Update ink! caller integration test (#1294)

integration/substrate/ink/caller/Cargo.toml

@@ -5,10 +5,10 @@ authors = ["Cyrill Leutwiler <cyrill@parity.io>"]
 edition = "2021"
 
 [dependencies]
-ink = { version = "4.0.0-rc", default-features = false }
+ink = { version = "4.2", default-features = false }
 
 scale = { package = "parity-scale-codec", version = "3", default-features = false, features = ["derive"] }
-scale-info = { version = "2.3", default-features = false, features = ["derive"], optional = true }
+scale-info = { version = "2.6", default-features = false, features = ["derive"], optional = true }
 
 [lib]
 name = "caller"

integration/substrate/inkee.sol

@@ -1,6 +1,24 @@
+// ink! contracts are expected to return a value of type `Result<T, LangError>` (Rust's "Result" type).
+// The ink! call builder only supports decoding the result into this `Result` type.
+// This works fine for ink! because ink! contracts are supposed to always return this type.
+// 
+// In Solidity, not only does this type not exist, but also the concept of a "LangError" is inexistent.
+// Hence we mock it, so we can (ab)use the ink! call builder for setting up the call to our contract.
+// In SCALE, a preceding null-byte will decode to the `Ok` variant.
+struct InkResult {
+    uint8 ok;
+    uint32 value;
+}
+
+// Helper function to convert the result value into an ink! compatible result type.
+function ink_result(uint32 _value) pure returns (InkResult) {
+    return InkResult({ok: 0, value: _value});
+}
+
 contract Inkee {
+    // We use this selector just for convenience
     @selector([1, 2, 3, 4])
-    function echo(uint32 v) public pure returns (bool, uint32) {
-        return (false, v);
+    function echo(uint32 v) public pure returns (InkResult) {
+        return ink_result(v);
     }
 }