Hello,

Adding mut in front of a function parameter makes the parameter (or more accurately variable) itself mutable within the function body, exactly like in a variable declaration.

So you can think of:

fn fill_vec(mut vec: Vec<i32>) -> Vec<i32> {
    vec.push(88);

    vec
}

As:

fn fill_vec(vec: Vec<i32>) -> Vec<i32> {
    let mut vec = vec;
    vec.push(88);

    vec
}

Without the ability to add mut to function parameters you would have to manually shadow them if you wanted to modify them, using the let mut x = x pattern.