async: switch to async-fn-in-traits, release v0.2.0-alpha.0

[Dirbaio]

Latest Rust nightlies have somewhat usable async-fn-in-trait support already! 🎉

embassy-nrf updated here embassy-rs/embassy#974

Paprecuts encountered:

• there's this annoying error playground, workaround is to use the concrete type instead of Self::Error. This is a limitation of all async fns, not just in traits, but it hits especially hard within traits, so I dunno if there's plans to improve it.

async fn return type cannot contain a projection or Self that references lifetimes from a parent scope

• The SpiDevice trait ICEs, issue filed async fn in trait: ICE with 2 generic params in method rust-lang/rust#102310
• default methods don't work, but there's a PR already async fn in trait: trait method with default impl doesn't work rust-lang/rust#102308

Due to the last 2 I've left SpiDevice alone for now.

[rust-highfive]

r? @eldruin

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[korken89]

It seems the 2 issues with SpiDevice has been fixed in nighly now.

[Dirbaio]

converted all traits, embedded-hal-async no longer explodes when compiling now! I encountered another rustc bug rust-lang/rust#103457 but thankfully we don't need the + 'a anymore, so just removing it worked...

I haven't tried updating embassy yet, it's still possible it explodes with actual use.

[korken89]

How are we on this, is this ready to merge?
I'm about to start writing some example drivers and would love this :D

If not I'll use a git-dep to this branch.

[Dirbaio]

Since AFIT support is very recent in rustc we should check that it works end-to-end first. embedded-hal-async itself builds without ICEs now, but I haven't tried implementing it in a HAL and using it from a driver yet, it's still possible these things ICE. I wanted to update embassy but haven't found time for it yet. If you want to help test it out by using it as a git dep please go ahead! :)

When we know it works well we can merge it and release it as embedded-hal-async 0.2 :)

[korken89]

Super, then I know where it stands!
I'll git-dep for now and start hacking :)

[Dirbaio]

No longer builds in nightly-2022-10-29 and newer. Filed rust-lang/rust#103850

[Dirbaio]

Also, side note: A few places use T::Error instead of Self::Error because Self is not allowed in async fn return if it has lifetimes. Not a blocker since it can always be workarounded by writing the full Self type, but it can be annoying if it has lots of generics. Will likely be fixed in rust-lang/rust#103491

[Dirbaio]

Status update!

• nightly-2022-11-22 contains fixes for all the blockers
• Ported embassy-nrf here Switch to async-fn-in-trait embassy-rs/embassy#974 . Builds fine, runs fine. The only issue is that nightly also introduces a TAIT regression, but that can be fixed by porting the affected GAT+TAIT traits to AFIT.

So, I'm fairly certain this is ready to go now!

[eldruin]

Looks good to me, thank you!
bors r+

[bors]

Build succeeded:

• clippy
• fmt
• test (1.54.0, thumbv6m-none-eabi)
• test (1.54.0, thumbv7m-none-eabi)
• test (1.54.0, x86_64-unknown-linux-gnu)
• test (nightly, thumbv6m-none-eabi)
• test (nightly, thumbv7m-none-eabi)
• test (nightly, x86_64-unknown-linux-gnu)
• test (stable, thumbv6m-none-eabi)
• test (stable, thumbv7m-none-eabi)
• test (stable, x86_64-unknown-linux-gnu)