Comment by jason_oster

11 days ago

Therefore explaining why the bug hadn't been hit in practice for 11 years.

11 comments

jason_oster

On further reading, the Drop impl is a red herring. The bug is unrelated to Drop. It's there in the repro to show the effects of invalid assignment, but even without impl Drop the bug is still there.

Xirdus 11 days ago

The bug in the OP is about UNconditional impl. And honestly, looks like very standard code, definitely something that could've been written for real. I can't even explain why it passes checks, it clearly shouldn't.

dwattttt 11 days ago

This code, from the bug submission?

  impl<T: 'static + Send> Drop for Q<T> {
      fn drop(&mut self) { if let Some(t) = self.0.take() { KEPT.lock().unwrap().push(Box::new(t)); } }
  }

The bound on T in the Drop impl means Drop is not implemented for all T, only where T: 'static + Send. Q<T> also having the same bound doesn't make a difference to the Drop impl.

NobodyNada 11 days ago

You cannot write a type which conditionally implements Drop: https://play.rust-lang.org/?version=stable&mode=debug&editio...

    struct Q<T>(Option<T>);

    impl<T: 'static> Drop for Q<T> {
        fn drop(&mut self) {}
    }


    error[E0367]: `Drop` impl requires `T: 'static` but the struct it is implemented for does not
     --> src/main.rs:3:9
      |
    3 | impl<T: 'static> Drop for Q<T> {
      |         ^^^^^^^
      |
    note: the implementor must specify the same requirement
     --> src/main.rs:1:1
      |
    1 | struct Q<T>(Option<T>);
      | ^^^^^^^^^^^

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