It's been in the works for years and there's no end in sight. IIRC sound specialization might not be possible until some precursor work for GATs is complete.

Last I checked (a month or two ago), the specialization feature warning even included a bit about how it can crash the compiler (the crashes were the reason I gave up on specialization two years ago)

Edit: Turns out theres been some progress since I used it last! Feature `min_specialization` seems to not be a non-crashing subset of specialization [1]

[1] https://github.com/rust-lang/rust/pull/68970