rls no longer builds after rust-lang/rust#84310

[rust-highfive]

Hello, this is your friendly neighborhood mergebot.
After merging PR #84310, I observed that the tool rls no longer builds.
A follow-up PR to the repository https://github.com/rust-lang/rls is needed to fix the fallout.

cc @RalfJung, do you think you would have time to do the follow-up work?
If so, that would be great!

[RalfJung]

The fix should be to use the const_fn_trait_bound and/or const_fn_unsize feature gates instead of const_fn. Let me know if/how I can help with that.