[RFC 03/13] rust/cell: add get_mut() method for BqlCell

[Zhao Liu <zhao1.liu@intel.com>]

The get_mut() is useful when doing compound assignment operations, e.g.,
*c.get_mut() += 1.

Implement get_mut() for BqlCell by referring to Cell.

Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
---
 rust/qemu-api/src/cell.rs | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/rust/qemu-api/src/cell.rs b/rust/qemu-api/src/cell.rs
index 07b636f26266..95f1cc0b3eb5 100644
--- a/rust/qemu-api/src/cell.rs
+++ b/rust/qemu-api/src/cell.rs
@@ -324,6 +324,31 @@ impl<T> BqlCell<T> {
     pub const fn as_ptr(&self) -> *mut T {
         self.value.get()
     }
+
+    /// Returns a mutable reference to the underlying data.
+    ///
+    /// This call borrows `BqlCell` mutably (at compile-time) which guarantees
+    /// that we possess the only reference.
+    ///
+    /// However be cautious: this method expects `self` to be mutable, which is
+    /// generally not the case when using a `BqlCell`. If you require interior
+    /// mutability by reference, consider using `BqlRefCell` which provides
+    /// run-time checked mutable borrows through its [`borrow_mut`] method.
+    ///
+    /// [`borrow_mut`]: BqlRefCell::borrow_mut()
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use qemu_api::cell::BqlCell;;
+    ///
+    /// let mut c = BqlCell::new(5);
+    /// *c.get_mut() += 1;
+    ///
+    /// assert_eq!(c.get(), 6);
+    pub fn get_mut(&mut self) -> &mut T {
+        self.value.get_mut()
+    }
 }
 
 impl<T: Default> BqlCell<T> {
-- 
2.34.1