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Feature Name: expanded_impl_trait
Start Date: 2017-03-12
RFC PR: rust-lang/rfcs#1951
Rust Issue: rust-lang/rust#42183

Summary

This RFC proposes several steps forward for impl Trait:

Settling on a particular syntax design, resolving questions around the some/any proposal and others.
Resolving questions around which type and lifetime parameters are considered in scope for an impl Trait.
Adding impl Trait to argument position.

The first two proposals, in particular, put us into a position to stabilize the current version of the feature in the near future.

Motivation

To recap, the current impl Trait feature allows functions to write a return type like impl Iterator<Item = u64> or impl Fn(u64) -> bool, which says that the function's return type satisfies the given trait bounds, but nothing more about it can be assumed. It's useful to impose an abstraction barrier and to avoid writing down complex (or un-nameable) types. The current feature was designed very conservatively, and only allows impl Trait to be used in function return position on inherent or free functions.

The core motivation for this RFC is to pave the way toward stabilization of impl Trait; from that perspective, it inherits the motivation of the previous RFC. Making progress on this front falls clearly under the rubric of the productivity and learnability goals for the 2017 roadmap.

Stabilization is currently blocked on three inter-related questions:

Will impl Trait ever be usable in argument position? With what semantics?
Will we want to distinguish between some and any, that is, between existential types (where the callee chooses the type) and universal types (where the caller chooses)? Or is it enough to deduce the desired meaning from context?
When you use impl Trait, what lifetime and type parameters are in scope for the hidden, concrete type that will be returned? Can you customize this set?

This RFC is aimed squarely at resolving these questions. However, by resolving some of them, it also unlocks the door to an expansion of the feature to new locations (arguments, traits, trait impls), as we'll see.

Motivation for expanding to argument position

This RFC proposes to allow impl Trait to be used in argument position, with "universal" (aka generics-style) semantics. There are three lines of argument in favor of doing so, given here along with rebuttals from the lang team.

Argument from learnability

There's been a lot of discussion around universals vs. existentials (in today's Rust, generics vs impl Trait). The RFC makes a few assumptions:

Most programmers won't come to Rust with a crisp understanding of the distinction.
Even when people learn the distinction, it's often confusing and hard to remember with precision.
But, on the other hand, programmers have a very deep intuition around the difference between arguments and return values, and "who" provides which (amongst caller and callee).

Now, consider a new Rust programmer, who has learned about generics:

fn take_iter<T: Iterator>(t: T)

What happens when they want to return an unstated iterator instead? It's pretty natural to reach for:

fn give_iter<T: Iterator>() -> T

if you don't have a crisp understanding of the unversal/existential distinction. If we only allowed impl Trait in return position, we'd have to say: when returning an unknown type, please use a completely different mechanism.

By contrast, a programmer who first learned:

fn take_iter(t: impl Iterator)

and then tried:

fn give_iter() -> impl Iterator

would be successful, without any rigorous understanding that they just transitioned from a universal to an existential.

What's at play here is who gets to pick a type? And as above, programmers have a strong intuition about callers providing arguments, and callees providing return values. The proposed impl Trait extension to argument aligns with this intuition (and with what is most definitely the common case in practice), so that:

If you pick the value, you also pick the type

Thus in fn f(x: impl Foo) -> impl Bar, the caller picks the value of x and so picks the type for impl Foo, but the function picks the return value, so it picks the type for impl Bar.

This intuitive basis lets you get a lot of work done without learning the deeper distinction; you can fake it 'til you make it. If we, in addition, have an explicit syntax, you can eventually come to a fully rigorous understanding in terms of that syntax. And then you can go back to mostly operating intuitively with impl Trait, reaching for the fine distinctions only when you need them (the "post-rigorous" stage of learning).

@solson did a great job of laying this kind of argument out.

Argument from ergonomics

Ergonomics is rarely about raw character count, and the argument here isn't about shaving off a few characters. Rather, it's about how much you have to hold in your head.

Generic syntax requires you to introduce a name for an argument's type, and to separate information about that type from the argument itself:

fn map<U, F: FnOnce(T) -> U>(self, f: F) -> Option<U>

To read this signature, you have to first parse the type parameters and bounds, then remember which ones applied to F, and then see where F shows up in the argument.

By contrast:

fn map<U>(self, f: impl FnOnce(T) -> U) -> Option<U>

Here, there are no additional names or indirections to hold in your head, and the relevant information about the argument type is located right next to the argument's name. Even better:

fn map<U>(self, f: FnOnce(T) -> U) -> Option<U>

Also, when programming at speed, the fact that you can use the same impl Trait syntax in argument and return position -- and it almost always has the meaning you want -- means less pausing to think "hm, am I dealing with an existential here?"

Argument from familiarity

Finally, there's an argument from familiarity, which was given eloquently by @withoutboats:

The proposal is (syntactically) more like Java. In Java, non-static methods aren't parametric; generics are used at the type level, and you just use interfaces at the method level.

We'd end up with APIs that look very similar to Java or C#:
impl<T> Option<T> {
    fn map<U>(self, f: impl FnOnce(T) -> U) -> Option<U> { ... }
}
I think this is a good thing from the pre-rigorous/rigourous/post-rigourous sense: you have this incremental onboarding experience in which at first blush it is quite similar to what you're used to. What I like even more, though, is that under the hood its all parametric polymorphism. In Java you actually have inheritance, and interfaces, and generics, and they all interact but not in a very unified way. In Rust, this is just a syntactic easement into a unitary polymorphism system which is fundamentally one idea: parametric polymorphism with trait constraints.

Critique from the lang team

@nrc argued that there's also a learnability downside, because Rust programmers now have one additional syntax for generic arguments to learn.

Rebuttal: I agree that there's an additional syntax to learn, but a key here is that there's no genuine complexity addition: it's pure sugar. In other words, it's not a new concept, and learning that there's an alternative, more verbose and expressive syntax tends to be a relatively easy step to take in practice. In addition, treating it as "anonymous generics" (for arguments) makes it pretty easy to understand the relationship.

@nrc argued that there would also be stylistic overhead: when to use impl Trait vs generics vs where clauses? And won't you often end up having to use where clauses anyway, when things get longer?

Rebuttal: @withoutboats pointed out that impl Trait can actually help ease such style questions:

fn foo<
    T: Whatever + SomethingElse,
    U: Whatever,
>(
    t: T,
    u: U,
)

// vs

fn foo<T, U>(t: T, u: U) where
    T: Whatever + SomethingElse,
    U: Whatever,

// vs

fn foo(
    t: impl Whatever + SomethingElse,
    u: impl Whatever,
)

It seems plausible that impl Trait syntax should simply always be used whenever it can be, since expanding out an argument list into multiple lines tends to be preferable to expanding out a where clause to multiple lines (and even more so, expanding out a generics list).

@joshtriplett raised concerns about the purported learnability benefits absent having an explicit syntax for the "rigorous" stage.

Rebuttal: the RFC takes as a basic assumption that we will eventually have such a syntax. But I think it's worth diving into greater detail on the learnability tradeoffs here. I think that if we offered an explicit syntax that was similar to today's generic syntax, it could help tell a coherent, intuitive story.

@nrc raised his point about auto traits.

Rebuttal: the auto trait story here is essentially the same as with generics:

fn foo(t: impl Trait) -> impl Trait { t }
fn foo<T: Trait>(t: T) -> T { t }

In both of these functions, if you pass in an argument that is Send, you will be able to rely on Send in the return value.

Detailed design

The proposal in a nutshell

Expand impl Trait to allow use in arguments, where it behaves like an anonymous generic parameter. This will be separately feature-gated.
Stick with the impl Trait syntax, rather than introducing a some/any distinction.
Treat all type parameters as in scope for the concrete "witness" type underlying a use of impl Trait.
Treat any explicit lifetime bounds (as in impl Trait + 'a) as bringing those lifetimes into scope, and no other lifetime parameters are explicitly in scope. However, type parameters may mention lifetimes which are hence indirectly in scope.

Background

Before diving more deeply into the design, let's recap some of the background that's emerged over time for this corner of the language.

Universals (`any`) versus existentials (`some`)

There are basically two ways to talk about an "unknown type" in something like a function signature:

Universal quantification, i.e. "for any type T", i.e. "caller chooses". This is how generics work today. When you write fn foo<T>(t: T), you're saying that the function will work for any choice of T, and leaving it to your caller to choose the T.
Existential quantification, i.e. "for some type T", i.e. "callee chooses". This is how impl Trait works today (which is in return position only). When you write fn foo() -> impl Iterator, you're saying that the function will produce some type T that implements Iterator, but the caller is not allowed to assume anything else about that type.

When it comes to functions, we usually want any T for arguments, and some T for return values. However, consider the following function:

fn thin_air<T: Default>() -> T {
    T::default()
}

The thin_air function says it can produce a value of type T for any T the caller chooses---so long as T: Default. The collect function works similarly. But this pattern is relatively uncommon.

As we'll see later, there are also considerations for higher-order functions, i.e. when you take another function as an argument.

In any case, one longstanding proposal for impl Trait is to split it into two distinct features: some Trait and any Trait. Then you'd have:

// These two are equivalent
fn foo<T: MyTrait>(t: T)
fn foo(t: any MyTrait)

// These two are equivalent
fn foo() -> impl Iterator
fn foo() -> some Iterator

// These two are equivalent
fn foo<T: Default>() -> T
fn foo() -> any Default

Scoping for lifetime and type parameters

There's a subtle issue for the semantics of impl Trait: what lifetime and type parameters are considered "in scope" for the underlying concrete type that implements Trait?

Type parameters and type equalities

It's easiest to understand this issue through examples where it matters. Suppose we have the following function:

fn foo<T>(t: T) -> impl MyTrait { .. }

Here we're saying that the function will yield some type back, whose identity we don't know, but which implements MyTrait. But, in addition, we have the type parameter T. The question is: can the return type of the function depend on T?

Concretely, we expect at least the following to work:

vec![
    foo(0u8),
    foo(1u8),
]

because we expect both expressions to have the same type, and hence be eligible to place into a single vector. That's because, although we don't know the identity of the return type, everything it could depend on is the same in both cases: T is instantiated with u8. (Note: there are "generative" variants of existentials for which this is not the case; see Unresolved questions);

But what about the following:

vec![
    foo(0u8),
    foo(0u16),
]

Here, we're making different choices of T in the two expressions; can that affect what return type we get? The impl Trait semantics needs to give an answer to that question.

Clearly there are cases where the return type very much depends on type parameters, for example the following:

fn buffer<T: Write>(t: T) -> impl Write {
    BufWriter::new(t)
}

But there are also cases where there isn't a dependency, and tracking that information may be important for type equalities like the vectors above. And this applies equally to lifetime parameters as well.

Lifetime parameters

It's vital to know what lifetime parameters might be used in the concrete type underlying an impl Trait, because that information will affect lifetime inference.

For concrete types, we're pretty used to thinking about this. Let's take slices:

impl<T> [T] {
    fn len(&self) -> usize { ... }
    fn first(&self) -> Option<&T> { ... }
}

A seasoned Rustacean can read the ownership story directly from these two signatures. In the case of len, the fact that the return type does not involve any borrowed data means that the borrow of self is only used within len, and doesn't need to persist afterwards. For first, by contrast, the return value contains &T, which will extend the borrow of self for at least as long as that return value is kept around by the caller.

As a caller, this difference is quite apparent:

{
    let len = my_slice.len(); // the borrow of `my_slice` lasts only for this line
    *my_slice[0] = 1;         // ... so this mutable borrow is allowed
}

{
    let first = my_slice.first(); // the borrow of `my_slice` lasts for the rest of this scope
    *my_slice[0] = 1;             // ... so this mutable borrow is *NOT* allowed
}

Now, the issue is that for impl Trait, we're not writing down the concrete return type, so it's not obvious what borrows might be active within it. In other words, if we write:

impl<T> [T] {
    fn bork(&self) -> impl SomeTrait { ... }
}

it's not clear whether the function is more like len or more like first.

This is again a question of what lifetime parameters are in scope for the actual return type. It's a question that needs a clear answer (and some flexibility) for the impl Trait design.

Core assumptions

The design in this RFC is guided by several assumptions which are worth laying out explicitly.

Assumption 1: we will eventually have a fully expressive and explicit syntax for existentials

The impl Trait syntax can be considered an "implicit" or "sugary" syntax in that it (1) does not introduce a name for the existential type and (2) does not allow you to control the scope in which the underlying concrete type is known.

Moreover, some versions of the design (including in this RFC) impose further limitations on the power of the feature for the sake of simplicity.

This is done under the assumption that we will eventually introduce a fully expressive, explicit syntax for existentials. Such a syntax is sketched in an appendix to this RFC.

Assumption 2: treating all type variables as in scope for `impl Trait` suffices for the vast majority of cases

The background section discussed scoping issues for impl Trait, and the main implication for type parameters (as opposed to lifetimes) is what type equalities you get for an impl Trait return type. We're making two assumptions about that:

In practice, you usually need to close over most of all of the type parameters.
In practice, you usually don't care much about type equalities with impl Trait.

This latter point means, for example, that it's relatively unusual to do things like construct the vectors described in the background section.

Assumption 3: there should be an explicit marker when a lifetime could be embedded in a return type

As mentioned in a recent blog post, one regret we have around lifetime elision is the fact that it applies when leaving off a lifetime for a non-& type constructor that expects one. For example, consider:

impl<T> SomeType<T> {
    fn bork(&self) -> Ref<T> { ... }
}

To know whether the borrow of self persists in the return value, you have to know that Ref takes a lifetime parameter that's being left out here. This is a tad too implicit for something as central as ownership.

Now, we also don't want to force you to write an explicit lifetime. We'd instead prefer a notation that says "there is a lifetime here; it's the usual one from elision". As a purely strawman syntax (an actual RFC on the topic is upcoming), we might write:

impl<T> SomeType<T> {
    fn bork(&self) -> Ref<'_, T> { ... }
}

In any case, to avoid compounding the mistake around elision, there should be some marker when using impl Trait that a lifetime is being captured.

Assumption 4: existentials are vastly more common in return position, and universals in argument position

As discussed in the background section, it's possible to make sense of some Trait and any Trait in arbitrary positions in a function signature. But experience with the language strongly suggests that some Trait semantics is virtually never wanted in argument position, and any Trait semantics is rarely used in return position.

Assumption 5: we may be interested in eventually pursuing a bare `fn foo() -> Trait` syntax rather than `fn foo() -> impl Trait`

Today, traits can be used directly as (unsized) types, so that you can write things like Box<MyTrait> to designate a trait object. However, with the advent of impl Trait, there's been a desire to repurpose that syntax, and instead write Box<dyn Trait> or some such to designate trait objects.

That would, in particular, allow syntax like the following when taking a closure:

fn map<U>(self, f: FnOnce(T) -> U) -> Option<U>

The pros, cons, and logistics of such a change are out of scope for this RFC. However, it's taken as an assumption that we want to keep the door open to such a syntax, and so shouldn't stabilize any variant of impl Trait that lacks a good story for evolving into a bare Trait syntax later on.

Sticking with the `impl Trait` syntax

This RFC proposes to stabilize the impl Trait feature with its current syntax, while also expanding it to encompass argument position. That means, in particular, not introducing an explicit some/any distinction.

This choice is based partly on the core assumptions:

Assumption 1, we'll have a fully expressive syntax later.
Assumption 4, we can use the some semantics in return position and any in argument position, and almost always be right.
Assumption 5, we may want bare Trait syntax, which would not give "syntactic space" for a some/any distinction.

One important question is: will people find it easier to understand and use impl Trait, or something like some Trait and any Trait? Having an explicit split may make it easier to understand what's going on. But on the other hand, it's a somewhat complicated distinction to make, and while you usually know intuitively what you want, being forced to spell it out by choosing the correct choice of some or any seems like an unnecessary burden, especially if the choice is almost always dictated by the position.

Pedagogically, if we have an explicit syntax, we retain the option of explaining what's going on with impl Trait by "desugaring" it into that syntax. From that standpoint, impl Trait is meant purely for ergonomics, which means not just what you type, but also what you have to remember. Having impl Trait "just do the right thing" seems pretty clearly to be the right choice ergonomically.

Expansion to arguments

This RFC proposes to allow impl Trait in function arguments, in addition to return position, with the any Trait semantics (as per assumption 4). In other words:

// These two are equivalent
fn map<U>(self, f: impl FnOnce(T) -> U) -> Option<U>
fn map<U, F>(self, f: F) -> Option<U> where F: FnOnce(T) -> U

However, this RFC also proposes to disallow use of impl Trait within Fn trait sugar or higher-ranked bounds, i.e. to disallow examples like the following:

fn foo(f: impl Fn(impl SomeTrait) -> impl OtherTrait)
fn bar() -> (impl Fn(impl SomeTrait) -> impl OtherTrait)

While we will eventually want to allow such uses, it's likely that we'll want to introduce nested universal quantifications (i.e., higher-ranked bounds) in at least some cases; we don't yet have the ability to do so. We can revisit this question later on, once higher-ranked bounds have gained full expressiveness.

Explicit instantiation

This RFC does not propose any means of explicitly instantiating an impl Trait in argument position. In other words:

fn foo<T: Trait>(t: T)
fn bar(t: impl Trait)

foo::<u32>(0) // this is allowed
bar::<u32>(0) // this is not

Thus, while impl Trait in argument position in some sense "desugars" to a generic parameter, the parameter is treated fully anonymously.

Scoping for type and lifetime parameters

In argument position, the type fulfilling an impl Trait is free to reference any types or lifetimes whatsoever. So in a signature like:

fn foo(iter: impl Iterator<Item = u32>);

the actual argument type may contain arbitrary lifetimes and mention arbitrary types. This follows from the desugaring to "anonymous" generic parameters.

For return position, things are more nuanced.

This RFC proposes that all type parameters are considered in scope for impl Trait in return position, as per Assumption 2 (which claims that this suffices for most use-cases) and Assumption 1 (which claims that we'll eventually provide an explicit syntax with finer-grained control).

The lifetimes in scope include only those mentioned "explicitly" in a bound on the impl Trait. That is:

For impl SomeTrait + 'a, the 'a is in scope for the concrete witness type.
For impl SomeTrait + '_, the lifetime that elision would imply is in scope (this is again using the strawman shorthand syntax for an elided lifetime).

Note, however, that the witness type can freely mention type parameters, which may themselves involve embedded lifetimes. Consider, for example:

fn transform(iter: impl Iterator<Item = u32>) -> impl Iterator<Item = u32>

Here, if the actual argument type was SomeIter<'a>, the return type can mention SomeIter<'a>, and therefore can indirectly mention 'a.

In terms of Assumption 3 -- the constraint that lifetime embedding must be explicitly marked -- we clearly get that for the explicitly in-scope variables. For indirect mentions of lifetimes, it follows from whatever is provided for the type parameters, much like the following:

fn foo<T>(v: Vec<T>) -> vec::IntoIter<T>

In this example, the return type can of course reference any lifetimes that T does, but this is apparent from the signature. Likewise with impl Trait, where you should assume that all type parameters could appear in the return type.

Relationship to trait objects

It's worth noting that this treatment of lifetimes is related but not identical to the way they're handled for trait objects.

In particular, Box<SomeTrait> imposes a 'static requirement on the underlying object, while Box<SomeTrait + 'a> only imposes a 'a constraint. The key difference is that, for impl Trait, in-scope type parameters can appear, which indirectly mention additional lifetimes, so impl SomeTrait imposes 'static only if those type parameters do:

// In these cases, we know that the concrete return type is 'static
fn foo() -> impl SomeTrait;
fn foo(x: u32) -> impl SomeTrait;
fn foo<T: 'static>(t: T) -> impl SomeTrait;

// In the following case, the concrete return type may embed lifetimes that appear in T:
fn foo<T>(t: T) -> impl SomeTrait;

// ... whereas with Box, the 'static constraint is imposed
fn foo<T>(t: T) -> Box<SomeTrait>;

This difference is a natural one when you consider the difference between generics and trait objects in general -- which is precisely that with generics, the actual types are not erased, and hence auto traits like Send work transparently, as do lifetime constraints.

How We Teach This

Generics and traits are a fundamental aspect of Rust, so the pedagogical approach here is really important. We'll outline the basic contours below, but in practice it's going to take some trial and error to find the best approach.

One of the hopes for impl Trait, as extended by this RFC, is that it aids learnability along several dimensions:

It makes it possible to meaningfully work with traits without visibly using generics, which can provide a gentler learning curve. In particular, signatures involving closures are much easier to understand. This effect would be further heightened if we eventually dropped the need for impl, so that you could write fn map<U>(self, f: FnOnce(T) -> U) -> Option<U>.
It provides a greater degree of analogy between static and dynamic dispatch when working with traits. Introducing trait objects is easier when they can be understood as a variant of impl Trait, rather than a completely different approach. This effect would be further heightened if we moved to dyn Trait syntax for trait objects.
It provides a more intuitive way of working with traits and static dispatch in an "object" style, smoothing the transition to Rust's take on the topic.
It provides a more uniform story for static dispatch, allowing it to work in both argument and return position.

There are two ways of teaching impl Trait:

Introduce it prior to bounded generics, as the first way you learn to "consume" traits. That works particularly well with teaching Iterator as one of the first real traits you see, since impl Trait is a strong match for working with iterators. As mentioned above, this approach also provides a more intuitive stepping stone for those coming from OO-ish languages. Later, bounded generics can be introduced as a more powerful, explicit syntax, which can also reveal a bit more about the underlying semantic model of impl Trait. In this approach, the existential use case doesn't need a great deal of ceremony---it just follows naturally from the basic feature.
Alternatively, introduce it after bounded generics, as (1) a sugar for generics and (2) a separate mechanism for existentials. This is, of course, the way all existing Rust users will come to learn impl Trait. And it's ultimately important to understand the mechanism in this way. But it's likely not the ideal way to introduce it at first.

In either case, people should learn impl Trait early (since it will appear often) and in particular prior to learning trait objects. As mentioned above, trait objects can then be taught using intuitions from impl Trait.

There's also some ways in which impl Trait can introduce confusion, which we'll cover in the drawbacks section below.

Drawbacks

It's widely recognized that we need some form of static existentials for return position, both to be able to return closures (which have un-nameable types) and to ergonomically return things like iterator chains.

However, there are two broad classes of drawbacks to the approach taken in this RFC.

Relatively inexpressive sugary syntax

This RFC is built on the idea that we'll eventually have a fully expressive explicit syntax, and so we should tailor the "sugary" impl Trait syntax to the most common use cases and intuitions.

That means, however, that we give up an opportunity to provide more expressive but still sugary syntax like some Trait and any Trait---we certainly don't want all three.

That syntax is further discussed in Alternatives below.

Potential for confusion

There are two main avenues for confusion around impl Trait:

Because it's written where a type would normally go, one might expect it to be usable everywhere a type is accepted (e.g., within struct definitions and impl headers). While it's feasible to allow the feature to be used in more locations, the semantics is tricky, and in any case it doesn't behave like a normal type, since it's introducing an existential. The approach in this RFC is to have a very clear line: impl Trait is a notation for function signatures only, and there's a separate explicit notation (TBD) that can be used to provide more general existentials (which can then be used as if they were normal types).
You can use impl Trait in both argument and return position, but the meaning is different in the two cases. On the one hand, the meaning is generally the intuitive one---it behaves as one would likely expect. But it blurs the line a bit between the some and any meanings, which could lead to people trying to use generics for existentials. We may be able to provide some help through errors, or eventually provide a syntax like <out T> for named existentials.

There's also the fact that impl Trait introduces "yet another" way to take a bounded generic argument (in addition to <T: Trait> and <T> where T: Trait). However, these ways of writing a signature are not semantically distinct ways; they're just stylistically different. It's feasible that rustfmt could even make the choice automatically.

Alternatives

There's been a lot of discussion about the impl Trait feature and various alternatives. Let's look at some of the most prominent of them.

Limiting to return position forever. A particularly conservative approach would be to treat impl Trait as used purely for existentials and limit its use to return position in functions (and perhaps some other places where we want to allow for existentials). Limiting the feature in this way, however, loses out on some significant ergonomic and pedagogical wins (previously discussed in the RFC), and risks confusion around the "special case" treatment of return types.
Finer grained sugary syntax. There are a couple options for making the sugary syntax more powerful:
- some/any notation, which allows selecting between universals and existentials at will. The RFC has already made some argument for why it does not seem so important to permit this distinction for impl Trait. And doing so has some significant downsides: it demands a more sophisticated understanding of the underlying type theory, which precludes using impl Trait as an early teaching tool; it seems easy to get confused and choose the wrong variant; and we'd almost certainly need different keywords (that don't mirror the existing Some and Any names), but it's not clear that there are good choices.
- impl<...> Trait syntax, as a way of giving more precise control over which type and lifetime parameters are in scope. The idea is that the parameters listed in the <...> are in scope, and nothing else is. This syntax, however, is not forward-compatible with a bare Trait syntax. It's also not clear how to get the right defaults without introducing some inconsistency; if you leave off the <> altogether, we'd presumably like something like the defaults proposed in this RFC (otherwise, the feature would be very unergonomic). But that would mean that, when transitioning from no <> to including a <> section, you go from including all type parameters to including only the listed set, which is a bit counterintuitive.

Unresolved questions

Full evidence for core assumptions. The assumptions in this RFC are stated with anecdotal and intuitive evidence, but the argument would be stronger with more empirical evidence. It's not entirely clear how best to gather that, though many of the assumptions could be validated by using an unstable version of the proposed feature.

The precedence rules around impl Trait + 'a need to be nailed down.

The RFC assumes that we only want "applicative" existentials, which always resolve to the same type when in-scope parameters are the same:

fn foo() -> impl SomeTrait { ... }

fn bar() {
    // valid, because we know the underlying return type will be the same in both cases:
    let v = vec![foo(), foo()];
}

However, it's also possible to provide "generative" existentials, which give you a fresh type whenever they are unpacked, even when their arguments are the same---which would rule out the example above. That's a powerful feature, because it means in effect that you can generate a fresh type for every dynamic invocation of a function, thereby giving you a way to hoist dynamic information into the type system.

As one example, generative existentials can be used to "bless" integers as being in bounds for a particular slice, so that bounds checks can be safely elided. This is currently possible to encode in Rust by using callbacks with fresh lifetimes (see Section 6.3 of @Gankro's thesis, but generative existentials would provide a much more natural mechanism.

We may want to consider adding some form of generative existentials in the future, but would almost certainly want to do so via the fully expressive/explicit syntax, rather than through impl Trait.

Appendix: a sketch of a fully-explicit syntax

This section contains a brief sketch of a fully-explicit syntax for existentials. It's a strawman proposal based on many previously-discussed ideas, and should not be bikeshedded as part of this RFC. The goal is just to give a flavor of how the full system could eventually fit together.

The basic idea is to introduce an abstype item for declaring abstract types:

abstype MyType: SomeTrait;

This construct would be usable anywhere items currently are. It would declare an existential type whose concrete implementation is known within the item scope in which it is declared, and that concrete type would be determined by inference based on the same scope. Outside of that scope, the type would be opaque in the same way as impl Trait.

So, for example:

mod example {
    static NEXT_TOKEN: Cell<u64> = Cell::new(0);

    pub abstype Token: Eq;
    pub fn fresh() -> Token {
        let r = NEXT_TOKEN.get();
        NEXT_TOKEN.set(r + 1);
        r
    }
}

fn main() {
    assert!(example::fresh() != example::fresh());

    // fails to compile, because in this scope we don't know that `Token` is `u64`
    let _ = example::fresh() + 1;
}

Of course, in this particular example we could just as well have used fn fresh() -> impl Eq, but abstype allows us to use the same existential type in multiple locations in an API:

mod example {
    pub abstype Secret: SomeTrait;

    pub fn foo() -> Secret { ... }
    pub fn bar(s: Secret) -> Secret { ... }

    pub struct Baz {
        quux: Secret,
        // ...
    }
}

Already abstype gives greater expressiveness than impl Trait in several respects:

It allows existentials to be named, so that the same one can be referred to multiple times within an API.
It allows existentials to appear within structs.
It allows existentials to appear within function arguments.
It gives tight control over the "scope" of the existential---what portion of the code is allowed to know what the concrete witness type for the existential is. For impl Trait, it's always just a single function.

But we also wanted more control over scoping of type and lifetime parameters. For this, we can introduce existential type constructors:

abstype MyIter<'a>: Iterator<Item = u32>;

impl SomeType<T> {
    // we know that 'a is in scope for the return type, but *not* `T`
    fn iter<'a>(&'a self, ) -> MyIter<'a> { ... }
}

(These type constructors raise various issues around inference, which I believe are tractable, but are out of scope for this sketch).

It's worth noting that there's some relationship between abstype and the "newtype deriving" concept: from an external perspective, abstype introduces a new type but automatically delegates any of the listed trait bounds to the underlying witness type.

Finally, a word on syntax:

Why abstype Foo: Trait; rather than type Foo = impl Trait;?
- Two reasons. First, to avoid confusion about impl Trait seeming to be like a type, when it is actually an existential. Second, for forward compatibility with bare Trait syntax.
Why not type Foo: Trait?
- That may be a fine syntax, but for clarity in presenting the idea I preferred to introduce a new keyword.

There are many detailed questions that would need to be resolved to fully specify this more expressive syntax, but the hope here is to show that (1) there's a plausible direction to take here and (2) give a sense for how impl Trait and a more expressive form could fit together.

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