Auto merge of #65933 - crgl:vec-deque-truncate, r=alexcrichton

Use ptr::drop_in_place for VecDeque::truncate and VecDeque::clear

This commit allows `VecDeque::truncate` to take advantage of its (largely) contiguous memory layout and is consistent with the change in #64432 for `Vec`. As with the change to `Vec::truncate`, this changes both:

- the drop order, from back-to-front to front-to-back
- the behavior when dropping an element panics

For consistency, it also changes the behavior when dropping an element panics for `VecDeque::clear`.

These changes in behavior can be observed. This example ([playground](https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=d0b1f2edc123437a2f704cbe8d93d828))
```rust
use std::collections::VecDeque;

fn main() {
    struct Bomb(usize);
    impl Drop for Bomb {
        fn drop(&mut self) {
            panic!(format!("{}", self.0));
        }
    }
    let mut v = VecDeque::from(vec![Bomb(0), Bomb(1)]);
    std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| {
        v.truncate(0);
    }));
    std::mem::forget(v);
}
```
panics printing `1` today and succeeds. `v.clear()` panics printing `0` today and succeeds. With the change, `v.clear()`, `v.truncate(0)`, and dropping the `VecDeque` all panic printing `0` first and then abort with a double-panic printing `1`.

The motivation for this was making `VecDeque::truncate` more efficient since it was used in the implementation of `VecDeque::clone_from` (#65069), but it also makes behavior more consistent within the `VecDeque` and with `Vec` if that change is accepted (this probably doesn't make sense to merge if not).

This might need a crater run and an FCP as well.

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.
@@ -855,8 +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);
+            }
         }
     }
 
@@ -1117,7 +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

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;