How do you create a dynx?

In the previous section, we showed how a #[dyn(identity)] function must return "something that can be converted into a dynx struct", and we showed that a case of returning a Pin<Box<impl Future, A>> type. But what are the general rules for constructing a dynx struct? You're asking the right question, but that's a part of the design we haven't bottomed out yet.

In short, there are two "basic" approaches we could take. One of them is more conservative, in that it doesn't change much about Rust today, but it's also much more complex, because dyn dealing with all the "corner cases" of dyn is kind of complicated. The other is more radical, but may result in an overall smoother, more coherent design.

Apart from that tantalizing tidbit, we are intentionally not providing the details here, because this document is long enough as it is! The next document dives into this question, along with a related question, which is how dynx and sealed traits interact.

This is actually a complex question with (at least) two possible answers.

Alternative A: P must deref to something that implements Bounds

The pointer type P must implement IntoRawPointer (along with various other criteria) and its referent must implement Bounds.

IntoRawPointer trait


#![allow(unused)]
fn main() {
// Not pseudocode, will be added to the stdlib and implemented
// by various types, including `Box` and `Pin<Box>`.

unsafe trait IntoRawPointer: Deref {
    /// Convert this pointer into a raw pointer to `Self::Target`. 
    ///
    /// This raw pointer must be valid to dereference until `drop_raw` (below) is invoked;
    /// this trait is unsafe because the impl must ensure that to be true.
    fn into_raw(self) -> *mut Self::Target;
    
    /// Drops the smart pointer itself as well as the contents of the pointer.
    /// For example, when `Self = Box<T>`, this will free the box as well as the
    /// `T` value.
    unsafe fn drop_raw(this: *mut Self::Target);
}
}

Other conditions pointer must meet

  • Must be IntoRawPointer which ensures:
    • Deref and DerefMut are stable, side-effect free and all that
    • they deref to the same memory as into_raw
  • If Bounds includes a &mut self method, P must be DerefMut
  • If Bounds includes a &self method, P must be Deref
  • If Bounds includes Pin<&mut Self>, P must be Unpin ... and ... something something DerefMut? how do you get from Pin<P> to Pin<&mut P::Target>?
  • If Bounds includes Pin<&Self>, P must be Unpin ... and ... something something DerefMut? how do you get from Pin<P> to Pin<&mut P::Target>?
  • If Bounds includes an auto trait AutoTrait, P must implement AutoTrait
    • and: dynx Bounds implements the auto trait AutoTrait (in general, dynx Bounds implements all of Bounds)
  • Bounds must be "dyn safe"

Alternative B: P must implement Bounds

Alternatively, we could declare that the pointer type P must implement Bounds. This is much simpler to express, but it has some issues. For example, if you have


#![allow(unused)]
fn main() {
trait Foo { 

}
}

then we could not construct a dynx Foo from a Box<dyn Foo> because there is no impl Foo for Box<dyn Foo>. It would be nice if those impls could be added automatically or at least more easily.


#![allow(unused)]
fn main() {
// Not pseudocode, will be added to the stdlib and implemented
// by various types, including `Box` and `Pin<Box>`.

unsafe trait IntoRawPointer: Deref {
    /// Convert this pointer into a raw pointer to `Self::Target`. 
    ///
    /// This raw pointer must be valid to dereference until `drop_raw` (below) is invoked;
    /// this trait is unsafe because the impl must ensure that to be true.
    fn into_raw(self) -> *mut Self::Target;
    
    /// These methods would be used by compiler to convert back so we can invoke the original
    /// impls.
    unsafe fn from_ref(this: &*mut Self::Target) -> &Self;
    unsafe fn from_mut_ref(this: &mut *mut Self::Target) -> &mut Self;
    ...

    /// Drops the smart pointer itself as well as the contents of the pointer.
    /// For example, when `Self = Box<T>`, this will free the box as well as the
    /// `T` value.
    unsafe fn drop_raw(this: *mut Self::Target);
}
}