rebase and rewrite and fix issues raised by @hawkw

fix clippy warnings

fix clippy warnings and address the issues raised by @Darksonn

fix more clippy warnings

Update tokio/src/runtime/task/core.rs

Co-authored-by: Eliza Weisman <eliza@buoyant.io>

Update tokio/src/runtime/task/mod.rs

Co-authored-by: Eliza Weisman <eliza@buoyant.io>

fix nits raised by @hawkw

tokio/src/runtime/context.rs

@@ -1,4 +1,5 @@
 use crate::runtime::coop;
+use crate::runtime::task::Id;
 
 use std::cell::Cell;
 
@@ -17,6 +18,7 @@ struct Context {
     /// Handle to the runtime scheduler running on the current thread.
     #[cfg(feature = "rt")]
     handle: RefCell<Option<scheduler::Handle>>,
+    current_task_id: Cell<Option<Id>>,
 
     /// Tracks if the current thread is currently driving a runtime.
     /// Note, that if this is set to "entered", the current scheduler
@@ -41,6 +43,7 @@ tokio_thread_local! {
             /// accessing drivers, etc...
             #[cfg(feature = "rt")]
             handle: RefCell::new(None),
+            current_task_id: Cell::new(None),
 
             /// Tracks if the current thread is currently driving a runtime.
             /// Note, that if this is set to "entered", the current scheduler
@@ -107,6 +110,14 @@ cfg_rt! {
 
     pub(crate) struct DisallowBlockInPlaceGuard(bool);
 
+    pub(crate) fn set_current_task_id(id: Option<Id>) -> Option<Id> {
+        CONTEXT.try_with(|ctx| ctx.current_task_id.replace(id)).unwrap_or(None)
+    }
+
+    pub(crate) fn current_task_id() -> Option<Id> {
+        CONTEXT.try_with(|ctx| ctx.current_task_id.get()).unwrap_or(None)
+    }
+
     pub(crate) fn try_current() -> Result<scheduler::Handle, TryCurrentError> {
         match CONTEXT.try_with(|ctx| ctx.handle.borrow().clone()) {
             Ok(Some(handle)) => Ok(handle),

tokio/src/runtime/task/abort.rs

@@ -67,7 +67,7 @@ impl AbortHandle {
     #[cfg(tokio_unstable)]
     #[cfg_attr(docsrs, doc(cfg(tokio_unstable)))]
     pub fn id(&self) -> super::Id {
-        self.id.clone()
+        self.id
     }
 }
 

tokio/src/runtime/task/core.rs

@@ -11,6 +11,7 @@
 
 use crate::future::Future;
 use crate::loom::cell::UnsafeCell;
+use crate::runtime::context;
 use crate::runtime::task::raw::{self, Vtable};
 use crate::runtime::task::state::State;
 use crate::runtime::task::{Id, Schedule};
@@ -157,6 +158,26 @@ impl<T: Future> CoreStage<T> {
     }
 }
 
+/// Set and clear the task id in the context when the future is executed or
+/// dropped, or when the output produced by the future is dropped.
+pub(crate) struct TaskIdGuard {
+    parent_task_id: Option<Id>,
+}
+
+impl TaskIdGuard {
+    fn enter(id: Id) -> Self {
+        TaskIdGuard {
+            parent_task_id: context::set_current_task_id(Some(id)),
+        }
+    }
+}
+
+impl Drop for TaskIdGuard {
+    fn drop(&mut self) {
+        context::set_current_task_id(self.parent_task_id);
+    }
+}
+
 impl<T: Future, S: Schedule> Core<T, S> {
     /// Polls the future.
     ///
@@ -183,6 +204,7 @@ impl<T: Future, S: Schedule> Core<T, S> {
                 // Safety: The caller ensures the future is pinned.
                 let future = unsafe { Pin::new_unchecked(future) };
 
+                let _guard = TaskIdGuard::enter(self.task_id);
                 future.poll(&mut cx)
             })
         };
@@ -236,6 +258,7 @@ impl<T: Future, S: Schedule> Core<T, S> {
     }
 
     unsafe fn set_stage(&self, stage: Stage<T>) {
+        let _guard = TaskIdGuard::enter(self.task_id);
         self.stage.stage.with_mut(|ptr| *ptr = stage)
     }
 }

tokio/src/runtime/task/error.rs

@@ -128,7 +128,7 @@ impl JoinError {
     #[cfg(tokio_unstable)]
     #[cfg_attr(docsrs, doc(cfg(tokio_unstable)))]
     pub fn id(&self) -> Id {
-        self.id.clone()
+        self.id
     }
 }
 

tokio/src/runtime/task/harness.rs

@@ -454,13 +454,12 @@ fn cancel_task<T: Future, S: Schedule>(core: &Core<T, S>) {
         core.drop_future_or_output();
     }));
 
-    let id = core.task_id.clone();
     match res {
         Ok(()) => {
-            core.store_output(Err(JoinError::cancelled(id)));
+            core.store_output(Err(JoinError::cancelled(core.task_id)));
         }
         Err(panic) => {
-            core.store_output(Err(JoinError::panic(id, panic)));
+            core.store_output(Err(JoinError::panic(core.task_id, panic)));
         }
     }
 }
@@ -492,7 +491,7 @@ fn poll_future<T: Future, S: Schedule>(core: &Core<T, S>, cx: Context<'_>) -> Po
         Ok(Poll::Ready(output)) => Ok(output),
         Err(panic) => {
             core.scheduler.unhandled_panic();
-            Err(JoinError::panic(core.task_id.clone(), panic))
+            Err(JoinError::panic(core.task_id, panic))
         }
     };
 

tokio/src/runtime/task/join.rs

@@ -267,7 +267,7 @@ impl<T> JoinHandle<T> {
             raw.ref_inc();
             raw
         });
-        super::AbortHandle::new(raw, self.id.clone())
+        super::AbortHandle::new(raw, self.id)
     }
 
     /// Returns a [task ID] that uniquely identifies this task relative to other
@@ -282,7 +282,7 @@ impl<T> JoinHandle<T> {
     #[cfg(tokio_unstable)]
     #[cfg_attr(docsrs, doc(cfg(tokio_unstable)))]
     pub fn id(&self) -> super::Id {
-        self.id.clone()
+        self.id
     }
 }
 

tokio/src/runtime/task/mod.rs

@@ -139,6 +139,8 @@
 // unstable. This should be removed once `JoinSet` is stabilized.
 #![cfg_attr(not(tokio_unstable), allow(dead_code))]
 
+use crate::runtime::context;
+
 mod core;
 use self::core::Cell;
 use self::core::Header;
@@ -190,9 +192,13 @@ use std::{fmt, mem};
 /// # 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.
+/// 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()`](create::task::JoinHandle::id()`) function.
 ///
 /// **Note**: This is an [unstable API][unstable]. The public API of this type
 /// may break in 1.x releases. See [the documentation on unstable
@@ -201,10 +207,46 @@ use std::{fmt, mem};
 /// [unstable]: crate#unstable-features
 #[cfg_attr(docsrs, doc(cfg(all(feature = "rt", tokio_unstable))))]
 #[cfg_attr(not(tokio_unstable), allow(unreachable_pub))]
-// TODO(eliza): there's almost certainly no reason not to make this `Copy` as well...
-#[derive(Clone, Debug, Hash, Eq, PartialEq)]
+#[derive(Clone, Copy, Debug, Hash, Eq, PartialEq)]
 pub struct Id(u64);
 
+/// Returns the [`Id`] of the currently running task.
+///
+/// # Panics
+///
+/// This function panics if called from outside a task. Please note that calls
+/// to `block_on` do not have task IDs, so the method will panic if called from
+/// within a call to `block_on`. For a version of this function that doesn't
+/// panic, see [`task::try_id()`](crate::runtime::task::try_id()).
+/// **Note**: This is an [unstable API][unstable]. The public API of this type
+/// may break in 1.x releases. See [the documentation on unstable
+/// features][unstable] for details.
+///
+/// [task ID]: crate::task::Id
+#[cfg_attr(not(tokio_unstable), allow(unreachable_pub))]
+#[track_caller]
+pub fn id() -> Id {
+    context::current_task_id().expect("Can't get a task id when not inside a task")
+}
+
+/// Returns the [`Id`] of the currently running task, or `None` if called outside
+/// of a task.
+///
+/// This function is similar to  [`task::id()`](crate::runtime::task::id()), except
+/// that it returns `None` rather than panicking if called outside of a task
+/// context.
+///
+/// **Note**: This is an [unstable API][unstable]. The public API of this type
+/// may break in 1.x releases. See [the documentation on unstable
+/// features][unstable] for details.
+///
+/// [task ID]: crate::task::Id
+#[cfg_attr(not(tokio_unstable), allow(unreachable_pub))]
+#[track_caller]
+pub fn try_id() -> Option<Id> {
+    context::current_task_id()
+}
+
 /// An owned handle to the task, tracked by ref count.
 #[repr(transparent)]
 pub(crate) struct Task<S: 'static> {
@@ -284,7 +326,7 @@ cfg_rt! {
         T: Future + 'static,
         T::Output: 'static,
     {
-        let raw = RawTask::new::<T, S>(task, scheduler, id.clone());
+        let raw = RawTask::new::<T, S>(task, scheduler, id);
         let task = Task {
             raw,
             _p: PhantomData,

tokio/src/task/mod.rs

@@ -318,7 +318,7 @@ cfg_rt! {
     pub mod join_set;
 
     cfg_unstable! {
-        pub use crate::runtime::task::Id;
+        pub use crate::runtime::task::{Id, id, try_id};
     }
 
     cfg_trace! {