Rollup merge of #123107 - avandesa:vec_pop_if, r=joboet

Implement `Vec::pop_if`

This PR adds `Vec::pop_if` to the public API, behind the `vec_pop_if` feature.

```rust
impl<T> Vec<T> {
    pub fn pop_if<F>(&mut self, f: F) -> Option<T>
        where F: FnOnce(&mut T) -> bool;
}
```

Tracking issue: #122741

## Open questions

- [ ] Should the first unit test be split up?
- [ ] I don't see any guidance on ordering of methods in impl blocks, should I move the method elsewhere?

1 files changed, 25 insertions, 0 deletions

diff --git a/library/alloc/src/vec/mod.rs b/library/alloc/src/vec/mod.rs
index 3062f8664b7..8ca8046dac5 100644
--- a/library/alloc/src/vec/mod.rs
+++ b/library/alloc/src/vec/mod.rs
@@ -2058,6 +2058,31 @@ impl<T, A: Allocator> Vec<T, A> {
         }
     }
 
+    /// Removes and returns the last element in a vector if the predicate
+    /// returns `true`, or [`None`] if the predicate returns false or the vector
+    /// is empty.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(vec_pop_if)]
+    ///
+    /// let mut vec = vec![1, 2, 3, 4];
+    /// let pred = |x: &mut i32| *x % 2 == 0;
+    ///
+    /// assert_eq!(vec.pop_if(pred), Some(4));
+    /// assert_eq!(vec, [1, 2, 3]);
+    /// assert_eq!(vec.pop_if(pred), None);
+    /// ```
+    #[unstable(feature = "vec_pop_if", issue = "122741")]
+    pub fn pop_if<F>(&mut self, f: F) -> Option<T>
+    where
+        F: FnOnce(&mut T) -> bool,
+    {
+        let last = self.last_mut()?;
+        if f(last) { self.pop() } else { None }
+    }
+
     /// Moves all the elements of `other` into `self`, leaving `other` empty.
     ///
     /// # Panics