[NLL] Tidy up error messages when borrowing locals

[spastorino]

Given this test which is included in #48592 and when merged will be located here https://github.com/rust-lang/rust/tree/master/src/test/ui/nll/borrowed-local-error.rs

// compile-flags: -Znll-dump-cause

#![feature(nll)]

fn gimme(x: &(u32,)) -> &u32 {
    &x.0
}

fn main() {
    let x = gimme({
        let v = (22,);
        &v
        //~^ ERROR `v` does not live long enough [E0597]
    });
    println!("{:?}", x);
}

With #48592 merged, the output looks like ...

[santiago@archlinux rust1 (borrowed_value_error)]$ RUST_BACKTRACE=1 rustc +stage1 src/test/ui/nll/borrowed-local-error.rs -Znll-dump-cause
error[E0597]: `v` does not live long enough
  --> src/test/ui/nll/borrowed-local-error.rs:22:9
   |
20 |       let x = gimme({
   |  _____________-
21 | |         let v = (22,);
22 | |         &v
   | |         ^^ borrowed value does not live long enough
23 | |         //~^ ERROR `v` does not live long enough [E0597]
24 | |     });
   | |_____-- borrow later used here
   |       |
   |       borrowed value only lives until here

error: aborting due to previous error

If you want more information on this error, try using "rustc --explain E0597"

@nikomatsakis said that the output should look like ...

This is what I think it should look like:

error[E0597]: `v` does not live long enough
  --> borrowed-local-error.rs:20:9
     |
  18 |       let x = gimme({
     |               ----- borrow later used here, during the call
  19 |           let v = (22,);
  20 |           &v
     |           ^^ borrowed value does not live long enough
  21 |       });
     |       - borrowed value only lives until here

Previous discussion #48592 (comment)

[pnkfelix]

While I agree that @nikomatsakis 's suggested revision would be an improvement, the output currently produced by NLL seems competitive with the AST-borrowck diagnostic.

That is, AST-borrowck also fails to identify the role of the function call, as shown here:

error[E0597]: `v` does not live long enough
  --> src/main.rs:9:10
   |
9  |         &v
   |          ^ borrowed value does not live long enough
10 |         //~^ ERROR `v` does not live long enough [E0597]
11 |     });
   |     - `v` dropped here while still borrowed
12 |     println!("{:?}", x);
13 | }
   | - borrowed value needs to live until here

error: aborting due to previous error

(I assume that this is why @nikomatsakis removed this from the "NLL: diagnostic parity" milestone a month ago.)

[pnkfelix]

Tagging as NLL-deferred to reflect the fact that this is much lower priority than other NLL work items.