Lesson #3: Borrow Checker Challenge Nested Borrows

lesson3.rs

// Nested borrows are a common source of trouble. This example is a bit contrived
// but illustrates the challenge.
fn main() {
    let mut numbers = vec![1,2,3,4];
    // Because push() needs a mutable borrow and len() needs an immutable borrow,
    // the borrow checker lets you know you've broken the rules.
    numbers.push(numbers.len());
    println!("numbers has length {}", numbers.len());
}

/*
fn main() {
    let mut numbers = vec![1,2,3,4];
    // When nested incompatible borrows are needed, consider introducing a new binding.
    // The type signature of Vec.len is fn len(&self) -> usize
    // It needs an immutable borrow of self but returns a value you take ownership of.
    // Therefore a new binding fits the bill nicely.
    let length = numbers.len();
    numbers.push(length);
    println!("numbers has length {}", numbers.len());
    // Further Reading: https://internals.rust-lang.org/t/accepting-nested-method-calls-with-an-mut-self-receiver/4588
}*/

https://play.rust-lang.org/?181f0fd1fe583f5183484e5fa0d2cb35gist=