Overload Iterator::last() for IntoIter.

[hbina]

Simply use the fact that IntoIter stores the end pointer and we
can perform pointer arithmetic.

NOTE: Where is the test for this?

[rust-highfive]

r? @joshtriplett

(rust_highfive has picked a reviewer for you, use r? to override)

[ecstatic-morse]

This is unsound. The vec::IntoIter will be dropped when the function returns, in turn dropping every element including the one you just copied into the return place. unsafe should be used with care and only when there are no viable alternatives.

[hbina]

I changed the implementation a bit copying what next() does except it marches to the end of the iterator immediately (which makes it similar to folding until the last element calling next() each time)

[ollie27]

This was attempted before in #57246 but as pointed out in #57246 (comment) this will change the drop order so may be considered a breaking change.

EDIT: looking at this closer the drop order should be the same so this should be fine.

[hbina]

I am quite new to raw Rust but AFAIK, but shouldn't the Drop order not be changed because the destructor is handled here by DropGuard

rust/library/alloc/src/vec.rs

Lines 2762 to 2781 in db534b3

	#[stable(feature = "rust1", since = "1.0.0")]
	unsafe impl<#[may_dangle] T> Drop for IntoIter<T> {
	fn drop(&mut self) {
	struct DropGuard<'a, T>(&'a mut IntoIter<T>);
	impl<T> Drop for DropGuard<'_, T> {
	fn drop(&mut self) {
	// RawVec handles deallocation
	let _ = unsafe { RawVec::from_raw_parts(self.0.buf.as_ptr(), self.0.cap) };
	}
	}
	let guard = DropGuard(self);
	// destroy the remaining elements
	unsafe {
	ptr::drop_in_place(guard.0.as_raw_mut_slice());
	}
	// now `guard` will be dropped and do the rest
	}
	}

I am not sure what this is for, tho

rust/library/alloc/src/vec.rs

Line 2777 in db534b3

ptr::drop_in_place(guard.0.as_raw_mut_slice());

[ollie27]

Yeah, the Drop impl for IntoIter drops the remaining items front to back which is the same order as you'd get from repeatedly calling .next() so in this case it does match.

[bors]

☔ The latest upstream changes (presumably #70793) made this pull request unmergeable. Please resolve the merge conflicts.

[Dylan-DPC-zz]

@hbina any updates on this?

[hbina]

@Dylan-DPC I have rebased to latest and accepted the change requested by @pickfire
Is there anything else you need me to do?

[ollie27]

As I mentioned in #76087 (comment) all last needs to do is call self.next_back().

[m-ou-se]

NOTE: Where is the test for this?

Just testing for functionality can be done in library/alloc/tests/vec.rs. If you also want to test that .last() generates more efficient code than iterating over the entire range, a codegen test can be added in src/test/codegen, but that's probably not worth the effort.

[Dylan-DPC-zz]

r? @KodrAus

[m-ou-se]

Does this make any difference in performance or binary size?

Both the previous and the proposed implementation end up iterating through all elements to Drop them.

[bors]

☔ The latest upstream changes (presumably #80530) made this pull request unmergeable. Please resolve the merge conflicts.