Fix: cairo_native swaps the high and low fields of U256 compared to Cairo's u256

src/starknet.rs

@@ -23,8 +23,8 @@ pub struct Felt252Abi(pub [u8; 32]);
 #[cfg_attr(feature = "with-serde", derive(serde::Serialize, serde::Deserialize))]
 #[repr(C, align(16))]
 pub struct U256 {
-    pub hi: u128,
     pub lo: u128,
+    pub hi: u128,
 }
 
 #[derive(Debug, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]

tests/tests/starknet/contracts/test_u256_order.cairo

@@ -0,0 +1,27 @@
+#[starknet::interface]
+trait IKeccak<TContractState> {
+    fn cairo_keccak_test(self: @TContractState) -> felt252;
+}
+
+#[starknet::contract]
+mod Keccak {
+    use core::clone::Clone;
+    use array::{Array, ArrayTrait};
+    use core::traits::Into;
+
+    #[storage]
+    struct Storage {}
+
+    #[abi(embed_v0)]
+    impl Keccak of super::IKeccak<ContractState> {
+        fn cairo_keccak_test(self: @ContractState) -> felt252 {
+            let input : Array::<u64> = array![1,2,4,5,6,6,7,2,3,4,4,5,5,6,7,7,2];
+            let output = starknet::syscalls::keccak_syscall(input.span()).unwrap();
+
+            if output.low == 0x9293867273ef341e81577655f28aeade && output.high == 0xf70cba9bb86caa97b086fdfa3df602ed {
+                panic_with_felt252('arguments swapped');
+            }
+            output.low.into()
+        }
+    }
+}

tests/tests/starknet/mod.rs

@@ -1,5 +1,6 @@
 mod keccak;
 mod secp256;
+mod u256;
 
 #[cfg(feature = "with-cheatcode")]
 mod syscalls;