Comment by 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.

8 comments

dwattttt

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>);
      | ^^^^^^^^^^^

dwattttt 11 days ago
Even if the bounds are the same on the struct and the impl, it's still true that placing bounds on the Drop impl means it's not implemented for all T (just all T that also satisfy Q).
You also don't have to place those bounds on the Drop impl just because they're on Q, that's not an error.
- Xirdus 11 days ago
  
  The Drop isn't implemented for T at all. It's only implemented for Q<T>. And it's implemented for all Q<T>, in the sense that for every value of type Q<T>, this Drop impl applies.
  And I'm still not any closer to understanding what the bug is about.
  
  5 replies →