These rules are the same as those for `-> impl Trait` in inherent impls.
When desugaring, captured parameters from the method are reflected as generic parameters on the `$` associated type. Furthermore, the `$` associated type brings whatever where clauses are declared on the method into scope, excepting those which reference parameters that are not captured.
This transformation is precisely the same as the one which is applied to other forms of `-> impl Trait`, except that it applies to an associated type and not a top-level type alias.
Example:
```rust
trait RefIterator for Vec<u32> {
type Item<'me>
where
Self: 'me;
fn iter<'a>(&'a self) -> impl Iterator<Item = Self:Item<'a>>;
}
// Since 'a is named in the bounds, it is captured.
// `RefIterator` thus becomes:
trait RefIterator for Vec<u32> {
type Item<'me>
where
Self: 'me;
type $<'a>: impl Iterator<Item = Self::Item<'a>>
where
Self: 'a; // Implied bound from fn
fn iter<'a>(&'a self) -> Self::$<'a>;
}
```
## Validity constraint on impls
Given a trait method where `impl Trait` appears in return position,
```rust
trait Trait {
fn method() -> impl T_0 + ... + T_m;
}
```
where `T_0 + ... + T_m` are bounds, for any impl of that trait to be valid, the following conditions must hold:
* The return type named in the corresponding impl method must implement all bounds `T_0 + ... + T_m` specified in the trait.
* Either the impl method must have `#[refine]`,[^refine] OR
* The impl must use `impl Trait` syntax to name the corresponding type, and
* The return type in the trait must implement all bounds `I_0 + ... + I_n` specified in the impl return type. (Taken with the first outer bullet point, we can say that the bounds in the trait and the bounds in the impl imply each other.)
[^refine]: `#[refine]` was added in [RFC 3245: Refined trait implementations](https://rust-lang.github.io/rfcs/3245-refined-impls.html). This feature is not yet stable. Examples in this RFC requiring the use of `#[refine]` will not work until that feature is stabilized.
Additionally, using `-> impl Trait` notation in an impl is only legal if the trait also uses that notation.
```rust
trait NewIntoIterator {
type Item;
fn into_iter(self) -> impl Iterator<Item = Self::Item>;
}
// OK:
impl NewIntoIterator for Vec<u32> {
type Item = u32;
fn into_iter(self) -> impl Iterator<Item = u32> {
self.into_iter()
}
}
// OK:
impl NewIntoIterator for Vec<u32> {
type Item = u32;
#[refine]
fn into_iter(self) -> impl Iterator<Item = u32> + DoubleEndedIterator {
self.into_iter()
}
}
// OK:
impl NewIntoIterator for Vec<u32> {
type Item = u32;
#[refine]
fn into_iter(self) -> std::vec::IntoIter<u32> {
self.into_iter()
}
}
// Not OK (requires `#[refine]`):
impl NewIntoIterator for Vec<u32> {
type Item = u32;
fn into_iter(self) -> std::vec::IntoIter<u32> {
self.into_iter()
}
}
```
### Interaction with `async fn` in trait
This RFC modifies the “Static async fn in traits” RFC so that async fn in traits may be satisfied by implementations that return `impl Future<Output = ...>` as long as the return-position impl trait type matches the async fn's desugared impl trait with the same rules as above.
```rust
trait Trait {
async fn async_fn();
async fn async_fn_refined();
}
impl Trait for MyType {
fn async_fn() -> impl Future<Output = ()> + '_ { .. }
#[refine]
fn async_fn_refined() -> BoxFuture<'_, ()> { .. }
}
```
Similarly, the equivalent `-> impl Future` signature in a trait can be satisfied by using `async fn` in an impl of that trait.
## Legal positions for `impl Trait` to appear
`impl Trait` can appear in the return type of a trait method in all the same positions as it can in a free function.
For example, return position impl trait in traits may be nested in associated types bounds:
```rust
trait Nested {
fn deref(&self) -> impl Deref<Target = impl Display> + '_;
}
// This desugars into:
trait Nested {
type $1<'a>: Deref<Target = Self::$2> + 'a;
type $2: Display;
fn deref(&self) -> Self::$1<'_>;
}
```
It may also be used in type argument position of a generic type, including tuples:
```rust
trait Foo {
fn bar(&self) -> (impl Debug, impl Debug);
}
// This desugars into:
trait Foo {
type $1: Debug;
type $2: Debug;
fn bar(&self) -> (Self::$1, Self::$2);
}
```
But following the same rules as the allowed positions for return-position impl trait, it is not allowed to be nested in trait generics:
``` rust
trait Nested {
fn deref(&self) -> impl AsRef<impl Sized>; // ❌
}
```
## Dyn safety
To start, traits that use `-> impl Trait` will not be considered dyn safe, *unless the method has a `where Self: Sized` bound*. This is because dyn types currently require that all associated types are named, and the `$` type cannot be named. The other reason is that the value of `impl Trait` is often a type that is unique to a specific impl, so even if the `$` type *could* be named, specifying its value would defeat the purpose of the `dyn` type, since it would effectively identify the dynamic type.
On the other hand, if the method has a `where Self: Sized` bound, the method will not exist on `dyn Trait` and therefore there will be no type to name.
### Dyn safety for `async fn` in trait
This RFC modifies the "Static async fn in traits" RFC to allow traits with `async fn` to be dyn-safe if the method has a `where Self: Sized` bound. This is consistent with how `async fn foo()` desugars to `fn foo() -> impl Future`.
# Drawbacks
[drawbacks]: #drawbacks
This section discusses known drawbacks of the proposal as presently designed and (where applicable) plans for mitigating them in the future.
## Cannot migrate off of impl Trait
In this RFC, if you use `-> impl Trait` in a trait definition, you cannot "migrate away" from that without changing all impls. In other words, we cannot evolve:
```rust
trait NewIntoIterator {
type Item;
fn into_iter(self) -> impl Iterator<Item = Self::Item>;
}
```
into
```rust
trait NewIntoIterator {
type Item;
type IntoIter: Iterator<Item = Self::Item>;
fn into_iter(self) -> Self::IntoIter;
}
```
without breaking semver compatibility for your trait. The [future possibilities](#future-possibilities) section discusses one way to resolve this, by permitting impls to elide the definition of associated types whose values can be inferred from a function return type.
## Clients of the trait cannot name the resulting associated type, limiting extensibility
[As @Gankra highlighted in a comment on a previous RFC][gankra], the traditional `IntoIterator` trait permits clients of the trait to name the resulting iterator type and apply additional bounds:
[gankra]: https://github.com/rust-lang/rfcs/pull/3193#issuecomment-965505149
```rust
fn is_palindrome<Iter, T>(iterable: Iter) -> bool
where
Iter: IntoIterator<Item = T>,
Iter::IntoIter: DoubleEndedIterator,
T: Eq;
```
The `NewIntoIterator` trait used as an example in this RFC, however, doesn't support this kind of usage, because there is no way for users to name the `IntoIter` type (and, as discussed in the previous section, there is no way for users to migrate to a named associated type, either!). The same problem applies to async functions in traits, which sometimes wish to be able to [add `Send` bounds to the resulting futures](https://rust-lang.github.io/async-fundamentals-initiative/evaluation/challenges/bounding_futures.html).
The [future possibilities](#future-possibilities) section discusses a planned extension to support naming the type returned by an impl trait, which could work to overcome this limitation for clients.
# Rationale and alternatives
[rationale-and-alternatives]: #rationale-and-alternatives
### Does auto trait leakage still occur for `-> impl Trait` in traits?
Yes, so long as the compiler has enough type information to figure out which impl you are using. In other words, given a trait function `SomeTrait::foo`, if you invoke a function `<T as SomeTrait>::foo()` where the self type is some generic parameter `T`, then the compiler doesn't really know what impl is being used, so no auto trait leakage can occur. But if you were to invoke `<u32 as SomeTrait>::foo()`, then the compiler could resolve to a specific impl, and hence a specific [impl trait type alias][tait], and auto trait leakage would occur as normal.
[tait]: https://rust-lang.github.io/impl-trait-initiative/explainer/tait.html
### Can traits migrate from a named associated type to `impl Trait`?
Not compatibly, no, because they would no longer have a named associated type.
### Can traits migrate from `impl Trait` to a named associated type?
Generally yes, but all impls would have to be rewritten to include the definition of the associated type.
### Would there be any way to make it possible to migrate from `impl Trait` to a named associated type compatibly?
Potentially! There have been proposals to allow the values of associated types that appear in function return types to be inferred from the function declaration. So the trait has `fn method(&self) -> Self::Iter` and the impl has `fn method(&self) -> impl Iterator`, then the impl would also be inferred to have `type Iter = impl Iterator` (and the return type rewritten to reference it). This may be a good idea, but it is not proposed as part of this RFC.
### What about using a named associated type?
One alternative under consideration was to use a named associated type instead of the anonymous `$` type. The name could be derived by converting "snake case" methods to "camel case", for example. This has the advantage that users of the trait can refer to the return type by name.
We decided against this proposal:
* Introducing a name by converting to camel-case feels surprising and inelegant.
* Return position impl Trait in other kinds of functions doesn't introduce any sort of name for the return type, so it is not analogous.
* We would like to allow `-> impl Trait` methods to work with dynamic dispatch (see [Future possibilities][future-possibilities]). `dyn` types typically require naming all associated types of a trait. That would not be desirable for this feature, and these associated types would therefore not be consistent with other named associated types.
There is a need to introduce a mechanism for naming the return type for functions that use `-> impl Trait`; we plan to introduce a second RFC addressing this need uniformly across all kinds of functions.
As a backwards compatibility note, named associated types could likely be introduced later, although there is always the possibility of users having introduced associated types with the same name.
### What about using an explicit associated type?
Giving users the ability to write an explicit `type Foo = impl Bar;` is already covered as part of the `type_alias_impl_trait` feature, which is not yet stable at the time of writing, and which represents an extension to the Rust language both inside and outside of traits. This RFC is about making trait methods consistent with normal free functions and inherent methods.
There are different situations where you would want to use an explicit associated type:
1. The type is central to the trait and deserves to be named.
1. You want to give users the ability to use concrete types without `#[refine]`.
1. You want to give generic users of your trait the ability specify a particular type, instead of just bounding it.
1. You want to give users the ability to easily name and bound the type without using (to-be-RFC'd) special syntax to name the type.
1. You want the trait to work with dynamic dispatch today.
1. In the future, you want the associated type to be specified as part of `dyn Trait`, instead of using dynamic dispatch itself.
Using our hypothetical `NewIntoIterator` example, most of these are not met for the `IntoIter` type:
1. While the `Item` type is pretty central to users of the trait, the specific iterator type `IntoIter` is usually not.
1. The concrete type of an impl may or may not be useful, but usually what's important is the specific extra bounds like `ExactSizeIterator` that a user can use. Using `#[refine]` to explicitly choose to expose this (or a fully concrete type) is not overly burdensome.
1. Rarely does a function taking `impl IntoIterator` specify a particular iterator type; it would be rare to see a function like this, for example:
```rust
fn iterate_over_anything_as_long_as_it_is_vec<T>(
vec: impl IntoIterator<IntoIter = std::vec::IntoIter<T>, Item = T>
)
```
1. Bounding the iterator by adding extra bounds like `DoubleEndedIterator` is useful, but not the common case for `IntoIterator`. It therefore shouldn't be overly burdensome to use a (reasonably ergonomic) special syntax in the cases where it's needed.
1. Using `IntoIterator` with dynamic dispatch would be surprising; more common would be to call `.into_iter()` using static dispatch and then pass the resulting iterator to a function that uses dynamic dispatch.
1. If we did use `IntoIterator` with dynamic dispatch, the resulting iterator being dynamically dispatched would make the most sense.
Therefore, if we were writing `IntoIterator` today, it would probably use `-> impl Trait` in return position instead of having an explicit `IntoIter` type.
The same is not true for `Iterator::Item`: because `Item` is so central to what an `Iterator` is, and because it rarely makes sense to use an opaque type for the item, it would remain an explicit associated type.
# Prior art
[prior-art]: #prior-art
There are a number of crates that do desugaring like this manually or with procedural macros. One notable example is [real-async-trait](https://crates.io/crates/real-async-trait).
# Unresolved questions
[unresolved-questions]: #unresolved-questions
- None.
# Future possibilities
[future-possibilities]: #future-possibilities
### Naming return types
We expect to introduce a mechanism for naming the result of `-> impl Trait` return types in a follow-up RFC.
### Dynamic dispatch
Similarly, we expect to introduce language extensions to address the inability to use `-> impl Trait` types with dynamic dispatch. These mechanisms are needed for async fn as well. A good writeup of the challenges can be found on the "challenges" page of the [async fundamentals initiative](https://rust-lang.github.io/async-fundamentals-initiative/evaluation/challenges/dyn_traits.html).
### Migration to associated type
It would be possible to introduce a mechanism that allows users to migrate from an `impl Trait` to a named associated type.
Existing users of the trait won't specify an associated type bound for the new associated type, nor will existing implementers of the trait specify the type. This can be fixed with [associated type defaults](https://github.com/rust-lang/rfcs/blob/master/text/2532-associated-type-defaults.md). So given a trait like `NewIntoIterator`, we could choose to introduce an associated type for the iterator like so:
```rust
// Now old again!
trait NewIntoIterator {
type Item;
type IntoIter = impl Iterator<Item = Self::Item>;
fn into_iter(self) -> Self::IntoIter;
}
```
The only problem remaining is with `#[refine]`. If an existing implementation refined its return value of an RPITIT method, we would need the existing `#[refine]` attribute to stand in for an overriding of the associated type default.
Whatever rules we decide to make this work, they will interact with some ongoing discussions of proposals for `#[defines]` or `#[defined_by]` attributes on `type_alias_impl_trait`. We therefore leave the details of this to a future RFC.
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