If "Rust reports potentially useful cfg-disabled items in errors" works remotely as well as I'm imagining it, that's going to be insanely useful.
It was this very day, that I wasted several minutes wondering how the hell url::Url does not implement the serde-traits, when those were simply behind a feature flag.
This will also hopefully limit the number of issues opened that are resolved with a "you must enable X feature."
A lot of nice QoL changes (checking for missing feature flags, for example) but the thing that stood out to me the most was impl Sync for mpsc::Sender. This has always been a pain point in my opinion for the standard library's channels, but now that they're using crossbeam-channel internally, there's no need to add it as a dependency anymore.
I think some people will be upset by them dropping support for older Windows versions. I can see why they would not want to continue support for them though, it takes extra work to maintain compatibility for those old OS versions and the vast majority of users (by percentage) are on 10/11 now. Still, a shame.