Rust Platform

Moving ownership everywhere gets tedious. What if you just want to look at a value without taking it?

In Python, everything is passed by reference automatically:

def print_length(items):
    print(len(items))  # just reads, doesn't modify

names = ["Alice", "Bob"]
print_length(names)
print(names)  # still valid

In Rust, you borrow with &:

fn print_length(items: &Vec<String>) {
    println!("{}", items.len());
}

let names = vec!["Alice".to_string(), "Bob".to_string()];
print_length(&names);  // lend with &
println!("{:?}", names);  // still ours

The borrowing rules

A shared reference (&T) is read-only. You can have as many as you want:

let text = String::from("hello");
let r1 = &text;
let r2 = &text;
let r3 = &text;
println!("{} {} {}", r1, r2, r3);  // all valid

But you cannot modify through a shared reference:

fn try_modify(s: &String) {
    // s.push_str(" world");  // ERROR: cannot borrow as mutable
}

Why this matters

Shared references guarantee that the data won't change while …