Add Iterator::array_windows()

[rossmacarthur]

This has been similarly implemented as .tuple_windows() in itertools as Itertools::tuple_windows(). But it makes more sense with arrays since all elements are the same type.

I will update stability attributes with a tracking issue if accepted.

See also #92393

[rust-highfive]

r? @dtolnay

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[mominul]

Suggested change

	/// let mut iter = "rust".chars().array_windows();
	/// let mut iter = "rust".chars().array_windows::<2>();

I think you missed it?

[rossmacarthur]

It's not required because of type inference

[the8472]

This involves a lot of clone and rotate_left operations, so this has significantly different performance characteristics compared to [T]::array_windows. Since clone operations can be arbitrarily expensive offering something like this under the same name as a fairly cheap method could be misleading if someone tried to substitute one for the other.

[rossmacarthur]

Thanks @the8472, you make some good points. I do think in a lot of cases it might be more performant to collect into a Vec and then use [T]::array_windows, so you think this API is ever something we would want add to Iterator even under a different name?

[the8472]

Perhaps reviving #82413 would be a better option since the map closure can take a reference to the internal iterator state and thus incur no cloning overhead. Additionally it amortizes the rotation cost by increasing the internal buffer size.

[TennyZhuang]

Is there some way to assert it in compile time?

[the8472]

https://blog.rust-lang.org/2021/12/02/Rust-1.57.0.html#panic-in-const-contexts

[rossmacarthur]

It would be ideal but I couldn't get it to work. I tried it for #92393 and get the following error

error[E0401]: can't use generic parameters from outer function
  --> library/core/src/iter/adapters/array_chunks.rs:86:31
   |
81 | impl<I, const N: usize> ArrayChunks<I, N>
   |               - const parameter from outer function
...
86 |         const _: () = assert!(N != 0, "chunk size must be non-zero");
   |                               ^ use of generic parameter from outer function

For more information about this error, try `rustc --explain E0401`.

And if I make the const part of the impl block it doesn't result in a compile time error.

[rossmacarthur]

Based on the discussion above I'm closing this. I might look at reviving #82413 instead.