Implement Rust object API defaults (#6444)

* Implment Rust object API defaults

* satisfy return analysis

* git clang format

Co-authored-by: Casper Neo <cneo@google.com>

samples/monster_generated.rs

@@ -698,7 +698,7 @@ impl std::fmt::Debug for Monster<'_> {
   }
 }
 #[non_exhaustive]
-#[derive(Debug, Clone, PartialEq, Default)]
+#[derive(Debug, Clone, PartialEq)]
 pub struct MonsterT {
   pub pos: Option<Vec3T>,
   pub mana: i16,
@@ -710,6 +710,21 @@ pub struct MonsterT {
   pub equipped: EquipmentT,
   pub path: Option<Vec<Vec3T>>,
 }
+impl Default for MonsterT {
+  fn default() -> Self {
+    Self {
+      pos: None,
+      mana: 150,
+      hp: 100,
+      name: None,
+      inventory: None,
+      color: Color::Blue,
+      weapons: None,
+      equipped: EquipmentT::NONE,
+      path: None,
+    }
+  }
+}
 impl MonsterT {
   pub fn pack<'b>(
     &self,
@@ -864,11 +879,19 @@ impl std::fmt::Debug for Weapon<'_> {
   }
 }
 #[non_exhaustive]
-#[derive(Debug, Clone, PartialEq, Default)]
+#[derive(Debug, Clone, PartialEq)]
 pub struct WeaponT {
   pub name: Option<String>,
   pub damage: i16,
 }
+impl Default for WeaponT {
+  fn default() -> Self {
+    Self {
+      name: None,
+      damage: 0,
+    }
+  }
+}
 impl WeaponT {
   pub fn pack<'b>(
     &self,