More IS_ZST in library

[scottmcm]

I noticed that post_inc_start and pre_dec_end were doing this check in different ways

rust/library/core/src/slice/iter/macros.rs

Lines 76 to 93 in d19b64f

	if mem::size_of::<T>() == 0 {
	zst_shrink!(self, offset);
	self.ptr.as_ptr()
	} else {
	let old = self.ptr.as_ptr();
	// SAFETY: the caller guarantees that `offset` doesn't exceed `self.len()`,
	// so this new pointer is inside `self` and thus guaranteed to be non-null.
	self.ptr = unsafe { NonNull::new_unchecked(self.ptr.as_ptr().add(offset)) };
	old
	}
	}
	// Helper function for moving the end of the iterator backwards by `offset` elements,
	// returning the new end.
	// Unsafe because the offset must not exceed `self.len()`.
	#[inline(always)]
	unsafe fn pre_dec_end(&mut self, offset: usize) -> * $raw_mut T {
	if T::IS_ZST {

so started making this PR, then added a few more I found since I was already making changes anyway.

[rustbot]

r? @Mark-Simulacrum

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[the8472]

@bors r+ rollup

[bors]

📌 Commit 56613f8 has been approved by the8472

It is now in the queue for this repository.

[scottmcm]

Oops, pushed commits to the wrong branch, sorry.

I've forced it back to the one in the bors approval.