The 1.78.0 pre-release is ready for testing. The release is scheduled for May 2. Release notes can be found here.
You can try it out locally by running:
RUSTUP_DIST_SERVER=https://dev-static.rust-lang.org rustup update stable
The index is https://dev-static.rust-lang.org/dist/2024-04-28/index.html.
The release team is also thinking about changes to our pre-release process: we'd love your feedback on this GitHub issue.
The release notes include:
Should that actually be included? As far as I can tell, it is about an improvement to trait object upcasting, which is still an unstable feature overall.
I also asked 
I'm told that the trait upcasting functionality that is still unstable is only for casts that have to modify the vtable. Since auto traits can't have methods, an upcast is trivial (I guess it still changes the vtable but to a statically known one, so not that interesting).
It also allows casting from dyn Trait to dyn Trait + Auto (when Auto is a supertrait), which is analogous to a downcast.
Is Tracking import use types for more accurate redundant import checking by surechen · Pull Request #117772 · rust-lang/rust · GitHub (currently listed in the release notes) one of the changes that was reverted on beta and nightly?
Happy to drop it if so, I'm not sure where that change landed. My recollection was that parts of it were reverted, but not the full thing. That might be totally false though.
It doesn't really feel like a change that is critical to not put in the notes even if it actually will land a release later, so I'm not particularly worried about getting this wrong though.
this is apparently just entirely wrong 
we only support dyn Trait to dyn Trait + Auto.
dyn Trait to dyn Auto apparently doesn't even work with with feature(trait_upcasting) enabled. Opened `trait Trait: Send`: upcasting from `dyn Trait` to `dyn Send` does not work · Issue #126313 · rust-lang/rust · GitHub for that.
Fixing my mistake in the relnotes in fix RELEASES: we do not support upcasting to auto traits by lcnr · Pull Request #126314 · rust-lang/rust · GitHub
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