NLL: missing note that "borrowed value needs to live until here" (sometimes?)

[pnkfelix]

Consider: https://github.com/rust-lang/rust/blob/master/src/test/ui/span/issue-11925.nll.stderr

Compare it to: https://github.com/rust-lang/rust/blob/master/src/test/ui/span/issue-11925.stderr

The former is missing a note that "borrowed value needs to live until here"

---

Oftentimes, NLL diagnostics do include a note of the form "needs to live until use here." But apparently for this case (due to closures? Or destructors? Or both?) we do not have such a use to point at.

Anyway, this represents a regression in the informativeness of our diagnostics under NLL.

[nikomatsakis]

The reason for this is indeed the closure, I think. We detect the error when analyzing the closure body, so it is hard for us to report a span that is outside the closure. I think this will be pretty tricky to fix and is maybe not that important. What we could probably do is take a different tactic: we could explain that x is owned by the closure and yet we are returning a reference to it (just as we would a top-level function, perhaps).

I'm imagining:

error[E0597]: `x` is owned by the closure, so you cannot return a reference to it
  --> $DIR/issue-11925.rs:18:35
   |
LL |         let f = to_fn_once(move|| &x); //~ ERROR does not live long enough
   |                                   ^
   |                                   |
   |                                   |
| cannot return reference to data owned by the closure

for bonus points, we can highlight the move keyword and/or the use of x that caused it to a move in the first place (we have that data available somewhere).

[nikomatsakis]

I think that fixing this would also fix the examples here:

#51027

[nikomatsakis]

Actually I'm marking this as a sub-issue of #51027