[ChihweiLHBird]

A UsmBox could be freed while a copy or fill that uses it was still running, which breaks the precondition of sycl::free that no enqueued or in-progress command uses the memory. Safe code could do this by dropping an allocation before its zero fill finished, or by dropping the source or destination of a copy before the copy finished.

Have each UsmBox record the events of the copies and fills that use it, and wait for them before freeing the memory. If waiting fails, the memory is leaked instead, since a command may still be using it.

Fixes #63

[Copilot]

🟢 Approval recommended

No active review issues are reported for the changes to track pending operations before USM memory deallocation.

No open issues 🎉

What changed in this PR

Addresses #63 by making UsmBox wait for tracked copies and fills before freeing its memory, preventing deallocation while those operations still use it.

Changes:

• Track copy and fill events on each affected allocation.
• Wait for pending events during drop; leak the allocation if waiting fails.
• Update safety documentation and the changelog to describe the blocking drop behavior.

File	Description
sycl/​sycl-rs/​src/​usmbox.rs	Tracks pending events and waits before deallocation.
sycl/​sycl-rs/​src/​queue.rs	Registers fill events and copy events with affected allocations.
sycl/​sycl-rs/​src/​lib.rs	Updates the documented safety model and removes the zero-cost claim.
CHANGELOG.md	Records the new wait-before-free behavior.

_{Review effort: Balanced}