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The Rust project is currently working towards a slate of 41 project goals, with 13 of them designated as Flagship Goals. This post provides selected updates on our progress towards these goals (or, in some cases, lack thereof). The full details for any particular goal are available in its associated tracking issue on the rust-project-goals repository.

Flagship goals

“Beyond the `&`”

1 detailed update available.

Comment by @frank-king posted on 2025-10-22:

Status update:

Regarding the TODO list in the next 6 months, here is the current status:

Introduce &pin mut|const place borrowing syntax

• [x] parsing: #135731, merged.
• [ ] lowering and borrowck: not started yet.

I’ve got some primitive ideas about borrowck, and I probably need to confirm with someone who is familiar with MIR/borrowck before starting to implement.

A pinned borrow consists two MIR statements:

1. a borrow statement that creates the mutable reference,
2. and an adt aggregate statement that put the mutable reference into the Pin struct.

I may have to add a new borrow kind so that pinned borrows can be recognized. Then traverse the dataflow graph to make sure that pinned places cannot been moved.

Pattern matching of &pin mut|const T types

In the past few months, I have struggled with the !Unpin stuffs (the original design sketch Alternative A), trying implementing it, refactoring, discussing on zulips, and was constantly confused; luckily, we have finally reached a new agreement of the Alternative B version.

• [ ] #139751 under review (reimplemented regarding Alternative B).

Support drop(&pin mut self) for structurally pinned types

• [ ] adding a new Drop::pin_drop(&pin mut self) method: draft PR #144537

Supporting both Drop::drop(&mut self) and Drop::drop(&pin mut self) seems to introduce method-overloading to Rust, which I think might need some more general ways to handle (maybe by a rustc attribute?). So instead, I’d like to implemenent this via a new method Drop::pin_drop(&pin mut self) first.

Introduce &pin pat pattern syntax

Not started yet (I’d prefer doing that when pattern matching of &pin mut|const T types is ready).

Support &pin mut|const T -> &|&mut T coercion (requires T: Unpin of &pin mut T -> &mut T)

Not started yet. (It’s quite independent, probably someone else can help with it)

Support auto borrowing of &pin mut|const place in method calls with &pin mut|const self receivers

Seems to be handled by Autoreborrow traits?

TL;DR.

There have been lots of internal developments since the last update:

• field representing types and chained projections have received a fundamental overhaul: disallowing field paths and requiring projections to decompose. Additionally we explored how const generics could emulate FRTs.
• we discussed a potential solution to having only a single project operator & trait through a decay operation with special borrow checker treatment.
• we were able to further simplify the project trait moving the generic argument of the represented field to the project function. We also discovered the possibility that FRTs are not fundamentally necessary for field projections – however, they are still very useful in other applications and my gut feeling is that they are also right for field projections. So we will continue our experiment with them.
• we talked about making Project::project be a safe function by introducing a new kind of type.

Next Steps:

• we’re still planning to merge https://github.com/rust-lang/rust/pull/146307, after I have updated it with the new FRT logic and it has been reviewed
• once that PR lands, I plan to update the library experiment to use the experimental FRTs
• then the testing using that library can begin in the Linux kernel and other projects (this is where anyone interested in trying field projections can help out!)

4 detailed updates available.

Comment by @BennoLossin posted on 2025-10-23:

Decomposing Projections

A chained projection operation should naturally decompose, so foo.[Ber Clausen][].[Baz Shkara][] should be the same as writing (foo.[Ber Clausen][]).[Baz Shkara][]. Until now, the different parenthesizing would have allowed different outcomes. This behavior is confusing and also makes many implementation details more complicated than they need to be.

Field Representing Types

Since projections now decompose, we have no need from a design perspective for multi-level FRTs. So field_of!(Foo, bar.baz) is no longer required to work. Thus we have decided to restrict FRTs to only a single field and get rid of the path. This simplifies the implementation in the compiler and also avoids certain difficult questions such as the locality of FRTs (if we had a path, we would have to walk the path and it is local, if all structs included in the path are local). Now with only a single field, the FRT is local if the struct is.

We also discovered that it is a good idea to make FRTs inhabited (they still are ZSTs), since then it allows the following pattern to work:

fn project_free_standing<F: Field>(_: Field, r: &F::Base) -> &F::Type { ... }
// can now call the function without turbofish:
let my_field = project_free_standing(field_of!(MyStruct, my_field), &my_struct);

FRTs via const Generics

We also spent some time thinking about const generics and FRTs on zulip:

• https://rust-lang.zulipchat.com/#narrow/channel/144729-t-types/topic/const.20generics.3A.20implementing.20field.20representing.20types/with/544617587
• https://rust-lang.zulipchat.com/#narrow/channel/144729-t-types/topic/field.20representing.20values.20.26.20.60Field.3Cconst.20F.3A.20.3F.3F.3F.3E.60.20trait/with/542855620

In short, this won’t be happening any time soon. However, it could be a future implementation of the field_of! macro depending on how reflection through const generics evolves (but also only in the far-ish future).

Comment by @BennoLossin posted on 2025-10-23:

Single Project Operator & Trait via Exclusive Decay

It would be great if we only had to add a single operator and trait and could obtain the same features as we have with two. The current reason for having two operators is to allow both shared and exclusive projections. If we could have another operation that decays an exclusive reference (or custom, exclusive smart-pointer type) into a shared reference (or the custom, shared version of the smart pointer). This decay operation would need borrow checker support in order to have simultaneous projections of one field exclusively and another field shared (and possibly multiple times).

This goes into a similar direction as the reborrowing project goal https://github.com/rust-lang/rust-project-goals/issues/399, however, it needs extra borrow checker support.

fn add(x: cell::RefMut<'_, i32>, step: i32) {
*x = *x + step;
}
struct Point {
x: i32,
y: i32,
}
fn example(p: cell::RefMut<'_, Point>) {
let y: cell::Ref<'_, i32> = coerce_shared!(p.[@y][]);
let y2 = coerce_shared!(p.[@y][]); // can project twice if both are coerced
add(p.[Devon Peticolas][], *y);
add(p.[Devon Peticolas][], *y2);
assert_eq!(*y, *y2); // can still use them afterwards
}

Problems:

• explicit syntax is annoying for these “coercions”, but

• we cannot make this implicit:

• if this were an implicit operation, only the borrow checker would know when one had to coerce,

• this operation is allowed to change the type,

• this results in borrow check backfeeding into typecheck, which is not possible or at least extremely difficult

Syntax

Not much movement here, it depends on the question discussed in the previous section, since if we only have one operator, we could choose .@, -> or ~; if we have to have two, then we need additional syntax to differentiate them.

Comment by @BennoLossin posted on 2025-10-23:

Simplifying the Project trait

There have been some developments in pin ergonomics https://github.com/rust-lang/rust/issues/130494: “alternative B” is now the main approach which means that Pin<&mut T> has linear projections, which means that it doesn’t change its output type depending on the concrete field (really depending on the field, not only its type). So it falls into the general projection pattern Pin<&mut Struct> -> Pin<&mut Field> which means that Pin doesn’t need any where clauses when implementing Project.

Additionally we have found out that RCU also doesn’t need where clauses, as we can also make its projections linear by introducing a MutexRef<'_, T> smart pointer that always allows projections and only has special behavior for T = Rcu<U>. Discussed on zulip after this message.

For this reason we can get rid of the generic argument to Project and mandate that all types that support projections support them for all fields. So the new Project trait looks like this:

// still need a common super trait for `Project` & `ProjectMut`
pub trait Projectable {
type Target: ?Sized;
}
pub unsafe trait Project: Projectable {
type Output<F: Field<Base = Self::Target>>;
unsafe fn project<F: Field<Base = Self::Target>>(
this: *const Self,
) -> Self::Output<F>;
}

Are FRTs even necessary?

With this change we can also think about getting rid of FRTs entirely. For example we could have the following Project trait:

pub unsafe trait Project: Projectable {
type Output<F>;
unsafe fn project<const OFFSET: usize, F>(
this: *const Self,
) -> Self::Output<F>;
}

There are other applications for FRTs that are very useful for Rust-for-Linux. For example, storing field information for intrusive data structures directly in that structure as a generic.

More concretely, in the kernel there are workqueues that allow you to run code in parallel to the currently running thread. In order to insert an item into a workqueue, an intrusive linked list is used. However, we need to be able to insert the same item into multiple lists. This is done by storing multiple instances of the Work struct. Its definition is:

pub struct Work<T, const ID: u64> { ... }

Where the ID generic must be unique inside of the struct.

struct MyDriver {
data: Arc<MyData>,
main_work: Work<Self, 0>,
aux_work: Work<Self, 1>,
// more fields ...
}
// Then you call a macro to implement the unsafe `HasWork` trait safely.
// It asserts that there is a field of type `Work<MyDriver, 0>` at the given field
// (and also exposes its offset).
impl_has_work!(impl HasWork<MyDriver, 0> for MyDriver { self.main_work });
impl_has_work!(impl HasWork<MyDriver, 1> for MyDriver { self.aux_work });
// Then you implement `WorkItem` twice:
impl WorkItem<0> for MyDriver {
type Pointer = Arc<Self>;
fn run(this: Self::Pointer) {
println!("doing the main work here");
}
}
impl WorkItem<1> for MyDriver {
type Pointer = Arc<Self>;
fn run(this: Self::Pointer) {
println!("doing the aux work here");
}
}
// And finally you can call `enqueue` on a `Queue`:
let my_driver = Arc::new(MyDriver::new());
let queue: &'static Queue = kernel::workqueue::system_highpri();
queue.enqueue::<_, 0>(my_driver.clone()).expect("my_driver is not yet enqueued for id 0");
// there are different queues
let queue = kernel::workqueue::system_long();
queue.enqueue::<_, 1>(my_driver.clone()).expect("my_driver is not yet enqueued for id 1");
// cannot insert multiple times:
assert!(queue.enqueue::<_, 1>(my_driver.clone()).is_err());

FRTs could be used instead of this id, making the definition be Work<F: Field> (also merging the T parameter).

struct MyDriver {
data: Arc<MyData>,
main_work: Work<field_of!(Self, main_work)>,
aux_work: Work<field_of!(Self, aux_work)>,
// more fields ...
}
impl WorkItem<field_of!(MyDriver, main_work)> for MyDriver {
type Pointer = Arc<Self>;
fn run(this: Self::Pointer) {
println!("doing the main work here");
}
}
impl WorkItem<field_of!(MyDriver, aux_work)> for MyDriver {
type Pointer = Arc<Self>;
fn run(this: Self::Pointer) {
println!("doing the aux work here");
}
}
let my_driver = Arc::new(MyDriver::new());
let queue: &'static Queue = kernel::workqueue::system_highpri();
queue
.enqueue(my_driver.clone(), field_of!(MyDriver, main_work))
//                          ^ using Gary's idea to avoid turbofish
.expect("my_driver is not yet enqueued for main_work");
let queue = kernel::workqueue::system_long();
queue
.enqueue(my_driver.clone(), field_of!(MyDriver, aux_work))
.expect("my_driver is not yet enqueued for aux_work");
assert!(queue.enqueue(my_driver.clone(), field_of!(MyDriver, aux_work)).is_err());

This makes it overall a lot more readable (by providing sensible names instead of magic numbers), and maintainable (we can add a new variant without worrying about which IDs are unused). It also avoids the unsafe HasWork trait and the need to write the impl_has_work! macro for each Work field.

I still think that having FRTs is going to be the right call for field projections as well, so I’m going to keep their experiment going. However, we should fully explore their necessity and rationale for a future RFC.

Comment by @BennoLossin posted on 2025-10-23:

Making Project::project safe

In the current proposal the Project::project function is unsafe, because it takes a raw pointer as an argument. This is pretty unusual for an operator trait (it would be the first). Tyler Mandry thought about a way of making it safe by introducing “partial struct types”. This new type is spelled Struct.F where F is an FRT of that struct. It’s like Struct, but with the restriction that only the field represented by F can be accessed. So for example &Struct.F would point to Struct, but only allow one to read that single field. This way we could design the Project trait in a safe manner:

// governs conversion of `Self` to `Narrowed<F>` & replaces Projectable
pub unsafe trait NarrowPointee {
type Target;
type Narrowed<F: Field<Base = Self::Target>>;
}
pub trait Project: NarrowPointee {
type Output<F: Field<Base = Self::Type>>;
fn project(narrowed: Self::Narrowed<F>) -> Self::Output<F>;
}

The NarrowPointee trait allows a type to declare that it supports conversions of its Target type to Target.F. For example, we would implement it for RefMut like this:

unsafe impl<'a, T> NarrowPointee for RefMut<'a, T> {
type Target = T;
type Narrowed<F: Field<Base = T>> = RefMut<'a, T.F>;
}

Then we can make the narrowing a builtin operation in the compiler that gets prepended on the actual coercion operation.

However, this “partial struct type” has a fatal flaw that Oliver Scherer found (edit by oli: it was actually boxy who found it): it conflicts with mem::swap, if Struct.F has the same layout as Struct, then writing to such a variable will overwrite all bytes, thus also overwriting field that aren’t F. Even if we make an exception for these types and moves/copies, this wouldn’t work, as a user today can rely on the fact that they write size_of::<T>() bytes to a *mut T and thus have a valid value of that type at that location. Tyler Mandry suggested we make it !Sized and even !MetaSized to prevent overwriting values of that type (maybe the Overwrite trait could come in handy here as well). But this might make “partial struct types” too weak to be truly useful. Additionally this poses many more questions that we haven’t yet tackled.

1 detailed update available.

Comment by @aapoalas posted on 2025-10-22:

Initial implementation of a Reborrow trait for types with only lifetimes with exclusive reference semantics is working but not yet upstreamed not in review. CoerceShared implementation is not yet started.

Proper composable implementation will likely require a different tactic than the current one. Safety and validity checks are currently absent as well and will require more work.

“Flexible, fast(er) compilation”

1 detailed update available.

No detailed updates available.

No detailed updates available.

No detailed updates available.

“Higher-level Rust”

3 detailed updates available.

Comment by @nikomatsakis posted on 2025-10-07:

I posted this blog post that proposes that we ought to name the trait Handle and define it as a trait where clone produces an “entangled” value – i.e., a second handle to the same underlying value.

Before that, there’s been a LOT of conversation that hasn’t made its way onto this tracking issue. Trying to fix that! Here is a brief summary, in any case:

• It began with the first Rust Project Goals program in 2024H2, where Jonathan Kelley from Dioxus wrote a thoughtful blog post about a path to high-level Rust that eventually became a 2024H2 project goal towards ergonomic ref-counting.
• I wrote a series of blog posts about a trait I called Claim.
• Josh Triplett and I talked and Josh Triplett opened RFC #3680[], which proposed a use keyword and use || closures. Reception, I would say, was mixed; yes, this is tackling a real problem, but there were lots of concerns on the approach. I summarized the key points here.
• Santiago Pastorino implemented experimental support for (a variant of) RFC #3680[] as part of the 2025H1 project goal.
• I authored a 2025H2 project goal proposing that we create an alternative RFC focused on higher-level use-cases which prompted Josh Triplett and I have to have a long and fruitful conversation in which he convinced me that this was not the right approach.
• We had a lang-team design meeting on 2025-08-27 in which I presented this survey and summary of the work done thus far.
• And then at the RustConf 2025 Unconf we had a big group discussion on the topic that I found very fruitful, as well as various follow-up conversations with smaller groups. The name Handle arose from this and I plan to be posting further thoughts as a result.

RFC #3680: https://github.com/rust-lang/rfcs/pull/3680

Comment by @nikomatsakis posted on 2025-10-09:

I wrote up a brief summary of my current thoughts on Zulip; I plan to move this content into a series of blog posts, but I figured it was worth laying it out here too for those watching this space:

09:11 (1) I don’t think clones/handles are categorically different when it comes to how much you want to see them made explicit; some applications want them both to be explicit, some want them automatic, some will want a mix – and possibly other kinds of categorizations.

09:11 (2) But I do think that if you are making everything explicit, it’s useful to see the difference between a general purpose clone and a handle.

09:12 (3) I also think there are many classes of software where there is value in having everything explicit – and that those classes are often the ones most in Rust’s “sweet spot”. So we should make sure that it’s possible to have everything be explicit ergonomically.

09:12 (4) This does not imply that we can’t make automatic clones/handles possible too – it is just that we should treat both use cases (explicit and automatic) as first-class in importance.

09:13 (5) Right now I’m focused on the explicit case. I think this is what the use-use-everywhere was about, though I prefer a different proposal now – basically just making handle and clone methods understood and specially handled by the compiler for optimization and desugaring purposes. There are pros and cons to that, obviously, and that’s what I plan to write-up in more detail.

09:14 (6) On a related note, I think we also need explicit closure captures, which is a whole interesting design space. I don’t personally find it “sufficient” for the “fully explicit” case but I could understand why others might think it is, and it’s probably a good step to take.

09:15 (7) I go back and forth on profiles – basically a fancy name for lint-groups based on application domain – and whether I think we should go that direction, but I think that if we were going to go automatic, that’s the way I would do it: i.e., the compiler will automatically insert calls to clone and handle, but it will lint when it does so; the lint can by deny-by-default at first but applications could opt into allow for either or both.

I previously wanted allow-by-default but I’ve decided this is a silly hill to die on, and it’s probably better to move in smaller increments.

Comment by @nikomatsakis posted on 2025-10-22:

Update:

There has been more discussion about the Handle trait on Zulip and elsewhere. Some of the notable comments:

• Downsides of the current name: it’s a noun, which doesn’t follow Rust naming convention, and the verb handle is very generic and could mean many things.
• Alternative names proposed: Entangle/entangle or entangled, Share/share, Alias/alias, or Retain/retain. if we want to seriously hardcore on the science names – Mitose/mitose or Fission/fission.
• There has been some criticism pointing out that focusing on handles means that other types which might be “cheaply cloneable” don’t qualify.

For now I will go on using the term Handle, but I agree with the critique that it should be a verb, and currently prefer Alias/alias as an alternative.

---

I’m continuing to work my way through the backlog of blog posts about the conversations from Rustconf. The purposes of these blog posts is not just to socialize the ideas more broadly but also to help myself think through them. Here is the latest post:

https://smallcultfollowing.com/babysteps/blog/2025/10/13/ergonomic-explicit-handles/

The point of this post is to argue that, whatever else we do, Rust should have a way to create handles/clones (and closures that work with them) which is at once explicit and ergonomic.

To give a preview of my current thinking, I am working now on the next post which will discuss how we should add an explicit capture clause syntax. This is somewhat orthogonal but not really, in that an explicit syntax would make closures that clone more ergonomic (but only mildly). I don’t have a proposal I fully like for this syntax though and there are a lot of interesting questions to work out. As a strawperson, though, you might imagine [this older proposal I wrote up](https://hackmd.io/Niko Matsakis/SyI0eMFXO?type=view), which would mean something like this:

let actor1 = async move(reply_tx.handle()) {
reply_tx.send(...);
};
let actor2 = async move(reply_tx.handle()) {
reply_tx.send(...);
};

This is an improvement on

let actor1 = {
let reply_tx = reply_tx.handle();
async move(reply_tx.handle()) {
reply_tx.send(...);
}
};

but only mildly.

The next post I intend to write would be a variant on “use, use everywhere” that recommends method call syntax and permitting the compiler to elide handle/clone calls, so that the example becomes

let actor1 = async move {
reply_tx.handle().send(...);
//       -------- due to optimizations, this would capture the handle creation to happen only when future is *created*
};

This would mean that cloning of strings and things might benefit from the same behavior:

let actor1 = async move {
reply_tx.handle().send(some_id.clone());
//                     -------- the `some_id.clone()` would occur at future creation time
};

The rationable that got me here is (a) minimizing perceived complexity and focusing on muscle memory (just add .clone() or .handle() to fix use-after-move errors, no matter when/where they occur). The cost of course is that (a) Handle/Clone become very special; and (b) it blurs the lines on when code execution occurs. Despite the .handle() occurring inside the future (resp. closure) body, it actually executes when the future (resp. closure) is created in this case (in other cases, such as a closure that implements Fn or FnMut and hence executes more than once, it might occur during each execution as well).

No detailed updates available.

“Unblocking dormant traits”

No detailed updates available.

1 detailed update available.

1 detailed update available.

Comment by @lcnr posted on 2025-10-23:

Since the last update we’ve fixed the hang in rayon in https://github.com/rust-lang/rust/pull/144991 and https://github.com/rust-lang/rust/pull/144732 which relied on https://github.com/rust-lang/rust/pull/143054 https://github.com/rust-lang/rust/pull/144955 https://github.com/rust-lang/rust/pull/144405 https://github.com/rust-lang/rust/pull/145706. This introduced some search graph bugs which we fixed in https://github.com/rust-lang/rust/pull/147061 https://github.com/rust-lang/rust/pull/147266.

We’re mostly done with the opaque type support now. Doing so required a lot of quite involved changes:

• https://github.com/rust-lang/rust/pull/145244 non-defining uses in borrowck
• https://github.com/rust-lang/rust/pull/145925 non-defining uses in borrowck closure support
• https://github.com/rust-lang/rust/pull/145711 non-defining uses in hir typeck
• https://github.com/rust-lang/rust/pull/140375 eagerly compute sub_unification_table again
• https://github.com/rust-lang/rust/pull/146329 item bounds
• https://github.com/rust-lang/rust/pull/145993 function calls
• https://github.com/rust-lang/rust/pull/146885 method selection
• https://github.com/rust-lang/rust/pull/147249 fallback

We also fixed some additional self-contained issues and perf improvements: https://github.com/rust-lang/rust/pull/146725 https://github.com/rust-lang/rust/pull/147138 https://github.com/rust-lang/rust/pull/147152 https://github.com/rust-lang/rust/pull/145713 https://github.com/rust-lang/rust/pull/145951

We have also migrated rust-analyzer to entirely use the new solver instead of chalk. This required a large effort mainly by Jack Huey Chayim Refael Friedman and Shoyu Vanilla. That’s some really impressive work on their end 🎉 See this list of merged PRs for an overview of what this required on the r-a side. Chayim Refael Friedman also landed some changes to the trait solver itself to simplify the integration: https://github.com/rust-lang/rust/pull/145377 https://github.com/rust-lang/rust/pull/146111 https://github.com/rust-lang/rust/pull/147723 https://github.com/rust-lang/rust/pull/146182.

We’re still tracking the remaining issues in https://github.com/orgs/rust-lang/projects/61/views/1. Most of these issues are comparatively simple and I expect us to fix most of them over the next few months, getting us close to stabilization. We’re currently doing another crater triage which may surface a few more issues.

1 detailed update available.

Comment by @lqd posted on 2025-10-22:

Here’s another summary of the most interesting developments since the last update:

• reviews and updates have been done on the polonius alpha, and it has since landed
• the last 2 trivial diagnostics failures were fixed
• we’ve done perf runs, crater runs, completed gathering stats on crates.io for avg and outliers in CFG sizes, locals, loan and region counts, dataflow framework behavior on unexpected graph shapes and bitset invalidations
• I worked on dataflow for borrowck: single pass analyses on acyclic CFGs, dataflow analyses on SCCs for cyclic CFGs
• some more pieces of amanda’s SCC rework have landed, with lcnr’s help
• lcnr’s opaque type rework, borrowcking of nested items, and so on, also fixed some issues we mentioned in previous updates with member constraints for computing when loans are going out of scope
• we also studied recent papers in flow-sensitive pointer analysis
• I also started the loans-in-scope algorithm rework, and also have reachability acceleration with the CFG SCCs
• the last 2 actual failures in the UI tests are soundness issues, related to liveness of captured regions for opaque types: some regions that should be live are not, which were done to help with precise capture and limit the impact of capturing unused regions that cannot be actually used in the hidden type. The unsoundness should not be observable with NLLs, but polonius alpha relies on liveness to propagate loans throughout the CFG: these dead regions prevent detecting some error-causing loan invalidations. The easiest fix would cause breakage in code that’s now accepted. niko, jack and I have another possible solution and I’m trying to implement it now

Goals looking for help

Other goal updates

No detailed updates available.

No detailed updates available.

No detailed updates available.

No detailed updates available.

1 detailed update available.

Comment by @nikomatsakis posted on 2025-10-22:

We had a design meeting on 2025-09-10, minutes available here, aiming at these questions:

There are a few concrete things I would like to get out of this meeting, listed sequentially in order of most to least important:

1. Would you be comfortable stabilizing the initial ADTs-only extensions?

   • This would be properly RFC’d before stabilization, this ask is just a “vibe check”.

2. Are you interested in seeing Per-Value Rejection for enums with undesirable variants?

3. How do you feel about the idea of Lossy Conversion as an approach in general, what about specifically for the References and Raw Pointers extensions?

4. How do you feel about the idea of dropping the One Equality ideal in general, what about specifically for -0.0 vs +0.0, what about specifically for NaN values?

The vibe checks on the first one were as follows:

Vibe check

The main ask:

Would you be comfortable stabilizing the initial ADTs-only extensions?

(plus the other ones)

nikomatsakis

I am +1 on working incrementally and focusing first on ADTs. I am supportive of stabilization overall but I don’t feel like we’ve “nailed” the way to talk or think about these things. So I guess my “vibe” is +1 but if this doc were turned into an RFC kind of “as is” I would probably wind up -1 on the RFC, I think more work is needed (in some sense, the question is, “what is the name of the opt-in trait and why is it named that”). This space is complex and I think we have to do better at helping people understand the fine-grained distinctions between runtime values, const-eval values, and type-safe values.

Niko: if we add some sort of derive of a trait name, how much value are we getting from the derive, what should the trait be named?

tmandry

I think we’ll learn the most by stabilizing ADTs in a forward compatible way (including an opt-in) now. So +1 from me on the proposed design.

It’s worth noting that this is a feature that interacts with many other features, and we will be considering extensions to the MVP for the foreseeable future. To some extent the lang team has committed to this already but we should know what we’re signing ourselves up for.

scottmcm

scottmcm: concern over the private fields restriction (see question below), but otherwise for the top ask, yes happy to just do “simple” types (no floats, no cells, no references, etc).

TC

As Niko said, +1 on working incrementally, and I too am supportive overall.

As a vibe, per-value rejection seems fairly OK to me in that we decided to do value-based reasoning for other const checks. It occurs to me there’s some parallel with that.

https://github.com/rust-lang/rust/pull/119044

As for the opt-in on types, I see the logic. I do have reservations about adding too many opt-ins to the language, and so I’m curious about whether this can be safely removed.

Regarding floats, I see the question on these as related to our decision about how to handle padding in structs. If it makes sense to normalize or otherwise treat -0.0 and +0.0 as the same, then it’d also make sense in my view to normalize or otherwise treat two structs with the same values but different padding (or where only one has initialized padding) as the same.

No detailed updates available.

2 detailed updates available.

1 detailed update available.

Comment by @icmccorm posted on 2025-10-25:

Here’s our first status update!

We’ve been experimenting with a few different ways of emitting retags in codegen, as well as a few different forms that retags should take at this level. We think we’ve settled on a set of changes that’s worth sending out to the community for feedback, likely as a pre-RFC. You can expect more engagement from us on this level in the next couple of weeks.

We’ve used these changes to create an initial working prototype for BorrowSanitizer that supports finding Tree Borrows violations in tiny, single-threaded Rust programs. We’re working on getting Miri’s test suite ported over to confirm that everything is working correctly and that we’ve quashed any false positives or false negatives.

This coming Monday, I’ll be presenting on BorrowSanitizer and this project goal at the Workshop on Supporting Memory Safety in LLVM. Please reach out if you’re attending and would like to chat more in person!

1 detailed update available.

Comment by @joshtriplett posted on 2025-10-22:

The work on this goal has led to many ongoing discussions on the current status of the Reference. Those discussions are still in progress.

Meanwhile, many people working on this goal have successfully written outlines or draft chapters, at various stages of completeness. There’s a broken-out status report at https://github.com/rust-lang/project-goal-reference-expansion/issues/11 .

No detailed updates available.

1 detailed update available.

Comment by @ZuseZ4 posted on 2025-10-22:

A longer update of the changes over the fall. We had two gsoc contributors and a lot of smaller improvements for std::autodiff. The first two improvements were already mentioned as draft PRs in the previous update, but got merged since. I also upstreamed more std::offload changes.

1. Marcelo Domínguez refactored the autodiff frontend to be a proper rustc intrinsic, rather than just hackend into the frontend like I first implemented it. This already solved multiple open issues, reduced the code size, and made it generally easier to maintain going forward.
2. Karan Janthe upstreamed a first implementation of “TypeTrees”, which lowers rust type and layout information to Enzyme, our autodiff backend. This makes it more likely that you won’t see compilation failures with the error message “Can not deduce type of “. We might refine in the future what information exactly we lower.
3. Karan Janthe made sure that std::autodiff has support for f16 and and f128 types.
4. One more of my offload PRs landed. I also figured out why the LLVM-IR generated by the std::offload code needed some manual adjustments in the past. We were inconsistent when communicating with LLVM’s offload module, about whether we’d want a magic, extra, dyn_ptr argument, that enables kernels to use some extra features. We don’t use these features yet, but for consistency we now always generate and expect the extra pointer. The bugfix is currently under review, once it lands upstream, rustc is able to run code on GPUs (still with a little help of clang).
5. Marcelo Domínguez refactored my offload frontend, again introducing a proper rustc intrinsic. That code will still need to go through review, but once it lands it will get us a lot closer to a usable frontend. He also started to generate type information for our offload backend to know how many bytes to copy to and from the devices. This is a very simplified version of our autodiff typetrees.
6. At RustChinaConf, I was lucky to run into the wild linker author David Lattimore, which helped me to create a draft PR that can dlopen Enzyme at runtime. This means we could ship it via rustup for people interested in std::autodiff, and don’t have to link it in at build time, which would increase binary size even for those users that are not interested in it. There are some open issues, so please reach out if you have time to get the PR ready!
7. @sgasho spend a lot of time trying to get Rust into the Enzyme CI. Unfortunately that is a tricky process due to Enzyme’s CI requirements, so it’s not merged yet.
8. I tried to simplify building std::autodiff by marking it as compatible with download-llvm-ci. Building LLVM from source was previously the by far slowest part of building rustc with autodiff, so this has a large potential. Unfortunately the CI experiments revealed some issues around this setting. We think we know why Enzyme’s Cmake causes issues here and are working on a fix to make it more reliable.
9. Osama Abdelkader and bjorn3 looked into automatically enabling fat-lto when autodiff is enabled. In the past, forgetting to enable fat-lto resulted in incorrect (zero) derivatives. The first approach unfortunately wasn’t able to cover all cases, so we need to see whether we can handle it nicely. If that turns out to be too complicated, we will revert it and instead “just” provide a nice error message, rather than returning incorrect derivatives.

All-in-all I spend a lot more time on infra (dlopen, cmake, download-llvm-ci, ...) then I’d like, but on the happy side there are only so many features left that I want to support here so there is an end in sight. I am also about to give a tech-talk at the upcoming LLVM dev meeting about safe GPU programming in Rust.

3 detailed updates available.

Comment by @tomassedovic posted on 2025-10-10:

• [merged] Sanitizers target modificators / https://github.com/rust-lang/rust/pull/138736

• [merged] Add assembly test for -Zreg-struct-return option / https://github.com/rust-lang/rust/pull/145382

• [merged] CI: rfl: move job forward to Linux v6.17-rc5 to remove temporary commits / https://github.com/rust-lang/rust/pull/146368

• -Zharden-sls / https://github.com/rust-lang/rust/pull/136597

• Waiting on review

• #![register_tool] / https://github.com/rust-lang/rust/issues/66079

• Waiting on https://github.com/rust-lang/rfcs/pull/3808

• -Zno-jump-tables / https://github.com/rust-lang/rust/pull/145974

• Active FCP, waiting on 2 check boxes

Comment by @tomassedovic posted on 2025-10-24:

-Cunsigned-char

We’ve discussed adding an option analogous to -funsigned-char in GCC and Clang, that would allow you to set whether std::ffi::c_char is represented by i8 or u8. Right now, this is platform-specific and should map onto whatever char is in C on the same platform. However, Linux explicitly sets char to be unsigned and then our Rust code conflicts with that. And isn this case the sign is significant.

Rust for Linux works around this this with their rust::ffi module, but now that they’ve switched to the standard library’s CStr type, they’re running into it again with the as_ptr method.

Tyler mentioned https://docs.rs/ffi_11/latest/ffi_11/ which preserves the char / signed char / unsigned char distinction.

Grouping target modifier flags

The proposed unsigned-char option is essentially a target modifier. We have several more of these (e.g. llvm-args, no-redzone) in the Rust compiler and Josh suggested we distinguish them somehow. E.g. by giving them the same prefix or possibly creating a new config option (right now we have -C and -Z, maybe we could add -T for target modifiers) so they’re distinct from the e.g. the codegen options.

Josh started a Zulip thread here: https://rust-lang.zulipchat.com/#narrow/channel/131828-t-compiler/topic/Grouping.20target.20modifier.20options.3F/with/546524232

#![register_tool] / rust#66079 / RFC#3808

Tyler looked at the RFC. The Crubit team started using register_tool but then moved to using an attribute instead. He proposed we could do something similar here, although it would require a new feature and RFC.

The team was open to seeing how it would work.

3 detailed updates available.

Comment by @tomassedovic posted on 2025-10-10:

Deref/Receiver

• Ding Xiang Fei keeps updating the PR: https://github.com/rust-lang/rust/pull/146095
• They’re also working on a document to explain the consequences of this split

Arbitrary Self Types

• https://github.com/rust-lang/rust/issues/44874
• Waiting on the Deref/Receiver work, no updates

derive(CoercePointee)

• https://github.com/rust-lang/rust/pull/133820
• Waiting on Arbitrary self types

Pass pointers to const in asm! blocks

• RFC: https://github.com/rust-lang/rfcs/pull/3848
• The Lang team went through the RFC with Alice Ryhl on 2025-10-08 and it’s in FCP now

Field projections

• Benno Lossin opened a PR here: https://github.com/rust-lang/rust/pull/146307
• Being reviewed by the compiler folks

Providing \0 terminated file names with #[track_caller]

• The feature has been implemented and stabilized with file_as_c_str as the method name: https://github.com/rust-lang/rust/pull/145664

Supertrait auto impl RFC

• Ding Xiang Fei opened the RFC and works with the reviewers: https://github.com/rust-lang/rfcs/pull/3851

Other

• Miguel Ojeda spoke to Linus about rustfmt and they came to agreement.

Comment by @tomassedovic posted on 2025-10-24:

Layout of core::any::TypeId

Danilo asked about the layout of TypeId – specifically its size and whether they can rely on it because they want to store it in a C struct. The struct’s size is currently 16 bytes, but that’s an implementation detail.

As a vibe check, Josh Triplett and Tyler Mandry were open to guaranteeing that it’s going to be at most 16 bytes, but they wanted to reserve the option to reduce the size at some point. The next step is to have the full Lang and Libs teams discuss the proposal.

Danilo will open a PR to get that discussion started.

rustfmt

Miguel brought up the “trailing empty comment” workaround for the formatting issue that made the rounds on the Linux kernel a few weeks ago. The kernel style places each import on a single line:

use crate::{
fmt,
page::AsPageIter,
};

rustfmt compresses this to:

use crate::{fmt, page::AsPageIter};

The workaround is to put an empty trailing comment at the end

use crate::{
fmt,
page::AsPageIter, //
};

This was deemed acceptable (for the time being) and merged into the mainline kernel: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=4a9cb2eecc78fa9d388481762dd798fa770e1971

Miguel is in contact with rustfmt to support this behaviour with