task: clarify the task ID reuse guarantees (#7577)

tokio/src/runtime/task/id.rs

@@ -5,16 +5,28 @@ use std::{fmt, num::NonZeroU64};
 /// An opaque ID that uniquely identifies a task relative to all other currently
 /// running tasks.
 ///
+/// A task's ID may be re-used for another task only once *both* of the
+/// following happen:
+/// 1. The task itself exits.
+/// 2. There is no active [`JoinHandle`] associated with this task.
+///
+/// A [`JoinHandle`] is considered active in the following situations:
+/// - You are explicitly holding a [`JoinHandle`], [`AbortHandle`], or
+///   `tokio_util::task::AbortOnDropHandle`.
+/// - The task is being tracked by a [`JoinSet`] or `tokio_util::task::JoinMap`.
+///
 /// # Notes
 ///
-/// - Task IDs are unique relative to other *currently running* tasks. When a
-///   task completes, the same ID may be used for another task.
 /// - Task IDs are *not* sequential, and do not indicate the order in which
 ///   tasks are spawned, what runtime a task is spawned on, or any other data.
 /// - The task ID of the currently running task can be obtained from inside the
 ///   task via the [`task::try_id()`](crate::task::try_id()) and
 ///   [`task::id()`](crate::task::id()) functions and from outside the task via
 ///   the [`JoinHandle::id()`](crate::task::JoinHandle::id()) function.
+///
+/// [`JoinHandle`]: crate::task::JoinHandle
+/// [`AbortHandle`]: crate::task::AbortHandle
+/// [`JoinSet`]: crate::task::JoinSet
 #[cfg_attr(docsrs, doc(cfg(all(feature = "rt"))))]
 #[derive(Clone, Copy, Debug, Hash, Eq, PartialEq, PartialOrd, Ord)]
 pub struct Id(pub(crate) NonZeroU64);

tokio/src/task/join_set.rs

@@ -50,6 +50,16 @@ use crate::util::IdleNotifiedSet;
 ///     }
 /// }
 /// ```
+///
+/// # Task ID guarantees
+///
+/// While a task is tracked in a `JoinSet`, that task's ID is unique relative
+/// to all other running tasks in Tokio. For this purpose, tracking a task in a
+/// `JoinSet` is equivalent to holding a [`JoinHandle`] to it. See the [task ID]
+/// documentation for more info.
+///
+/// [`JoinHandle`]: crate::task::JoinHandle
+/// [task ID]: crate::task::Id
 #[cfg_attr(docsrs, doc(cfg(feature = "rt")))]
 pub struct JoinSet<T> {
     inner: IdleNotifiedSet<JoinHandle<T>>,