[[rust-dot-vs-double-colon-syntax]]

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Mutable reference (&mut)

fn (&mut foo) {

}

Pass the function a mutable reference to foo.

The & indicates that this argument is a reference, which gives you a way to let multiple parts of your code access one piece of data without needing to copy that data into memory multiple times. References are a complex feature, and one of Rust’s major advantages is how safe and easy it is to use references. You don’t need to know a lot of those details to finish this program. For now, all you need to know is that like variables, references are immutable by default. Hence, you need to write &mut guess rather than &guess to make it mutable. – From The Rust Book