Non-lexical lifetimes - The Edition Guide

# Non-lexical lifetimes

The borrow checker has been enhanced to accept more code, via a mechanism called "non-lexical lifetimes." Consider this example:

    fn main() {
        let mut x = 5;

        let y = &x;

        let z = &mut x;
    }

In older Rust, this is a compile-time error:

    error[E0502]: cannot borrow `x` as mutable because it is also borrowed as immutable
     --> src/main.rs:5:18
      |
    4 |     let y = &x;
      |              - immutable borrow occurs here
    5 |     let z = &mut x;
      |                  ^ mutable borrow occurs here
    6 | }
      | - immutable borrow ends here

This is because lifetimes follow "lexical scope"; that is, the borrow from `y` is considered to be held until `y` goes out of scope at the end of `main`, even though we never use `y` again. This code is fine, but the borrow checker could not handle it.

Today, this code will compile just fine.

## Better errors

What if we did use `y`, like this?

    fn main() {
        let mut x = 5;
        let y = &x;
        let z = &mut x;

        println!("y: {}", y);
    }

Here's the error:

    error[E0502]: cannot borrow `x` as mutable because it is also borrowed as immutable
     --> src/main.rs:5:18
      |
    4 |     let y = &x;
      |              - immutable borrow occurs here
    5 |     let z = &mut x;
      |                  ^ mutable borrow occurs here
    ...
    8 | }
      | - immutable borrow ends here

With non-lexical lifetimes, the error changes slightly:

    error[E0502]: cannot borrow `x` as mutable because it is also borrowed as immutable
     --> src/main.rs:5:13
      |
    4 |     let y = &x;
      |             -- immutable borrow occurs here
    5 |     let z = &mut x;
      |             ^^^^^^ mutable borrow occurs here
    6 |
    7 |     println!("y: {}", y);
      |                       - borrow later used here

Instead of pointing to where `y` goes out of scope, it shows you where the conflicting borrow occurs. This makes these sorts of errors far easier to debug.