From cdb1a799f8793c4c858abc05b437fa2e569f51e6 Mon Sep 17 00:00:00 2001
From: Scott McMurray <scottmcm@users.noreply.github.com>
Date: Sat, 17 Nov 2018 00:31:34 -0800
Subject: [PATCH] Add VecDeque::resize_with

---
 src/liballoc/collections/vec_deque.rs | 40 ++++++++++++++++++++++++++-
 1 file changed, 39 insertions(+), 1 deletion(-)

diff --git a/src/liballoc/collections/vec_deque.rs b/src/liballoc/collections/vec_deque.rs
index 88e76033f273e..cbf104a8fcde9 100644
--- a/src/liballoc/collections/vec_deque.rs
+++ b/src/liballoc/collections/vec_deque.rs
@@ -19,7 +19,7 @@
 
 use core::cmp::Ordering;
 use core::fmt;
-use core::iter::{repeat, FromIterator, FusedIterator};
+use core::iter::{repeat, repeat_with, FromIterator, FusedIterator};
 use core::mem;
 use core::ops::Bound::{Excluded, Included, Unbounded};
 use core::ops::{Index, IndexMut, RangeBounds};
@@ -1920,6 +1920,44 @@ impl<T: Clone> VecDeque<T> {
             self.truncate(new_len);
         }
     }
+
+    /// Modifies the `VecDeque` in-place so that `len()` is equal to `new_len`,
+    /// either by removing excess elements from the back or by appending
+    /// elements generated by calling `generator` to the back.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(vec_resize_with)]
+    ///
+    /// use std::collections::VecDeque;
+    ///
+    /// let mut buf = VecDeque::new();
+    /// buf.push_back(5);
+    /// buf.push_back(10);
+    /// buf.push_back(15);
+    /// assert_eq!(buf, [5, 10, 15]);
+    ///
+    /// buf.resize_with(5, Default::default);
+    /// assert_eq!(buf, [5, 10, 15, 0, 0]);
+    ///
+    /// buf.resize_with(2, || unreachable!());
+    /// assert_eq!(buf, [5, 10]);
+    ///
+    /// let mut state = 100;
+    /// buf.resize_with(5, || { state += 1; state });
+    /// assert_eq!(buf, [5, 10, 101, 102, 103]);
+    /// ```
+    #[unstable(feature = "vec_resize_with", issue = "41758")]
+    pub fn resize_with(&mut self, new_len: usize, generator: impl FnMut()->T) {
+        let len = self.len();
+
+        if new_len > len {
+            self.extend(repeat_with(generator).take(new_len - len))
+        } else {
+            self.truncate(new_len);
+        }
+    }
 }
 
 /// Returns the index in the underlying buffer for a given logical element index.