alloc_uninit is in the latest_stable_rust already, it just doesn't do anything if extern_crate_alloc is not also enabled, and latest_stable_rust doesn't include extern_crate_alloc (explicitly).

For future¹ features which depend both on higher MSRV and alloc/std, I see two options:

1. Have them be in latest_stable_rust, and have them just do nothing if extern_crate_alloc/extern_crate_std is not enabled (status quo for alloc_uninit)
2. Have them not in latest_stable_rust, and have them depend on extern_crate_alloc/extern_crate_std
   a. Perhaps have new latest_stable_rust_alloc = ["latest_stable_rust", "extern_crate_alloc", "alloc_uninit", ...], latest_stable_rust_std = ["latest_stable_rust_alloc", "extern_crate_std", ...] features that enable all latest-stable features including ones that require alloc/std.

I looked, and it seems like other than transparentwrapper_extra, all other existing, sound, non-nightly, non-extern_crate_alloc/std features are included in latest_stable_rust, so I'll make a PR to add transparentwrapper_extra to latest_stable_rust.