Changes with unclear timing

Associated items

Anonymous, unboxed return types (aka impl Trait types)

Default type parameters

We are already using this in a few places (e.g. HashMap), but it's feature-gated.

Compile-time function evaluation (CTFE)

https://github.com/mozilla/rust/issues/11621

Improved constant folding

https://github.com/rust-lang/rust/issues/7834