provide project method on &mut Pin<&mut Self>

[ijackson]

AFAICT it is not possible, given a function which takes Pin<&mut Sefl>, to make two function calls in turn which take Pin<&mut Self>. This is because the borrow checker's special logic for permitting this for &mut Self does not apply to Pin<&mut Self>.

I think it would be possible to solve this problem if pin_project provided something like this:

    fn project_mut(self: &mut Pin<&mut Self>) -> ....

It seems that this was previously done, but this was changed in #89. Obviously I'm not suggesting it ought to be changed back. But I don't see any reason why it wouldn't be sensible to provide both. I think this should be sound because the outer &mut guarantees unique ownership of the inner &mut.

Would you welcome a MR to provide this other method, perhaps optionally?

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=a538875119bf28650f67540fe94a440f

[taiki-e]

As discussed in #89, you can use .as_mut() to reborrow Pin<&mut Self>.

-      self.project().field; // ERROR *self` moved due to this method call
+      self.as_mut().project().field;

- pin_ref.takes_pin();
- pin_ref.takes_pin(); // ERROR value used here after move
+ pin_ref.as_mut().takes_pin();
+ pin_ref.takes_pin();

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=826520ca2b49581ce0a8d9a89a4c734e

[ijackson]

Oh. Thank you for educating me and sorry for wasting your time!