Ban manual implementation of TryFuture and TryStream

[taiki-e]

Future<Output = Result<T, E>> automatically implements Tryfuture. Also it can call .await and FutureExt's methods without TryFuture::into_future.

Also, we may want to rewrite this trait to inherit Future in the future, so I think it makes sense to ban manual implementation for forward compatibility.
https://github.com/rust-lang-nursery/futures-rs/blob/78c5d852ec24edcf630a4c569c1234d078416f4f/futures-core/src/future/mod.rs#L61-L63

I'm thinking of using the approach that SliceIndex uses, but there may be better ways.

[taiki-e]

cc @cramertj @Nemo157

[Nemo157]

I think adding Future as a super trait combined with the existing impl TryFuture for impl Future would also accomplish this. Is there a reason to not have that supertrait bound now?

[taiki-e]

Is there a reason to not have that supertrait bound now?

Nope, there is no particular reason. It's probably a better option.
(I thought that when TryFuture was defined, someone might did this intentionally.)

[cramertj]

Yeah, @aturon and I hit a bug w/ cycles in the trait checker when attempting to do that:

trait Future { 
    type Output;
}

trait TryFuture: Future<Output = Result<Self::Ok, Self::Err>> {
    type Ok;
    type Err;
}

gives

error[E0391]: cycle detected when computing the supertraits of `TryFuture`
 --> src/main.rs:5:1
  |
5 | trait TryFuture: Future<Output = Result<Self::Ok, Self::Err>> {
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: ...which again requires computing the supertraits of `TryFuture`, completing the cycle
note: cycle used when collecting item types in top-level module
 --> src/main.rs:5:1
  |
5 | trait TryFuture: Future<Output = Result<Self::Ok, Self::Err>> {
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to previous error

IIRC @nikomatsakis said this was fixed in chalk, but there's no ETA for the fix as far as I know.

[taiki-e]

Seems it can be avoided by using <Self as TryFuture>::Ok. EDIT: see #1776 (comment)

trait Future { 
    type Output;
}

trait TryFuture: Future<Output = Result<<Self as TryFuture>::Ok, <Self as TryFuture>::Err>> {
    type Ok;
    type Err;
}

https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=a9d4353ff24b3f2fa6a9c9281889dd86

[taiki-e]

Nope, this still doesn't work.

error[E0275]: overflow evaluating the requirement `<Self as std::future::Future>::Output`
  --> src/lib.rs:7:1
   |
7  | / pub trait TryFuture:
8  | |     Future<Output = Result<<Self as TryFuture>::Ok, <Self as TryFuture>::Error>>
9  | | {
10 | |     type Ok;
...  |
14 | |     fn try_poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<Self::Ok, Self::Error>>;
15 | | }
   | |_^
   |
   = note: required because of the requirements on the impl of `TryFuture` for `Self`
   = note: required because of the requirements on the impl of `TryFuture` for `Self`

https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=fad9b6f6899e6748118abec9fb199d26

[taiki-e]

So I believe #1777 is a good way at this time.