From 3834e7b7393bf1a0d7df02ccd1d2e896c1465769 Mon Sep 17 00:00:00 2001
From: The8472 <git@infinite-source.de>
Date: Mon, 29 Mar 2021 04:22:34 +0200
Subject: [PATCH 1/2] add testcase for double-drop during Vec in-place
 collection

---
 library/alloc/tests/vec.rs | 38 +++++++++++++++++++++++++++++++++++++-
 1 file changed, 37 insertions(+), 1 deletion(-)

diff --git a/library/alloc/tests/vec.rs b/library/alloc/tests/vec.rs
index 5c7ff67bc621..4cdb7eefcdf1 100644
--- a/library/alloc/tests/vec.rs
+++ b/library/alloc/tests/vec.rs
@@ -954,7 +954,7 @@ 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();
@@ -977,6 +977,42 @@ 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)"];
-- 
2.31.1


From 8e2706343e1ce1c5a2d3a2ceaaaa010aaeb21d93 Mon Sep 17 00:00:00 2001
From: The8472 <git@infinite-source.de>
Date: Mon, 29 Mar 2021 04:22:48 +0200
Subject: [PATCH 2/2] fix double-drop in in-place collect specialization

---
 library/alloc/src/vec/into_iter.rs          | 27 ++++++++++++++-------
 library/alloc/src/vec/source_iter_marker.rs |  4 +--
 2 files changed, 20 insertions(+), 11 deletions(-)

diff --git a/library/alloc/src/vec/into_iter.rs b/library/alloc/src/vec/into_iter.rs
index f131d06bb18f..74adced53f6d 100644
--- a/library/alloc/src/vec/into_iter.rs
+++ b/library/alloc/src/vec/into_iter.rs
@@ -85,20 +85,29 @@ fn as_raw_mut_slice(&mut self) -> *mut [T] {
         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);
+        }
     }
 }
 
diff --git a/library/alloc/src/vec/source_iter_marker.rs b/library/alloc/src/vec/source_iter_marker.rs
index 8c0e95559fa1..9301f7a5184e 100644
--- a/library/alloc/src/vec/source_iter_marker.rs
+++ b/library/alloc/src/vec/source_iter_marker.rs
@@ -78,9 +78,9 @@ impl<T, I> SpecFromIter<T, I> for Vec<T>
         }
 
         // 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 {
             let len = dst.offset_from(dst_buf) as usize;
-- 
2.31.1