[NLL] borrowed-universal-test.rs output is not great

[spastorino]

The output of this test ...

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

#![feature(nll)]
#![allow(warnings)]

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

fn foo<'a>(x: &'a (u32,)) -> &'a u32 {
    let v = 22;
    gimme(&(v,))
    //~^ ERROR borrowed value does not live long enough [E0597]
}

fn main() {}

is not great

[santiago@archlinux rust1 (borrowed_value_error)]$ rustc +stage1 src/test/ui/nll/borrowed-universal-error.rs -Znll-dump-cause
error[E0597]: borrowed value does not live long enough
  --> src/test/ui/nll/borrowed-universal-error.rs:22:12
   |
22 |     gimme(&(v,))
   |            ^^^^ temporary value does not live long enough
23 |     //~^ ERROR borrowed value does not live long enough [E0597]
24 | }
   | - temporary value only lives until here
   |
note: borrowed value must be valid for the lifetime 'a as defined on the function body at 20:1...
  --> src/test/ui/nll/borrowed-universal-error.rs:20:1
   |
20 | fn foo<'a>(x: &'a (u32,)) -> &'a u32 {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to previous error

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

cc @nikomatsakis

[nikomatsakis]

So, I wanted this opened because:

borrowed value must be valid for the lifetime 'a as defined on the function body at 20:1...

doesn't seem great to me. Not terrible, but not great. I'd prefer to see us tracking whether the value is returned or escapes in some other way, and pointing at the spot where the escape occurs with a nice message. Something like this maybe?

error[E0597]: borrowed value does not live long enough
  --> src/test/ui/nll/borrowed-universal-error.rs:22:12
   |
22 |     gimme(&(v,))
   |      ----- ^^^^ temporary is created here, and then borrowed
   |      |
   |      reference is returned here
24 | }
   | - temporary will be freed here, when this block exits

[nikomatsakis]

This was spawned from this conversation. I'm going to copy over @estebank's last comment in particular:

For named we currently only point at the signature of the scope that defines it, while for anonymous we point at the entire block on its own span note. We could change this to point at the end where it must live, but that doesn't seem to me entirely different to what we do now, and would lose some context... Of course, this only applies to LL.

I think the best bet is still to continue handling specific cases with more targeted suggestions/explanations, as gaurikholkar was doing. These can get increasingly niche, but those are the cases where rustc holding your hand is most impactful and can be a great teachable opportunity.

[matthewjasper]

Current output:

error[E0515]: cannot return value referencing temporary value
  --> <source>:12:5
   |
12 |     gimme(&(v,))
   |     ^^^^^^^----^
   |     |      |
   |     |      temporary value created here
   |     returns a value referencing data owned by the current function
error: aborting due to previous error
For more information about this error, try `rustc --explain E0515`.
Compiler returned: 1

nominating for discussion on whether this should be kept open

[nikomatsakis]

Discussed in the NLL meeting:

• New output has very nice content
  • It could make be made more clear by emphasizing that the value will be freed
• However, the formatting is kind of confusing, particularly due to the overlapping spans
  • maybe something like this would help?

We decided to keep it open for now but mark it P-low.

[Aaron1011]

This now produces the same error message both with and without #![feature(nll)]