Keyword mut

A mutable variable, reference, or pointer.

mut can be used in several situations. The first is mutable variables, which can be used anywhere you can bind a value to a variable name. Some examples:

// A mutable variable in the parameter list of a function.
fn foo(mut x: u8, y: u8) -> u8 {
    x += y;
    x
}

// Modifying a mutable variable.
let mut a = 5;
a = 6;

assert_eq!(foo(3, 4), 7);
assert_eq!(a, 6);

The second is mutable references. They can be created from mut variables and must be unique: no other variables can have a mutable reference, nor a shared reference.

// Taking a mutable reference.
fn push_two(v: &mut Vec<u8>) {
    v.push(2);
}

// A mutable reference cannot be taken to a non-mutable variable.
let mut v = vec![0, 1];
// Passing a mutable reference.
push_two(&mut v);

assert_eq!(v, vec![0, 1, 2]);

let mut v = vec![0, 1];
let mut_ref_v = &mut v;
#[allow(unused)]
let ref_v = &v;
mut_ref_v.push(2);

Mutable raw pointers work much like mutable references, with the added possibility of not pointing to a valid object. The syntax is *mut Type.

More information on mutable references and pointers can be found in the Reference.