Higher-ranked closure error messages should avoid "anonymous lifetime #2 defined on the body"

[arielb1]

When a higher-ranked closure is misused, as in:

fn give_any<F: for<'r> FnOnce(&'r ())>(f: F) {
    f(&());
}

fn main() {
    let x = None;
    give_any(|y| x = Some(y));
}

The compiler gives an obscure error message:

error[E0495]: cannot infer an appropriate lifetime due to conflicting requirements
 --> src/main.rs:7:27
  |
7 |     give_any(|y| x = Some(y));
  |                           ^
  |
note: first, the lifetime cannot outlive the anonymous lifetime #2 defined on the body at 7:14...
 --> src/main.rs:7:14
  |
7 |     give_any(|y| x = Some(y));
  |              ^^^^^^^^^^^^^^^
note: ...so that expression is assignable (expected &(), found &())
 --> src/main.rs:7:27
  |
7 |     give_any(|y| x = Some(y));
  |                           ^
note: but, the lifetime must be valid for the expression at 7:14...
 --> src/main.rs:7:14
  |
7 |     give_any(|y| x = Some(y));
  |              ^^^^^^^^^^^^^^^
note: ...so type `[closure@src/main.rs:7:14: 7:29 x:&mut std::option::Option<&()>]` of expression is valid during the expression
 --> src/main.rs:7:14
  |
7 |     give_any(|y| x = Some(y));
  |              ^^^^^^^^^^^^^^^

error: aborting due to previous error

The problem isn't that complicated: give_any was declared as taking F: for<'r> FnOnce(&'r ()), so it can pass a y can be of type &'α () for any lifetime 'α, which might not live long enough to be assigned to x.

However, the error message talks about an "anonymous lifetime #2 defined on the body", which is not something someone can understand without knowing the compiler. It should at least mention the problem argument.

cc @cengizio @estebank

[arielb1]

cc @nikomatsakis

[cengiz-io]

I'd like to work on this if noone is already interested.

@nikomatsakis

[nikomatsakis]

So this specific case:

• If we see one bound that is a free region scoped to a closure expression;
• and the other bound is outside the closure expression,

probably we should say something like

error[E0495]: borrowed data cannot be used outside closure
 --> src/main.rs:7:27
  |
7 |     give_any(|y| x = Some(y));
  |                           ^ data from the parameter `y` is only usable inside the closure
  |

It should be pretty easy to use the routines that @gaurikholkar added to identify which parameter the bound region comes from, at least much of the time (i.e., y). We probably won't always be able to say for sure, but that's ok, we can fallback to "data is only usable inside the closure" or something.

Not perfect. Thoughts on how to improve? I want to say "cannot escape the closure" but that sounds like jargon.

Maybe "data is only borrowed during the closure body"?

[estebank]

I wonder if it is worth it to also point at the closure as well:

error[E0495]: borrowed data cannot be used outside closure
 --> src/main.rs:7:27
  |
7 |     give_any(|y| x = Some(y));
  |              -------------^-
  |              |            |
  |              |            data from the parameter `y` is only usable inside its closure
  |              ...this closure

[nikomatsakis]

@estebank do we have some way to just highlight the |y| part?

[estebank]

@nikomatsakis I believe there was a recent change that either added or merely used that, so it is possible.

[nikomatsakis]

@estebank if so, I'd be in favor. Otherwise, we're just setting ourselves up for the "overlapping spans" problem.

[cengiz-io]

@estebank @nikomatsakis I was pretty much away from keyboard for the last couple days. Finally I'm getting into the issue

[estebank]

I believe #45927 might be the PR that I'd seen pointing at the closure's arg block.

[estebank]

The current output is still sub-par, but it is slightly clearer on what is going on:

error[E0495]: cannot infer an appropriate lifetime due to conflicting requirements
 --> src/main.rs:7:27
  |
7 |     give_any(|y| x = Some(y));
  |                           ^
  |
note: first, the lifetime cannot outlive the anonymous lifetime #2 defined on the body at 7:14...
 --> src/main.rs:7:14
  |
7 |     give_any(|y| x = Some(y));
  |              ^^^^^^^^^^^^^^^
note: ...so that expression is assignable (expected &(), found &())
 --> src/main.rs:7:27
  |
7 |     give_any(|y| x = Some(y));
  |                           ^
note: but, the lifetime must be valid for the block suffix following statement 0 at 6:5...
 --> src/main.rs:6:5
  |
6 | /     let x = None;
7 | |     give_any(|y| x = Some(y));
8 | | }
  | |_^
note: ...so that variable is valid at time of its declaration
 --> src/main.rs:6:9
  |
6 |     let x = None;
  |         ^

---

I now think that the output should point at the closure, the outside binding being affected and its declaration:

error[E0495]: borrowed data cannot be used outside closure
 --> src/main.rs:7:27
  |
6 |     let x = None;
  |         - binding declared outside of the closure
7 |     give_any(|y| x = Some(y));
  |              --- -        ^ data from the parameter `y` is only usable inside its closure
  |              |   |
  |              |   you're assigning to this binding declared outside of the closure
  |              the closure you can't escape