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

[Andrew Ballance]

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

Signed-off-by: Andrew Ballance <andrewjballance@xxxxxxxxx>
---
 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..75e9feebb81f 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;
+        }
+
+        // [new_len, len) is guaranteed to be valid because [0, len) is guaranteed to be valid
+        let drop_range = len..self.len();
+
+        // SAFETY:
+        // we can safely ignore the bounds check because we already did our own check
+        let ptr: *mut [T] = unsafe { self.get_unchecked_mut(drop_range) };
+
+        // SAFETY:
+        // it is safe to shrink the length because the new length is
+        // guaranteed to be less than the old length
+        unsafe { self.set_len(len) };
+
+        // SAFETY:
+        // - the dropped values are valid `T`s
+        // - we are allowed to invalidate [new_len, old_len) because we just changed the len
+        unsafe { ptr::drop_in_place(ptr) };
+    }
 }
 
 impl<T: Clone, A: Allocator> Vec<T, A> {
-- 
2.48.1