Document guarantee: `sync_channel` `send()` ⇒ immediate `try_recv()` visibility.

Location (URL)

Maybe https://doc.rust-lang.org/std/sync/mpsc/fn.sync_channel.html

Summary

Request: Make explicit in std::sync::mpsc::sync_channel docs that after Sender::send returns Ok(()), the message is enqueued such that an immediate Receiver::try_recv() on the same channel cannot return Empty.

Rationale: This is relied on for tests (where code calls send() and the test immediately calls try_recv() on the same channel to assert a message was produced). Implementation appears to already behave this way, but the guarantee isn’t stated. It's conceivable that in a different environment that an alternate implementation might be used that introduces the potential for messaging latency.

Ask: Treat this as a normative docs change.

(Reference: https://stackoverflow.com/questions/79753705/rust-mpscsync-channel-after-send-returns-can-try-recv-still-return-empty )

Start with the linked std::sync::mpsc::sync_channel documentation and inspect the surrounding Sender::send and Receiver::try_recv documentation. Update the normative text to state the requested post-send visibility guarantee, then check the surrounding standard-library documentation for consistency.