footnote on dropping futures

Author: Manishearth

library/core/src/pin.rs

@@ -126,7 +126,7 @@
 //! for such a type to be implemented soundly, the pointer which points into `self`'s data must be
 //! proven valid whenever it is accessed. But if that value is *moved*, the pointer will still
 //! point to the old address where the value was located and not into the new location of `self`,
-//! thus becoming invalid. A key example of such self-referrential types are the state machines
+//! thus becoming invalid. A key example of such self-referential types are the state machines
 //! generated by the compiler to implement [`Future`] for `async fn`s.
 //!
 //! Such types that have an *address-sensitive* state usually follow a lifecycle
@@ -141,7 +141,7 @@
 //!     * e.g. subsequent calls to [`poll`]
 //! 4. Before the value is invalidated (e.g. deallocated), it is *dropped*, giving it a chance to
 //! notify anything with pointers to itself that those pointers will be invalidated
-//!     * e.g. [`drop`]ping the [`Future`]
+//!     * e.g. [`drop`]ping the [`Future`] [^pin-drop-future]
 //!
 //! There are two possible ways to ensure the invariants required for 2. and 3. above (which
 //! apply to any address-sensitive type, not just self-referrential types) do not get broken.
@@ -196,6 +196,9 @@
 //! discussed with the second option by building a *shared contractual language* around the
 //! guarantees of "pinning" data.
 //!
+//! [^pin-drop-future]: Futures themselves do not ever need to notify other bits of code that
+//! they are being dropped, however data structures like stack-based intrusive linked lists do.
+//!
 //! ## Using [`Pin<Ptr>`] to pin values
 //!
 //! In order to pin a value, we wrap a *pointer to that value* (of some type `Ptr`) in a