Rollup merge of #82292 - SkiFire13:fix-issue-82291, r=m-ou-se

Prevent specialized ZipImpl from calling `__iterator_get_unchecked` twice with the same index

Fixes #82291

It's open for review, but conflicts with #82289, wait before merging. The conflict involves only the new test, so it should be rather trivial to fix.

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

@@ -13,9 +13,10 @@
 pub struct Zip<A, B> {
     a: A,
     b: B,
-    // index and len are only used by the specialized version of zip
+    // index, len and a_len are only used by the specialized version of zip
     index: usize,
     len: usize,
+    a_len: usize,
 }
 impl<A: Iterator, B: Iterator> Zip<A, B> {
     pub(in crate::iter) fn new(a: A, b: B) -> Zip<A, B> {
@@ -110,6 +111,7 @@ impl<A, B> ZipImpl<A, B> for Zip<A, B>
             b,
             index: 0, // unused
             len: 0,   // unused
+            a_len: 0, // unused
         }
     }
 
@@ -184,8 +186,9 @@ impl<A, B> ZipImpl<A, B> for Zip<A, B>
     B: TrustedRandomAccess + Iterator,
 {
     fn new(a: A, b: B) -> Self {
-        let len = cmp::min(a.size(), b.size());
-        Zip { a, b, index: 0, len }
+        let a_len = a.size();
+        let len = cmp::min(a_len, b.size());
+        Zip { a, b, index: 0, len, a_len }
     }
 
     #[inline]
@@ -197,7 +200,7 @@ fn next(&mut self) -> Option<(A::Item, B::Item)> {
             unsafe {
                 Some((self.a.__iterator_get_unchecked(i), self.b.__iterator_get_unchecked(i)))
             }
-        } else if A::MAY_HAVE_SIDE_EFFECT && self.index < self.a.size() {
+        } else if A::MAY_HAVE_SIDE_EFFECT && self.index < self.a_len {
             let i = self.index;
             self.index += 1;
             self.len += 1;
@@ -262,6 +265,7 @@ fn next_back(&mut self) -> Option<(A::Item, B::Item)>
                     for _ in 0..sz_a - self.len {
                         self.a.next_back();
                     }
+                    self.a_len = self.len;
                 }
                 let sz_b = self.b.size();
                 if B::MAY_HAVE_SIDE_EFFECT && sz_b > self.len {
@@ -273,6 +277,7 @@ fn next_back(&mut self) -> Option<(A::Item, B::Item)>
         }
         if self.index < self.len {
             self.len -= 1;
+            self.a_len -= 1;
             let i = self.len;
             // SAFETY: `i` is smaller than the previous value of `self.len`,
             // which is also smaller than or equal to `self.a.len()` and `self.b.len()`

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

@@ -265,3 +265,26 @@ fn overflowed_zip(arr: &[i32]) -> impl Iterator<Item = (i32, &())> {
         panic!();
     }
 }
+
+#[test]
+fn test_issue_82291() {
+    use std::cell::Cell;
+
+    let mut v1 = [()];
+    let v2 = [()];
+
+    let called = Cell::new(0);
+
+    let mut zip = v1
+        .iter_mut()
+        .map(|r| {
+            called.set(called.get() + 1);
+            r
+        })
+        .zip(&v2);
+
+    zip.next_back();
+    assert_eq!(called.get(), 1);
+    zip.next();
+    assert_eq!(called.get(), 1);
+}