Replace an assert! with debug_assert! in u256::shr

The implementation came from the `compiler_builtins` port but this
should be weakened to match other integer types.

src/math/support/big.rs

@@ -109,7 +109,10 @@ impl ops::Shr<u32> for u256 {
     type Output = Self;
 
     fn shr(self, rhs: u32) -> Self::Output {
-        assert!(rhs < Self::BITS, "attempted to shift right with overflow");
+        debug_assert!(rhs < Self::BITS, "attempted to shift right with overflow");
+        if rhs >= Self::BITS {
+            return Self::ZERO;
+        }
 
         if rhs == 0 {
             return self;

src/math/support/big/tests.rs

@@ -108,3 +108,22 @@ fn shr_u128() {
     }
     assert!(errors.is_empty());
 }
+
+#[test]
+#[should_panic]
+#[cfg(debug_assertions)]
+// FIXME(ppc): ppc64le seems to have issues with `should_panic` tests.
+#[cfg(not(all(target_arch = "powerpc64", target_endian = "little")))]
+fn shr_u256_overflow() {
+    // Like regular shr, panic on overflow with debug assertions
+    let _ = u256::MAX >> 256;
+}
+
+#[test]
+#[cfg(not(debug_assertions))]
+fn shr_u256_overflow() {
+    // No panic without debug assertions
+    assert_eq!(u256::MAX >> 256, u256::ZERO);
+    assert_eq!(u256::MAX >> 257, u256::ZERO);
+    assert_eq!(u256::MAX >> u32::MAX, u256::ZERO);
+}