Auto merge of #143741 - connortsui20:oneshot, r=joboet

`oneshot` Channel

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](rust-lang/libs-team#610) it would be

# Unresolved Questions

- [x] ~~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<T> {}`?~~
- [x] ~~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`)

Author: bors

library/std/src/sync/mod.rs

@@ -184,6 +184,8 @@ pub use alloc_crate::sync::{Arc, Weak};
 #[unstable(feature = "mpmc_channel", issue = "126840")]
 pub mod mpmc;
 pub mod mpsc;
+#[unstable(feature = "oneshot_channel", issue = "143674")]
+pub mod oneshot;
 
 pub(crate) mod once; // `pub(crate)` for the `sys::sync::once` implementations and `LazyLock`.
 

library/std/src/sync/mpmc/mod.rs

@@ -654,7 +654,7 @@ impl<T> Clone for Sender<T> {
 #[unstable(feature = "mpmc_channel", issue = "126840")]
 impl<T> fmt::Debug for Sender<T> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        f.pad("Sender { .. }")
+        f.debug_struct("Sender").finish_non_exhaustive()
     }
 }
 
@@ -1380,7 +1380,7 @@ impl<T> Clone for Receiver<T> {
 #[unstable(feature = "mpmc_channel", issue = "126840")]
 impl<T> fmt::Debug for Receiver<T> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        f.pad("Receiver { .. }")
+        f.debug_struct("Receiver").finish_non_exhaustive()
     }
 }
 

library/std/src/sync/mpsc.rs

@@ -1114,8 +1114,10 @@ impl<T> error::Error for SendError<T> {}
 impl<T> fmt::Debug for TrySendError<T> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match *self {
-            TrySendError::Full(..) => "Full(..)".fmt(f),
-            TrySendError::Disconnected(..) => "Disconnected(..)".fmt(f),
+            TrySendError::Full(..) => f.debug_tuple("TrySendError::Full").finish_non_exhaustive(),
+            TrySendError::Disconnected(..) => {
+                f.debug_tuple("TrySendError::Disconnected").finish_non_exhaustive()
+            }
         }
     }
 }