[PATCH 2/5] rust: alloc: add Vec::pop

[Alice Ryhl <aliceryhl@google.com>]

This introduces a basic method that our custom Vec is missing. I expect
that it will be used in many places, but at the time of writing, Rust
Binder has six calls to Vec::pop.

Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
 rust/kernel/alloc/kvec.rs | 31 +++++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

diff --git a/rust/kernel/alloc/kvec.rs b/rust/kernel/alloc/kvec.rs
index 95e752ed27395fce72d372976b74fb1b0e957194..9943358c70aa63f5ad7ed9782cb8879d7a80a8fb 100644
--- a/rust/kernel/alloc/kvec.rs
+++ b/rust/kernel/alloc/kvec.rs
@@ -302,6 +302,37 @@ pub fn push(&mut self, v: T, flags: Flags) -> Result<(), AllocError> {
         Ok(())
     }
 
+    /// Removes the last element from a vector and returns it, or `None` if it is empty.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let mut v = KVec::new();
+    /// v.push(1, GFP_KERNEL)?;
+    /// v.push(2, GFP_KERNEL)?;
+    /// assert_eq!(&v, &[1, 2]);
+    ///
+    /// assert_eq!(v.pop(), Some(2));
+    /// assert_eq!(v.pop(), Some(1));
+    /// assert_eq!(v.pop(), None);
+    /// # Ok::<(), Error>(())
+    /// ```
+    pub fn pop(&mut self) -> Option<T> {
+        let Some(len_sub_1) = self.len.checked_sub(1) else {
+            return None;
+        };
+
+        // INVARIANT: If the first `len` elements are valid, then the first `len-1` elements are
+        // valid.
+        self.len = len_sub_1;
+
+        // INVARIANT: This invalidates a value in this vector's allocation, but the Vec invariants
+        // do not require it to be valid because `self.len <= len_sub_1`.
+        // SAFETY: Since `len_sub_1` is less than the value `self.len` had at the beginning of
+        // `pop`, this index holds a valid value.
+        Some(unsafe { self.as_mut_ptr().add(len_sub_1).read() })
+    }
+
     /// Creates a new [`Vec`] instance with at least the given capacity.
     ///
     /// # Examples

-- 
2.49.0.rc1.451.g8f38331e32-goog