Rollup merge of #99434 - timvermeulen:skip_next_non_fused, r=scottmcm

Fix `Skip::next` for non-fused inner iterators

`iter.skip(n).next()` will currently call `nth` and `next` in succession on `iter`, without checking whether `nth` exhausts the iterator. Using `?` to propagate a `None` value returned by `nth` avoids this.

library/core/src/iter/adapters/skip.rs

@@ -33,7 +33,7 @@ where
     #[inline]
     fn next(&mut self) -> Option<I::Item> {
         if unlikely(self.n > 0) {
-            self.iter.nth(crate::mem::take(&mut self.n) - 1);
+            self.iter.nth(crate::mem::take(&mut self.n) - 1)?;
         }
         self.iter.next()
     }

library/core/tests/iter/adapters/skip.rs

@@ -1,5 +1,7 @@
 use core::iter::*;
 
+use super::Unfuse;
+
 #[test]
 fn test_iterator_skip() {
     let xs = [0, 1, 2, 3, 5, 13, 15, 16, 17, 19, 20, 30];
@@ -190,3 +192,12 @@ fn test_skip_nth_back() {
     it.by_ref().skip(2).nth_back(10);
     assert_eq!(it.next_back(), Some(&1));
 }
+
+#[test]
+fn test_skip_non_fused() {
+    let non_fused = Unfuse::new(0..10);
+
+    // `Skip` would previously exhaust the iterator in this `next` call and then erroneously try to
+    // advance it further. `Unfuse` tests that this doesn't happen by panicking in that scenario.
+    let _ = non_fused.skip(20).next();
+}