`oneshot` Channel #143741

connortsui20 · 2025-07-10T17:06:19Z

Tracking Issue: #143674

This PR adds an experimental oneshot module.

Before talking about the API itself, I would prefer to get some of these questions below out of the way first. And as discussed in the ACP it would be

Unresolved Questions

Why exactly is it okay for Sender to be Sync? Or basically, how do we boil down the discussion in Implement Sync for mpsc::Sender #111087 into a comment for the unsafe impl<T: Send> Sync for Sender<T> {}?
~~Why is mpsc::Receiver !Sync but mpmc::Receiver is Sync? Should oneshot::Receiver be Sync or not?~~
Should this PR try to add an is_ready method as proposed in the tracking issue? If so, then the surface of this PR would likely need to increase to add a pub(crate) fn is_disconnected method to mpmc (might even be a good idea to add that to all 3 channel flavors).
In a similar vein to the previous question, should the first internal implementation simply be a wrapper around mpmc, or should it be a wrapper around the internal crossbeam implementation?
Should the Sender and Receiver operations be methods or associated methods? So sender.send(msg) or Sender::send(sender, msg)? The method syntax is more consistent with the rest of the ecosystem (namely tokio)

rustbot · 2025-07-10T17:09:38Z

r? @tgross35

rustbot has assigned @tgross35.
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tgross35 · 2025-07-10T19:19:44Z

Maybe want to ask the unresolved questions at the ACP thread? To make sure it's seen by those already involved.

(I'll take a closer look later)

RustyYato · 2025-07-10T20:14:17Z

Why exactly is it okay for Sender to be Sync? Or basically, how do we boil down the discussion in #111087 into a comment for the unsafe impl<T: Send> Sync for Sender {}?

Why is mpsc::Receiver !Sync but mpmc::Receiver is Sync? Should oneshot::Receiver be Sync or not?

Because mpsc is multiple producer single consumer, and does the receiver operation via &self, so it would be incorrect to allow recv to be shared across threads because that would allow two threads to recv at the same time, which isn't allowed for a mpsc.

But mpmc is multiple producer multiple consumer, so it doesn't matter how many threads recv at the same time.

For this one, they can both be unconditionally Sync, since there is no API to go from &Sender<T> or &Reciever<T> to &T. (even via traits like Debug or Clone). In fact it looks like &Sender<T> and &Receiver<T> are entirely useless (only a Debug impl, which doesn't provide any information), which is just like Exclusive.. This is more evidence that both Sender<T> and Receiver<T> can be unconditionally Sync

connortsui20 · 2025-07-10T20:29:50Z

Thank you for the answer!

Please let me know if I'm interpreting this right: If all methods where synchronization must occur consume self, we can say that both Sender and Receiver are Sync. For example, if threads share a &oneshot::Receiver, then none of them can call recv anyways. And this would still be compatible with the proposed is_ready method discussed in the tracking issue.

library/std/tests/sync/oneshot.rs

tgross35 · 2025-07-29T03:29:05Z

I haven't gotten a chance to look into this unfortunately

r? libs

connortsui20 · 2025-08-23T13:55:59Z

r? libs (based on this comment)

bors · 2025-10-21T08:25:48Z

☔ The latest upstream changes (presumably #147928) made this pull request unmergeable. Please resolve the merge conflicts.

tgross35 · 2025-10-21T20:38:37Z

It's been a little while so
r? @joboet

Feel free to reroll if you want but you've reviewed a lot of sync module things.

connortsui20 · 2026-01-04T00:41:34Z

For reference I just changed the doctest to this:

#![feature(oneshot_channel)]
use std::sync::oneshot::{self, RecvTimeoutError};
use std::thread;
use std::time::Duration;

let (sender, receiver) = oneshot::channel();

let send_failure = thread::spawn(move || {
    // Simulate a long computation that takes longer than our timeout.
    thread::sleep(Duration::from_millis(250));

    // This will likely fail to send because we drop the receiver in the main thread.
    sender.send("Goodbye!".to_string()).unwrap();
});

// Try to receive the message with a short timeout.
match receiver.recv_timeout(Duration::from_millis(10)) {
    Ok(msg) => println!("Received: {}", msg),
    Err(RecvTimeoutError::Timeout(rx)) => {
        println!("Timed out waiting for message!");

        // Note that you can reuse the receiver without dropping it.
        drop(rx);
    },
    Err(RecvTimeoutError::Disconnected) => println!("Sender dropped!"),
}

send_failure.join().unwrap_err();

@rustbot ready

joboet · 2026-01-04T18:02:46Z

library/std/src/sync/oneshot.rs

+#[unstable(feature = "oneshot_channel", issue = "143674")]
+impl<T> fmt::Debug for TryRecvError<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        "oneshot::TryRecvError(..)".fmt(f)


Somethings gone wrong here, this issue is back...

connortsui20 · 2026-01-04T22:42:04Z

argh I accidentally dropped the commit instead of squashing it... Thank you for catching that

The `oneshot` channel is gated under the `oneshot_channel` feature. Signed-off-by: Connor Tsui <connor.tsui20@gmail.com>

Tests inspired by tests in the `oneshot` third-party crate.

rustbot · 2026-01-04T22:51:39Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

connortsui20 · 2026-01-04T22:52:37Z

@rustbot ready

joboet · 2026-01-05T10:31:11Z

Great, thank you!
@bors r+

bors · 2026-01-05T10:31:14Z

📌 Commit b481ecd has been approved by joboet

It is now in the queue for this repository.

bors · 2026-01-05T11:35:46Z

⌛ Testing commit b481ecd with merge b7bcaa5...

bors · 2026-01-05T14:54:07Z

☀️ Test successful - checks-actions
Approved by: joboet
Pushing b7bcaa5 to main...

github-actions · 2026-01-05T14:57:14Z

What is this?

This is an experimental post-merge analysis report that shows differences in test outcomes between the merged PR and its parent PR.

Comparing 9e8a0c5 (parent) -> b7bcaa5 (this PR)

Test differences

Show 67 test diffs

Stage 1

oneshot::drop_sender_then_recv: [missing] -> pass (J1)
oneshot::message_in_channel_dropped_on_receiver_drop: [missing] -> pass (J1)
oneshot::non_send_type_can_be_used_on_same_thread: [missing] -> pass (J1)
oneshot::recv_before_send: [missing] -> pass (J1)
oneshot::recv_deadline_passed: [missing] -> pass (J1)
oneshot::recv_no_time: [missing] -> pass (J1)
oneshot::recv_then_drop_sender: [missing] -> pass (J1)
oneshot::recv_time_passed: [missing] -> pass (J1)
oneshot::recv_timeout_before_send: [missing] -> pass (J1)
oneshot::send_before_recv: [missing] -> pass (J1)
oneshot::send_before_recv_timeout: [missing] -> pass (J1)
oneshot::send_before_try_recv: [missing] -> pass (J1)
oneshot::send_error: [missing] -> pass (J1)
oneshot::send_error_drops_message_correctly: [missing] -> pass (J1)
oneshot::send_error_drops_message_correctly_on_extract: [missing] -> pass (J1)
oneshot::send_never_deadline: [missing] -> pass (J1)
oneshot::sender_drop: [missing] -> pass (J1)
oneshot::try_recv_empty: [missing] -> pass (J1)
oneshot::try_recv_then_drop_receiver: [missing] -> pass (J1)

Stage 2

oneshot::drop_sender_then_recv: [missing] -> pass (J0)
oneshot::message_in_channel_dropped_on_receiver_drop: [missing] -> pass (J0)
oneshot::non_send_type_can_be_used_on_same_thread: [missing] -> pass (J0)
oneshot::recv_before_send: [missing] -> pass (J0)
oneshot::recv_deadline_passed: [missing] -> pass (J0)
oneshot::recv_no_time: [missing] -> pass (J0)
oneshot::recv_then_drop_sender: [missing] -> pass (J0)
oneshot::recv_time_passed: [missing] -> pass (J0)
oneshot::recv_timeout_before_send: [missing] -> pass (J0)
oneshot::send_before_recv: [missing] -> pass (J0)
oneshot::send_before_recv_timeout: [missing] -> pass (J0)
oneshot::send_before_try_recv: [missing] -> pass (J0)
oneshot::send_error: [missing] -> pass (J0)
oneshot::send_error_drops_message_correctly: [missing] -> pass (J0)
oneshot::send_error_drops_message_correctly_on_extract: [missing] -> pass (J0)
oneshot::send_never_deadline: [missing] -> pass (J0)
oneshot::sender_drop: [missing] -> pass (J0)
oneshot::try_recv_empty: [missing] -> pass (J0)
oneshot::try_recv_then_drop_receiver: [missing] -> pass (J0)

Additionally, 29 doctest diffs were found. These are ignored, as they are noisy.

Job group index

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard b7bcaa5c715ed07af2d74dac7ddb8786abeb4299 --output-dir test-dashboard

And then open test-dashboard/index.html in your browser to see an overview of all executed tests.

Job duration changes

dist-loongarch64-linux: 5266.7s -> 6269.6s (+19.0%)
dist-aarch64-apple: 7844.4s -> 9045.2s (+15.3%)
dist-x86_64-netbsd: 4832.3s -> 5415.7s (+12.1%)
dist-ohos-armv7: 4086.1s -> 4519.0s (+10.6%)
dist-armv7-linux: 4967.4s -> 5453.1s (+9.8%)
dist-i686-msvc: 8076.1s -> 8795.6s (+8.9%)
x86_64-msvc-ext1: 7886.9s -> 7191.3s (-8.8%)
x86_64-gnu-llvm-20-3: 6686.9s -> 6125.0s (-8.4%)
dist-x86_64-apple: 9550.0s -> 8749.7s (-8.4%)
dist-apple-various: 4738.0s -> 4358.9s (-8.0%)

How to interpret the job duration changes?

Job durations can vary a lot, based on the actual runner instance
that executed the job, system noise, invalidated caches, etc. The table above is provided
mostly for t-infra members, for simpler debugging of potential CI slow-downs.

rust-timer · 2026-01-05T15:44:30Z

Finished benchmarking commit (b7bcaa5): comparison URL.

Overall result: ❌ regressions - no action needed

@rustbot label: -perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	0.1%	[0.1%, 0.1%]	1
Regressions ❌ (secondary)	0.5%	[0.0%, 0.9%]	4
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.1%	[0.1%, 0.1%]	1

Max RSS (memory usage)

Results (primary 2.4%, secondary 2.4%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	2.4%	[2.4%, 2.4%]	1
Regressions ❌ (secondary)	2.4%	[2.4%, 2.4%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.4%	[2.4%, 2.4%]	1

Cycles

Results (primary -2.1%, secondary -2.6%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-2.1%	[-2.1%, -2.1%]	1
Improvements ✅ (secondary)	-2.6%	[-2.6%, -2.6%]	1
All ❌✅ (primary)	-2.1%	[-2.1%, -2.1%]	1

Binary size

Results (primary -0.0%, secondary -0.0%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.0%	[-0.0%, -0.0%]	4
Improvements ✅ (secondary)	-0.0%	[-0.1%, -0.0%]	13
All ❌✅ (primary)	-0.0%	[-0.0%, -0.0%]	4

Bootstrap: 474.474s -> 474.563s (0.02%)
Artifact size: 390.78 MiB -> 390.70 MiB (-0.02%)

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Jul 10, 2025

connortsui20 marked this pull request as ready for review July 10, 2025 17:09

rustbot assigned tgross35 Jul 10, 2025

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Jul 10, 2025

connortsui20 force-pushed the oneshot branch from 2934b19 to 90740d3 Compare July 10, 2025 17:20

connortsui20 mentioned this pull request Jul 10, 2025

Tracking Issue for oneshot_channel #143674

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connortsui20 force-pushed the oneshot branch from 91df320 to 1d9d8fd Compare July 10, 2025 20:50

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connortsui20 force-pushed the oneshot branch 2 times, most recently from c7e668b to 1a515fd Compare July 17, 2025 16:08

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ACP: Add a oneshot channel rust-lang/libs-team#610

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connortsui20 force-pushed the oneshot branch 3 times, most recently from e32a0d2 to c9cf24e Compare January 4, 2026 00:39

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Jan 4, 2026

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connortsui20 added 2 commits January 5, 2026 09:47

add experimental oneshot channel

9ba5b5e

The `oneshot` channel is gated under the `oneshot_channel` feature. Signed-off-by: Connor Tsui <connor.tsui20@gmail.com>

add oneshot tests

b481ecd

Tests inspired by tests in the `oneshot` third-party crate.

connortsui20 force-pushed the oneshot branch from c9cf24e to b481ecd Compare January 4, 2026 22:51

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Jan 4, 2026

bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jan 5, 2026

bors added the merged-by-bors This PR was explicitly merged by bors. label Jan 5, 2026

bors merged commit b7bcaa5 into rust-lang:main Jan 5, 2026
12 checks passed

rustbot added this to the 1.94.0 milestone Jan 5, 2026

connortsui20 deleted the oneshot branch January 5, 2026 19:22

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oneshot Channel #143741

oneshot Channel #143741

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Test differences

Stage 1

Stage 2

Job duration changes

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rust-timer commented Jan 5, 2026

Overall result: ❌ regressions - no action needed

Instruction count

Max RSS (memory usage)

Cycles

Binary size

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`oneshot` Channel #143741

`oneshot` Channel #143741

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