Tracking Issue for `pin_deref_mut`

[jonhoo]

Feature gate: #![feature(pin_deref_mut)]

This is a tracking issue for Pin::as_deref_mut.

The feature adds the method as_deref_mut to Pin, which allow a safe transformation from a Pin<&mut Pin<P>> to a Pin<&mut P::Target>.

Public API

impl<Ptr> Pin<Ptr> {
    pub fn as_deref_mut(self: Pin<&mut Pin<Ptr>>) -> Pin<&mut Ptr::Target>
    where
        Ptr: DerefMut;
}

Steps / History

• Remove P: Unpin bound on impl Future for Pin #81363
• Put Pin::as_deref_mut in impl Pin<Ptr> / rearrange Pin methods #129449
• Stabilize Pin::as_deref_mut() #129424

Unresolved Questions

• None yet.

[RalfJung]

from a Pin<P>

It's from a Pin<&'a mut Pin<P>>, right? The nesting is a bit odd. But I guess it is what comes up in practice?

[jonhoo]

Ah, yes, sorry — fixed!

[coolreader18]

Are there any remaining blockers on this? It's a pretty useful function for blanket impls for traits with poll methods (e.g. Future already uses it in its impl<P> Future for Pin<P> impl).

[jonhoo]

Not that I know of — given it touches the Pin machinery, I suspect it should go through an actual FCP, but I'll let others be the judge of that :)

[dtolnay]

Currently this method is located in its own solitary impl block:

impl<'a, Ptr: DerefMut> Pin<&'a mut Pin<Ptr>> {
    pub fn as_deref_mut(self) -> Pin<&'a mut Ptr::Target>;
}

I'd like to consider changing it to:

impl<Ptr> Pin<Ptr> {
    pub fn as_deref_mut(self: Pin<&mut Pin<Ptr>>) -> Pin<&mut Ptr::Target>
    where
        Ptr: DerefMut;
}

Some advantages:

• Synergy with the existing as_ref and as_mut signatures (stable since Rust 1.33)
• Lifetime elision reduces noise in the signature
• Turbofish less verbose: Pin::<&mut T>::as_deref_mut vs Pin::<&mut Pin<&mut T>>::as_deref_mut

impl<Ptr> Pin<Ptr> {
    pub fn as_ref(&self) -> Pin<&Ptr::Target>
    where
        Ptr: Deref;

    pub fn as_mut(&mut self) -> Pin<&mut Ptr::Target>
    where
        Ptr: DerefMut;
}

[thynson]

Any concern blocks this from stablization?

[jonhoo]

@dtolnay I agree — that is better 👍