Implement VecDeque::truncate_front()

library/alloc/src/collections/vec_deque/mod.rs

@@ -1188,6 +1188,73 @@ impl<T, A: Allocator> VecDeque<T, A> {
         }
     }
 
+    /// Shortens the deque, keeping the last `len` elements and dropping
+    /// the rest.
+    ///
+    /// If `len` is greater or equal to the deque's current length, this has
+    /// no effect.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # #![feature(vec_deque_truncate_front)]
+    /// use std::collections::VecDeque;
+    ///
+    /// let mut buf = VecDeque::new();
+    /// buf.push_front(5);
+    /// buf.push_front(10);
+    /// buf.push_front(15);
+    /// assert_eq!(buf, [15, 10, 5]);
+    /// assert_eq!(buf.as_slices(), (&[15, 10, 5][..], &[][..]));
+    /// buf.truncate_front(1);
+    /// assert_eq!(buf.as_slices(), (&[5][..], &[][..]));
+    /// ```
+    #[unstable(feature = "vec_deque_truncate_front", issue = "140667")]
+    pub fn truncate_front(&mut self, len: usize) {
+        /// Runs the destructor for all items in the slice when it gets dropped (normally or
+        /// during unwinding).
+        struct Dropper<'a, T>(&'a mut [T]);
+
+        impl<'a, T> Drop for Dropper<'a, T> {
+            fn drop(&mut self) {
+                unsafe {
+                    ptr::drop_in_place(self.0);
+                }
+            }
+        }
+
+        unsafe {
+            if len >= self.len {
+                // No action is taken
+                return;
+            }
+
+            let (front, back) = self.as_mut_slices();
+            if len > back.len() {
+                // The 'back' slice remains unchanged.
+                // front.len() + back.len() == self.len, so 'end' is non-negative
+                // and end < front.len()
+                let end = front.len() - (len - back.len());
+                let drop_front = front.get_unchecked_mut(..end) as *mut _;
+                self.head += end;
+                self.len = len;
+                ptr::drop_in_place(drop_front);
+            } else {
+                let drop_front = front as *mut _;
+                // 'end' is non-negative by the condition above
+                let end = back.len() - len;
+                let drop_back = back.get_unchecked_mut(..end) as *mut _;
+                self.head = self.to_physical_idx(self.len - len);
+                self.len = len;
+
+                // Make sure the second half is dropped even when a destructor
+                // in the first one panics.
+                let _back_dropper = Dropper(&mut *drop_back);
+                ptr::drop_in_place(drop_front);
+            }
+        }
+    }
+
     /// Returns a reference to the underlying allocator.
     #[unstable(feature = "allocator_api", issue = "32838")]
     #[inline]

library/alloctests/tests/lib.rs

@@ -37,6 +37,7 @@
 #![feature(str_as_str)]
 #![feature(strict_provenance_lints)]
 #![feature(vec_deque_pop_if)]
+#![feature(vec_deque_truncate_front)]
 #![feature(unique_rc_arc)]
 #![feature(macro_metavar_expr_concat)]
 #![allow(internal_features)]

library/alloctests/tests/vec_deque.rs

@@ -1686,6 +1686,40 @@ fn truncate_leak() {
     assert_eq!(unsafe { DROPS }, 7);
 }
 
+#[test]
+#[cfg_attr(not(panic = "unwind"), ignore = "test requires unwinding support")]
+fn truncate_front_leak() {
+    static mut DROPS: i32 = 0;
+
+    struct D(bool);
+
+    impl Drop for D {
+        fn drop(&mut self) {
+            unsafe {
+                DROPS += 1;
+            }
+
+            if self.0 {
+                panic!("panic in `drop`");
+            }
+        }
+    }
+
+    let mut q = VecDeque::new();
+    q.push_back(D(false));
+    q.push_back(D(false));
+    q.push_back(D(false));
+    q.push_back(D(false));
+    q.push_back(D(false));
+    q.push_front(D(true));
+    q.push_front(D(false));
+    q.push_front(D(false));
+
+    catch_unwind(AssertUnwindSafe(|| q.truncate_front(1))).ok();
+
+    assert_eq!(unsafe { DROPS }, 7);
+}
+
 #[test]
 #[cfg_attr(not(panic = "unwind"), ignore = "test requires unwinding support")]
 fn test_drain_leak() {
@@ -1863,3 +1897,38 @@ fn test_collect_from_into_iter_keeps_allocation() {
         assert_eq!(v.capacity(), 13);
     }
 }
+
+#[test]
+fn test_truncate_front() {
+    let mut v = VecDeque::with_capacity(13);
+    v.extend(0..7);
+    assert_eq!(v.as_slices(), ([0, 1, 2, 3, 4, 5, 6].as_slice(), [].as_slice()));
+    v.truncate_front(10);
+    assert_eq!(v.len(), 7);
+    assert_eq!(v.as_slices(), ([0, 1, 2, 3, 4, 5, 6].as_slice(), [].as_slice()));
+    v.truncate_front(7);
+    assert_eq!(v.len(), 7);
+    assert_eq!(v.as_slices(), ([0, 1, 2, 3, 4, 5, 6].as_slice(), [].as_slice()));
+    v.truncate_front(3);
+    assert_eq!(v.as_slices(), ([4, 5, 6].as_slice(), [].as_slice()));
+    assert_eq!(v.len(), 3);
+    v.truncate_front(0);
+    assert_eq!(v.as_slices(), ([].as_slice(), [].as_slice()));
+    assert_eq!(v.len(), 0);
+
+    v.clear();
+    v.extend(0..7);
+    assert_eq!(v.as_slices(), ([0, 1, 2, 3, 4, 5, 6].as_slice(), [].as_slice()));
+    v.push_front(9);
+    v.push_front(8);
+    v.push_front(7);
+    assert_eq!(v.as_slices(), ([7, 8, 9].as_slice(), [0, 1, 2, 3, 4, 5, 6].as_slice()));
+    v.truncate_front(12);
+    assert_eq!(v.as_slices(), ([7, 8, 9].as_slice(), [0, 1, 2, 3, 4, 5, 6].as_slice()));
+    v.truncate_front(10);
+    assert_eq!(v.as_slices(), ([7, 8, 9].as_slice(), [0, 1, 2, 3, 4, 5, 6].as_slice()));
+    v.truncate_front(8);
+    assert_eq!(v.as_slices(), ([9].as_slice(), [0, 1, 2, 3, 4, 5, 6].as_slice()));
+    v.truncate_front(5);
+    assert_eq!(v.as_slices(), ([2, 3, 4, 5, 6].as_slice(), [].as_slice()));
+}