[PATCH 7.2.y] rust: num: reject Bounded::shr overshifts at build time

[Miguel Ojeda <ojeda@kernel.org>]

From: Eliot Courtney <ecourtney@nvidia.com>

Make `shr` reject shifts of at least the type's bit width at build
time, instead of panicking or masking the shift amount at runtime.

[ This implies we can break the type invariant, which in turn means
  we can trigger UB via `Deref`, e.g.:

      rust_kernel: panicked at rust/kernel/num/bounded.rs:528:22:
      unsafe precondition(s) violated: hint::unreachable_unchecked must never be reached

    - Miguel ]

Signed-off-by: Eliot Courtney <ecourtney@nvidia.com>
Acked-by: Alexandre Courbot <acourbot@nvidia.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Danilo Krummrich <dakr@kernel.org>
Cc: stable@vger.kernel.org
Fixes: c59a2d14cd24 ("rust: num: add `shr` and `shl` methods to `Bounded`")
Link: https://patch.msgid.link/20260810-pramin-split-v2-2-65a00b3c7309@nvidia.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
(cherry picked from commit 223aa25aee82e188ddf043a8703b16e5fdfc37d8)
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
---
 rust/kernel/num/bounded.rs | 1 +
 1 file changed, 1 insertion(+)

diff --git a/rust/kernel/num/bounded.rs b/rust/kernel/num/bounded.rs
index dafe77782d79..92a17d5aec32 100644
--- a/rust/kernel/num/bounded.rs
+++ b/rust/kernel/num/bounded.rs
@@ -485,6 +485,7 @@ pub fn cast<U>(self) -> Bounded<U, N>
     /// assert_eq!(v_shifted.get(), 0xff);
     /// ```
     pub fn shr<const SHIFT: u32, const RES: u32>(self) -> Bounded<T, RES> {
+        const_assert!(SHIFT < T::BITS);
         const { assert!(RES + SHIFT >= N) }
 
         // SAFETY: We shift the value right by `SHIFT`, reducing the number of bits needed to
-- 
2.55.0