Chapter 8.1: Explanation for listing 8-7 seems to be incorrect

[pszpetkowski]

Recall the rule that states you can’t have mutable and immutable references in the same scope. That rule applies in Listing 8-7, where we hold an immutable reference to the first element in a vector and try to add an element to the end, which won’t work.

It's quite interesting, because I've tried it out with Rust 1.32 and pushing new element to vector seems to work just fine:

let mut v = vec![1, 2, 3, 4, 5];
let first = &v[0];
println!("v: {:?}", v);
v.push(6);
println!("v: {:?}", v);

Output:

v: [1, 2, 3, 4, 5]
v: [1, 2, 3, 4, 5, 6]

In my case it's printing variable first after pushing to vector that causes the error.

[eddyp]

@piotr-szpetkowski I think this is due to non lexical lifetimes and the fact you don't try to use 'first' after the push and the same code will not compile if you don't use 2018 edition semantics.

Can you try removing the edition field from Cargo.toml to check?

[pszpetkowski]

@eddyp Yeah, it looks like you're right. In case of 2015 edition it fails on push with cannot borrow 'v' as mutable because it is also borrowed as immutable

[eddyp]

@piotr-szpetkowski OK, going back to 2018, I think it will fail with non lexical lifetimes if you try to use 'first', say by println!("first = {}", first) after the v.push. Can try that, too?

[pszpetkowski]

@eddyp Sure, I've already tried that before when I've been experimenting with the listing from the book and it does cause an error which is presented in Listing 8.7 of the book.

So, basically it seems like Listing has been updated for the 2018 edition, but the explanation hasn't.