You can check out the release notes of Rust 1.39.0 on github .
Key features
- include async/.await,
- shared references to by-move bindings in match guards
- attributes on function parameters.
The functions mentioned below are const fn:
- Vec::new, String::new, and LinkedList::new
- str::len, [T]::len, and str::as_bytes
- abs, wrapping_abs, and overflowing_abs
Rust blog 