[gf000]

This is more of a toy example for how a set of atomic changes can still end up in an inconsistent state, e.g. setting January the 31st and February 3rd in quick succession from two or more different threads may result in Feb 31st being visible from a third thread. This is not solved by Rust and your struct will even get the Sync trait automatically, which may be not be applicable as in this case.

[brabel]

Given your example, I am convinced you've never written any Rust. Of course it does stop you doing shit like that. But in this example, even Java does it properly, since the constructor runs to completion before any Object is accessible to any Thread, not just the one creating it. You need to validate the state of the object in the constructor to prevent that, but TBH why are we talking about this, it's almost completely unrelated to concurrency models.

[gf000]

Of course if you are creating a new object and you have an atomic handle to it, it is trivial to solve. Like, having immutable objects solves a lot of these problems.

But what I'm quite obviously talking about is a Rust struct with 3 atomic fields. Just because I can safely race on any of its fields, doesn't mean that the whole struct can safely be shared, yet it will be inferred to be Sync.

[tialaramex]

Object mutability isn't relevant here. A Date type which is mutable can ensure that all mutations are valid, it just can't do so while retaining this clumsy "LOL I'm just a D-M-Y tuple" API.

We can see immediately that your type is broken because it allows us to directly set the date to February 31st, there's no concurrency bug needed, the type was always defective.

[gf000]

  void setDate(int month, int day) {
    if (notValidDate(month, date)) { throw; }

    this.month = month; // atomic
    this.day = day // atomic
  }

Yet the whole function is not "atomic"/transactional/consistent, and two threads running simultaneously may surface the above error.

Of course it can ensure that it is consistent, C code can also just ensure that it is memory safe. This is just not an inherent property, and in general you will mess it up.

The only difference is that we can reliably solve memory safety issues (GC, Rusty's ownership model), but we have absolutely no way to solve concurrency issues in any model. The only solution is.. having a single thread.

[tialaramex]

But you were critiquing Rust's model, yet you've written C++ here. I agree it's perfectly easy to write the bug in C++.

In Rust this improved type doesn't have the defect, to call Rust's analogue of your setDate function you must have the exclusive mutable reference, which means there's no concurrency problem.

You have to do a whole lot of extra work to write the bug and why would you, just write what you meant and it behaves correctly.

[gf000]

It's called pseudo-code, and some extra attempt on your part to deliberately miss the point.

Give it another go at understanding what I'm saying, cheers!