Auto merge of #96350 - austinabell:skip_optimization, r=Mark-Simulacrum

fix(iter::skip): Optimize `next` and `nth` implementations of `Skip`

This avoids calling nth/next or nth/nth to first skip elements and then get the next one (unless necessary due to usize overflow).

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

@@ -33,21 +33,32 @@ 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))
+        } else {
+            self.iter.next()
         }
-        self.iter.next()
     }
 
     #[inline]
     fn nth(&mut self, n: usize) -> Option<I::Item> {
-        // Can't just add n + self.n due to overflow.
         if self.n > 0 {
-            let to_skip = self.n;
-            self.n = 0;
-            // nth(n) skips n+1
-            self.iter.nth(to_skip - 1)?;
+            let skip: usize = crate::mem::take(&mut self.n);
+            // Checked add to handle overflow case.
+            let n = match skip.checked_add(n) {
+                Some(nth) => nth,
+                None => {
+                    // In case of overflow, load skip value, before loading `n`.
+                    // Because the amount of elements to iterate is beyond `usize::MAX`, this
+                    // is split into two `nth` calls where the `skip` `nth` call is discarded.
+                    self.iter.nth(skip - 1)?;
+                    n
+                }
+            };
+            // Load nth element including skip.
+            self.iter.nth(n)
+        } else {
+            self.iter.nth(n)
         }
-        self.iter.nth(n)
     }
 
     #[inline]

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

@@ -201,3 +201,34 @@ fn test_skip_non_fused() {
     // advance it further. `Unfuse` tests that this doesn't happen by panicking in that scenario.
     let _ = non_fused.skip(20).next();
 }
+
+#[test]
+fn test_skip_non_fused_nth_overflow() {
+    let non_fused = Unfuse::new(0..10);
+
+    // Ensures that calling skip and `nth` where the sum would overflow does not fail for non-fused
+    // iterators.
+    let _ = non_fused.skip(20).nth(usize::MAX);
+}
+
+#[test]
+fn test_skip_overflow_wrapping() {
+    // Test to ensure even on overflowing on `skip+nth` the correct amount of elements are yielded.
+    struct WrappingIterator(usize);
+
+    impl Iterator for WrappingIterator {
+        type Item = usize;
+
+        fn next(&mut self) -> core::option::Option<Self::Item> {
+            <Self as Iterator>::nth(self, 0)
+        }
+
+        fn nth(&mut self, nth: usize) -> core::option::Option<Self::Item> {
+            self.0 = self.0.wrapping_add(nth.wrapping_add(1));
+            Some(self.0)
+        }
+    }
+
+    let wrap = WrappingIterator(0);
+    assert_eq!(wrap.skip(20).nth(usize::MAX), Some(20));
+}