[PATCH v2 3/4] rust: alloc: refactor `Vec::truncate` using `dec_len`

[Tamir Duberstein <tamird@gmail.com>]

Use `checked_sub` to satisfy the safety requirements of `dec_len` and
replace nearly the whole body of `truncate` with a call to `dec_len`.

Signed-off-by: Tamir Duberstein <tamird@gmail.com>
---
 rust/kernel/alloc/kvec.rs | 29 +++++++++++------------------
 1 file changed, 11 insertions(+), 18 deletions(-)

diff --git a/rust/kernel/alloc/kvec.rs b/rust/kernel/alloc/kvec.rs
index 97cc5ab11e2a..6f4dc89ef7f8 100644
--- a/rust/kernel/alloc/kvec.rs
+++ b/rust/kernel/alloc/kvec.rs
@@ -489,25 +489,18 @@ pub fn reserve(&mut self, additional: usize, flags: Flags) -> Result<(), AllocEr
     /// # Ok::<(), Error>(())
     /// ```
     pub fn truncate(&mut self, len: usize) {
-        if len >= self.len() {
-            return;
+        match self.len().checked_sub(len) {
+            None => {}
+            Some(count) => {
+                // SAFETY: `count` is `self.len() - len` so it is guaranteed to be less than or
+                // equal to `self.len()`.
+                let tail = unsafe { self.dec_len(count) };
+
+                // SAFETY: the contract of `dec_len` guarantees that the elements in `tail` are
+                // valid elements whose ownership has been transferred to the caller.
+                unsafe { ptr::drop_in_place(ptr) };
+            }
         }
-
-        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) };
     }
 }
 

-- 
2.48.1