use a default deny lint for preventing cycles

[thestinger]

A type like MutexArc could be annotated with an attribute enabling a cycle detection lint:

#[cycle_check]
struct MutexArc<T> { ... }

The lint would do a walk of the type and check for a reference to itself. This isn't good enough for an immutable type like Rc because it only needs to warn if the type can contain itself and T is non-Freeze, but that's easy enough to solve. There's also the issue of how Rc cycles may interact with the current managed pointer annihilator and a future garbage collector.

[thestinger]

This doesn't really work out very well thanks to trait objects and closures.