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rust-bors merged 2 commits into from
Sep 11, 2026
+43 −30
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Clarify examples related to Sync and SyncView #162621

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Clarify examples related to `Sync` and `SyncView`
tisonkun Sep 11, 2026
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tisonkun Sep 11, 2026
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73 changes: 43 additions & 30 deletions library/core/src/sync/sync_view.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -26,49 +26,62 @@ use core::task::{Context, Poll};
///
/// ## Examples
///
/// Using a non-`Sync` future prevents the wrapping struct from being `Sync`:
/// A non-`Sync` field prevents the wrapping struct from being `Sync`:
///
/// ```compile_fail
/// use core::cell::Cell;
/// ```compile_fail,E0277
/// use std::sync::mpsc::{self, Receiver};
///
/// async fn other() {}
/// fn assert_sync<T: Sync>(t: T) {}
/// struct State<F> {
/// future: F
/// struct Inbox {
/// name: &'static str,
/// receiver: Receiver<u32>,
/// }
///
/// assert_sync(State {
/// future: async {
/// let cell = Cell::new(1);
/// let cell_ref = &cell;
/// other().await;
/// let value = cell_ref.get();
/// }
/// });
/// fn require_send<T: Send>() {}
/// fn require_send_sync<T: Send + Sync>() {}
///
/// require_send::<Inbox>(); // compiled
/// require_send_sync::<Inbox>(); // compile-failed
/// ```
///
/// `SyncView` ensures the struct is `Sync` without stripping the future of its
/// `SyncView` makes the value `Sync` without stripping the struct of its
/// functionality:
///
/// ```
/// ```ignore-wasm
Comment on lines -53 to +49

@tgross35 tgross35 Sep 15, 2026

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This gives a "This example is not tested on wasm" warning in the docs https://doc.rust-lang.org/nightly/std/sync/struct.SyncView.html#examples which is a bit weird. Perhaps this and the original Atomic example should gate on target_has_threads instead?

This isn't super accurate anyway since we have wasm targets with threads and non-wasm targets without threads, it just happens that the wasm target in CI is the only thing we flag.

@tisonkun tisonkun Sep 15, 2026

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Thanks for your suggestion.

My concern is that target_has_threads doesn't guarantee that std::thread::spawn is supported.

For example, on wasm32-unknown-unknown, enabling +atomics makes singlethread() return false, which enables target_has_threads. However, std still selects the unsupported thread backend, where Thread::new always returns UNSUPPORTED_PLATFORM.

@tgross35 tgross35 Sep 15, 2026

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That's a good point too. Opened a discussion at #t-libs > Skipping doctests without thread::spawn

/// #![feature(exclusive_wrapper)]
/// use core::cell::Cell;
/// use core::sync::SyncView;
///
/// async fn other() {}
/// fn assert_sync<T: Sync>(t: T) {}
/// struct State<F> {
/// future: SyncView<F>
/// use std::sync::SyncView;
/// use std::sync::mpsc::{self, Receiver};
/// use std::thread;
///
/// struct Inbox {
/// name: &'static str,
/// receiver: SyncView<Receiver<u32>>,
/// }
///
/// impl Inbox {
/// fn name(&self) -> &'static str {
/// self.name
/// }
///
/// fn recv(&mut self) -> u32 {
/// self.receiver.as_mut().recv().unwrap()
/// }
/// }
///
/// assert_sync(State {
/// future: SyncView::new(async {
/// let cell = Cell::new(1);
/// let cell_ref = &cell;
/// other().await;
/// let value = cell_ref.get();
/// })
/// let (sender, receiver) = mpsc::channel();
/// let mut inbox = Inbox { name: "jobs", receiver: SyncView::new(receiver) };
/// sender.send(42).unwrap();
/// drop(sender);
///
/// thread::scope(|scope| {
/// let reader = scope.spawn(|| inbox.name());
/// assert_eq!(inbox.name(), "jobs");
/// assert_eq!(reader.join().unwrap(), "jobs");
/// });
///
/// let message = thread::spawn(move || inbox.recv()).join().unwrap();
/// assert_eq!(message, 42);
/// println!("Shared Inbox across threads, then moved it to a worker and received 42");
/// ```
///
/// ## Parallels with a mutex
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