Don't suggest .into_iter() on iterators

[dianne]

This makes the the suggestion to call .into_iter() only consider unsatisfied Iterator bounds for the receiver type itself. That way, it ignores predicates generated by trying to auto-ref the receiver (the result of which usually won't implement Iterator).

Fixes #127511

Unfortunately, the error in that case is still confusing: it labels Iterator as an unsatisfied bound because &impl Iterator: Iterator can't be satisfied, despite that not being required or helpful. I'd like to handle that in a separate PR. I'm hoping fixing #124802 will fix it too. It doesn't look connected to that issue. Still, I think it'd be clearest to visually distinguish unsatisfied predicates from different attempts at pick_method; I'll make a PR for that soon.

[rustbot]

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[lcnr]

this should not be necessary 🤔 why is the impl Iterator: Iterator in the unsatisfied_predicates. I feel like there's something bigger going on which likely also affects other diagnostics.

[lcnr]

ah, the reason is that unsatisfied_predicates also contains all the unsatisfied predicates from auto-ref and the deref-steps, e.g.

trait Trait {
    fn method(self)
}

trait Bound {}
impl<T: Bound> Trait for T {
    fn method(self) {}
}

fn main() {
    0u32.method()
}

results in

error[E0599]: the method `method` exists for type `u32`, but its trait bounds were not satisfied
  --> src/main.rs:11:10
   |
11 |     0u32.method()
   |          ^^^^^^ method cannot be called on `u32` due to unsatisfied trait bounds
   |
note: the following trait bounds were not satisfied:
      `&mut u32: Bound`
      `&u32: Bound`
      `u32: Bound

as you can see in the new error message for this issue, it's talking about the following unsatisfied obligations:

      `&impl Iterator: Iterator`
      `&impl Iterator: Missing`
      `&mut impl Iterator: Missing`
      `impl Iterator: Missing`

so instead of checking whether ty implements iterator, modify is_iterator_predicate to only trigger if the self-type of the iterator predicate is ty itself

[dianne]

nice catch! I was thinking along those lines for making that error less confusing, but I totally missed that it could be applied to is_iterator_predicate too. it's much nicer now; thank you

[lcnr]

@bors r+ rollup

[bors]

📌 Commit cea82ed has been approved by lcnr

It is now in the queue for this repository.