From 18c5f4e7f27aa3180d2b21f8383f3b0ba4f2bc73 Mon Sep 17 00:00:00 2001
From: Charles Gleason <charles_gleason@alumni.brown.edu>
Date: Mon, 28 Oct 2019 17:52:50 -0400
Subject: [PATCH 1/4] Match docs for VecDeque truncate to Vec truncate

---
 src/liballoc/collections/vec_deque.rs | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/src/liballoc/collections/vec_deque.rs b/src/liballoc/collections/vec_deque.rs
index 8f3dfabd8886d..61a2f620d295c 100644
--- a/src/liballoc/collections/vec_deque.rs
+++ b/src/liballoc/collections/vec_deque.rs
@@ -835,7 +835,8 @@ impl<T> VecDeque<T> {
         }
     }
 
-    /// Shortens the `VecDeque`, dropping excess elements from the back.
+    /// Shortens the `VecDeque`, keeping the first `len` elements and dropping
+    /// the rest.
     ///
     /// If `len` is greater than the `VecDeque`'s current length, this has no
     /// effect.

From aa893535c4232176c0c05e4e8fb8c85a2a855a1f Mon Sep 17 00:00:00 2001
From: Charles Gleason <charles_gleason@alumni.brown.edu>
Date: Mon, 28 Oct 2019 17:53:03 -0400
Subject: [PATCH 2/4] Use ptr::drop_in_place in VecDeque truncate

---
 src/liballoc/collections/vec_deque.rs | 27 +++++++++++++++++++++++++--
 1 file changed, 25 insertions(+), 2 deletions(-)

diff --git a/src/liballoc/collections/vec_deque.rs b/src/liballoc/collections/vec_deque.rs
index 61a2f620d295c..fffd3a9dedae9 100644
--- a/src/liballoc/collections/vec_deque.rs
+++ b/src/liballoc/collections/vec_deque.rs
@@ -856,8 +856,31 @@ impl<T> VecDeque<T> {
     /// ```
     #[stable(feature = "deque_extras", since = "1.16.0")]
     pub fn truncate(&mut self, len: usize) {
-        for _ in len..self.len() {
-            self.pop_back();
+        // Safe because:
+        //
+        // * Any slice passed to `drop_in_place` is valid; the second case has
+        //   `len <= front.len()` and returning on `len > self.len()` ensures
+        //   `begin <= back.len()` in the first case
+        // * The head of the VecDeque is moved before calling `drop_in_place`,
+        //   so no value is dropped twice if `drop_in_place` panics
+        unsafe {
+            if len > self.len() {
+                return;
+            }
+            let num_dropped = self.len() - len;
+            let (front, back) = self.as_mut_slices();
+            if len > front.len() {
+                let begin = len - front.len();
+                let drop_back = back.get_unchecked_mut(begin..) as *mut _;
+                self.head = self.wrap_sub(self.head, num_dropped);
+                ptr::drop_in_place(drop_back);
+            } else {
+                let drop_back = back as *mut _;
+                let drop_front = front.get_unchecked_mut(len..) as *mut _;
+                self.head = self.wrap_sub(self.head, num_dropped);
+                ptr::drop_in_place(drop_front);
+                ptr::drop_in_place(drop_back);
+            }
         }
     }
 

From 71191bc3e9c592e6f52a646414fd9e337e3f3407 Mon Sep 17 00:00:00 2001
From: Charles Gleason <charles_gleason@alumni.brown.edu>
Date: Mon, 28 Oct 2019 18:50:07 -0400
Subject: [PATCH 3/4] Add test for VecDeque truncate

---
 src/liballoc/collections/vec_deque/tests.rs | 35 +++++++++++++++++++++
 1 file changed, 35 insertions(+)

diff --git a/src/liballoc/collections/vec_deque/tests.rs b/src/liballoc/collections/vec_deque/tests.rs
index d578ee0dac493..09009ff516ac8 100644
--- a/src/liballoc/collections/vec_deque/tests.rs
+++ b/src/liballoc/collections/vec_deque/tests.rs
@@ -384,6 +384,41 @@ fn test_clone_from() {
     }
 }
 
+#[test]
+fn test_vec_deque_truncate_drop() {
+    static mut DROPS: u32 = 0;
+    #[derive(Clone)]
+    struct Elem(i32);
+    impl Drop for Elem {
+        fn drop(&mut self) {
+            unsafe {
+                DROPS += 1;
+            }
+        }
+    }
+
+    let v = vec![Elem(1), Elem(2), Elem(3), Elem(4), Elem(5)];
+    for push_front in 0..=v.len() {
+        let v = v.clone();
+        let mut tester = VecDeque::with_capacity(5);
+        for (index, elem) in v.into_iter().enumerate() {
+            if index < push_front {
+                tester.push_front(elem);
+            } else {
+                tester.push_back(elem);
+            }
+        }
+        assert_eq!(unsafe { DROPS }, 0);
+        tester.truncate(3);
+        assert_eq!(unsafe { DROPS }, 2);
+        tester.truncate(0);
+        assert_eq!(unsafe { DROPS }, 5);
+        unsafe {
+            DROPS = 0;
+        }
+    }
+}
+
 #[test]
 fn issue_53529() {
     use crate::boxed::Box;

From 27e0ab578cc0fc4c72da54bbeb42c0c44d848207 Mon Sep 17 00:00:00 2001
From: Charles Gleason <charles_gleason@alumni.brown.edu>
Date: Tue, 29 Oct 2019 10:45:54 -0400
Subject: [PATCH 4/4] Use truncate(0) in VecDeque clear

---
 src/liballoc/collections/vec_deque.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/liballoc/collections/vec_deque.rs b/src/liballoc/collections/vec_deque.rs
index fffd3a9dedae9..7795083e0580c 100644
--- a/src/liballoc/collections/vec_deque.rs
+++ b/src/liballoc/collections/vec_deque.rs
@@ -1141,7 +1141,7 @@ impl<T> VecDeque<T> {
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
     pub fn clear(&mut self) {
-        self.drain(..);
+        self.truncate(0);
     }
 
     /// Returns `true` if the `VecDeque` contains an element equal to the