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Edit the schedule here: https://github.com/orgs/rust-lang/projects/31/views/7.
(Meeting attendees, feel free to add items here!)
TC: For any guests who are present, please note in this section if you're attending for the purposes of any items on (or off) the agenda in particular.
TC: As we've been doing recently, due to the impressive backlog, I'm going to push the pace a bit. If it's ever too fast or you need a moment before we move on, please raise a hand and we'll pause.
TC: Remember that we have a design/planning meeting that starts half an hour after this call ends.
We're next meeting with RfL on 2024-09-25 to review the status of RfL project goals.
https://github.com/rust-lang/rfcs/pull/3614
Recent LWN articles:
Project board: https://github.com/orgs/rust-lang/projects/43/views/5
None.
TC: We have tracking issues for the Rust 2024 aspects of every item queued for the edition:
https://github.com/rust-lang/rust/issues?q=label%3AA-edition-2024+label%3AC-tracking-issue
For each item, we've identified an owner.
Our motivating priorities are:
The current timeline is:
| Date | Version | Edition stage |
|---|---|---|
| 2024-09-05 | Release v1.81 | Checking off items… |
| 2024-10-11 | Branch v1.83 | Go / no go on all items |
| 2024-10-17 | Release v1.82 | Rust 2024 nightly beta |
| 2024-11-22 | Branch v1.84 | Prepare to stabilize… |
| 2024-11-28 | Release v1.83 | Stabilize Rust 2024 on master |
| 2025-01-03 | Branch v1.85 | Cut Rust 2024 to beta |
| 2025-01-09 | Release v1.84 | Announce Rust 2024 is pending! |
| 2025-02-20 | Release v1.85 | Release Rust 2024 |
All lang priority items are ready for Rust 2024. The remaining not-ready lang items are:
Link: https://github.com/rust-lang/rust/issues/123076
TC: Nadri is working on a stabilization proposal for Rule 1C and Rule 2C:
https://hackmd.io/zUqs2ISNQ0Wrnxsa9nhD0Q#RFC-3627-nano
Link: https://github.com/rust-lang/rust/issues/123735
Link: https://github.com/rust-lang/rust/issues/123739
else" rust#124085Link: https://github.com/rust-lang/rust/issues/124085
expr macro fragment specifier for Rust 2024" rust#123742Link: https://github.com/rust-lang/rust/issues/123742
Link: https://github.com/rust-lang/rust/issues/123758
tail_expr_drop_order lint today gives a lot of false positives" rust#130836Link: https://github.com/rust-lang/rust/issues/130836
TC: Ding's question here for us is about the acceptance criteria of the lint. What do we want to see as a minimum to stabilize shorter_tail_lifetimes in Rust 2024?
So far, the lint has been developed with best-effort approach. We will give the best shot possible to give analysis and diagnosis as precise as possible. If we have to decide go/no go, what is the general criterion for the lint to be acceptable? Knowing that, I can make a better judgement here.
TC: What can we tell him?
Josh: I'd be happy to ship this without a lint and any lint (that doesn't have) is better than nothing. We could always make a better lint later.
tmandry: I'm maybe skeptical. A mutex guarding data outside of a mutex is a necessary pattern.
Josh: You have to have something like that (Mutex<()>), and then write code that's very specific about how it uses tail expressions and how long it expects the mutex to be held.
NM: I'm not sure yet where I fall, probably somewhere between these. I'd like to see some cases of true positives here.
scottmcm: Maybe we could lint about specific types. Not sure if that's practical or not.
NM: In the past we've often been able to make good progress by being specific about what destructors users might care about. I think I'm mostly where Josh is, but I'd like to see what we can do with a lint here.
Josh: As scottmcm suggested, maybe we can narrow this to some specific types.
NM: We'd look for "significant drops" and union that with a specific list of types.
pnkfelix: I'm not opposed to flipping the polarity, but the advantage of it being opt-out rather than opt-in is that ecosystem code benefits for this too. If we make this opt-in, then it seems like we should offer a way for ecosystem crates to opt-in to this.
NM: (Another option is include parking_lot and other known candidates.)
TC: I'm where Josh and Niko are on this also. I think it's OK to expect that people read the edition guide.
extended_varargs_abi_support" rust#116161Link: https://github.com/rust-lang/rust/pull/116161
TC: This stabilization was nominated for us, with pnkfelix commenting:
Just to add on to @cjgillot 's comment above: @wesleywiser and I could not remember earlier today whether T-lang wants to own FCP'ing changes like this that are restricted to extending the set of calling-conventions (i.e. the
convinextern "conv" fn foo(...)), which is largely a detail about what platforms one is interoperating with, and not about changing the expressiveness of the Rust language as a whole in the abstract.(My own gut reaction is that T-compiler is a more natural owner for this than T-lang, but I wasn't certain and so it seems best to let the nomination stand and let the two teams duke it out.)
TC: What do we think about this stabilization?
(I suggest we defer the broader question of whether we want to own all such stabilizations, as the call on this stabilization is probably easy, but the broader discussion may not be.)
NM: Do it.
Josh: Ship it.
Josh: On the policy question I think this is lang, but I suspect it will often be an easy decision.
NM: As an example, "C-unwind" needed to go through us.
tmandry: Anyone know whether we're stabilizing "*-unwind" variants of all of these?
TC: OK, consensus, we'll ship this ABI (and ask about the unwind variants), and ABI decisions still need to go through us.
include! macro" rust#125205Link: https://github.com/rust-lang/rust/pull/125205
TC: Chris Denton asks us:
On Windows, the following code can fail if the
OUT_DIRenvironment variable is a verbatim path (i.e. begins with\\?\):include!(concat!(env!("OUT_DIR"), "/src/repro.rs"));This is because verbatim paths treat
/literally, as if it were just another character in the file name.The good news is that the standard library already has code to fix this. We can simply use
componentsto normalize the path so it works as intended.I think it could just be considered a bug fix but it might also be considered a change in language.
TC: What do we think, is this a bug fix or a change to the language, and if the latter, do we want it?
Josh: For the specific case of verbatim paths, I feel like we should do this. It is theoretically a behavior change, but probably in most cases this will make things work more correctly. There are cases of Windows paths where this wouldn't be correct, e.g. certain kinds of driver paths.
scottmcm: I'm staring at this wondering whether there are any caveats about here that we should worry about.
pnkfelix: Is it not weird we don't have path_concat! or something?
tmandry: I'd echo what pnkfelix said. That sounds better, and I do get nervous when I see special casing around paths like this.
pnkfelix: This no longer seems like an easy decision.
Josh: I feel like we should still make this change, to fix existing code that may have expected the other thing, and also do the concat_paths!.
tmandry: I propose that we ask in the issue about whether there is any way that people could have used this correctly, with forward slashes.
Josh: I'll propose FCP merge also.
Link: https://github.com/rust-lang/rust/pull/128784
TC: This comes to us as a proposed FCW for cases like:
// These raise E0570
extern "thiscall" fn foo() {}
extern "thiscall" { fn bar() }
// This did not raise any error
fn baz(f: extern "thiscall" fn()) { f() }
There's been extensive analysis using crater here. CE says it looks good to him at this point. RalfJ says this should be relatively uncontroversial.
TC: What do we think?
Josh: I don't think this an easy decision. There's some context here for what is or isn't allowed.
CE: This is extending the existing error that we have for invalid ABIs, ABIs not compatible with the target architecture.
Josh: OK, that makes sense and seems reasonable. Sounds great, ship it.
NM: +1.
tmandry: Sounds fine, we did a crater run.
NM: Let's FCP this.
Josh: I'll start an FCP.
TC: On the next two items below, I'm going to file concerns since we should talk through them, but we didn't quite get to them, and they'd otherwise complete FCP.
Josh: An the author of one of those RFCs, full support for filing such a concern to give time for discussion.
(The meeting ended here.)
Link: https://github.com/rust-lang/rfcs/pull/3678
TC: This RFC just went into FCP. I'd like to check my box, as I want something like this, but there are some things I'd like to talk though.
This RFC is about restricting where trait methods can be implemented, e.g. preventing any but default impls.
There's an existing accepted RFC 3323 for restricting where traits can be implemented, e.g.:
pub impl(crate) trait Tr {
fn method(&self) { .. }
}
…and where fields can be mutated…
pub struct S {
pub mut(crate) inner: u8,
}
…so the original draft of the RFC extended this "restrictions" RFC to support where methods can be implemented, e.g.:
pub trait Tr {
impl(crate) fn method1(&self) { .. }
impl(trait) fn method2(&self) { .. }
}
(RFC 3323 itself left the exact syntax as an open question, and this RFC would have inherited that by building on RFC 3323.)
Later drafts updated this to use the final keyword:
pub trait Tr {
final fn method(&self) { .. }
}
Things to talk through:
TC: In the issue, we've received feedback that an attribute would feel more consistent to some, e.g.:
pub trait Tr {
#[final]
fn method(&self) { .. }
}
The argument is that it feels closer to something like #[non_exhaustive] than to async, const, or pub.
I wonder how much having the keyword reserved should change our thinking here. If we didn't, it feels likely we wouldn't have thought twice about this being an attribute.
final too OO, and is the analogy correct?TC: Probably also – admittedly very subjectively – the final keyword to me feels a bit too "OO" for Rust. A trait isn't a base class, but that's kind of what my brain sees with final there.
Less subjectively, given its meaning for inheritance, it suggests another plausible meaning in Rust. Traits can "extend" other traits, i.e. be subtraits ("typeclass inheritance"?). So, arguable, especially in the context of supertrait item shadowing…
https://github.com/rust-lang/rfcs/pull/3624
…where we're preferring subtrait methods of the same name, one might expect final to be a designation on the supertrait that indicates that a subtrait cannot "override" the method. This is actually a more intuitive meaning, given what final means in OO systems, than meaning some restriction about how an "interface" (to use the OO term) can be implemented.
TC: I wonder, given how a crate providing traits often provides various blanket impls, are there really no use cases for a restriction that allows for overriding the method within the crate but not otherwise? If there are, it seems a bit strange to me to break parity with RFC 3323.
(If we don't like the syntax of RFC 3323, we can still change that.)
TC: I had assumed that final (in whatever form) would guarantee devirtualization so that it could be used for cases like the next method for AsyncGen, but now I see discussion that we may never want to do that (for reasonable reasons) and such methods should continue to live on separate traits with blanket impls. This seems a major class of use cases we'd be cutting out. I wonder if we want a way for users to control this.
Cutting this out also causes me to reconsider the distribution of use cases. Guaranteed devirtualization is a good reason for restricting impls to the trait and not to the rest of the crate. Without this, I start to imagine again reasons why one might want such crate-local but not trait-local impls.
Link: https://github.com/rust-lang/rfcs/pull/3624
TC: This RFC is in FCP. One of the main motivations for this RFC is so that libs-api can stabilize methods whose names match those used by itertools. Since the FCP started, one of the maintainers of itertools chimed in to say:
tmandry said:
We'll be creating a slightly bad experience for users of itertools with the lint, and that will only be resolved by the crate releasing a new semver-incompatible version or a new trait (or perhaps doing rustc version hacks, if the signatures are compatible).
This is going to create a slightly bad experience for the maintainers of itertools, too. We will get issues filed by users who would like us to remove now-shadowing methods for their toolchain because of the lint. (If we're very unlucky, those users will have also discovered that the rationale for the lint is to avoid supply chain attacks.) We won't be able to readily remove these methods, because that conflicts with our longstanding policy of maintaining a conservative MSRV. The only way we'll be able to reconcile these interests is by using
build.rs-rustc-version-detection hacks, something we've intentionally avoided becausebuild.rsis a vector for supply chain attacks.Please consider making this lint allow-by-default.
I nominated so we could consider that feedback. We could of course handle the setting of the lint level during stabilization, but it seems worth discussing this feedback while it's fresh.
TC: What do we think?
result_ffi_guarantees" rust#130628Link: https://github.com/rust-lang/rust/pull/130628
TC: Originally this came to us as a request to stabilize FFI guarantees for Result as we do for Option, following on from RFC 3391:
https://github.com/rust-lang/rfcs/pull/3391
However, it's since been observed that we earlier…
https://github.com/rust-lang/rust/pull/60300#issuecomment-488798080
…decided to make these guarantees not just for Option<T>, but for any Option-like enum.
So the question now to us is whether we actually want to make these guarantees for any Result<T, U>-like enum. Specifically, RalfJ proposes these rules as being symmetric with the Option<T> ones:
An enum
Eis guaranteed to be layout- and ABI-compatible with typeTif
Ehas exactly two variantsOne variant has exactly one field, of type
T
Tis arustc_nonnull_optimization_guaranteedtypeThe other variant either has no field, or exactly one field of type
Uthat satisfies all of the following:
Uis a 1-ZSTUmust have no fieldsUmust not have the#[non_exhaustive]attribute
…and proposes that we strengthen the guarantee by dropping the last two bullet points.
TC: What do we think?
Link: https://github.com/rust-lang/rust/pull/127731
TC: RalfJ nominates for us:
There's only 2 crates.io failures and they are both spurious. And the github failures I checked were also all spurious. So looking pretty good I would say. :)
Seems like the next step process-wise is to check back in with the lang team, now that we have crater results. (That's based on this message.) @rust-lang/lang this aims to fix #116558, which is part of the discussion we had in rust-lang/lang-team#235. The way it is fixed is by making it a hard error to call or declare an
extern "C"function that passes a SIMD type by-value without enabling the target feature that enables SIMD vectors of the appropriate size. The reason it is made a hard error is that these functions end up having a different ABI depending on whether that target feature is enabled or not. "Rust" ABI functions are not affected as those functions pass SIMD vectors indirectly (they do that precisely to avoid these ABI issues).There are two points we'd like your take on:
- How to stage this. Should it become a hard error immediately, or first be reported as a future-compat lint (I would say we can make it show up in dependencies immediately)? Crater found no regressions (probably because
extern "C"functions are generally fairly rare), but of course crater doesn't see all the codebases out there.- What exactly should be checked? Right now, the check is extremely low-level and looks at the computed effective ABI of the function, ensuring it doesn't end up with a too large by-value SIMD vector argument. The check is done during monomorphization, each time a function is instantiated, to make sure that we can even know the actual effective ABI. More mono-time checks are unfortunate, but there's no good way to test this in generic code since we can't know whether a generic argument is a SIMD vector or not. Give that it should be rather rare to run into this, hopefully a mono-time check is acceptable.
The other concern is semver compatibility: if a library changes a public type, e.g., from being an array ofi32to being a SIMD vector instead, that can now break downstream code. However, it can only break downstream code that passes types of this library by-value across anextern "C". If that is an FFI boundary, this is already extremely sketchy to do due to all the concerns around FFI safety and ABI compatibility. The most realistic scenario I could come up with is code usingextern "C"for Rust-to-Rust calls; not sure how much we worry about such code – I presume it is fairly rare. Still, we could try to mitigate this by making the check some sort of overapproxomation that generally refuses to pass types from other crates across anextern "C"boundary if those types could in the future be changed such that the ABI ends up with a by-value SIMD vector (i.e.,(i32, ForeignType)would be okay butTransparentWrapper<ForeignType>would not). I personally am not convinced that is worth the effort.
TC: What do we think?
Link: https://github.com/rust-lang/rust/pull/128351
TC: CE nominates for us a proposal by ChayimFriedman2:
Conversion from
&to&mutare and always were immediate UB, and we already lint against them, but until now the lint did not catch the case were the reference was in a field.Conversion from
&to&UnsafeCellis more nuanced: Stacked Borrows makes it immediate UB, but in Tree Borrows it is sound.However, even in Tree Borrows it is UB to write into that reference (if the original value was
Freeze). In all cases crater found where the lint triggered, the reference was written into.
More details here:
https://github.com/rust-lang/rust/pull/128351#issue-2435783651
TC: What do we think?
optimize attribute applied to things other than methods/functions/c…" rust#128943Link: https://github.com/rust-lang/rust/pull/128943
TC: jieyouxu nominates for us:
This needs a T-lang FCP since it diverges from the original RFC 2412 specified behavior. cc @rust-lang/lang
Summary for T-lang
This PR proposes to elevate
#[optimize]attribute (as specified in RFC 2412) applied to unsupported HIR nodes from a warn-by-defaultunused_attributeslint to a hard error.
More details are here:
https://github.com/rust-lang/rust/pull/128943#issuecomment-2282357407
TC: What do we think?
extern crate with #[macro_use]" rust#52043Link: https://github.com/rust-lang/rust/issues/52043
TC: Code like this today isn't idiomatic, since we would use use to import any macros, but it doesn't lint:
#[macro_use]
extern crate rayon;
fn main() { }
TC: Josh nominates as it seems perhaps beyond time that we do so. What do we think?
static mut" rust#53639Link: https://github.com/rust-lang/rust/issues/53639
TC: Long ago, eddyb opened an issue to consider deprecating static mut. This is because handling long-lived references to static mut items is hard to do right.
We decided for Rust 2024 to lint at deny-by-default against long-lived references to static mut items, thus apparently solving the problem that motivated opening the issue. And there seem to be use cases for static mut. As RalfJ says:
Sometimes, you need global mutable state. We already had several cases of people being quite confused and concerned by the lints we started emitting, and it turns out that what they were doing is completely fine and we basically told them to add a bit of noise to their code to silence the lint. Those people will be even more confused if we now tell them that
static mutwill be banned entirely, and I am not convinced that replacingaddr_of_mut!(STATIC)withSTATIC.get()is really doing anything for ensuring the safety of global mutable state.Many people that use
static mutnow are entirely aware of what they are doing, they didn't accidentally stumble into global mutable state. People that just mindlessly run intostatic mutwill equally mindlessly start usingSyncUnsafeCell(probably by copying some online resource telling them how to silence that annoying borrow checker, or by following helpful compiler hints).So overall I am not convinced that the churn caused on existing code, and the overhead caused for people that know what they are doing, is balanced by a sufficient risk reduction for people that don't know what they are doing.
Given all this, it's hard to me to imagine that we've soon depracet static mut, so I propose FCP close.
TC: What do we think?
Link: https://github.com/rust-lang/rust/issues/81391
TC: T-compiler nominates for us:
Nominating for both T-lang and T-compiler discussion: there is a request for the stabilization of the
C-cmse-nonsecure-callABI (posted above at #81391 (comment)), there's also the seemingly related#[cmse_nonsecure_entry]attribute (#75835). This is mostly nominated for several reasons stated below.…
For T-lang:
Procedure questions:
- Does this need T-lang sign-off, like a T-lang FCP or a joint T-lang/T-compiler FCP?
For both T-compiler and T-lang:
- Is the cmse-related efforts being tracked anywhere?
TC: What do we think?
Link: https://github.com/rust-lang/rust/issues/120141
TC: There's a proposed FCP merge for us:
https://github.com/rust-lang/rust/issues/120141#issuecomment-2161507356
TC: What do we think?
Link: https://github.com/rust-lang/rust/issues/128464
TC: Josh nominates for us the question:
Nominating this for lang to discuss the question of whether we should support use of
constinasm!for things that can't just be textually substituted, or whether we should give that a different name.@Amanieu, any input you'd like to provide would be helpful.
To which Amanieu replies:
After thinking about it a bit, I think it's probably fine to add this functionality to
const. I'm a bit bothered about the duplication withsym, which is already stable.
TC: What do we think?
derive(SmartPtr)" rust#129104Link: https://github.com/rust-lang/rust/issues/129104
TC: We need to resolve the open question on:
…regarding what name to use.
Tracking:
TC: What do we think?
target(...) compact feature" rust#130780Link: https://github.com/rust-lang/rust/pull/130780
TC: Urgau suggests that we remove the cfg_target_compact unstable feature. Its tracking issue is:
https://github.com/rust-lang/rust/issues/96901
TC: What do we think?
count, ignore, index, and length (macro_metavar_expr)" rust#122808Link: https://github.com/rust-lang/rust/pull/122808
TC: c410-f3r proposes the following for stabilization:
Stabilization proposal
This PR proposes the stabilization of a subset of
#![feature(macro_metavar_expr)]or more specifically, the stabilization ofcount,ignore,indexandlength.What is stabilized
Count
The number of times a meta variable repeats in total.
macro_rules! count_idents {
( $( $i:ident ),* ) => {
${count($i)}
};
}
fn main() {
assert_eq!(count_idents!(a, b, c), 3);
}
Ignore
Binds a meta variable for repetition, but expands to nothing.
macro_rules! count {
( $( $i:stmt ),* ) => {{
0 $( + 1 ${ignore($i)} )*
}};
}
fn main() {
assert_eq!(count!(if true {} else {}, let _: () = (), || false), 3);
}
Index
The current index of the inner-most repetition.
trait Foo {
fn bar(&self) -> usize;
}
macro_rules! impl_tuple {
( $( $name:ident ),* ) => {
impl<$( $name, )*> Foo for ($( $name, )*)
where
$( $name: AsRef<[u8]>, )*
{
fn bar(&self) -> usize {
let mut sum: usize = 0;
$({
const $name: () = ();
sum = sum.wrapping_add(self.${index()}.as_ref().len());
})*
sum
}
}
};
}
impl_tuple!(A, B, C, D);
fn main() {
}
Length
The current index starting from the inner-most repetition.
macro_rules! array_3d {
( $( $( $number:literal ),* );* ) => {
[
$(
[
$( $number + ${length()}, )*
],
)*
]
};
}
fn main() {
assert_eq!(array_3d!(0, 1; 2, 3; 4, 5), [[2, 3], [4, 5], [6, 7]]);
}
Motivation
Meta variable expressions not only facilitate the use of macros but also allow things that can't be done today like in the
$indexexample.An initial effort to stabilize this feature was made in #111908 but ultimately reverted because of possible obstacles related to syntax and expansion.
Nevertheless, #83527 (comment) tried to address some questions and fortunately the lang team accept #117050 the unblocking suggestions.
Here we are today after ~4 months so everything should be mature enough for wider use.
What isn't stabilized
$$is not being stabilized due to unresolved concerns.
TC: I asked WG-macros for feedback on this here:
TC: Josh proposed FCP merge on this stabilization.
Link: https://github.com/rust-lang/rfcs/pull/3680
TC: Josh nominates a new RFC for us. What do we think?
Link: https://github.com/rust-lang/rfcs/pull/3681
TC: This RFC adds a way to set field defaults, e.g.:
struct Point {
x: u8 = 0,
y: u8 = 0,
}
let p = Point { .. };
But, unfortunately, the interaction with Default here is a bit weird. If we write:
struct Point {
x: u8,
y: u8,
// We just want to override the default `None` on this field,
// but we can leave the others alone because `Default::default`
// for those is fine, right?
z: Option<u8> = Some(0),
}
let p = Point { .. }; //~ ERROR
let p = Point::default(); //~ OK!
In general, under this RFC, Point { .. } and Point::default() can each work when the other can fail and can have arbitrarily different behaviors. So it's in that context that I nominate, suggestion a nightly experiment, and propose a design meeting.
https://github.com/rust-lang/lang-team/issues/291
TC: What do we think?
macro_rules! attribute macros" rfcs#3697Link: https://github.com/rust-lang/rfcs/pull/3697
TC: Josh proposes an RFC for us:
Many crates provide attribute macros. Today, this requires defining proc macros, in a separate crate, typically with several additional dependencies adding substantial compilation time, and typically guarded by a feature that users need to remember to enable.
However, many common cases of attribute macros don't require any more power than an ordinary
macro_rules!macro. Supporting these common cases would allow many crates to avoid defining proc macros, reduce dependencies and compilation time, and provide these macros unconditionally without requiring the user to enable a feature.
E.g.:
macro_rules! main {
attr() ($func:item) => { make_async_main!($func) };
attr(threads = $threads:literal) ($func:item) => { make_async_main!($threads, $func) };
}
#[main]
async fn main() { ... }
#[main(threads = 42)]
async fn main() { ... }
TC: What do we think?
macro_rules! derive macros" rfcs#3698Link: https://github.com/rust-lang/rfcs/pull/3698
TC: Josh proposes an RFC for us:
Many crates support deriving their traits with
derive(Trait). Today, this requires defining proc macros, in a separate crate, typically with several additional dependencies adding substantial compilation time, and typically guarded by a feature that users need to remember to enable.However, many common cases of derives don't require any more power than an ordinary
macro_rules!macro. Supporting these common cases would allow many crates to avoid defining proc macros, reduce dependencies and compilation time, and provide these macros unconditionally without requiring the user to enable a feature.
E.g.:
trait Answer { fn answer(&self) -> u32; }
#[macro_derive]
macro_rules! Answer {
// Simplified for this example
(struct $n:ident $_:tt) => {
impl Answer for $n {
fn answer(&self) -> u32 { 42 }
}
};
}
#[derive(Answer)]
struct Struct;
fn main() {
let s = Struct;
assert_eq!(42, s.answer());
}
TC: What do we think?
use Trait::func" rfcs#3591Link: https://github.com/rust-lang/rfcs/pull/3591
This RFC would add support for:
use Default::default;
struct S {
a: HashMap<i32, i32>,
}
impl S {
fn new() -> S {
S {
a: default()
}
}
}
Josh has proposed FCP merge and has nominated. There are outstanding concerns about the handling of turbofish and associated constants.
TC: What do we think?
start feature" rust#29633Link: https://github.com/rust-lang/rust/issues/29633
TC: Nils proposes to us that we delete the unstable #[start] attribute:
I think this issue should be closed and
#[start]should be deleted. It's nothing but an accidentally leaked implementation detail that's a not very useful mix between "portable" entrypoint logic and bad abstraction.I think the way the stable user-facing entrypoint should work (and works today on stable) is pretty simple:
std-using cross-platform programs should usefn main(). the compiler, together withstd, will then ensure that code ends up atmain(by having a platform-specific entrypoint that gets directed throughlang_startinstdtomain- but that's just an implementation detail)no_stdplatform-specific programs should use#![no_main]and define their own platform-specific entrypoint symbol with#[no_mangle], likemain,_start,WinMainormy_embedded_platform_wants_to_start_here. most of them only support a single platform anyways, and need cfg for the different platform's ways of passing arguments or other things anyways
#[start]is in a super weird position of being neither of those two. It tries to pretend that it's cross-platform, but its signature is a total lie. Those arguments are just stubbed out to zero on Windows, for example. It also only handles the platform-specific entrypoints for a few platforms that are supported bystd, like Windows or Unix-likes.my_embedded_platform_wants_to_start_herecan't use it, and neither could a libc-less Linux program. So we have an attribute that only works in some cases anyways, that has a signature that's a total lie (and a signature that, as I might want to add, has changed recently, and that I definitely would not be comfortable giving any stability guarantees on), and where there's a pretty easy way to get things working without it in the first place.Note that this feature has not been RFCed in the first place.
TC: What do we think?
Link: https://github.com/rust-lang/rust/issues/56604
TC: It's currently UB to write:
fn main() {
let x = &mut 0;
let y: *const i32 = x;
unsafe { *(y as *mut i32) = 1; }
assert_eq!(*x, 1);
}
This is due to the fact that we implicitly first create a shared reference when coercing a &mut to a *const. See:
TC: What do we think about this?
anonymous_lifetime_in_impl_trait" rust#107378Link: https://github.com/rust-lang/rust/pull/107378
TC: We unnominated this back in October 2023 as more analysis seemed to be needed. Since then, nikomatsakis and tmandry have posted substantive analysis that it seems we should discuss.
Link: https://github.com/rust-lang/rust/pull/120193
TC: Apparently our unstable likely and unlikely intrinsics don't work. There's a proposal to do some work on fixing that and stabilizing a solution here. The nominated question is whether we want to charter this as an experiment.
Link: https://github.com/rust-lang/rust/pull/121270
TC: pnkfelix nominates this for us:
This PR that addresses some ICEs arising for the unstable
feature(unnamed_fields), by conservatively mapping the ICE'ing cases to static errors instead.The T-compiler team wants to know the opinion of T-lang of whether
feature(unnamed_fields)is sufficiently likely, in the near future, to be removed (or significantly reworked) to such a degree that it would make more sense to close this PR rather than have contributors spend further time on it.(See also the context established by https://hackmd.io/7r0i-EWyR8yO6po2LnS2rA#Tracking-issue-for-RFC-2102-Unnamed-fields-of-struct-and-union-type-rust49804 (where I think there was supposed to be an eventual writeup of the concerns people had with
feature(unnamed_fields)) and #49804 (comment) )
On 2024-05-12, Josh said:
It sounds like, from the minutes, that some folks would like to see this feature designed differently than it was when it was previously accepted. It wouldn't be the first or last feature to need some design adjustments when lang design met compiler reality. Happy to help with that, so that we can find a design that meets the requirements in the original RFC and any new issues that have arisen since then.
This is about RFC 2102:
https://github.com/rust-lang/rfcs/pull/2102
We discussed this in the meeting on 2024-06-12 without consensus.
Some felt that this was still needed, others first wanted to look for a more minimal approach.
We left this for further discussion.
TC: What do we think?
match is too complex" rust#122685Link: https://github.com/rust-lang/rust/pull/122685
TC: Nadri nominates this for us and describes the situation:
Dear T-lang, this PR adds a warning that cannot be silenced, triggered when a match takes a really long time to analyze (in the order of seconds). This is to help users figure out what's taking so long and fix it.
We could make the limit configurable or the warning
allowable. I argue that's not necessary because crater showed zero regressions with the current limit, and it's be pretty easy in general to split up amatchinto smallermatches to avoid blowup.We're still figuring out the exact limit, but does the team approve in principle?
(As an aside, awhile back someone showed how to lower SAT to exhaustiveness checking with match. Probably that would hit this limit.)
TC: What do we think?
Link: https://github.com/rust-lang/rust/issues/116907
TC: nnethercote has implemented most of RFC 3349 ("Mixed UTF-8 literals") and, based on implementation experience, argues that the remainder of the RFC should not be implemented:
I have a partial implementation of this RFC working locally (EDIT: now at #120286). The RFC proposes five changes to literal syntax. I think three of them are good, and two of them aren't necessary.
TC: What do we think?
i128/u128 from the improper_ctypes lint" lang-team#255Link: https://github.com/rust-lang/lang-team/issues/255
TC: Trevor Gross describes the situation:
For a while, Rust's 128-bit integer types have been incompatible with those from C. The original issue is here rust-lang/rust#54341, with some more concise background information at the MCP here rust-lang/compiler-team#683
The current Beta of 1.77 will have rust-lang/rust#116672, which manually sets the alignment of
i128to make it ABI-compliant with any version of LLVM (clangdoes something similar now). 1.78 will have LLVM18 as the vendored version which fixes the source of this error.Proposal: now that we are ABI-compliant, do not raise
improper_ctypeson our 128-bit integers. I did some testing with abi-cafe and a more isolated https://github.com/tgross35/quick-abi-check during the time https://reviews.llvm.org/D86310 was being worked on, and verified everything lines up. (It would be great to have some fork of abi-cafe in tree, but that is a separate discussion.)@joshtriplett mentioned that changing this lint needs a lang FCP https://rust-lang.zulipchat.com/#narrow/stream/187780-t-compiler.2Fwg-llvm/topic/LLVM.20alignment.20of.20i128/near/398422037. cc @maurer
Reference change from when I was testing rust-lang/rust@c742908
TC: Josh nominates this for our discussion. What do we think?
is operator for pattern-matching and binding" rfcs#3573Link: https://github.com/rust-lang/rfcs/pull/3573
TC: Josh proposes for us that we should accept:
if an_option is Some(x) && x > 3 {
println!("{x}");
}
And:
func(x is Some(y) && y > 3);
TC: The main topic discussed in the issue thread so far has been the degree to which Rust should have "two ways to do things". Probably the more interesting issue is how the binding and drop scopes for this should work.
TC: In the 2024-02-21 meeting (with limited attendance), we discussed how we should prioritize stabilizing let chains, and tmandry suggested we may want to allow those to settle first.
TC: What do we think, as a gut check?
Link: https://github.com/rust-lang/rfcs/pull/3458
TC: Nearly ten years ago, on 2014-10-09, pnkfelix proposed unsafe fields in RFC 381:
https://github.com/rust-lang/rfcs/issues/381
On 2017-05-04, Niko commented:
I am pretty strongly in favor of unsafe fields at this point. The only thing that holds me back is some desire to think a bit more about the "unsafe" model more generally.
Then, in 2023, Jacob Pratt refreshed this proposal with RFC 3458. It proposes that:
Fields may be declared
unsafe. Unsafe fields may only be mutated (excluding interior mutability) or initialized in an unsafe context. Reading the value of an unsafe field may occur in either safe or unsafe contexts. An unsafe field may be relied upon as a safety invariant in other unsafe code.
E.g.:
struct Foo {
safe_field: u32,
/// Safety: Value must be an odd number.
unsafe unsafe_field: u32,
}
// Unsafe field initialization requires an `unsafe` block.
// Safety: `unsafe_field` is odd.
let mut foo = unsafe {
Foo {
safe_field: 0,
unsafe_field: 1,
}
};
On 2024-05-21, Niko nominated this for us:
I'd like to nominate this RFC for discussion. I've not read the details of the thread but I think the concept of unsafe fields is something that comes up continuously and some version of it is worth doing.
TC: What do we think?
Link: https://github.com/rust-lang/rfcs/pull/3556
TC: This seems to be about making the following work:
// kind is optional if it's been specified elsewhere, e.g. via the `-l` flag to rustc
#[link(name="ext", kind="static")]
extern {
#[no_mangle]
pub fn foo();
#[no_mangle]
pub static bar: std::ffi::c_int;
}
There are apparently use cases for this.
What's interesting is that apparently it already does, but we issue a warning that is wrong:
warning: `#[no_mangle]` has no effect on a foreign function
--> src/lib.rs:21:5
|
21 | #[no_mangle]
| ^^^^^^^^^^^^ help: remove this attribute
22 | pub fn foo_rfc3556_pub_with_no_mangle();
| ---------------------------------------- foreign function
|
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: symbol names in extern blocks are not mangled
TC: One of the author's asks of us is that we don't make this into a hard error (e.g. with the new edition).
TC: What do we think?
Link: https://github.com/rust-lang/rust/pull/118939
TC: The idea here seems to be to improve some diagnostics around macro_rules, but this seems to be done by way of reserving the macro_rules token more widely, which is a breaking change. Petrochenkov has objected to it on that basis, given that reserving macro_rules minimally has been the intention since we hope it will one day disappear in favor of macro. What do we think?
clippy::invalid_null_ptr_usage lint" rust#119220Link: https://github.com/rust-lang/rust/pull/119220
TC: Urgau proposes this for us:
This PR aims at uplifting the
clippy::invalid_null_ptr_usagelint into rustc, this is similar to theclippy::invalid_utf8_in_uncheckeduplift a few months ago, in the sense that those two lints lint on invalid parameter(s), here a null pointer where it is unexpected and UB to pass one.
invalid_null_ptr_usages(deny-by-default)
The
invalid_null_ptr_usageslint checks for invalid usage of null pointers.Example
// Undefined behavior
unsafe { std::slice::from_raw_parts(ptr::null(), 0); }
// Not Undefined behavior
unsafe { std::slice::from_raw_parts(NonNull::dangling().as_ptr(), 0); }
Produces:
error: calling this function with a null pointer is undefined behavior, even if the result of the function is unused, consider using a dangling pointer instead
--> $DIR/invalid_null_ptr_usages.rs:14:23
|
LL | let _: &[usize] = std::slice::from_raw_parts(ptr::null(), 0);
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^-----------^^^^
| |
| help: use a dangling pointer instead: `core::ptr::NonNull::dangling().as_ptr()`
Explanation
Calling methods who's safety invariants requires non-null pointer with a null pointer is undefined behavior.
The lint use a list of functions to know which functions and arguments to checks, this could be improved in the future with a rustc attribute, or maybe even with a
#[diagnostic]attribute.
TC: What do we think?
clippy::double_neg lint as double_negations" rust#126604Link: https://github.com/rust-lang/rust/pull/126604
TC: This proposes to lint against cases like this:
fn main() {
let x = 1;
let _b = --x; //~ WARN use of a double negation
}
TC: What do we think?
Link: https://github.com/rust-lang/rust/issues/129030
TC: We use SipHash-1-3-128 in Rust for hashing types to form TypeIds. If these TypeIds collide in a single program, UB may result.
If SipHash-1-3-128 is a secure PRF, then the probability of such collisions happening accidentally in a program that contains an enormous 1M types is one in 2^-89.
But, if someone wanted to brute-force a collision – that is, find two entirely random types that would have the same TypeId – the work factor for that is no more than about 2^64 on average.
The question being nominated for lang is whether we consider that good enough for soundness, for now.
TC: What do we think?
const {} blocks, and const { assert!(...) }" lang-team#251Link: https://github.com/rust-lang/lang-team/issues/251
TC: This issue was raised due to discussion in a T-libs-api call. Josh gives the context:
In discussion of rust-lang/libs-team#325 (a proposal for a compile-time assert macro), the idea came up to allow
const {}blocks at item level, and then have people useconst { assert!(...) }.@rust-lang/libs-api would like some guidance from @rust-lang/lang about whether lang is open to toplevel
const { ... }blocks like this, which would influence whether we want to add a compile-time assert macro, as well as what we want to call it (e.g.static_assert!vsconst_assert!vs some other name).Filing this issue to discuss in a lang meeting. This issue is not seeking any hard commitment to add such a construct, just doing a temperature check.
CAD97 noted:
To ensure that it's noted: if both item and expression
constblocks are valid in the same position (i.e. in statement position), a rule to disambiguate would be needed (like for statement versus expressionif-else). IMO it would be quite unfortunate for item-levelconstblocks to be evaluated pre-mono if that sameconstblock but statement-level would be evaluated post-mono.Additionally: since
const { assert!(...) }is post-mono (due to using the generic context), it's potentially desirable to push people towards usingconst _: () = assert!(...);(which is pre-mono) whenever possible (not capturing generics).
TC: What do we think?
Link: https://github.com/rust-lang/rust/pull/118833
TC: In the 2024-01-03 call, we developed a tentative consensus to lint against direct function pointer comparison and to push people toward using ptr::fn_addr_eq. We decided to ask T-libs-api to add this. There's now an open proposal for that here:
https://github.com/rust-lang/libs-team/issues/323
One question that has come up is whether we would expect this to work like ptr::addr_eq and have separate generic parameters, e.g.:
/// Compares the *addresses* of the two pointers for equality,
/// ignoring any metadata in fat pointers.
///
/// If the arguments are thin pointers of the same type,
/// then this is the same as [`eq`].
pub fn addr_eq<T: ?Sized, U: ?Sized>(p: *const T, q: *const U) -> bool { .. }
Or whether we would prefer that fn_addr_eq enforced type equality of the function pointers. Since we're the ones asking for this, we probably want to develop a consensus here. We discussed this in the call on 2024-01-10, then we opened a Zulip thread:
TC: On this subject, scottmcm raised this point, with which pnkfelix seemed to concur:
I do feel like if I saw code that had
fn1.addr() == fn2.addr()(ifFnPtrwere stabilized), I'd write a comment saying "isn't that whatfn_addr_eqis for?"If the answer ends up being "no, actually, because I have different types", that feels unfortunate even if it's rare.
(Like how
addr_eq(a, b)is nice even if with strict provenance I could writea.addr() == b.addr()anyway.)
TC: scottmcm also asserted confidence that allowing mixed-type pointer comparisons is correct for ptr::addr_eq since comparing the addresses of *const T, *const [T; N], and *const [T] are all reasonable. I pointed out that, if that's reasonable, then ptr::fn_addr_eq is the higher-ranked version of that, since for the same use cases, it could be reasonable to compare function pointers that return those three different things or accept them as arguments.
TC: Adding to that, scottmcm noted that comparing addresses despite lifetime differences is also compelling, e.g. comparing fn(Box<T>) -> &'static mut T with for<'a> fn(Box<T>) -> &'a mut T.
TC: Other alternatives we considered were not stabilizing ptr::fn_addr_eq at all and instead stabilizing FnPtr so people could write ptr::addr_eq(fn1.addr(), fn2.addr()), or expecting that people would write instead fn1 as *const () == fn2 as *const ().
TC: Recently CAD97 raised an interesting alternative:
From the precedent of
ptr::eqandptr::addr_eq, I'd expect a "ptr::fn_eq" to have one generic type and a "ptr::fn_addr_eq" to have two. Even ifptr::fn_eq's implementation is just an address comparison, it still serves as a documentation point to call out the potential pitfalls with comparing function pointers.
TC: What do we think?
TC: Separately, on the 2024-01-10 call, we discussed some interest use cases for function pointer comparison, especially when it's indirected through PartialEq. We had earlier said we didn't want to lint when such comparisons were indirected through generics, but we did address the non-generic case of simply composing such comparisons.
One example of how this is used is in the standard library, in Waker::will_wake:
https://doc.rust-lang.org/core/task/struct.Waker.html#method.will_wake
It's comparing multiple function pointers via a #[derive(PartialEq)] on the RawWakerVTable.
We decided on 2024-01-01 that this case was interesting and we wanted to think about it further. We opened a discussion thread about this:
Since then, another interesting use case in the standard library was raised, in the formatting machinery:
https://doc.rust-lang.org/src/core/fmt/rt.rs.html
What do we think about these, and would we lint on derived PartialEq cases like these or no?
Link: https://github.com/rust-lang/rfcs/pull/2375
TC: We had a design meeting on 2023-09-12 about inherent trait impls. In that meeting, I proposed a use syntax for this:
In the discussion above, we had left two major items unresolved.
- How do we make blanket trait impls inherent?
- How can we allow only some items from the trait impl to be made inherent?
- This is especially tricky for associated functions and methods with a default implementation.
(Part of the motivation for wanting to allow only some items to be made inherent is to prevent or to fix breakage caused when a trait later adds a new method with a default implementation whose name conflicts with the name of an existing inherent method.)
Coming up with a syntax for these that combines well with the
#[inherent]attribute could be challenging.One alternative that would make solving these problems straightforward is to add some syntax to the inherent
implblock for the type. Given the desugaring in the RFC, there is some conceptual appeal here. (quaternic proposed this arrangement; TC is proposing the concrete syntax.)We can use
usesyntax to make this concise and intuitive.Here's an example:
trait Trait1<Tag, T> {
fn method0(&self) -> u8 { 0 }
fn method1(&self) -> u8 { 1 }
}
trait Trait2<Tag, T> {
fn method2(&self) -> u8 { 2 }
fn method3(&self) -> u8 { 3 }
fn method4(&self) -> u8 { 4 }
}
struct Tag;
struct Foo<T>(T);
impl<T> Foo<T> {
// All methods and associated items of Trait1 become inherent,
// except for `method0`. The inherent items are only visible
// within this crate.
pub(crate) use Trait1<Tag, T>::*;
// Only `method2` and `method3` on Trait2 become inherent.
pub use Trait2<Tag, T>::{method2, method3};
fn method0(&self) -> u64 { u64::MAX }
}
impl<T> Trait1<Tag, T> for Foo<T> {}
impl<U: Trait1<Tag, T>, T> Trait2<Tag, T> for U {}
This solves another problem that we discussed above. How do we prevent breakage in downstream crates when a trait later adds a new method with a default implementation? Since a downstream crate might have made an impl of this trait for some local type inherent and might have an inherent method with a conflicting name, this could be breaking.
We already handle this correctly for
usedeclarations with wildcards. Any locally-defined items override an item that would otherwise be brought into scope with a wildcard import. We can reuse that same behavior and intuition here. When a wildcard is used to make all items in the trait inherent, any locally-defined inherent items in theimplprevent those items from the trait with the same name from being made inherent.Advantages:
- It provides a syntax for adopting as inherent a blanket implementation of a trait for the type.
- It provides a syntax for specifying which methods should become inherent, including methods with default implementations.
- The wildcard import (
use Trait::*) makes it very intuitive what exactly is happening and what exactly your API is promising.- The
usesyntax makes it natural for a locally-defined item to override an item from the wildcard import because that's exactly how otherusedeclarations work.rust-analyzerwould probably support expanding a wildcarduse Trait::*to an explicituse Trait::{ .. }just as it does for otherusedeclarations, which would help people to avoid breakage.- We can support any visibility (e.g.
use,pub use,pub(crate) use, etc.) for the items made inherent.Disadvantages:
- There's some redundancy, especially when the items to make inherent are specifically named.
During the meeting, this emerged as the presumptive favorite, and we took on a TODO item to updated the RFC.
After follow-on discussion in Zulip, Niko agreed, and also raised a good question:
Per the discussion on zulip, I have become convinced that it would be better to make this feature use the syntax
use, like:
impl SomeType {
pub use SomeTrait::*; // re-export the methods for the trait implementation
}
This syntax has a few advantages:
- We can give preference to explicit method declared in the impl blocks over glob re-exports, eliminating one source of breakage (i.e., trait adds a method with a name that overlaps one of the inherent methods defined on
SomeType)- Can make just specific methods (not all of them) inherent.
- Easier to see the inherent method when scanning source.
- You can re-export with different visibility levels (e.g.,
pub(crate))- It would work best if we planned to permit
use SomeTrait::some_method;as a way to import methods as standalone fns, but I wish we did that.However, in writing this, I realize an obvious disadvantage – if the trait has more generics and things, it's not obvious how those should map. i.e., consider
struct MyType<T> {
}
impl<T> MyType<T> {
pub use MyTrait::foo;
}
impl<T: Debug> MyTrait for MyType<T> {
fn foo(&self) { }
}
This would be weird – is this an error, because the impl block says it's for all
T? And what if it weretrait MyTRait<X>?
TC: My sense is that we've just been awaiting someone digging in and updating the RFC here.
Link: https://github.com/rust-lang/rfcs/pull/3098
TC: We've at various times discussed that we had earlier decided that if we wanted to use a new keyword within an edition, we would write it as k#keyword, and for that reason, we prefer to not speculatively reserve keywords ahead of an edition (except, perhaps, when it's clear we plan to use it in the near future).
TC: Somewhat amusingly, however, we never in fact accepted that RFC. Back in 2021, we accepted scottmcm's proposal to cancel:
We discussed this RFC again in the lang team triage meeting today.
For the short-term goal of the reservation for the edition, we'll be moving forward on #3101 instead. As such, we wanted to leave more time for conversations about this one, and maybe use crater results from 3101 to make design changes,
@rfcbot cancel
Instead we accepted RFC 3101 that reserved ident#foo, ident"foo", ident'f', and ident#123 starting in the 2023 edition.
Reading through the history, here's what I see:
k#keyword, but it's another to actually do it in the face of certain criticism about that being e.g. unergonomic. Would we follow through?TC: What do we think?
Link: https://github.com/rust-lang/rfcs/pull/3437
TC: We discussed this in the lang planning meeting in June, and it looks like there have been updates since we last looked at this, so it's time for us to have another look since we seemed interested in this happening.
TC: What do we think?
Link: https://github.com/rust-lang/rust/issues/62569
TC: Prior to main() being executed, the Rust startup code makes a syscall to change the handling of SIGPIPE. Many believe that this is wrong thing for a low-level language like Rust to do, because 1) it makes it impossible to recover what the original value was, and 2) means things like seccomp filters must be adjusted for this.
It's also just, in a practical sense, wrong for most CLI applications.
This seems to have been added back when Rust had green threads and then forgotten about. But it's been an ongoing footgun.
Making a celebrity appearance, Rich Felker, the author of MUSL libc, notes:
As long as Rust is changing signal dispositions inside init code in a way that the application cannot suppress or undo, it is fundamentally unusable to implement standard unix utilities that run child processes or anything that needs to preserve the signal dispositions it was invoked with and pass them on to children. Changing inheritable process state behind the application's back is just unbelievably bad behavior and does not belong in a language runtime for a serious language…
As an example, if you implement
findin Rust, the-execoption will invoke its commands withSIGPIPEset toSIG_IGN, so that they will not properly terminate on broken pipe. But if you just made it setSIGPIPEtoSIG_DFLbefore invoking the commands, now it would be broken in the case where the invoking user intentionally setSIGPIPEtoSIG_IGNso that the commands would not die on broken pipe.
There was discussion in 2019 about fixing this over an edition, but nothing came of it.
Are we interested in fixing it over this one?
Strawman (horrible) proposal: We could stop making this pre-main syscall in Rust 2024 and have cargo fix insert this syscall at the start of every main function.
(In partial defense of the strawman, it gets us directly to the arguably best end result while having an automatic semantics-preserving edition migration and it avoids the concerns about lang/libs coupling that Mara raised. The edition migration could add a comment above this inserted code telling people under what circumstances they should either keep or delete the added line.)
Link: https://github.com/rust-lang/rust/issues/116557
TC: nikomatsakis nominated this:
We had some discussion about types/lang team interaction. We concluded a few things:
- Pinging the team like @rust-lang/lang is not an effective way to get attention. Nomination is the only official way to get attention.
- It's ok to nominate things in an "advisory" capacity but not block (e.g., landing a PR), particularly as most any action can ultimately be reversed. But right now, triagebot doesn't track closed issues, so that's a bit risky.
Action items:
- We should fix triagebot to track closed issues.
TC: What do we think?
core::marker::Freeze in bounds" rfcs#3633Link: https://github.com/rust-lang/rfcs/pull/3633
TC: There's a proposal on the table for the stabilization of the Freeze trait in bounds.
We discussed this in our design meeting on 2024-07-24.
TC: What's next here?
PartialOrd and Ord for Discriminant" rust#106418Link: https://github.com/rust-lang/rust/pull/106418
TC: We discussed this last in the meeting on 2024-03-13. scottmcm has now raised on concern on the issue and is planning to make a counter-proposal:
I remain concerned about exposing this with no opt-out on an unrestricted generic type @rfcbot concern overly-broad
I'm committing to making an alternative proposal because I shouldn't block without one. Please hold my feet to the fire if that's no up in a week.
Basically, I have an idea for how we might be able to do this, from #106418 (comment)
- Expose the variant ordering privately, only accessible by the type owner/module.
Solution 2. is obviously more desirable, but AFAIK Rust can't do that and there is no proposal to add a feature like that.
https://github.com/rust-lang/rust/pull/106418#issuecomment-1994833151
Link: https://github.com/rust-lang/rust/issues/121708
TC: We discussed this in the meeting on 2024-03-13. The feelings expressed included:
TC: tmandry volunteered to draft a policy proposal.
Link: https://github.com/rust-lang/rust/issues/122301
TC: The8472 asks whether this code, which compiles today, can be relied upon:
const fn panic<T>() {
struct W<T>(T);
impl<T> W<T> {
const C: () = panic!();
}
W::<T>::C
}
struct Invoke<T, const N: usize>(T);
impl<T, const N: usize> Invoke<T, N> {
const C: () = match N {
0 => (),
// Not called for `N == 0`, so not monomorphized.
_ => panic::<T>(),
};
}
fn main() {
let _x = Invoke::<(), 0>::C;
}
The8472 notes that this is a useful property and that there are use cases for this in the compiler and the standard library, at least unless or until we adopt something like const if:
https://github.com/rust-lang/rfcs/issues/3582
RalfJ has pointed out to The8472 that the current behavior might not be intentional and notes:
It's not opt-dependent, but it's also unclear how we want to resolve the opt-dependent issue. Some proposals involve also walking all items "mentioned" in a const. That would be in direct conflict with your goal here I think. To be clear I think that's a weakness of those proposals. But if that turns out to be the only viable strategy then we'll have to decide what we want more: using
consttricks to control what gets monomorphized, or not having optimization-dependent errors.One crucial part of this construction is that everything involved is generic. If somewhere in the two "branches" you end up calling a monomorphic function, then that may have its constants evaluated even if it is in the "dead" branch – or it may not, it depends on which functions are deemed cross-crate-inlinable. That's basically what #122814 is about.
TC: The question to us is whether we want to guarantee this behavior. What do we think?
Link: https://github.com/rust-lang/rust/issues/122759
TC: In the call on 2024-03-13, we discussed this issue raised by tmandry:
"Fallout from expansion of redundant import checking"
https://github.com/rust-lang/rust/issues/121708
During the call, the thoughts expressed included:
TC: tmandry volunteered to draft a policy proposal. He's now written up this proposal in this issue.
TC: What do we think?
Link: https://github.com/rust-lang/rust/issues/127436
TC: We decided recently to unblock progress on attributes on expressions (RFC 16) by allowing attributes on blocks. We have a proposed FCP to this effect.
After we did this, the question came up what we want to do about attributes in list contexts, e.g.:
call(#[foo] { block1 }, #[bar] { block2 })
…in particular, macro attributes.
Petrochenkov says:
It needs to be decided how proc macros see the commas, or other separators in similar cases.
Ideally proc macros should be able to turn 1 expression into multiple (including 0) expressions in this position, similarly to
cfgs or macros in list contexts without separators. So it would be reasonable if the separators were included into both input and output tokens streams (there are probably other alternatives, but they do not fit into the token-based model as well). The "reparse context" bit from #61733 (comment) is likely relevant to this case as well.
We filed a concern to figure this all out.
We discussed this on 2024-07-24 and came up with these options:
Options ordered from least to most conservative (and then from most to least expressive):
- Option A: Punt this case and don't support attributes in this position without parens (e.g.
call((#[attr] arg), (#[attr] arg2)))- Option B (exactly one): Specify that, for now, if you use a macro attribute on an expression, that macro can only expand to a single expresion (not zero tokens, and no tokens following in the output).
- Option C (zero or one): Specify that, for now, if you use a macro attribute on an expression, that macro can only expand to zero tokens or an expression with nothing following (extra tokens, including
,, are an error for now)- Option D (zero or more): Specify that an attribute in this position can expand to tokens that may include a
,, and that if they expand to zero tokens then we elide the comma.- Option E (flexible): include comma, let macro decide, etc
- We find it surprising that comma would be included.
In discussion, we seemed generally interested in allowing at least zero and 1. We weren't sure about N, and we weren't sure about the handling of the comma in the input.
TC: What do we think?
Link: https://github.com/rust-lang/rust/issues/128129
In the special design meeting 2024-07-23, we discussed five syntaxes for async closures:
F: async FnMut() -> TF: AsyncFnMut() -> TF: async mut fn() -> TF: async mut () -> TF: async mut || -> TOur current straw poll is:
| name | async Fn*() -> T |
AsyncFn*() -> T |
async fn * () -> T |
async * () -> T |
async * |A, B| -> T |
|---|---|---|---|---|---|
| nikomatsakis | +1 | +0 | -1 | -1 | -1 |
| tmandry | +1 | +1 | -0.5 | -1 | -0.5 |
| Josh | -1 | +0.75 | +0.5 | +1 | +0.5 |
| pnkfelix | +0 | -0 | -0.5 | +0 | -1 |
| scottmcm | |||||
| TC | +1 | -0.5 | -1 | -1 | -1 |
| CE | +1 | -1 | -1 | -0.5 | -1 |
| eholk | +1 | +0 | -1 | -0.75 | -0.5 |
We agreed to:
There was substantial discussion of this after the design meeting itself in the thread here:
TC: This is just an FYI for those that didn't follow this closely.
jiff due to ambiguous_negative_literals" rust#128287Link: https://github.com/rust-lang/rust/issues/128287
We recently adopted a lint againstn ambiguous_negative_literals like:
assert_eq!(-1.abs(), -1);
However, immediately after this landed, BurntSushi reported that this effectively breaks a pattern he had used in the API of his jiff datetime library:
This completely broke Jiff once the change landed. Because it specifically supports negating spans like
-1.hour(). In the case of Jiff, there is no ambiguity because-1.hour()and(-1).hour()and-(1.hour())are all precisely equivalent.
This seems worth our consideration.
One option proposed is that, perhaps, we should have a way to notate functions that are sensitive to the order of operations here, and lint on calls to those with unparenthesized negative literals.
Another option proposed is to go the other way, and introduce some way to notate functions that are not sensitive to the order of operations.
TC: What do we think?
Link: https://github.com/rust-lang/rust/pull/128985
Link: https://github.com/rust-lang/rust/issues/129577
None.
Link: https://github.com/rust-lang/lang-team/pull/236
Link: https://github.com/rust-lang/lang-team/pull/237
Link: https://github.com/rust-lang/lang-team/pull/258
Link: https://github.com/rust-lang/lang-team/pull/267
Link: https://github.com/rust-lang/lang-team/pull/290
None.
S-waiting-on-teamLink: https://github.com/rust-lang/rust/pull/129972
extended_varargs_abi_support" rust#116161Link: https://github.com/rust-lang/rust/pull/116161
include! macro" rust#125205Link: https://github.com/rust-lang/rust/pull/125205
Link: https://github.com/rust-lang/rust/pull/128784
dyn Trait principal" rust#126660Link: https://github.com/rust-lang/rust/pull/126660
Link: https://github.com/rust-lang/rust/pull/129670
ManuallyDrop" rust#130279Link: https://github.com/rust-lang/rust/pull/130279
Link: https://github.com/rust-lang/rust/pull/127731
optimize attribute applied to things other than methods/functions/c…" rust#128943Link: https://github.com/rust-lang/rust/pull/128943
repr(tag = ...) for type aliases" rfcs#3659Link: https://github.com/rust-lang/rfcs/pull/3659
count, ignore, index, and length (macro_metavar_expr)" rust#122808Link: https://github.com/rust-lang/rust/pull/122808
match is too complex" rust#122685Link: https://github.com/rust-lang/rust/pull/122685
Link: https://github.com/rust-lang/rust/pull/118939
AsyncIterator back to Stream, introduce an AFIT-based AsyncIterator trait" rust#119550Link: https://github.com/rust-lang/rust/pull/119550
#[deny] inside #[forbid] as a no-op" rust#121560Link: https://github.com/rust-lang/rust/pull/121560
Link: https://github.com/rust-lang/rust/pull/128985
Check your boxes!
result_ffi_guarantees" rust#130628Link: https://github.com/rust-lang/rust/pull/130628
repr(tag = ...) for type aliases" rfcs#3659Link: https://github.com/rust-lang/rfcs/pull/3659
static mut" rust#53639Link: https://github.com/rust-lang/rust/issues/53639
Link: https://github.com/rust-lang/rust/issues/120141
count, ignore, index, and length (macro_metavar_expr)" rust#122808Link: https://github.com/rust-lang/rust/pull/122808
Link: https://github.com/rust-lang/rfcs/pull/3681
use Trait::func" rfcs#3591Link: https://github.com/rust-lang/rfcs/pull/3591
anonymous_lifetime_in_impl_trait" rust#107378Link: https://github.com/rust-lang/rust/pull/107378
Link: https://github.com/rust-lang/rfcs/pull/3632
PartialOrd and Ord for Discriminant" rust#106418Link: https://github.com/rust-lang/rust/pull/106418
Link: https://github.com/rust-lang/rust/issues/122759
Link: https://github.com/rust-lang/rust/issues/127436
Link: https://github.com/rust-lang/rfcs/pull/3288
Link: https://github.com/rust-lang/rfcs/pull/3379
#[export_ordinal(n)] attribute" rfcs#3641Link: https://github.com/rust-lang/rfcs/pull/3641
Link: https://github.com/rust-lang/rust/pull/120700
Link: https://github.com/rust-lang/rust/pull/129972
Link: https://github.com/rust-lang/rfcs/pull/3678
Link: https://github.com/rust-lang/rfcs/pull/3624
dyn Trait principal" rust#126660Link: https://github.com/rust-lang/rust/pull/126660
Link: https://github.com/rust-lang/rust/pull/129670
cfg predicates" rfcs#3695Link: https://github.com/rust-lang/rfcs/pull/3695
Link: https://github.com/rust-lang/rust/pull/128778
Link: https://github.com/rust-lang/lang-team/issues/286
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