`std::iter::Skip::count` can overflow even if the iterator would return less that `usize::max_value()` items

[ollie27]

std::iter::Skip::count currently calls count on the inner iterator but that can overflow even if the Skip itself doesn't return more that usize::max_value() items.

For example:

(0..usize::max_value()).chain(0..10).skip(usize::max_value()).count()

This should return 10 but currently triggers an attempt to add with overflow (or returns 0 if overflow checks are disabled).

This is caused by

rust/src/libcore/iter/adapters/mod.rs

Lines 1817 to 1820 in 1389494

	#[inline]
	fn count(self) -> usize {
	self.iter.count().saturating_sub(self.n)
	}

which was added in #25035 which was merged between versions 1.0 and 1.1.

[clarfonthey]

This could probably be fixed by calling self.iter.nth(self.n.saturating_sub(1)) and then relying on self.iter.count().

[JohnTitor]

@clarfon I think it doesn't fix because nth(usize::max_value() - 1) is very slow, right?

[clarfonthey]

Should be overridden for ranges though.