Variability of variables#

let#

In Rust, you declare variables using the let keyword as follows.

fn main() {
    let apple = 10;
}

However, since let is fundamentally immutable, trying to change the value of apple as shown below will result in an error.

fn main() {
    let apple = 10;
    apple = 20;
}

error[E0384]: cannot assign twice to immutable variable `apple`

let mut#

To modify a value, you need to use the let mut keyword as follows.
Here, mut stands for mutable.