Implement VecDeque::pop_front_if & VecDeque::pop_back_if

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

@@ -1735,6 +1735,52 @@ impl<T, A: Allocator> VecDeque<T, A> {
         }
     }
 
+    /// Removes and returns the first element from the deque if the predicate
+    /// returns `true`, or [`None`] if the predicate returns false or the deque
+    /// is empty (the predicate will not be called in that case).
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(vec_deque_pop_if)]
+    /// use std::collections::VecDeque;
+    ///
+    /// let mut deque: VecDeque<i32> = vec![0, 1, 2, 3, 4].into();
+    /// let pred = |x: &mut i32| *x % 2 == 0;
+    ///
+    /// assert_eq!(deque.pop_front_if(pred), Some(0));
+    /// assert_eq!(deque, [1, 2, 3, 4]);
+    /// assert_eq!(deque.pop_front_if(pred), None);
+    /// ```
+    #[unstable(feature = "vec_deque_pop_if", issue = "135889")]
+    pub fn pop_front_if(&mut self, predicate: impl FnOnce(&mut T) -> bool) -> Option<T> {
+        let first = self.front_mut()?;
+        if predicate(first) { self.pop_front() } else { None }
+    }
+
+    /// Removes and returns the last element from the deque if the predicate
+    /// returns `true`, or [`None`] if the predicate returns false or the deque
+    /// is empty (the predicate will not be called in that case).
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(vec_deque_pop_if)]
+    /// use std::collections::VecDeque;
+    ///
+    /// let mut deque: VecDeque<i32> = vec![0, 1, 2, 3, 4].into();
+    /// let pred = |x: &mut i32| *x % 2 == 0;
+    ///
+    /// assert_eq!(deque.pop_back_if(pred), Some(4));
+    /// assert_eq!(deque, [0, 1, 2, 3]);
+    /// assert_eq!(deque.pop_back_if(pred), None);
+    /// ```
+    #[unstable(feature = "vec_deque_pop_if", issue = "135889")]
+    pub fn pop_back_if(&mut self, predicate: impl FnOnce(&mut T) -> bool) -> Option<T> {
+        let first = self.back_mut()?;
+        if predicate(first) { self.pop_back() } else { None }
+    }
+
     /// Prepends an element to the deque.
     ///
     /// # Examples

library/alloc/tests/lib.rs

@@ -38,6 +38,7 @@
 #![feature(str_as_str)]
 #![feature(strict_provenance_lints)]
 #![feature(vec_pop_if)]
+#![feature(vec_deque_pop_if)]
 #![feature(unique_rc_arc)]
 #![feature(macro_metavar_expr_concat)]
 #![allow(internal_features)]

library/alloc/tests/vec_deque.rs

@@ -80,6 +80,45 @@ fn test_parameterized<T: Clone + PartialEq + Debug>(a: T, b: T, c: T, d: T) {
     assert_eq!(deq[3].clone(), d.clone());
 }
 
+#[test]
+fn test_pop_if() {
+    let mut deq: VecDeque<_> = vec![0, 1, 2, 3, 4].into();
+    let pred = |x: &mut i32| *x % 2 == 0;
+
+    assert_eq!(deq.pop_front_if(pred), Some(0));
+    assert_eq!(deq, [1, 2, 3, 4]);
+
+    assert_eq!(deq.pop_front_if(pred), None);
+    assert_eq!(deq, [1, 2, 3, 4]);
+
+    assert_eq!(deq.pop_back_if(pred), Some(4));
+    assert_eq!(deq, [1, 2, 3]);
+
+    assert_eq!(deq.pop_back_if(pred), None);
+    assert_eq!(deq, [1, 2, 3]);
+}
+
+#[test]
+fn test_pop_if_empty() {
+    let mut deq = VecDeque::<i32>::new();
+    assert_eq!(deq.pop_front_if(|_| true), None);
+    assert_eq!(deq.pop_back_if(|_| true), None);
+    assert!(deq.is_empty());
+}
+
+#[test]
+fn test_pop_if_mutates() {
+    let mut v: VecDeque<_> = vec![-1, 1].into();
+    let pred = |x: &mut i32| {
+        *x *= 2;
+        false
+    };
+    assert_eq!(v.pop_front_if(pred), None);
+    assert_eq!(v, [-2, 1]);
+    assert_eq!(v.pop_back_if(pred), None);
+    assert_eq!(v, [-2, 2]);
+}
+
 #[test]
 fn test_push_front_grow() {
     let mut deq = VecDeque::new();