Tracking issue for Box::into_pin (feature `box_into_pin`)

[Nemo157]

This is a tracking issue for Box::into_pin implemented in #57313

impl<T: ?Sized> Box<T> {
    pub fn into_pin(boxed: Box<T>) -> Pin<Box<T>>;
}

[Nemo157]

I don't think this is really a useful feature to specialize in this way. Can't you just use the hyper fish operator to disambiguate?
— @czipperz (#57313 (comment))

There are currently 2 unambiguous stable ways to convert a Box<T> into a Pin<Box<T>> without additional inference constraints

let foo: Box<T> = ...;
let bar = <Pin<Box<_>>>::from(foo);
let bar: Pin<Box<_>> = foo.into();

Also, at the moment let bar = Pin::from(foo); works because there is only one trait implementation that can satisfy that call, but because Pin and Box are #[fundamental] it's possible for an external library to add an additional From implementation that makes that ambiguous (playground).

In a lot of cases where you need to convert into a Pin<Box<_>> you're likely to have inference available to guide the .into() call as well, but not always.

[czipperz]

Well there's also Box::pin(foo) right?

[czipperz]

Mmh but that takes a T and wraps it into Pin<Box<T>>. I think with that context Box::into_pin makes a lot of sense.

[joshtriplett]

Shall we stabilize Box::into_pin?

@rfcbot merge

[rfcbot]

Team member @joshtriplett has proposed to merge this. The next step is review by the rest of the tagged team members:

• @Amanieu
• @BurntSushi
• @dtolnay
• @joshtriplett
• @m-ou-se
• @yaahc

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

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[BurntSushi]

Also, at the moment let bar = Pin::from(foo); works because there is only one trait implementation that can satisfy that call, but because Pin and Box are #[fundamental] it's possible for an external library to add an additional From implementation that makes that ambiguous (playground).

It seems like this is something that should be mentioned in the docs of into_pin, specifically as something that differentiates it from Pin::from.

[rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔