Fix double-drop in Vec::from_iter(vec.into_iter()) specialization when items drop during panic

library/alloc/src/vec/into_iter.rs

@@ -85,20 +85,29 @@ impl<T, A: Allocator> IntoIter<T, A> {
         ptr::slice_from_raw_parts_mut(self.ptr as *mut T, self.len())
     }
 
-    pub(super) fn drop_remaining(&mut self) {
-        unsafe {
-            ptr::drop_in_place(self.as_mut_slice());
-        }
-        self.ptr = self.end;
-    }
+    /// Drops remaining elements and relinquishes the backing allocation.
+    ///
+    /// This is roughly equivalent to the following, but more efficient
+    ///
+    /// ```
+    /// # let mut into_iter = Vec::<u8>::with_capacity(10).into_iter();
+    /// (&mut into_iter).for_each(core::mem::drop);
+    /// unsafe { core::ptr::write(&mut into_iter, Vec::new().into_iter()); }
+    /// ```
+    pub(super) fn forget_allocation_drop_remaining(&mut self) {
+        let remaining = self.as_raw_mut_slice();
 
-    /// Relinquishes the backing allocation, equivalent to
-    /// `ptr::write(&mut self, Vec::new().into_iter())`
-    pub(super) fn forget_allocation(&mut self) {
+        // overwrite the individual fields instead of creating a new
+        // struct and then overwriting &mut self.
+        // this creates less assembly
         self.cap = 0;
         self.buf = unsafe { NonNull::new_unchecked(RawVec::NEW.ptr()) };
         self.ptr = self.buf.as_ptr();
         self.end = self.buf.as_ptr();
+
+        unsafe {
+            ptr::drop_in_place(remaining);
+        }
     }
 }
 

library/alloc/src/vec/source_iter_marker.rs

@@ -69,9 +69,9 @@ where
         }
 
         // drop any remaining values at the tail of the source
-        src.drop_remaining();
         // but prevent drop of the allocation itself once IntoIter goes out of scope
-        src.forget_allocation();
+        // if the drop panics then we also leak any elements collected into dst_buf
+        src.forget_allocation_drop_remaining();
 
         let vec = unsafe { Vec::from_raw_parts(dst_buf, len, cap) };
 

library/alloc/tests/vec.rs

@@ -1027,7 +1027,7 @@ fn test_from_iter_specialization_head_tail_drop() {
 }
 
 #[test]
-fn test_from_iter_specialization_panic_drop() {
+fn test_from_iter_specialization_panic_during_iteration_drops() {
     let drop_count: Vec<_> = (0..=2).map(|_| Rc::new(())).collect();
     let src: Vec<_> = drop_count.iter().cloned().collect();
     let iter = src.into_iter();
@@ -1050,6 +1050,42 @@ fn test_from_iter_specialization_panic_drop() {
     );
 }
 
+#[test]
+fn test_from_iter_specialization_panic_during_drop_leaks() {
+    static mut DROP_COUNTER: usize = 0;
+
+    #[derive(Debug)]
+    enum Droppable {
+        DroppedTwice(Box<i32>),
+        PanicOnDrop,
+    }
+
+    impl Drop for Droppable {
+        fn drop(&mut self) {
+            match self {
+                Droppable::DroppedTwice(_) => {
+                    unsafe {
+                        DROP_COUNTER += 1;
+                    }
+                    println!("Dropping!")
+                }
+                Droppable::PanicOnDrop => {
+                    if !std::thread::panicking() {
+                        panic!();
+                    }
+                }
+            }
+        }
+    }
+
+    let _ = std::panic::catch_unwind(AssertUnwindSafe(|| {
+        let v = vec![Droppable::DroppedTwice(Box::new(123)), Droppable::PanicOnDrop];
+        let _ = v.into_iter().take(0).collect::<Vec<_>>();
+    }));
+
+    assert_eq!(unsafe { DROP_COUNTER }, 1);
+}
+
 #[test]
 fn test_cow_from() {
     let borrowed: &[_] = &["borrowed", "(slice)"];