[PATCH v4 7/7] rust: alloc: add Vec::insert_within_capacity

[Alice Ryhl <aliceryhl@google.com>]

This adds a variant of Vec::insert that does not allocate memory. This
makes it safe to use this function while holding a spinlock. Rust Binder
uses it for the range allocator fast path.

Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
 rust/kernel/alloc/kvec.rs | 39 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 39 insertions(+)

diff --git a/rust/kernel/alloc/kvec.rs b/rust/kernel/alloc/kvec.rs
index 0682108951675cbee05faa130e5a9ce72fc343ba..998afdcde47bec94b2c9d990ba3afbb3488ea99e 100644
--- a/rust/kernel/alloc/kvec.rs
+++ b/rust/kernel/alloc/kvec.rs
@@ -355,6 +355,45 @@ pub unsafe fn push_within_capacity_unchecked(&mut self, v: T) {
         unsafe { self.inc_len(1) };
     }
 
+    /// Inserts an element at the given index in the [`Vec`] instance.
+    ///
+    /// Fails if the vector does not have capacity for the new element. Panics if the index is out
+    /// of bounds.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let mut v = KVec::with_capacity(10, GFP_KERNEL)?;
+    /// for i in 0..10 {
+    ///     v.push_within_capacity(i).unwrap();
+    /// }
+    ///
+    /// assert!(v.push_within_capacity(10).is_err());
+    /// # Ok::<(), Error>(())
+    /// ```
+    pub fn insert_within_capacity(&mut self, index: usize, element: T) -> Result<(), T> {
+        let len = self.len();
+        assert!(index <= len);
+
+        if len >= self.capacity() {
+            return Err(element);
+        }
+
+        // SAFETY: This is in bounds since `index <= len < capacity`.
+        let p = unsafe { self.as_mut_ptr().add(index) };
+        // INVARIANT: This breaks the Vec invariants by making `index` contain an invalid element,
+        // but we restore the invariants below.
+        // SAFETY: Both the src and dst ranges end no later than one element after the length.
+        // Since the length is less than the capacity, both ranges are in bounds of the allocation.
+        unsafe { ptr::copy(p, p.add(1), len - index) };
+        // INVARIANT: This restores the Vec invariants.
+        // SAFETY: The pointer is in-bounds of the allocation.
+        unsafe { ptr::write(p, element) };
+        // SAFETY: Index `len` contains a valid element due to the above copy and write.
+        unsafe { self.inc_len(1) };
+        Ok(())
+    }
+
     /// Removes the last element from a vector and returns it, or `None` if it is empty.
     ///
     /// # Examples

-- 
2.49.0.901.g37484f566f-goog