slice: Remove some uses of unsafe in first/last chunk methods

[okaneco]

Remove unsafe split_at_unchecked and split_at_mut_unchecked in some slice split_first_chunk/split_last_chunk methods.
Replace those calls with the safe split_at and split_at_checked where applicable.

Add codegen tests to check for no panics when calculating the last chunk index using checked_sub and split_at.

Better viewed with whitespace disabled in diff view

---

The unchecked calls are mostly manual implementations of the safe methods, but with the safety condition negated from mid <= len to len < mid.

if self.len() < N {
    None
} else {
    // SAFETY: We manually verified the bounds of the split.
    let (first, tail) = unsafe { self.split_at_unchecked(N) };
    // Or for the last_chunk methods
    let (init, last) = unsafe { self.split_at_unchecked(self.len() - N) };

Unsafe is still needed for the pointer array casts. Their safety comments are unmodified.

[rustbot]

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[Amanieu]

The code for all of these can be made more concise by using ?:

Suggested change

	let Some((first, tail)) = self.split_at_checked(N) else { return None };
	let (first, tail) = self.split_at_checked(N)?;

[okaneco]

These functions are const fn so I don't believe ? is available to use.

That results in the following error:

error[E0015]: `?` is not allowed on `Option<(&[T], &[T])>` in constant functions
   --> library\core\src\slice\mod.rs:385:29
    |
385 |         let (first, tail) = self.split_at_checked(N)?;
    |                             ^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants

[Amanieu]

Good point! I'd forgotten about that.

[Amanieu]

@bors r+

[bors]

📌 Commit 59ca767 has been approved by Amanieu

It is now in the queue for this repository.