[PATCH 4/5] rust: alloc: add Vec::clear

[Alice Ryhl <aliceryhl@google.com>]

Our custom Vec type is missing the stdlib method `clear`, thus add it.
It will be used in the miscdevice sample.

Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
 rust/kernel/alloc/kvec.rs | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/rust/kernel/alloc/kvec.rs b/rust/kernel/alloc/kvec.rs
index ae9d072741cedbb34bed0be0c20cc75472aa53be..2d213ede2873cef87116a5527e8e24008c970a58 100644
--- a/rust/kernel/alloc/kvec.rs
+++ b/rust/kernel/alloc/kvec.rs
@@ -395,6 +395,33 @@ pub fn into_raw_parts(self) -> (*mut T, usize, usize) {
         (ptr, len, capacity)
     }
 
+    /// Clears the vector, removing all values.
+    ///
+    /// Note that this method has no effect on the allocated capacity
+    /// of the vector.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let mut v = kernel::kvec![1, 2, 3]?;
+    ///
+    /// v.clear();
+    ///
+    /// assert!(v.is_empty());
+    /// # Ok::<(), Error>(())
+    /// ```
+    #[inline]
+    pub fn clear(&mut self) {
+        let elems: *mut [T] = self.as_mut_slice();
+
+        // INVARIANT: This call changes the number of elements to zero.
+        self.len = 0;
+
+        // SAFETY: The values being dropped are valid values of type `T` by the type invariants.
+        // It's okay to invalidate them as we just changed the length to zero.
+        unsafe { ptr::drop_in_place(elems) };
+    }
+
     /// Ensures that the capacity exceeds the length by at least `additional` elements.
     ///
     /// # Examples

-- 
2.49.0.rc0.332.g42c0ae87b1-goog