recover subtype error from custom deserializer

rust/candid/src/error.rs

@@ -151,7 +151,12 @@ impl ser::Error for Error {
 
 impl de::Error for Error {
     fn custom<T: std::fmt::Display>(msg: T) -> Self {
-        Error::msg(format!("Deserialize error: {msg}"))
+        let msg = msg.to_string();
+        if let Some(msg) = msg.strip_prefix("Subtyping error: ") {
+            Error::Subtype(msg.to_string())
+        } else {
+            Error::msg(format!("Deserialize error: {msg}"))
+        }
     }
     fn invalid_type(_: de::Unexpected<'_>, exp: &dyn de::Expected) -> Self {
         Error::Subtype(format!("{exp}"))

rust/candid/tests/serde.rs

@@ -173,6 +173,20 @@ fn test_option() {
     // Deserialize \mu T.Option<T> to a non-recursive type
     let v: Option<Option<Option<i32>>> = Some(Some(None));
     test_decode(b"DIDL\x01\x6e\0\x01\0\x01\x01\0", &v);
+    // special opt rule
+    #[derive(CandidType, Deserialize, PartialEq, Debug)]
+    struct A {
+        canister_id: Option<candid::Principal>,
+    }
+    #[derive(CandidType, Deserialize, PartialEq, Debug)]
+    struct B {
+        canister_id: Option<i32>,
+    }
+    let bytes = encode(&B {
+        canister_id: Some(42),
+    });
+    let expected = A { canister_id: None };
+    test_decode(&bytes, &expected);
 }
 
 #[test]