KCL None needs more specific serialization (#3720)

Previously, many KCL values could be deserialized as KCL None values.
This isn't good -- the only thing you should be able to possibly
deserialize as KCL None is KCL None.

Solution was to add a private field in KCL None, serialize it with a
special magic number value, and then check for that magic number when
deserializing

src/wasm-lib/kcl/src/ast/types/none.rs

@@ -9,6 +9,8 @@ use crate::{
     executor::{KclValue, SourceRange, UserVal},
 };
 
+const KCL_NONE_ID: &str = "KCL_NONE_ID";
+
 /// KCL value for an optional parameter which was not given an argument.
 /// (remember, parameters are in the function declaration,
 /// arguments are in the function call/application).
@@ -20,6 +22,45 @@ pub struct KclNone {
     // TODO: Convert this to be an Option<SourceRange>.
     pub start: usize,
     pub end: usize,
+    #[serde(deserialize_with = "deser_private")]
+    #[ts(skip)]
+    #[schemars(skip)]
+    __private: Private,
+}
+
+impl KclNone {
+    pub fn new(start: usize, end: usize) -> Self {
+        Self {
+            start,
+            end,
+            __private: Private {},
+        }
+    }
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Bake, Default)]
+#[databake(path = kcl_lib::ast::types)]
+struct Private;
+
+impl Serialize for Private {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: serde::Serializer,
+    {
+        serializer.serialize_str(KCL_NONE_ID)
+    }
+}
+
+fn deser_private<'de, D>(deserializer: D) -> Result<Private, D::Error>
+where
+    D: serde::Deserializer<'de>,
+{
+    let s = String::deserialize(deserializer)?;
+    if s == KCL_NONE_ID {
+        Ok(Private {})
+    } else {
+        Err(serde::de::Error::custom("not a KCL none"))
+    }
 }
 
 impl From<&KclNone> for SourceRange {
@@ -57,3 +98,24 @@ impl KclNone {
         }
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn other_types_will_not_deserialize() {
+        // This shouldn't deserialize into a KCL None,
+        // because it's missing the special Private tag.
+        let j = r#"{"start": 0, "end": 0}"#;
+        let _e = serde_json::from_str::<KclNone>(j).unwrap_err();
+    }
+    #[test]
+    fn serialize_then_deserialize() {
+        // Serializing, then deserializing a None should produce the same value.
+        let before = KclNone::default();
+        let j = serde_json::to_string_pretty(&before).unwrap();
+        let after: KclNone = serde_json::from_str(&j).unwrap();
+        assert_eq!(before, after);
+    }
+}

src/wasm-lib/kcl/src/executor.rs

@@ -2080,10 +2080,7 @@ fn assign_args_to_params(
             if param.optional {
                 // If the corresponding parameter is optional,
                 // then it's fine, the user doesn't need to supply it.
-                let none = KclNone {
-                    start: param.identifier.start,
-                    end: param.identifier.end,
-                };
+                let none = KclNone::new(param.identifier.start, param.identifier.end);
                 fn_memory.add(
                     &param.identifier.name,
                     KclValue::from(&none),