FromStream impl for Option<T> + Revised impl for Vec<T> #265
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266: Changes Extend trait in order to allow streams that yield references r=yoshuawuyts a=sunjay This is not ready to merge yet. I am mainly opening it so we can discuss a change I had to make to the `Extend` trait. cc @yoshuawuyts @stjepang (and anyone else interested) ## Before this can be merged - [x] Discuss/Approve changes to `Extend` trait - [x] Change to using `for_each` after #264 is merged - [ ] (optional) Wait until a `copied()` method is added to `StreamExt` so that the `&char` impl can be finished. - We can also just comment out or remove the impl that uses `copied` until that is added ## Changes To The Extend Trait While writing the impls of the `Extend` trait for the `String` type, I noticed that certain impls weren't possible because there is no bound on `Extend` that guarantees that the type `A` being yielded from the stream actually lives long enough. We probably didn't run into this earlier because this usually isn't a problem for owned values since the compiler doesn't have to worry about whether they will out live the stream that they come from. I ran into this because of the `Extend` impls that consume streams of references. The difference between the async `Extend` and the standard library `Extend` is that the async `Extend` returns a value that still references the input stream. That means that if `A` is any reference type, the compiler needs to be able to guarantee that `A` will be around as long as the `Future` returned from the trait method is around. To fix this, I had to add the bound shown below: ```patch pub trait Extend<A> { /// Extends a collection with the contents of a stream. fn stream_extend<'a, T: IntoStream<Item = A> + 'a>( &'a mut self, stream: T, - ) -> Pin<Box<dyn Future<Output = ()> + 'a>>; + ) -> Pin<Box<dyn Future<Output = ()> + 'a>> where A: 'a; } ``` This guarantees that each value of type `A` will last at least as long as our boxed future does. The bound had to be in a where clause on the method (and not on the declaration of `A` because the lifetime `'a` isn't in scope at the trait level. I don't think there are any negative consequences of using a where clause like this, but that's why I wanted to bring it up for discussion. In addition to this, I had to ensure that when writing the `Extend` impls for `String` I appropriately bounded the lifetime of the references from the stream. You can see this in the code below with `where 'b: 'a`. ```rust impl<'b> Extend<&'b str> for String { fn stream_extend<'a, S: IntoStream<Item = &'b str> + 'a>( &'a mut self, stream: S, ) -> Pin<Box<dyn Future<Output = ()> + 'a>> where 'b: 'a { //TODO: This can just be: stream.into_stream().for_each(move |s| self.push_str(s)) Box::pin(stream.into_stream().fold((), move |(), s| self.push_str(s))) } } ``` I should note that initially I tried to make it work with just the impl shown above, without modifying the `Extend` trait. This doesn't work because it would be a stricter bound than what is found in the trait itself. Rust does not allow stricter bounds like that because it could potentially cause unsoundness when dealing with generics. Of course, I am totally open to being completely wrong in my understanding of how to resolve this issue. I tried to solve the problem with as minimal of a change as possible. Please let me know if you have some better ideas or other suggestions. ## `FromStream` impls for String The purpose of adding these `Extend` impls is to continue my work from #129 in adding the rest of the `FromStream` impls. The `Extend` impls are used directly to add all of the `FromStream` impls for `String`. Just like with #207 and #265, this adds a new `string` module that is unstable just like the other modules added for `FromStream`. Co-authored-by: Sunjay Varma <[email protected]>
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cc #129
The
FromStreamimpl forOption<T>is almost identical to the one forResult<T>(and thus also almost identical to the one in the standard library). Just like with #207, this adds a new module for option and leaves it unstable with the otherFromStreamimpls.I've also re-implemented
FromStreamforVec<T>in terms of the asyncExtendtrait. This better matches the standard library and also gives us some code re-use as the previous impl was essentially the same as the implementation ofExtend.I'm waiting on a few more methods to be added to stream before I can add the rest of the
FromStreamimpls. @montekki has started to work on that in #264. Thanks!