Make SYST.has_wrapped() take &self

[fralalonde]

While I understand the intention in making SYST.has_wrapped() take &mut self because of the side effect, I believe the trouble this causes are bigger than any pain it aims to save.

For example, implementing a shared clock, because of &mut requirement one now has to use a mutex before accessing has_wrapped(). The locking operation infers a non-negligible and unwelcome CPU load especially in a time-sensitive and oft-called method.

The register operation csr.read() backing has_wrapped() being inherently atomic in it's operation guarantees that the side effect will always be observed by a single thread. Also, this side-effect is mostly non-deterministic, i.e. one does not "expect" a particular has_wrapped() return value in any situation. It's something that has to be unconditionally checked by every thread and will thus not lead to a race condition.

In a sense, this should be similar to reading an event from a lock-free queue. &mut is more about enforcing exclusivity than mutability. In the case of has_wrapped the exclusivity is unwarranted.

Thus I contend that has_wrapped() should be changed to take &self. It could also be made unsafe to make one think about how it works and retain the original intention behind the current &mut.