"Doesn't live long enough" error can be further improved

[sophiajt]

From @nikomatsakis (based on a chat with @wycats) - we can further improve the "doesn't live long enough" error by talking a bit more about how the values flow through the error. Something like:

error: `x` does not live long enough
 --> <anon>:4:10
  |
4 |     p = &x;
  |          - borrow occurs here
5 | }
  | ^ `x` dropped here while still borrowed
  |
= note: values in a scope are dropped in the opposite order they are created

[mikhail-m1]

@jonathandturner

I'm looking for test case, I've found some

error: `x` does not live long enough
  --> src/test/compile-fail/regions-escape-loop-via-variable.rs:21:14
   |
21 |         p = &x; //~ ERROR `x` does not live long enough
   |              ^ does not live long enough
22 |     }
   |     - borrowed value only lives until here
23 | }
   | - borrowed value needs to live until here

just need to change text for line 21 and 22 and add note?

[sophiajt]

These are slightly different examples, but I think you have the right idea. Your example would look like:

error: `x` does not live long enough
  --> src/test/compile-fail/regions-escape-loop-via-variable.rs:21:14
   |
21 |         p = &x;
   |              - borrow occurs here
22 |     }
   |     ^ `x` dropped here while still borrowed
23 | }
   | - borrowed value needs to live until here

Notice also that the primary span (the ^^^ part) is on line 22 instead of 21. I don't think there's a note on this one since the note is shown if both the borrowed value and the borrower are dropped in the same scope.

[mikhail-m1]

Is it example for first output?

fn main() {
    let p;
    let a = 42;
    p = &a;
}

[sophiajt]

@mikhail-m1 - that looks like it. Want to add it to the test suite, if it's not already there?

[mikhail-m1]

@jonathandturner I did not find it in repo. I doubt about '^', I far as i know both '^' and '-' adds by by span_label, so need to change error place to change mark types, but error in line 4 not in 5 (in first output)

[sophiajt]

Great, please add the test if you're creating a PR for this.

The ^ and - are created by the labels. Labels that have the same span as the error message itself get ^, all other labels get -

[mikhail-m1]

@jonathandturner Let me explain in another way:
for my sample I have next calls

struct_span_error(line 4 at 'a', ....)
span_label(line 4 at 'a', "does not live long enough")
span_label(line 5 at '}', "a dropped before borrower")

for achieve target output it will be

struct_span_error(line 5 at '}, ....)
span_label(line 4 at 'a', "borrow occurs here")
span_label(line 5 at '}',  "a dropped here while still borrowed")

so error will be in line 5, is it OK?

[sophiajt]

Yup, you got it.

[mikhail-m1]

@jonathandturner I've explored code borrow checker, and found several cases for "does not live long enough":

1. closures,
2. with same scope, like my last and your first sample
3. with different scope, like my first sample

Should I change only second case or third too?

and I've found some uncommon construction in closure case

let db = struct_span_error(...)
...
db.span = MultiSpan::from_span(...) 

Can I use it?

[sophiajt]

Have you tried the closure case to do a before/after and see what it looks like?

I don't remember using that functionality much. Does it let you set labels?

[mikhail-m1]

@jonathandturner In closure case It works before but in other logical branch.
It works, but reset all already performed span_label calls. So it was unexpected for me. Span_label works well after it.

It my case it allows minimize changes but not makes code harder to read.

What about my first question?

[mikhail-m1]

@jonathandturner error with different lifetime ranges like

fn main() {
  let x;
  {
     let y = 42;
     x = &y;
  }
}

remain unchanged

error: `y` does not live long enough
 --> 1.rs:5:11
  |
5 |      x = &y;
  |           ^ does not live long enough
6 |    }
  |    - borrowed value only lives until here
7 | }
  | - borrowed value needs to live until here

should I fix it too?