Co-authored-by: Nthe8472 <the8472@users.noreply.github.com>

This addresses a TODO comment. The behavior of #[derive(Clone)]
*does* result in a T: Clone requirement.

Add a manual Clone implementation, matching Split and SplitInclusive.

`[].split_inclusive()` currently yields a single, empty slice. That's
different from `"".split_inslusive()`, which yields no output at
all. I think that makes the slice version harder to use.

The case where I ran into this bug was when writing code for
generating a diff between two slices of bytes. I wanted to prefix
removed lines with "-" and a added lines with "+". Due to
`split_inclusive()`'s current behavior, that means that my code prints
just a "-" or "+" for empty files. I suspect most existing callers
have similar "bugs" (which would be fixed by this patch).

Closes #89716.

Adds extensive test for all the [r]split* iterators.
Fixes size_hint upper bound for split_inclusive* iterators which was one higher than necessary for non-empty slices.
Fixes size_hint lower bound for [r]splitn* iterators when n==0, which was one too high.

Add TrustedRandomAccessNoCoerce supertrait without requirements or guarantees about subtype coercions

Update all the TrustedRandomAccess impls to also implement the new supertrait

This method on the Iterator trait is doc(hidden), and about half of
implementations were doc(hidden). This adds the attribute to the
remaining implementations.

Much like #72584.

As per #58957 there's no blocker for this, and I wanted to use this
today :-)

This commit adds `as_str` methods to `SplitWhitespace` and `SplitAsciiWhitespace`
str iterators. The methods return the remainder, similar to `as_str` methods on
`Chars` and other split iterators.

This commit also makes fields of some iterators `pub(crate)`.

Note that float methods in `core::intrinsics` weren't converted because
they are only defined in `std` (using language item hacks).

The `may_have_side_effect` is an implementation detail of `TrustedRandomAccess`
trait. It describes if obtaining an iterator element may have side effects. It
is currently implemented as an associated function.

Turn `may_have_side_effect` into an associated constant. This makes the
value immediately available to the optimizer.

Closes #72360.
Signed-off-by: NIan Jackson <ijackson@chiark.greenend.org.uk>

Allows getting the slices directly, rather than just through an iterator as in `array_chunks(_mut)`.  The constructors for those iterators are then written in terms of these methods, so the iterator constructors no longer have any `unsafe` of their own.

It's possible for method resolution to pick this method over a lower
priority stable method,  causing compilation errors. Since this method
is permanently unstable, give it a name that is very unlikely to be used
in user code.

Init false state in Split* constructors