[PATCH v4 2/2] rust: transmute: add `as_bytes_mut` method to `AsBytes` trait

[Alexandre Courbot]

Types that implement both `AsBytes` and `FromBytes` can be safely
modified as a slice of bytes. Add a `as_bytes_mut` method for that
purpose.

Signed-off-by: Alexandre Courbot <acourbot@xxxxxxxxxx>
---
 rust/kernel/transmute.rs | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/rust/kernel/transmute.rs b/rust/kernel/transmute.rs
index d541c9960904d8c7f755351f22d06e4e8dbd546a..000fda2a78f8e6e8378bbe93cddd8a5008db20cc 100644
--- a/rust/kernel/transmute.rs
+++ b/rust/kernel/transmute.rs
@@ -56,6 +56,19 @@ fn as_bytes(&self) -> &[u8] {
         // SAFETY: `data` is non-null and valid for reads over `len * sizeof::<u8>()` bytes.
         unsafe { core::slice::from_raw_parts(data, len) }
     }
+
+    /// Returns `self` as a mutable slice of bytes.
+    fn as_bytes_mut(&mut self) -> &mut [u8]
+    where
+        Self: FromBytes,
+    {
+        let data = core::ptr::from_mut(self).cast::<u8>();
+        let len = size_of_val(self);
+
+        // SAFETY: `data` is non-null and valid for read and writes over `len * sizeof::<u8>()`
+        // bytes. Since `Self` implements `FromBytes` it can be represented by any value.
+        unsafe { core::slice::from_raw_parts_mut(data, len) }
+    }
 }
 
 macro_rules! impl_asbytes {

-- 
2.50.1