Guide: specify that both shared and mutable borrows can be re-lent.

[jkleint]

@steveklabnik, you can re-lend both mutable and immutable borrows, and the rules for returning them are the same, no?

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @pcwalton (or someone else) soon.

[steveklabnik]

Hmmmm. I guess this is true. I thought you could only lend out a borrow immutably, but I guess if you have a mutable borrow, you can also lend it out mutably, once.

Can someone else review this too, please?

[Gankra]

Definitely more accurate to me. This is the mechanism that allows calling &mut self methods from &mut self methods. It's not exactly obscure or rare. It's quite important!

[jkleint]

Does this illustrate it?

fn borrow_mut(x: &mut int) {
    *x += 1;
    second_hand_mut(x);
}

fn second_hand_mut(x: &mut int) {
    *x += 2;
}

fn main() {
    let mut x = 5i;
    borrow_mut(&mut x);
    println!("{}", x);      // Prints 8
}

[alexcrichton]

@steveklabnik yeah you should be able to pass along mutable borrows (as @jkleint showed), are you ok with the wording?

[steveklabnik]

Yeah, after coming back to it, all seems well. Thank you!