[PATCH v2 2/5] rust: num: add `shr` and `shl` methods to `Bounded`

[Alexandre Courbot <acourbot@nvidia.com>]

Shifting a `Bounded` left or right changes the number of bits required
to represent the value. Add methods that perform the shift and return a
`Bounded` with the appropriately adjusted bit width.

These methods are particularly useful for bitfield extraction.

Suggested-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
---
 rust/kernel/num/bounded.rs | 40 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 40 insertions(+)

diff --git a/rust/kernel/num/bounded.rs b/rust/kernel/num/bounded.rs
index f870080af8ac..8782535770f1 100644
--- a/rust/kernel/num/bounded.rs
+++ b/rust/kernel/num/bounded.rs
@@ -470,6 +470,46 @@ pub fn cast<U>(self) -> Bounded<U, N>
         // `N` bits, and with the same signedness.
         Bounded::__new(value)
     }
+
+    /// Right-shifts `self` by `SHIFT` and returns the result as a `Bounded<_, { N - SHIFT }>`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use kernel::num::Bounded;
+    ///
+    /// let v = Bounded::<u32, 16>::new::<0xff00>();
+    /// let v_shifted: Bounded::<u32, 8> = v.shr::<8, _>();
+    ///
+    /// assert_eq!(v_shifted.get(), 0xff);
+    /// ```
+    pub fn shr<const SHIFT: u32, const RES: u32>(self) -> Bounded<T, RES> {
+        const { assert!(RES >= N - SHIFT) }
+
+        // SAFETY: we shift the value right by `SHIFT`, reducing the number of bits needed to
+        // represent the shifted value by as much, and just asserted that `RES == N - SHIFT`.
+        unsafe { Bounded::__new(self.0 >> SHIFT) }
+    }
+
+    /// Left-shifts `self` by `SHIFT` and returns the result as a `Bounded<_, { N + SHIFT }>`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use kernel::num::Bounded;
+    ///
+    /// let v = Bounded::<u32, 8>::new::<0xff>();
+    /// let v_shifted: Bounded::<u32, 16> = v.shl::<8, _>();
+    ///
+    /// assert_eq!(v_shifted.get(), 0xff00);
+    /// ```
+    pub fn shl<const SHIFT: u32, const RES: u32>(self) -> Bounded<T, RES> {
+        const { assert!(RES >= N + SHIFT) }
+
+        // SAFETY: we shift the value left by `SHIFT`, augmenting the number of bits needed to
+        // represent the shifted value by as much, and just asserted that `RES == N + SHIFT`.
+        unsafe { Bounded::__new(self.0 << SHIFT) }
+    }
 }
 
 impl<T, const N: u32> Deref for Bounded<T, N>

-- 
2.52.0