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Add T: ?Sized to the From<Box<T>> impl for Pin<Box<T>> #56256

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DutchGhost opened this issue Nov 26, 2018 · 1 comment · Fixed by #57313
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Add T: ?Sized to the From<Box<T>> impl for Pin<Box<T>> #56256

DutchGhost opened this issue Nov 26, 2018 · 1 comment · Fixed by #57313

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@DutchGhost
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DutchGhost commented Nov 26, 2018 •
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Currently, the implementation for From<Box<T>> for Pin<Box<T>> is defined as:

impl<T> From<Box<T>> for Pin<Box<T>>

https://doc.rust-lang.org/nightly/std/pin/struct.Pin.html#impl-From%3CBox%3CT%3E%3E

This does not allow trait objects to be pinned. It is however possible to call Box::pinned, and convert that into a trait object.

Is it intentional that T is required to be Sized on the From implementation?

Defining it as this:

impl<T: ?Sized> From<Box<T>> for Pin<Box<T>>

allows to call Pin::from(trait_object)
@DutchGhost DutchGhost mentioned this issue Nov 28, 2018
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@cramertj
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cramertj commented Nov 28, 2018 •
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Is it intentional that T is required to be Sized on the From implementation?

nope, and I'd happily r+ a PR fixing this, Arc, and Rc as well

@DutchGhost DutchGhost mentioned this issue Jan 1, 2019
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@Nemo157 Nemo157 mentioned this issue Jan 3, 2019
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kennytm added a commit to kennytm/rust that referenced this issue Jan 5, 2019
@kennytm
Rollup merge of rust-lang#57313 - Nemo157:box-to-pin, r=cramertj
676b0b0 
Improve Box<T> -> Pin<Box<T>> conversion

I found the `From` trait conversion for this very hard to find, having a named function for it is much more discoverable. Also fixes rust-lang#56256 as I need that in the place I'm using this.

Has a placeholder tracking issue, will file an issue once I get feedback.
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Improve Box<T> -> Pin<Box<T>> conversion Nemo157/rust
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@cramertj @DutchGhost