Announcing the Keyword Generics Initiative
— 2022-07-27

This post is a mirror of the post published to the Inside Rust blog. If you want to read an earlier iteration of the ideas described in this post, check out the post on Async Overloading, written about a year earlier.

We (Oli, Niko, and Yosh) are excited to announce the start of the Keyword Generics Initiative, a new initiative 1 under the purview of the language team. We're officially just a few weeks old now, and in this post we want to briefly share why we've started this initiative, and share some insight on what we're about.


Rust governance terminology can sometimes get confusing. An "initiative" in Rust parlance is different from a "working group" or "team". Initiatives are intentionally limited: they exist to explore, design, and implement specific pieces of work - and once that work comes to a close, the initiative will wind back down. This is different from, say, the lang team - which essentially carries a 'static lifetime - and whose work does not have a clearly defined end.

A missing kind of generic

One of Rust's defining features is the ability to write functions which are generic over their input types. That allows us to write a function once, leaving it up to the compiler to generate the right implementations for us.

Rust allows you to be generic over types - it does not allow you to be generic over other things that are usually specified by keywords. For example, whether a function is async, whether a function can fail or not, whether a function is const or not, etc.

The post "What color is your function" 2 describes what happens when a language introduces async functions, but with no way to be generic over them:

I will take async-await over bare callbacks or futures any day of the week. But we’re lying to ourselves if we think all of our troubles are gone. As soon as you start trying to write higher-order functions, or reuse code, you’re right back to realizing color is still there, bleeding all over your codebase.

This isn't just limited to async though, it applies to all modifier keywords - including ones we may define in the future. So we're looking to fill that gap by exploring something we call "keyword generics" 3: the ability to be generic over keywords such as const and async.


R. Nystrom, “What Color is Your Function?,” Feb. 01, 2015. (accessed Apr. 06, 2022).


The longer, more specific name would be: "keyword modifier generics". We've tried calling it that, but it's a bit of a mouthful. So we're just sticking with "keyword generics" for now, even if the name for this feature may end up being called something more specific in the reference and documentation.

To give you a quick taste of what we're working on, this is roughly how we imagine you may be able to write a function which is generic over "asyncness" in the future:

Please note that this syntax is entirely made up, just so we can use something in examples. Before we can work on syntax we need to finalize the semantics, and we're not there yet. This means the syntax will likely be subject to change over time.

async<A> trait Read {
    async<A> fn read(&mut self, buf: &mut [u8]) -> Result<usize>;
    async<A> fn read_to_string(&mut self, buf: &mut String) -> Result<usize> { ... }

/// Read from a reader into a string.
async<A> fn read_to_string(reader: &mut impl Read * A) -> std::io::Result<String> {
    let mut string = String::new();
    reader.read_to_string(&mut string).await?;

This function introduces a "keyword generic" parameter into the function of A. You can think of this as a flag which indicates whether the function is being compiled in an async context or not. The parameter A is forwarded to the impl Read, making that conditional on "asyncness" as well.

In the function body you can see a .await call. Because the .await keyword marks cancellation sites we unfortunately can't just infer them 4. Instead we require them to be written for when the code is compiled in async mode, but are essentially reduced to a no-op in non-async mode.


No really, we can't just infer them - and it may not be as simple as omitting all .await calls either. The Async WG is working through the full spectrum of cancellation sites, async drop, and more. But for now we're working under the assumption that .await will remain relevant going forward. And even in the off chance that it isn't, fallibility has similar requirements at the call site as async does.

We still have lots of details left to figure out, but we hope this at least shows the general feel of what we're going for.

A peek at the past: horrors before const

Rust didn't always have const fn as part of the language. A long long long long long time ago (2018) we had to write a regular function for runtime computations and associated const of generic type logic for compile-time computations. As an example, to add the number 1 to a constant provided to you, you had to write (playground):

trait Const<T> {
    const VAL: T;

/// `42` as a "const" (type) generic:
struct FourtyTwo;
impl Const<i32> for FourtyTwo {
    const VAL: i32 = 42;

/// `C` -> `C + 1` operation:
struct AddOne<C: Const<i32>>(C);
impl<C: Const<i32>> Const<i32> for AddOne<C> {
    const VAL: i32 = C::VAL + 1;


Today this is as easy as writing a const fn:

const fn add_one(i: i32) -> i32 {
    i + 1


The interesting part here is that you can also just call this function in runtime code, which means the implementation is shared between both const (CTFE5) and non-const (runtime) contexts.


CTFE stands for "Compile Time Function Execution": const functions can be evaluated during compilation, which is implemented using a Rust interpreter (miri).

Memories of the present: async today

People write duplicate code for async/non-async with the only difference being the async keyword. A good example of that code today is async-std, which duplicates and translates a large part of the stdlib's API surface to be async 6. And because the Async WG has made it an explicit goal to bring async Rust up to par with non-async Rust, the issue of code duplication is particularly relevant for the Async WG as well. Nobody on the Async WG seems particularly keen on proposing we add a second instance of just about every API currently in the stdlib.


Some limitations in async-std apply: async Rust is missing async Drop, async traits, and async closures. So not all APIs could be duplicated. Also async-std explicitly didn't reimplement any of the collection APIs to be async-aware, which means users are subject to the "sandwich problem". The purpose of async-std was to be a proving ground to test whether creating an async mirror of the stdlib would be possible: and it's proven that it is, as far as was possible with missing language features.

We're in a similar situation with async today as const was prior to 2018. Duplicating entire interfaces and wrapping them in block_on calls is the approach taken by e.g. the mongodb [async, non-async], postgres [async, non-async], and reqwest [async, non-async] crates:

// Async functionality like this would typically be exposed from a crate "foo":
async fn bar() -> Bar { 
    // async implementation goes here
// And a sync counterpart would typically be exposed from a crate
// named "blocking_foo" or a submodule on the original crate as
// "foo::blocking". This wraps the async code in a `block_on` call:
fn bar() -> Bar {

This situation is not ideal. Instead of using the host's synchronous syscalls, we're now going through an async runtime to get the same results - something which is often not zero-cost. But more importantly, it's rather hard to keep both a sync and async API version of the same crate in, err, sync with each other. Without automation it's really easy for the two APIs to get out of sync, leading to mismatched functionality.

The ecosystem has come up with some solutions to this, perhaps most notably the proc-macro based maybe-async crate. Instead of writing two separate copies of foo, it generates a sync and async variant for you:

async fn foo() -> Bar { ... }

While being useful, the macro has clear limitations with respect to diagnostics and ergonomics. That's absolutely not an issue with the crate, but an inherent property of the problem it's trying to solve. Implementing a way to be generic over the async keyword is something which will affect the language in many ways, and a type system + compiler will be better equipped to handle it than proc macros reasonably can.

A taste of trouble: the sandwich problem

A pervasive issue in existing Rust is the sandwich problem. It occurs when a type passed into an operation wants to perform control flow not supported by the type it's passed into. Thus creating a sandwich 7 The classic example is a map operation:


Not to be confused with the higher-order sandwich dilemma which is when you look at the sandwich problem and attempt to determine whether the sandwich is two slices of bread with a topping in between, or two toppings with a slice of bread in between. Imo the operation part of the problem feels more bready, but that would make for a weird-looking sandwich. Ergo: sandwich dilemma. (yes, you can ignore all of this.)

enum Option<T> {

impl<T> Option<T> {
    fn map<J>(self, f: impl FnOnce(T) -> J) -> Option<J> { ... }
}|x| x.await)

This will produce a compiler error: the closure f is not an async context, so .await cannot be used within it. And we can't just convert the closure to be async either, since fn map doesn't know how to call async functions. In order to solve this issue, we could provide a new async_map method which does provide an async closure. But we may want to repeat those for more effects, and that would result in a combinatorial explosion of effects. Take for example "can fail" and "can be async":

not asyncasync
infalliblefn mapfn async_map
falliblefn try_mapfn async_try_map

That's a lot of API surface for just a single method, and that problem multiplies across the entire API surface in the stdlib. We expect that once we start applying "keyword generics" to traits, we will be able to solve the sandwich problem. The type f would be marked generic over a set of effects, and the compiler would choose the right variant during compilation.

Affecting all effects

Both const and async share a very similar issue, and we expect that other "effects" will face the same issue. "fallibility" is particularly on our mind here, but it isn't the only effect. In order for the language to feel consistent we need consistent solutions.


Q: Is there an RFC available to read?

Rust initiatives are intended for exploration. The announcement of the Keyword Generics Initiative marks the start of the exploration process. Part of exploring is not knowing what the outcomes will be. Right now we're in the "pre-RFC" phase of design. What we hope we'll achieve is to enumerate the full problem space, design space, find a balanced solution, and eventually summarize that in the form of an RFC. Then after the RFC is accepted: implement it on nightly, work out the kinks, and eventually move to stabilize. But we may at any point during this process conclude that this initiative is actually infeasible and start ramping down.

But while we can't make any assurances about the outcome of the initiative, what we can share is that we're pretty optimistic about the initiative overall. We wouldn't be investing the time we are on this if we didn't think we'd be actually be able to see it through to completion.

Q: Will this make the language more complicated?

The goal of keyword generics is not to minimize the complexity of the Rust programming language, but to minimize the complexity of programming in Rust. These two might sound similar, but they're not. Our reasoning here is that by adding a feature, we will actually be able to significantly reduce the surface area of the stdlib, libraries, and user code - leading to a more streamlined user experience.

Choosing between sync or async code is a fundamental choice which needs to be made. This is complexity which cannot be avoided, and which needs to exist somewhere. Currently in Rust that complexity is thrust entirely on users of Rust, making them responsible for choosing whether their code should support async Rust or not. But other languages have made diferent choices. For example Go doesn't distinguish between "sync" and "async" code, and has a runtime which is able to remove that distinction.

In today's Rust application authors choose whether their application will be sync or async, and even after the introduction of keyword generics we don't really expect that to change. All generics eventually need to have their types known, and keyword generics are no different. What we're targeting is the choice made by library authors whether their library supports is sync or async. With keyword generics library authors will be able to support both with the help of the compiler, and leave it up to application authors to decide how they want to compile their code.

Q: Are you building an effect system?

The short answer is: kind of, but not really. "Effect systems" or "algebraic effect systems" generally have a lot of surface area. A common example of what effects allow you to do is implement your own try/catch mechanism. What we're working on is intentionally limited to built-in keywords only, and wouldn't allow you to implement anything like that at all.

What we do share with effect systems is that we're integrating modifier keywords more directly into the type system. Modifier keywords like async are often referred to as "effects", so being able to be conditional over them in composable ways effectively gives us an "effect algebra". But that's very different from "generalized effect systems" in other languages.

Q: Are you looking at other keywords beyond async and const?

For a while we were referring to the initiative as "modifier generics" or "modifier keyword generics", but it never really stuck. We're only really interested in keywords which modify how types work. Right now this is const and async because that's what's most relevant for the const-generics WG and async WG. But we're designing the feature with other keywords in mind as well.

The one most at the top of our mind is a future keyword for fallibility. There is talk about introducing try fn() {} or fn () -> throws syntax. This could make it so methods such as Iterator::filter would be able to use ? to break out of the closure and short-circuit iteration.

Our main motiviation for this feature is that without it, it's easy for Rust to start to feel disjointed. We sometimes joke that Rust is actually 3-5 languages in a trenchcoat. Between const rust, fallible rust, async rust, unsafe rust - it can be easy for common APIs to only be available in one variant of the language, but not in others. We hope that with this feature we can start to systematically close those gaps, leading to a more consistent Rust experience for all Rust users.

Q: What will the backwards compatibility story be like?

Rust has pretty strict backwards-compatibility guarantees, and any feature we implement needs to adhere to this. Luckily we have some wiggle room because of the edition mechanism, but our goal is to shoot for maximal backwards compat. We have some ideas of how we're going to make this work though, and we're cautiously optimistic we might actually be able to pull this off.

But to be frank: this is by far one of the hardest aspects of this feature, and we're lucky that we're not designing any of this just by ourselves, but have the support of the language team as well.

Q: Aren't implementations sometimes fundamentally different?

Const Rust can't make any assumptions about the host it runs on, so it can't do anything platform-specific. This includes using more efficient instructions of system calls which are only available in one platform but not another. In order to work around this, the const_eval_select intrinsic in the standard library enables const code to detect whether it's executing during CTFE or runtime, and execute different code based on that.

For async we expect to be able to add a similar intrinsic, allowing library authors to detect whether code is being compiled as sync or async, and do something different based on that. This includes: using internal concurrency, or switching to a different set of system calls. We're not sure whether an intrinsic is the right choice for this though; we may want to provide a more ergonomic API for this instead. But because keyword generics is being designed as a consistent feature, we expect that whatever we end up going with can be used consistently by all modifier keywords.


In this post we've introduced the new keyword generics initiatve, explained why it exists, and shown a brief example of what it might look like in the future.

The initiative is active on the Rust Zulip under t-lang/keyword-generics - if this seems interesting to you, please pop by!

Thanks to everyone who's helped review this post, but in particular: fee1-dead, Daniel Henry-Mantilla, and Ryan Levick