alloc: fix Debug implementation of ExtractIf

Author: joboet

library/alloc/src/vec/extract_if.rs

@@ -115,7 +115,20 @@ where
     A: Allocator,
 {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        let peek = if self.idx < self.end { self.vec.get(self.idx) } else { None };
+        let peek = if self.idx < self.end {
+            // This has to use pointer arithmetic as `self.vec[self.idx]` or
+            // `self.vec.get_unchecked(self.idx)` wouldn't work since we
+            // temporarily set the length of `self.vec` to zero.
+            //
+            // SAFETY:
+            // Since `self.idx` is smaller than `self.end` and `self.end` is
+            // smaller than `self.old_len`, `idx` is valid for indexing the
+            // buffer. Also, per the invariant of `self.idx`, this element
+            // has not been inspected/moved out yet.
+            Some(unsafe { &*self.vec.as_ptr().add(self.idx) })
+        } else {
+            None
+        };
         f.debug_struct("ExtractIf").field("peek", &peek).finish_non_exhaustive()
     }
 }

library/alloctests/tests/vec.rs

@@ -1635,6 +1635,17 @@ fn extract_if_unconsumed() {
     assert_eq!(vec, [1, 2, 3, 4]);
 }
 
+#[test]
+fn extract_if_debug() {
+    let mut vec = vec![1, 2];
+    let mut drain = vec.extract_if(.., |&mut x| x % 2 != 0);
+    assert!(format!("{drain:?}").contains("Some(1)"));
+    drain.next();
+    assert!(format!("{drain:?}").contains("Some(2)"));
+    drain.next();
+    assert!(format!("{drain:?}").contains("None"));
+}
+
 #[test]
 fn test_reserve_exact() {
     // This is all the same as test_reserve