maybe-async breaks crates

[xfbs]

Hey!

I just noticed something that seems kind of bad! The is_sync feature of maybe-async breaks stuff. Here's an example:

Create a new, empty crate, add these dependencies:

crates-index = { version = "2.1.1", features = ["git", "git-https"] }
rust-s3 = "0.33.0"

Building this will break hard. Disabling either one of the dependencies makes the build work.

Why? Both of these crates us maybe-async. The crates-index crate enables the is_sync feature. The presence of this feature breaks rust-s3.

My understanding is that this feature is not "additive".

Can someone look into this? In my project, I am able to work around this, but this issue currently makes doing cargo build on the workspace level broken. In my workspace I can work around it by always compiling crates individually. But maybe it would be a good idea to peek into this and see if there was some way to solve this.

Cheers!