Fix pinned boxed async functions

[raviqqe]

This change fixes pinned boxed async functions which were unable to compile.

[raviqqe]

I made the Attribute enum to pass information of if futures should be Send or not and reduce arguments of the async_inner() function. However, I don't know whether it's a correct approach.

[ngg]

It's working for me pretty well, can you please merge this?

[withoutboats]

Looks good, just want to clarify why the additional re-exports.

[withoutboats]

Aren't these already exported through the stable submodule?

[raviqqe]

It's better to put them into futures::prelude as normal Future and Stream so that users don't need to use them every time in modules which use #[async] stuff otherwise they'll see some error messages about missing methods of pin() or pin_local().

[ngg]

This code doesn't work for me (it works if one of the parameters is i32 instead of &i32)

#[async(pinned)]
fn bar2(x: &i32, y: &i32) -> Result<i32, i32> {
    Ok(x+y)
}

It fails with the following error:

error[E0226]: only a single explicit lifetime bound is permitted
  --> futures/tests/async_await/pinned.rs:16:45
   |
16 | fn bar2(x: &i32, y: &i32) -> Result<i32, i32> {
   |                                             ^

error[E0623]: lifetime mismatch
  --> futures/tests/async_await/pinned.rs:16:30
   |
16 | fn bar2(x: &i32, y: &i32) -> Result<i32, i32> {
   |                     ----     ^^^^^^^^^^^^^^^^
   |                     |        |
   |                     |        ...but data from `y` is returned here
   |                     this parameter and the return type are declared with different lifetimes...

EDIT:
I just realized it's not working with #[async], it might be independent from this.
When using #[async] instead of #[async(pinned)] the first error disappears but the second stays.

EDIT 2:
If I explicitly specify the same lifetime for both arguments then both the pinned and normal version seem to work.

[withoutboats]

@ngg's first error is because Box<Trait + 'a + 'b> isn't valid syntax. In contrast, impl Trait + 'a + 'b is valid syntax, but it doesn't work. (This seems pretty bad). The second error is the result of neither working, even if syntactically its allowed.

The expansion of lifetimes needs to be changed to instead of just adding + 'a + 'b ... to the return type, instead adding an additional lifetime and bounding every lifetime by that. So the expansion changes:

#[async] fn foo(x: &i32, y: &i32) -> Result<i32, !> { ... }

// currently:
fn foo<'_async0, '_async1>(x: &'_async0 i32, y: &'_async1 i32)
    -> impl Future<Item = i32, Error = !> + '_async0 + '_async1 { ... }

// changed to:
fn foo<'_ret, '_async0: '_ret, '_async1: '_ret>(x: &'_async0, y: &'_async1 i32)
    -> impl Future<Item = i32, Error = !> + '_ret { ... }

[withoutboats]

Its a separate issue from this PR though!

[Nemo157]

See rust-lang/rust#49431 for a rust issue about impl Trait + 'a + 'b.

[ngg]

Yeah I realized it's different but only after writing this here. (I wrongly remembered that it was working with normal #[async], and I tried it only with #[async(pinned)]).
Even if rust-lang/rust#49431 will be fixed it won't work with pinned as currently Box<T + 'a + 'b> and PinBox<T + 'a + 'b> are not permitted either.

[raviqqe]

Did any collaborators review @ngg's #932 which looks cleaner than mine?

[raviqqe]

It's better to put them into futures::prelude as normal Future and Stream so that users don't need to use them every time in modules which use #[async] stuff otherwise they'll see some error messages about missing methods of pin() or pin_local().

[ngg]

@raviqqe I reverted it pretty fast when I realized you already did it and that Nemo157@5c32c05 is based on your version. I actually prefer your solution with not creating gen_pinbox and gen_pinbox_local, but calling pin() and pin_local() inline, the other differences are quite irrelevant after #918.

[withoutboats]

Will merge if tests pass, thanks for this!

[withoutboats]

one failure, missing spawn_pinned method

[withoutboats]

Merged, thanks so much for doing this! 🎉