improve error message when returning iterator with reference into stack frame

[nikomatsakis]

This example (playground):

use std::rc::Rc;

fn iterate(data: Rc<Vec<u32>>) -> impl Iterator<Item = u32> {
    data.iter().cloned()
}

fn main() {
    for x in iterate(Rc::new(vec![1, 2, 3])) {
        println!("Hi! {}", x);
    }
}

gives this error:

error[E0597]: `data` would be dropped while still borrowed
 --> src/main.rs:4:5
   |
 4 |     data.iter().cloned()
   |     ^^^^ borrowed value does not live long enough
 5 | }
   | - borrowed value only lives until here
   |
   = note: borrowed value must be valid for the static lifetime...

I think we could do better. For example, we don't explain anything about how impl Iterator disallows lifetimes; and I think we can clarify why `data is being dropped.

I don't yet have a concrete suggestion, though, have to think about it.

Related to #52534 -- in some sense a specific instance of that.

[nikomatsakis]

With respect to impl Iterator, one thing I thought of is this

error[E0597]: reference to `data` would be used after `data` is freed
 --> src/main.rs:4:5
   |
 3 | fn iterate(data: Rc<Vec<u32>>) -> impl Iterator<Item = u32> {
   |                                   -------------------------
   |                                   |
   |                                   as written, this type does not permit borrowed data
   |                                   help: try `impl Iterator<Item = u32> + '_`
 4 |     data.iter().cloned()
   |     ^^^^ `data` is borrowed here
 5 | }
   | - borrowed value only lives until here
   |
   = note: borrowed value must be valid for the static lifetime...

But in a way -- in this particular case -- it doesn't matter anyway, as there is nothing you could change that signature too that would work.

[matthewjasper]

I've assigned myself since I've been working on things in this area recently, but other things have come up as higher priority, so I might not get around to this for a few weeks.

[Nadrieril]

The current error is:

error[E0597]: `data` does not live long enough
 --> src/main.rs:4:5
  |
3 | fn iterate(data: Rc<Vec<u32>>) -> impl Iterator<Item = u32> {
  |            ---- binding `data` declared here
4 |     data.iter().cloned()
  |     ^^^^----------------
  |     |
  |     borrowed value does not live long enough
  |     opaque type requires that `data` is borrowed for `'static`
5 | }
  | - `data` dropped here while still borrowed

For more information about this error, try `rustc --explain E0597`.

Which is already much better! I think "opaque type requires that" could point to the impl Iterator return type as Niko suggested, and "borrowed value does not live long enough" doesn't explain what it points to. My personal ideal would look like:

error[E0597]: `data` does not live long enough
 --> src/main.rs:4:5
  |
3 | fn iterate(data: Rc<Vec<u32>>) -> impl Iterator<Item = u32> {
  |            ----                   -------------------------
  |            |                      |
  |            |                      as written, this type does not permit borrowed data
  |            |                      help: try `impl Iterator<Item = u32> + '_`
  |            binding `data` declared here
4 |     data.iter().cloned()
  |     ^^^^----------------
  |     |
  |     `data` is borrowed here...
  |     ...and the borrow is captured by this whole expression
5 | }
  | ^ `data` dropped here while still borrowed

For more information about this error, try `rustc --explain E0597`.