rust - Matching on a Box -

this code shown in the rust programming language:

#![feature(box_syntax, box_patterns)]  fn main() {     let b = some(box 5);     match b {         some(box n) if n < 0 => {             println!("box contains negative number {}", n);         }         some(box n) if n >= 0 => {             println!("box contains non-negative number {}", n);         }         none => {             println!("no box");         }         _ => unreachable!(),     } } 

but when run it, following error occurs:

error[e0554]: #[feature] may not used on stable release channel 

i tried

fn main() {     let b = some(box 5); } 

error: box expression syntax experimental;  

is because version of rust not latest? how can content in box::new()? tried

fn main() {     let b = some(box::new(5));     match b {         some(box::new(y)) => print!("{:?}", y),           _ => print!("{:?}", 1),     } } 

error[e0164]: `box::new` not name tuple variant or tuple struct --> main.rs:6:14   | 6 |         some(box::new(y)) => print!("{:?}", y),    |              ^^^^^^^^^^^ not tuple variant or struct 

you using #[feature] , can used nightly rust compiler. don't think possible match against box in stable rust, nightly allows following way of doing (like attempted in beginning):

#![feature(box_patterns)]  fn main() {     let b = some(box::new(5));     match b {         some(box y) => print!("{:?}", y),         _ => print!("{:?}", 1),     } }