How does task_local! work?

[jjant]

If I understand correctly, task locals are like thread locals, but they handle:

• The same thread possibly interleaving many tasks (a ThreadLocal would be overwritten)
• The same task switching threads (the ThreadLocal being unset for the task)

Can anyone explain the technical details of Tokio's implementation of task_local!?

[Darksonn]

Most of the time, the value is stored inside the task local scope future. However, when you poll the scope future, it will temporarily move the value to a thread-local. When the poll on the scope future ends, the value is moved out of the thread-local.

This means that whenever you are inside a call to poll on the task-local scope future, the value will be in the right thread-local. This allows you to access it. You can't access the value outside of the scope future, but that's okay.