Welcome to the repository for the Rust Language Design Team. This page stores our administrative information, meeting minutes, as well as some amount of design constraints. It's also still a work-in-progress (insert omnipresent mid 90s logo for under construction here).

Chat platform

The lang team hangs out in the rust-lang Zulip in the #t-lang stream. There are also other #t-lang/* streams that might be of interest.


We have a lang-team calendar that shows the time for our various meetings. Meetings are generally open to anyone who wants to listen in. We also try to post minutes and recordings on a "best-effort" basis from our meetings.


The lang team has two standing meetings. The current timing for these can be found on the lang team calendar. We generally record our public meetings and post the recordings in our YouTube playlist (along with automatically created subtitles). We also publish notes and minutes in written form in this github repository.

Triage meeting

The weekly triage meeting is when we go over the newly filed project proposals along with issues that have been nominated for lang-team feedback. We also get regular updates from the active project groups so we can stay on top of what is going on.

Triage meeting minutes are available in this directory.

Nominating an issue, PR, RFC etc for the triage meeting

You can nominate an issue/PR/RFC etc for the triage meeting by adding the T-lang and I-nominated labels. When you do so, it's best if you leave a comment explaining what kind of feedback you would like. We try to go over all nominated issues and leave a comment of some kind in response.

Design meeting

The second weekly meeting is our design meeting. The design meeting topics are used for in-depth discussions on a particular topic, often in connection with an active working group.

Proposing a topic for a design meeting

You can propose a topic for a design meeting by opening an Design meeting proposal issue on the lang-team repository. During our weekly traige meetings, we scan open issues to try and decide whether to schedule meetings in the upcoming weeks.

Design meeting minutes are available in this directory.


This page describes the current lang team priorities and explains their motivations. This page is typically updated as part of the yearly Rust roadmap process. They are derived from a combination of the Rust survey results, feedback from users, and other soruces. When new project proposals are created, they can cite priorities listed in this page.

Each priority also lists lang-team members who typically prefer to liaison for issues in this area. This can give you an idea of who you might reach out to if you wish to discuss a project proposal.

Last updated: 2020-07-28

  • Async I/O
    • What? Continued improvements with ergonomics and productivity related to Async I/O.
    • Why? Shows up heavily on the survey, this is an obvious area where a lot of Rust developers are working.
    • Who: nikomatsakis, withoutboats, cramertj
  • C Parity, interop, and embedded -- these often overlap in 'low level capabilities'
    • What? Extending Rust's low-level capabilities to do "things otherwise only possible in C or assembly", as well as enabling smooth, ergonomic FFI between other languages and Rust.
    • Why? Embedded is a large factor in the survey.
    • Why? Our ability to act like "native C" is a differentiating capability for Rust. We've seen a lot of traction integrating into big companies on this basis, as a C++ replacement. It's clear that doing this requires the ability to do piecewise adoption.
    • Who: joshtriplett
  • Const generics and constant evaluation
  • Trait and type system extensions
    • What? Specifically impl Trait, GATs, and specialization
    • Why? Long-standing areas that affect a lot of domains, including async
    • Who: nikomatsakis
  • Error handling
    • What? Combination of library related improvements that consolidate "best practices" into standard library, documentation to describe how it works, as well as possible language improvements to leverage those changes (try blocks, yeet/throw keyword, etc).
    • Why? Cross-cutting productivity concern, and a persistent problem that makes working with Rust code more difficult than it should be.
    • Why? Anecdotally, something that comes up for a lot of people (see e.g. nrc's #rust2020 blog post)
    • Who: withoutboats, joshtriplett
  • Borrow checker expressiveness and other lifetime issues
    • What? Think Polonius, RFC 2229, RFC 66, and other ideas like knowing which fields of self are used by particular methods.
    • Why? Learning curve remains a stubborn problem, and the best way to improve it is to make the compiler smarter.
    • Who: nikomatsakis, pnkfelix
  • Unsafe code capabilities and reference material
    • What? Document the rules for legal unsafe code and add features that either add required capabilities or make correct code easier and more ergonomic to write.
    • Why? Growing base of unsafe code, changes here are getting harder, this represents a kind of "reputation risk". We really want to be "better than C" here.
    • Who: nikomatsakis, pnkfelix
  • Targeted ergonomic wins and extensions
    • What? Small additions or improvements to make Rust easier to use.
    • Why? These will never rise to the "top of the list", but they often have outsized impact on people's enjoyment of Rust.
    • Who: scottmcm
  • Soundness holes to try and correct
    • What?
    • Why? Rust's appeal rests on safety. We have to make steady progress on these points. It's often hard to prioritize them compared to "whiz-bang" features. Also, long-running safety issues can cause fallout when fixed, weakening our stability guarantees.
    • Who: nikomatsakis

Active project groups

Would you like to find out what the lang team is working on? Check out our github project board. It shows the active projects at each stage of development, along with an explanation of the various stages. Each project has a representative issue that links to more complete information. The stages of a project are as follows:

ProposalDeciding whether to start this project
ExplorationExploring the problem space and deciding what RFC(s) to write
RFCThere is a pending RFC describing the design
ImplementingThere is an accepted RFC, but implementation work is ongoing
EvaluationThe RFC is fully implemented and we are requesting public feedback
StabilizedThe feature is complete and available on stable compilers

If you'd like to read more about the stages of a project, or you would like to propose a new project, see the "proposing a new project" page.

Proposing a new project

Interested in extending the Rust language in some way? The language design team is experimenting with a new staged process.

First step: file a Project Proposal

The very first step is [file a project proposal issue]. A project proposal is a lightweight issue that describes your motivations, any ideas you've had thus far on the design, and other background information.

The primary role of the proposal is to request feedback on an idea from the lang team. Before creating a proposal, it's a good idea to create a thread on internals and float the idea to get immediate feedback on whether the problem is real, prior ideas, or related problems. However, you don't have to have a complete solution in mind -- and it's fine to have 2 or 3 ideas for how one might solve something.

Once you file the issue, a Zulip stream will be created and you may start to get feedback right away. The proposal will also be discussed in the triage meeting, and we'll try to post general feedback from that meeting.

The next step depends on the scope of the proposal and on whether a [lang-team liaison] is available who has bandwidth.

Smaller proposals that can be implemented directly

If a proposal is considered "small", then the lang-team will approve the proposal to go straight to PR. In this case, the next step is simply to create an implementation on the appropriate rust-lang repository. The PR can cite the proposal page in its description.

Proposals approved to go straight to PR are tagged as implementation-needed.

Larger proposals that require an RFC

Most proposals however require some sort of RFC to be implemented. In this case, we will create a project group to pursue the project. A project group is just a group of people who are authoring the RFC and pushing it through to stabilization (and ideally through implementation as well, although sometimes that involves very difficult people).

If we decide that a proposal requires an RFC, then once it is assigned a liaison, it will be tagged with charter-needed.

  • Propose a charter: The liaison will work with the author of the proposal (and any other interested parties) to create a project charter, based on the charter template. This is often just a copy of the MCP, updated with whatever new material came up during discussion. The charter should be added to the lang-team repository in a PR, and that PR can "close" the proposal issue.
    • This charter PR is approved by the lang-team with a @rustbot fcp merge command.
  • Exploration phase: Once the project is created, it enters the "exploration phase". We will create a Zulip stream, project issue, and potentially a repository as desired. The goal of this phase is to explore the design space and to come up with one or more RFCs that will actually solve the problem. The project should keep the lang team up to date on the designs being considered during this phase.
    • It is also permitted to land feature-gated code in the rust compiler during this phase, although any such change should be considered highly unstable (and should ideally be minimally disruptive).
    • Project groups are also encouraged to write regular blog posts in the Inside Rust blog documenting their progress.
  • RFC phase: The output of the exploration phase is one or more RFCs that describes the proposed design; these RFCs are typically written by the project group collaboratively. It's a good idea to create RFCs early because this they often receive much wider community feedback and may uncover problems in the design.
  • Implementation phase: Once the RFC is landed, implementation typically begins, although it sometimes happens that implementation occurs concurrently with authoring the RFC.
  • Evaluation phase: Once implementation of the RFC is complete, the project group should write a blog post requesting evaluation. They should keep the lang-team abreast of any feedback that is received.
  • Stabilization phase: Finally, the feature can be stabilized and released on stable Rust. This is done by authorizing a stabilization report, as described here.

Shortlisted proposals

In some cases, there is a liaison who really likes the idea of a proposal, but does not presently have the bandwidth to pursue the idea. In that case, it can be assigned to the liaison but tagged as shortlisted. The liaison should comment on when they expect to have bandwidth to help out; if other liaisons are available, they may want to pick up the project instead.

Frequently asked questions

What is the role of a lang-team liaison?

The lang-team liaison has the job of monitoring the progress on the project and posting updates for the rest of the team. These updates can be delivered in person at the triage meeting or as comments on the corresponding tracking issue for the project.

Liaisons should particularly focus on questions where the lang team can help -- for example, if the project has narrowed down the set of options to a few viable candidates, it would be good to seek lang team feedback on which option seems the most promising (often this would be a good point to request a design meeting).

Sometimes liaisons will also get actively involved in the design itself, or help to mediate the exploration of the design space and ensure that the project is pursuing all the possible designs.

How do I become a liaision?

Liaisons play an important role and hence must either members of the lang-team or people that the lang team selects for the role. If you'd be interested in serving as a liaison, feel free to ping the [lang team leads] to express your interest!

If you're interested in eventually becoming a member of the lang team, serving as a liaison is a necessary first step (but many people serve as liaisons without the intention to join the lang team).

What are the things the lang-team is looking for in a proposal?

When evaluating project proposals, the lang-team is looking to figure out the following:

  • Does this proposal solve a problem? Does that problem fit within our stated priorities?
  • Who are the people involved in this proposal? In particular, are there stakeholders who have explored this design but are not mentioned in the proposal?
  • What is the scope of work involved and is it proportional to the problem being solved? Sometimes there are problems that are really complex to solve and ultimately not that important.
  • How "contained" is this project? Sometimes, even narrow-sounding features can wind up affecting lots of things in the language and requiring lots of team bandwidth.
  • How controversial is this project and is it proportional to the problem being solved? Let's be honest. Lang team discussions can sometimes get quite heated. Sometimes we might decide that even if we think an idea is good, it's not worth the controversy that will result from pursuing it.
  • Do we have the right people involved? Some ideas require specific stakeholders to be a success. This might be because of specific knowledge (e.g., deep understanding of the type system), or because we want representatives for specific use cases, or because we want representatives from a wider variety of companies/backgrounds. If there have been big projects in this area, we might want representatives from all those projects.
  • What other things are going on? We are trying hard not to take on more work than we can handle. It's very tempting in open source to start a ton of different activities because there are often lots of people around with good ideas, but having too many things going on is very stressful for everyone.

This section contains "notes" about the design of various proposals. These are often just links to conversations, along with a few key ideas and summaries. Sometimes it includes other information, such as lang-team decisions about whether a particular proposal is viable.

Can we allow integer literals like 1 to be inferred to floating point type?


In rust today, an integer like 1 cannot be inferred to floating point type. This means that valid-looking numeric expressions like 22.5 + 1 will not compile, and one must instead write 22.5 + 1.0. Can/should we change this?


This was discussed on Zulip in May 2020. Some of the key highlights from the discussion were:

Generalized coroutines

Since even before Rust 1.0, users have desired the ability to yield like in other languages. The compiler infrastructure to achieve this, along with an unstable syntax, have existed for a while now. But despite a lot of debate, we've failed to polish the feature up enough to stabilize it. I've tried to write up a summary of the different design considerations and the past debate around them below:


  • The distinction between a "coroutine" and a "generator" can be a bit vague, varying from one discussion to the next.
  • In these notes a generator is anything which directly implements Iterator or Stream while a coroutine is anything which can take arbitrary input, yield arbitrary output, and later resume execution at the previous yield.
  • Thus, the "generator" syntax proposed in eRFC-2033 and currently implemented behind the "generator" feature is actually a coroutine syntax for the sake of these notes, not a true generator.
    • RFC-2996 defines a true generator syntax in "future additions".
  • Note also that "coroutines" here are really "semicoroutines" since they can only yield back to their caller.
  • I will continue to group the original eRFC text and the later generator resume arguments extension togther as "eRFC-2033". That way I only have 3 big proposals to deal with.
  • In rustc, a coroutine's "witness" is the space where stack-allocated values are stored if needed across yields. I'm borrowing this terminology here. Any such cross-yield bindings are said to be "witnessed".
// This is an example coroutine which might assist a streaming base64 encoder
|sextet, octets| {
    let a = sextet; // witness a, b, and c sextets for later use
    let b = sextet;
    octets.push(a << 2 | b >> 4); // aaaaaabb
    let c = sextet;
    octets.push((b & 0b1111) << 4 | c >> 2); // bbbbcccc
    octets.push((c & 0b11) << 6 | sextet) // ccdddddd

// This is an example generator which might be used in Iterator::flat_map.
gen {
  for item in inner {
    for mapped in func(item) {
      yield mapped;

// This is an example async generator which might be used in Stream::and_then
async gen {
  while let Some(item) = inner.next().await {
    yield func(item).await;

Coroutine trait

  • The coroutine syntax must produce implementations of some trait.
  • RFC-2781 and eRFC-2033 propose the Generator trait.
  • Note that Rust's coroutines and subroutines look the same from the outside: take input, mutate state, produce output.
  • Thus, MCP-49 proposes using the Fn* traits instead, including a new FnPin for immovable coroutines.
    • Hierarchy: Fn is FnMut + Unpin is FnPin is FnOnce.
      • May not be required at the trait level (someone may someday find a use to implementing FnMut + !FnPin) but all closures implement the traits in this order.

Coroutine syntax

  • The closure syntax is reused for coroutines by eRFC-2033, RFC-2781, and MCP-49.
  • Commentators have suggested that the differences between coroutines and closures under eRFC-2033 and RFC-2781 justify an entirely distinct syntax to reduce confusion.
  • MCP-49 fully reuses the semantics of closures, greatly simplifying the design space and making the shared syntax obvious.

Taking input

  • The major disagreement between past proposals is whether to use "yield expressions" or "magic mutation".
    • Yield expression: let after = yield output;
    • Magic mutation: let before = arg; yield output; let after = arg;
  • Many people have a strong gut preference for yield expressions.
    • In simple cases, Rust generally prefers to produce values as output from expressions rather than by mutation of state. "Yield expressions feel more Rusty."
    • However, magic mutation is likely correct, even though at first glance it feels surprising. In addition to reasons below, holding references to past resume args is rare, often a logic error. Rust can use mutation checks to catch and give feedback.
  • "Magic mutation" is a bit of a misnomer. The resume argument values are not themselves being mutated. The argument bindings are simply being reassigned across yields.
    • In a sense, argument bindings are reassigned in the exact same way across returns.
    • Previous arguments (if unmoved) are dropped prior to yielding and are reassigned after resuming.
    • People will get yelled at by the borrow checker if they try to hold borrows of arguments across yields. But the fix is generally easy: move the argument to a new binding before yielding.
=> |x| {
    let y = &x;
    dbg!(y, x);

error[E0506]: cannot pass new `x` because it is borrowed
 --> src/lib.rs:3:4
2 |     let y = &x;
  |             -- borrow of `x` occurs here
3 |     yield;
  |     ^^^^^ assignment to borrowed `x` occurs here
4 |     dbg!(y, x);
  |             - borrow later used here
  = help: consider moving `x` into a new binding before borrowing

=> |x| {
    let a = x;
    let y = &a;
    dbg!(y, x);
  • Magic mutation could be replaced by "magic shadowing" where new arguments shadow old ones at yield in order to allow easy borrowing of past argument values. But this is a huge footgun. See if you can spot the issue with the following code if ctx shadows its past value rather than overwriting it:
std::future::from_fn(|ctx| {
  if is_blocked() {
    yield Pending;

  while let Pending = task.poll(ctx) { .. }
  • "Yield expression" causes problems with first-resume input.
    • eRFC-2033 passes the first resume argument via a closure parameters while later arguments are produced by yield expressions.
    • This part of why it is so hard to unify generalized coroutines with a generator syntax like gen { } or gen fn. Where does the first input go? Where do you annotate the argument type even?
  • To increase clarity, users almost always want resume arguments to be named.
    • With magic mutation, all resume arguments are already named since they reuse the closures arguments on every resume. Any unmoved arguments are dropped just prior to yielding, so they are not witnessed and do not increase the coroutine size.
      • Also get multiple arguments for free if using the Fn* traits.
    • Yield expressions require users to repeatedly assign resume arguments to named bindings manually. Such bindings must be included in the closure state if they have any drop logic.

Borrowed resume arguments

  • What happens when a coroutine witnesses a borrow passed as a resume argument? For example:
let co = |x: &i32| {
  let mut values = Vec::new();
  loop {

// potentially ok:
let mut x = 0;

// must not be allowed:
x = 1;
  • As of writing, RFC-2781 leaves this as an unresolved question with a note to potentially restrict resume arguments to being 'static.
  • Since coroutines under MCP-49 act as much like closures as possible, and treat the witness and capture data the same whenever possible, the example above would fail in a similar way to the example below, giving a "borrowed data escapes into closure state" error or similar even if x is not mutated.
let mut values = Vec::new();
|x: &i32| {
  loop {
//  ^^^^^^^^^^^^^^ `x` escapes the closure body here
  • As of writing, eRFC-2033 appears to take a similar approach (although the error message is not super descriptive).
  • Ideally someday we'd do something nicer but any such solution would apply to both captured state and witnessed state in the same way.


  • Coroutines would eventually like to yield borrows of state to the caller. This is "lending" coroutine (sometimes also called an "attached" coroutine).
  • Using MCP-49, a lending coroutine might look like:
|| {
  let mut buffer = Vec::new();
  loop {
    let n = fill_buffer(&mut buffer);
    yield &buffer[..n];
  • None of the major proposals have made an effort to resolve this directly as far as I am aware.
    • RFC-2996 gets the closest with a mention of LendingStream and LendingIterator traits in "future additions".
    • We should probably get some experience with lending traits at the lib level before attempting to add language level support.
  • If lending closures were implemented, MCP-49 could immediately be used to build lending streams, iterators, etc so long as the respective traits have the needed GAT-ification.


  • RFC-2781 and eRFC-2033 propose that yield x should produce GeneratorState::Yielded(x) or equivalent as an output, in order to discriminate between yielded and returned values.
  • MCP-49 instead gives yield x and return x nearly identical semantics and output x directly, so the two must return the same type.
  • Enum-wrapping here is analogous to Ok-wrapping elsewhere. Similar debates result.
  • When using enum-wrapping, the syntax to specify distinct return/yield types is hotly debated.
  • Generators always want return and yield to have different types (() vs T) but a generator syntax on top of coroutines could be used to auto-insert enum wrappers around yield vs return arguments.
  • Auto-enum-wrapping can slightly improve type safety in some cases where return should be treated specially to avoid bugs.
  • No-enum-wrapping when combined with the impl Fn* choice of trait, allow the coroutine syntax to be used directly with existing higher-order methods on iterator, stream, collection types, async traits, etc.
  • Note these two approaches are "isomorphic": a coroutine that returns GeneratorState<T, T> could be wrapped to return T by some sort of combinator and a coroutine that only returns T can have yield and return values manually wrapped in GeneratorState. This is just about ergonomics:
// Without enum wrapping:
std::iter::from_fn(|| {
  yield Some(1);
  yield Some(2);
  yield Some(3);
}).map(|x| {
  yield -x;
  yield x;

// With enum wrapping:
std::iter::from_gen_fn(|| {
  yield 1;
  yield 2;
  yield 3;
}).map(unwrap_gen_state(|x| {
  yield -x;
  yield x;

// Needed for un-enum-wrapping when not desired.
// Could be replaced by sufficiently fancy !-casting?
fn unwrap_gen_state<T>(f: impl FnMut() -> GeneratorState<T, !>) -> T { ... }
fn merge_gen_state<T>(f: impl FnMut() -> GeneratorState<T, T>) -> T { ... }

// With no wrapping + generators:
(gen {
  yield 1;
  yield 2;
  yield 3;
}).map(|x| {
  yield -x;
  yield x;


  • All proposals want movability/impl Unpin to be inferred.
    • If we forbid "borrowed data escaping into closure state", the inference rules should be relatively simple: witnessing any borrow triggers immovability.
      • Dead borrows should not be witnessed.
    • But exact inference rules may only be well understood after an attempt at implementation.
  • Soundness of pin_mut! is a little tricky but seems to be fine no matter what.
    • If the resulting mutable borrow is witnessed ⇒ coroutine is !Unpin because of inference rules
    • If the pinned data is !Unpin and is witnessed ⇒ coroutine is !Unpin because witness contains !Unpin data
    • Thus, if the coroutine can be moved after resume, any data stack-pinned (really witness-pinned) by pin_mut! is not referenced and is Unpin.
  • Until inference is solved, the static keyword can be used as a modifier.
// movable via inference
|| {
  let x = 4;
  let y = &x;

// guaranteed movable (pending inference)
static || {

// immovable
|| {
  let x = 4;
  let y = &x;

"Once" coroutines

  • A lot of coroutines destroy captured data when run.
  • These coroutines (notably futures) can be resumed many times but can only be run through "once".
  • In contrast to non-yield FnOnce closures, this can not be solved at the type level because a coroutine can run out after an arbitrary, runtime-dependent number of resumptions.
    • Attempts to discriminate with enums tend to run up against Pin.
  • Coroutines must have the ability to block restart with a panic!.
    • Following return.
    • Following panic! and recovery.
    • The term "poison state" technically refers to only the later case. But here I will use it to mean any state at which the closure panics if resumed.
  • RFC-2781 and eRFC-2033 propose that all coroutines become poisoned after returning.
  • MCP-49 recommends that all non-capture-destroying coroutines resume at their initial state after returning.
    • This can be very handy in some situations. In fact, I use it several times in examples to increase readability. See anywhere I iter.map(coroutine) or the base64 encoder.
    • Similar question around generators: should they loop to save on a state or should they be fused-by-default?
    • If we do decide to panic-after-return, restart-after-return can still be emulated using loop { .. } as the coroutine body instead of simply { .. }. This is even zero-cost because unreachable poison states are eliminated.
  • MCP-49 also optionally proposes that capture-destroying closures should only implement FnOnce unless explicitly annotated, even if they should apparently be resumable several times.
    • mut || { drop(capture); } is recommended as the modifier, to hint that an FnMut impl is being requested when the closure in question would otherwise impl only FnOnce.
    • But the behavior of this modifier is probably too obscure and requires too much explanation vs "closures always impl FnMut/FnPin if they contain yield".

Async coroutines

  • I am aware of no strong proposal for an async version of generalized coroutines although a fair amount of discussion has taken place.
    • In the context of MCP-49, how should async || { ... yield ...} be handled in the very long-term? Error right now.
  • Async coroutines don't make much sense because of resume arguments. Async functions are already coroutines which take an &mut Context as a resume argument. How should additional arguments should be specified?
    • Do the additional args need to be passed every single poll or are they only needed when resuming after Ready?
    • If they are stored between Readys, how does that interact with the ban on witnessing external borrows? Badly.
    • On resume, the the coroutine might only take the additional arguments. It could then yield a future to take the async context and handle any Pending yields.
    • If so, how is the coroutine body broken up into distinct futures to be yielded?
    • What happens if a yielded future is destroyed early? Panic on resume?
  • Generators and async are both sugars on top of coroutines and are orthogonal to each other. But neither is orthogonal to the underlying coroutine feature:
// an async block
async {

// an async generator
async gen {
  yield "hello";

// an "async coroutine"
|ctx: &mut Context| {
  yield Poll::Ready("hello");
  • Taking the async context explicitly makes it cleaner to implement some complex async functions which take additional poll parameters.
    • An await_with! macro would be quite useful for implementing await loops on arbitrary Poll-returning functions.
      • Would be a good candidate for an .await(args..) syntax if very heavily used.
    • For example, an simple little checksumming async write wrapper might look like this:
|ctx: &mut Context, bytes: &[u8]| -> Poll<usize> {
  let mut checksum = 0;
  let mut count = 0;

  loop {
    let n = 4096 - count;
    if n == 0 {
      await_with!(writer.poll_write, ctx, &[checksum]);

    let part = &bytes[..bytes.len().min(n)];
    checksum = part.fold(checksum, |x, &y| x ^ y);
    await_with!(writer.poll_write, ctx, part);

    count += part.len();
    yield Ready(part.len());


  • All proposals work fine with the ? operator without even trying (haha).
  • Poll<Result<_, _>> and Poll<Option<Result<_, _>>> already implement Try!
  • Generators usually want a totally different ? desugar that does yield Some(Err(...)); return None; instead of return Err(...).
    • This comes up a lot in discussions of general coroutine syntaxes but just muddies things up because (say it with me) generators ≠ coroutines.
    • Sugar-free implementation is easy: yield Some(try { ... }); None
  • Try blocks in general are super useful for handing errors by moving into specific error-handeling states.

Language similarity

  • Rust's version of coroutines can be a bit unusual compared to other languages. But the reason for this is simple: you need arguments to resume Rusty coroutines.
  • Resume arguments in other languages can be passed just fine by sharing mutable data. So all they need to implement are generators, not true coroutines as defined here.
# Generator function takes a word list and name on construction.
# The shared list is mutated to make room for new words.
def write_greeting(name, words):
    if words.is_full:

// Function only needs name to construct coroutine.
// Coroutine gets mutable access to the word list each resume.
fn write_greeting(name: String) -> impl FnMut(&mut Vec<String>) {
  |words| {
    if words.len() == words.capacity() {

Language complexity

  • The main selling point of MCP-49 is that it avoids adding a whole new language feature with associated design questions. Instead, the common answer to questions regarding MCP-49 is that yield-closures simply do whatever closures do.
    • The syntax is the same.
    • Captures work the same way.
    • Arguments are passed the same way.
    • Return and yield both drop the latest arguments and then pop the call stack.
    • The only big difference is that once yield is involved, some variables get stored in a witness struct rather than in the stack frame. Plus the need for a poison state.
  • In fact, return behaves exactly like a simultaneous yield + break 'closure_body.
    • In a sense, every closure already has a single yield point at which it resumes after return.
    • A yield adds a second resume point: hence the need for a discriminant.
  • Under that proposal, anywhere a closure can be used, a coroutine can too. And vice versa.

Generator unification

  • So far in this proposal, I've been very careful to distinguish generators (as supported by the propane and async_stream crates, proposed by RFC-2996, etc) from the coroutines discussed here. They are treated as two separate language features.
  • Does Rust have "room" for both stream syntax and a generator syntax? Would it be better to find a single solution to both?
  • A single solution is difficult for a few reasons:
    • Taking resume arguments muddies the syntax. For example, what would be the syntax for a generator function which takes an explicit resume argument?
    • The closure syntax works great for coroutines which implement Fn* a la MCP-49! But reusing that syntax to magically implement Iterator or Stream would cause confusion.
    • On that note, generators definitely want to implement different traits vs coroutines. Iterator and Stream rather than Fn or (ironically) Generator.
    • As stated above, async coroutines don't make much sense: async interacts poorly with resume arguments.
    • Async generators are super important, don't care about resume arguments.
    • As mentioned in the section on try, generators and coroutines generally want different error handling. Or at lest, some more complex ? desugar is not so obvious for coroutines in general as it is for generators specifically.
  • Once generalized coroutines are in place, a generator syntax like the one in RFC-2996 is a trivial sugar on top:
gen {
  for item in inner {
    for mapped in func(item) {
      yield mapped;

// becomes

std::iter::from_fn(|| {
  for item in inner {
    for mapped in func(item) {
      yield Some(mapped);
async gen {
  while let Some(item) = inner.next().await {
    yield func(item).await;

// becomes

std::stream::from_fn(|ctx| {
  while let Some(item) = await_with!(inner.next(), ctx) {
    yield Ready(Some(await_with!(func(item), ctx)));
  • Proc-macro crates could provide very satisfactory gen and gen_async macros until we are sure of the need to support such a sugar directly in language as a keyword or in core as a first-party macro.

Past discussions

There are a lot of these. Dozens of internals threads, reddit posts, blog posts, draft RFCs, pre RFCs, actual RFCs, who knows what in Zulip, and so on. So this isn't remotely exhaustive:

  • https://github.com/CAD97/rust-rfcs/pull/1
  • https://github.com/rust-lang/lang-team/issues/49
  • https://github.com/rust-lang/rfcs/pull/2033
  • https://github.com/rust-lang/rfcs/pull/2781
  • https://github.com/rust-lang/rust/issues/43122
  • https://github.com/rust-lang/rust/pull/68524
  • https://internals.rust-lang.org/t/crazy-idea-coroutine-closures/1576
  • https://internals.rust-lang.org/t/no-return-for-generators/11138
  • https://internals.rust-lang.org/t/syntax-for-generators-with-resume-arguments/11456
  • https://internals.rust-lang.org/t/trait-generator-vs-trait-fnpin/10411
  • https://reddit.com/r/rust/comments/dvd3az/generalizing_coroutines/
  • https://samsartor.com/coroutines-2
  • https://smallcultfollowing.com/babysteps/blog/2020/03/10/async-interview-7-withoutboats/#async-fn-are-implemented-using-a-more-general-generator-mechanism
  • https://users.rust-lang.org/t/coroutines-and-rust/9058