Remove Iterator #[rustc_on_unimplemented]s that no longer apply.

[m-ou-se]

Now that IntoIterator is implemented for arrays, all the rustc_on_unimplemented for arrays of ranges (e.g. for _ in [1..3] {}) no longer apply, since they are now valid Rust.

Separated these from #85670, because we should discuss a potential new (clippy?) lint for these.

Until Rust 1.52, for _ in [1..3] {} produced:

error[E0277]: `[std::ops::Range<{integer}>; 1]` is not an iterator
 --> src/main.rs:2:14
  |
2 |     for _ in [1..3] {}
  |              ^^^^^^ if you meant to iterate between two values, remove the square brackets
  |
  = help: the trait `std::iter::Iterator` is not implemented for `[std::ops::Range<{integer}>; 1]`
  = note: `[start..end]` is an array of one `Range`; you might have meant to have a `Range` without the brackets: `start..end`
  = note: required by `std::iter::IntoIterator::into_iter`

But in Rust 1.53 and later, it compiles fine. It iterates over the array by value, for one iteration with the element 1..3.

This is probably a mistake, which is no longer caught. Should we have a lint for it? Should Clippy have a lint for it?

cc @estebank @flip1995

cc #84513

[flip1995]

This looks like a Clippy lint to me. I can't see many situations, where you can accidentally use the range inside the for loop without getting a type error if you meant to use an integer/the range item.

[CDirkx]

I can't see many situations, where you can accidentally use the range inside the for loop without getting a type error if you meant to use an integer/the range item.

The result doesn't have to be used, iterating over an integer range without using the value is a pretty common way to do fixed sized loops:

fn do_x_16_times() {
    for _ in [0..16] {
        x(); // will actually do x() only once
    }
}

[estebank]

Don't we need to keep this for 201x editions?

---

Yes, I think we should have a lint in clippy, if not in rustc because of how problematic this mistake could be.

[jyn514]

Don't we need to keep this for 201x editions?

@estebank arrays implement IntoIterator on all editions. 2021 only affects the method resolution of the into_iter() call. So this is dead code no matter what.

[estebank]

@bors r+

[bors]

📌 Commit caf6faf has been approved by estebank

[bors]

🌲 The tree is currently closed for pull requests below priority 1000. This pull request will be tested once the tree is reopened.

[cuviper]

This is probably a mistake, which is no longer caught. Should we have a lint for it? Should Clippy have a lint for it?

See rust-lang/rust-clippy#7125