Crate for pin macros

[MajorBreakfast]

I'd like to extract our pin macros into their own crate because they're useful for pinning in general and not tied to futures at all. Here's what I'd like to do:

• Remove the macros from the facade. Implementors can depend directly on the pin macro crate. #[macro_use] is going away and an additional line in Cargo.toml ins't a burden. So the difference is really just use pin_utils::pin_mut; instead of use futures::pin_mut;
• Keep the source within the futures repository
• Possible names: pin-utils (my favorite), pin-util, pin, pin-macros, pin-macro (one day we might want to include not-macro stuff in it as well, though)
• Independent versioning: Start with version 0.1.0

Thoughts?
@aturon
@cramertj
and anyone who's reading this 🙂

[Nemo157]

CC @withoutboats, @pythonesque and @ashfordneil since this includes unsafe pin accessors and they were discussing them on the Pin API tracking issue. It would be good to have just a single canonical utility library for making pinned types more ergonomic rather than splitting the effort.

@MajorBreakfast I assume this is just https://github.com/rust-lang-nursery/futures-rs/blob/master/futures-util/src/macros/pin.rs, or are there other related macros somewhere else?

pin-utils is my favourite as well (but it's going to make me mistype futures-utils even more often).

[MajorBreakfast]

@Nemo157

It would be good to have just a single canonical utility library for making pinned types more ergonomic rather than splitting the effort.

I agree. Although, in that case we should consider giving it its own repo.

or are there other related macros somewhere else?

It's just the one file. Some time ago there were more macros (also in this file), but I killed those a few weeks ago because we didn't need them 🙂

but it's going to make me mistype futures-utils even more often

I've mistyped this countless times as well ^^'

[withoutboats]

I don't think pin helper macros belong in the public API of futures, so this makes sense to me. :)

[aturon]

This sounds really great to me!

[MajorBreakfast]

Should we put the code in a new git repo?

[MajorBreakfast]

https://crates.io/crates/pin-utils

https://github.com/rust-lang-nursery/pin-utils

[MajorBreakfast]

With #1185 the futures crate now uses the macros from the pin-utils crate.

The change will be formerly announced in the blog post for alpha 3.