Rust Essentials - Second Edition

Match, struct, and ref

If you want to get a reference to a matched variable inside a match, use the ref keyword, as in the following example:

// see code in Chapter 7/code/ref.rs    
fn main() { 
   let n = 42; 
   match n { 
      ref r => println!("Got a reference to {}", r), 
   } 
 
   let mut m = 42; 
   match m { 
      ref mut mr => { 
         println!("Got a mutable reference to {}", mr); 
         *mr = 43; 
      }, 
   } 
   println!("m has changed to {}!", m); 
} 

This prints out the following output:

    Got a reference to 42
    Got a mutable reference to 42
    m has changed to 43!  

The r variable inside the match has the type &i32. In other words, the ref keyword creates a reference for use in the pattern. If you need a mutable reference, use ref mut.

We can also use ref to get a reference to a field of a struct ...