Spawn local without attribute

[usbalbin]

This is an alternative to #1108 for solving #1107 that does not require the local_task attribute.

Breaking change

Note this is a breaking change.

Inference will no longer accept

task1::spawn(Default::default()).unwrap();

So that would have to be changed to

task1::spawn(MyArgType::default()).unwrap();

// or

ctx.local_spawner.task1(Default::default()).unwrap();

Similarly with &mut T for task argument no longer auto converted into &T.

let x: &mut u8 = ctx.local.x;
task1_taking_shared_ref::spawn(x).unwrap();

instead do

let x: &mut u8 = ctx.local.x;
task1_taking_shared_ref::spawn(&*x).unwrap();

// or 

ctx.local_spawner.task1_taking_shared_ref(x).unwrap();

[usbalbin]

Well this breaks a lot of inference stuff, so probably not a great solution...

[usbalbin]

This is a breaking change. For some reason the compiler can not auto demote &mut T to &T when generics are involved (not sure of the rules here).

Without the above fix, we get

error[E0271]: type mismatch resolving `<&mut DoublyLinkedList<Waker> as Dummy>::T == &DoublyLinkedList<Waker>`
  --> examples/wait-queue.rs:31:28
   |
31 |         incrementer::spawn(cx.local.wait_queue).ok().unwrap();
   |         ------------------ ^^^^^^^^^^^^^^^^^^^ type mismatch resolving `<&mut DoublyLinkedList<Waker> as Dummy>::T == &DoublyLinkedList<Waker>`
   |         |
   |         required by a bound introduced by this call
   |
note: expected this to be `&'static DoublyLinkedList<Waker>`
  --> examples/wait-queue.rs:11:1

[usbalbin]

Copying AfoHT's link on matrix: https://doc.rust-lang.org/reference/type-coercions.html#r-coerce.types.mut-reborrow

[usbalbin]

Any suggestions for what to call this?

[usbalbin]

This is used because we can not have a function with a where clause which requires a concrete type to be Send if that type is not Send. The compiler will error out on us.
However hiding that behind a dummy trait which is only implemented for that same type then we defer the error to when the user tries to call the function

[usbalbin]

I went with TaskArg for now