In #25009 we're destabilizing the standard std::io::BufStream type. This is a breaking change for any users of this type as it will now no longer be accessible on the beta and stable channels.
This change is motivated by #17136 where it was pointed out that having totally separate buffering for each half of the stream can be surprising some situations and not always work as intended. The destabilization here is a reflection that we'd like some more time to work out what this primitive should look like and how it should work.
In the meantime I have published a bufstream crate on crates.io which is largely just a copy of the original source and builds on stable Rust, so you should be able to migrate to that if you're currently using std::io::BufStream.