Best summarized as a flood of unsafe blocks.

The src/lib.rs already looks somewhat suspicious. We have a thread local
containing a pointer that is unsafely accessed. It is set by transmuting a
local reference to a dynamic trait object which has its lifetime erased. This
particular portion might be fine if no code that uses the trait depends on
that lifetime. However, all of this is worryingly done without any statement
of purpose, any reference to the overall structure, and little to no
comments.

Then we look into src/scheduler.rs and find our worst nightmares come to
life. Here's a short list of unexplained stuff:

• uncommented impls of Sync and Send

• an adhoc implementation of an intrusive queue/linked-list, giving a blog
  post about C (!) as the source. Maybe one of the better sources, I do not
  know, but it appears somewhat unmotivated and is intertwined with the rest of
  the code.

• several different UnsafeCell that have been encapsulated with telepathy,
  apparently, since there are no comments explaining them nor their access.

• unsafe blocks spanning ~70 lines

• and the amazing unsafe fn ptr2arc(ptr: *const T) -> Arc containing

  let anchor = mem::transmute::<usize, Arc<T>>(0x10);
  

  without further explanation. Note that T is a type parameter that is
  exposed to the user through the type parameter P of CurrrentThread.
  This is restricted to tokio-executor::park::Park and provides an
  associated type which is later used as the T in question. Note that a
  public constructor exists.

  This is slightly unsound at best, if the node must be aligned to more than
  that function guarantees, and horrible with regards to how the pointer is
  used as a manual memoffset calculation afterwards.

• The part where Node is used to ignore the lifetime of a contained object
  and only its Drop implementation double panicking protects against using and
  dropping the object out of its own lifetime is the least worrying here. Also,
  since the comment refers to some type parameter T that doesn't exist in the
  impl.