[PATCH v2 1/3] rust: alloc: add Vec::truncate method

[Andrew Ballance <andrewjballance@gmail.com>]

implement the equivalent to the std's Vec::truncate
on the kernel's Vec type.

Signed-off-by: Andrew Ballance <andrewjballance@gmail.com>
---
 rust/kernel/alloc/kvec.rs | 36 ++++++++++++++++++++++++++++++++++++
 1 file changed, 36 insertions(+)

diff --git a/rust/kernel/alloc/kvec.rs b/rust/kernel/alloc/kvec.rs
index ae9d072741ce..18bcc59f0b38 100644
--- a/rust/kernel/alloc/kvec.rs
+++ b/rust/kernel/alloc/kvec.rs
@@ -452,6 +452,42 @@ pub fn reserve(&mut self, additional: usize, flags: Flags) -> Result<(), AllocEr
 
         Ok(())
     }
+
+    /// Shortens the vector, setting the length to `len` and drops the removed values.
+    /// If `len` is greater than or equal to the current length, this does nothing.
+    ///
+    /// This has no effect on the capacity and will not allocate.
+    /// # Examples
+    ///
+    /// ```
+    /// let mut v = kernel::kvec![1, 2, 3]?;
+    /// v.truncate(1);
+    /// assert_eq!(v.len(), 1);
+    /// assert_eq!(&v, &[1]);
+    ///
+    /// # Ok::<(), Error>(())
+    /// ```
+    pub fn truncate(&mut self, len: usize) {
+        if len >= self.len() {
+            return;
+        }
+
+        let drop_range = len..self.len();
+
+        // SAFETY: `drop_range` is a subrange of `[0, len)` by the bounds check above.
+        let ptr: *mut [T] = unsafe { self.get_unchecked_mut(drop_range) };
+
+        // SAFETY:
+        // - this will always shrink the vector because of the above bounds check
+        // - [`new_len`, `self.len`) will be dropped through the call to `drop_in_place` below
+        unsafe { self.set_len(len) };
+
+        // SAFETY:
+        // - the dropped values are valid `T`s by the type invariant
+        // - we are allowed to invalidate [`new_len`, `old_len`) because we just changed the
+        //   len, therefore we have exclusive access to [`new_len`, `old_len`)
+        unsafe { ptr::drop_in_place(ptr) };
+    }
 }
 
 impl<T: Clone, A: Allocator> Vec<T, A> {
-- 
2.48.1