master. It becomes part of the next
nightlyrelease. Since it's not yet stable we don't want people to accidentally use it (since it can still be seriously changed and then their code may break). So, one has to explicitly add a feature gate to the source file, like this:
#![feature(box_syntax)]. Normally, identifier for feature gate is the same as feature name in RFC.
nightlypeople collect feedback, resolve questions, check more chekboxes in the plan and if everything goes well, prepare features for stabilization. That involves creating one more document and one more Final Comment Period, oh well. Eventually, everyone is happy and the feature is considered stable.
Cargo.tomlusing something like
rust = "1.35".
#[stable(feature = "slice_ptr_range", since = "1.0.79")]