What I mean is that subroutine can observe its own state being changed even after a call of a non-sync marked function if that function directly or indirectly c

What I mean is that subroutine can observe its own state being changed even after a call of a non-sync marked function if that function directly or indirectly calls into a closure closing over that subroutine state. I.e. IMHO async offers very weak reentrancy guarantees that are better enforced via other means (rust-like lifetimes, immutability annotations, atomic contructs, etc).