[PATCH 03/12] rust: num: add Bounded::shr_exact

Add `shr_exact` in the vein of `try_shrink` which shifts a bounded right
only if it loses no set bits. This is useful for getting a shifted down
integer while simultaneously checking that it's aligned.

Signed-off-by: Eliot Courtney <[email protected]>
---
 rust/kernel/num/bounded.rs | 30 ++++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)

diff --git a/rust/kernel/num/bounded.rs b/rust/kernel/num/bounded.rs
index f263107f001e..2695a7858d8a 100644
--- a/rust/kernel/num/bounded.rs
+++ b/rust/kernel/num/bounded.rs
@@ -493,6 +493,36 @@ pub fn shr<const SHIFT: u32, const RES: u32>(self) -> Bounded<T, RES> {
         unsafe { Bounded::__new(self.0 >> SHIFT) }
     }
 
+    /// Right-shifts `self` by `SHIFT` if that loses no set bits, and returns the result as a
+    /// `Bounded<_, RES>`, where `RES >= N - SHIFT`.
+    ///
+    /// Returns [`None`] if any of the `SHIFT` least significant bits of `self` is set.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use kernel::num::Bounded;
+    ///
+    /// let v = Bounded::<u32, 16>::new::<0xff00>();
+    /// let v_shifted: Option<Bounded<u32, 8>> = v.shr_exact::<8, _>();
+    ///
+    /// assert_eq!(v_shifted.map(|v| v.get()), Some(0xff));
+    ///
+    /// // A set bit would be shifted out.
+    /// let v = Bounded::<u32, 16>::new::<0xff01>();
+    /// let v_shifted: Option<Bounded<u32, 8>> = v.shr_exact::<8, _>();
+    ///
+    /// assert!(v_shifted.is_none());
+    /// ```
+    pub fn shr_exact<const SHIFT: u32, const RES: u32>(self) -> Option<Bounded<T, RES>> {
+        let shifted = self.shr::<SHIFT, RES>();
+        if shifted.get() << SHIFT == self.0 {
+            Some(shifted)
+        } else {
+            None
+        }
+    }
+
     /// Left-shifts `self` by `SHIFT` and returns the result as a `Bounded<_, RES>`, where `RES >=
     /// N + SHIFT`.
     ///

-- 
2.55.0