The reference type

A reference is written in one of three ways: &, ref, or ref mut:

let mut var = 4; 
let ref_to_var = &var; 
let ref second_ref = var; 
let ref mut third_ref = var; 

The references are all equivalent here. Note, however, that the preceding code doesn't work as it is due to mutable reference rules. Rust allows several immutable reference to a thing, but if a mutable reference is taken, no other references may exist at the time. Therefore, the last line would not work, since there are already two active references to var.