From 5a88f7ed6940b316a0609aca66e257af9cca122d Mon Sep 17 00:00:00 2001
From: Jack Lloyd <jack.lloyd@dfinity.org>
Date: Mon, 2 Dec 2024 10:03:54 -0500
Subject: [PATCH] Add a test regarding structs with a new Option field

This checks that adding an Option field to a struct has the expected
behavior, where

* For an old writer and new reader, the struct deserializes with
  a field of None
* For a new writer and an old reader, the struct deserializes with
  the value of the Option ignored.
---
 rust/candid/tests/serde.rs | 50 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 50 insertions(+)

diff --git a/rust/candid/tests/serde.rs b/rust/candid/tests/serde.rs
index 87113a46..375b796a 100644
--- a/rust/candid/tests/serde.rs
+++ b/rust/candid/tests/serde.rs
@@ -193,6 +193,56 @@ fn test_option() {
     test_decode(&bytes, &expected);
 }
 
+#[test]
+fn test_struct_with_option() {
+    #[derive(CandidType, Deserialize, PartialEq, Debug)]
+    struct Old {
+        field: i32,
+    }
+
+    #[derive(CandidType, Deserialize, PartialEq, Debug)]
+    struct New {
+        field: i32,
+        new_field: Option<i32>,
+    }
+
+    let old_bytes = encode(&Old { field: 1 });
+    let new_bytes_with_none = encode(&New {
+        field: 2,
+        new_field: None,
+    });
+    let new_bytes_with_some = encode(&New {
+        field: 3,
+        new_field: Some(4),
+    });
+
+    test_decode(&old_bytes, &Old { field: 1 });
+    test_decode(&new_bytes_with_none, &Old { field: 2 });
+    test_decode(&new_bytes_with_some, &Old { field: 3 });
+
+    test_decode(
+        &old_bytes,
+        &New {
+            field: 1,
+            new_field: None,
+        },
+    );
+    test_decode(
+        &new_bytes_with_none,
+        &New {
+            field: 2,
+            new_field: None,
+        },
+    );
+    test_decode(
+        &new_bytes_with_some,
+        &New {
+            field: 3,
+            new_field: Some(4),
+        },
+    );
+}
+
 #[test]
 fn test_struct() {
     #[derive(PartialEq, Debug, Deserialize, CandidType)]