fix(flatbuffers): use manual impl Default for struct object types

src/idl_gen_rust.cpp

@@ -2939,13 +2939,48 @@ class RustGenerator : public BaseGenerator {
     // Generate Struct Object.
     if (parser_.opts.generate_object_based_api) {
       // Struct declaration
-      code_ += "#[derive(Debug, Clone, PartialEq, Default)]";
+      code_ += "#[derive(Debug, Clone, PartialEq)]";
       code_ += "{{ACCESS_TYPE}} struct {{STRUCT_OTY}} {";
       ForAllStructFields(struct_def, [&](const FieldDef& field) {
         (void)field;  // unused.
         code_ += "pub {{FIELD}}: {{FIELD_OTY}},";
       });
       code_ += "}";
+      // Manual impl Default to avoid issues with arrays > 32 elements
+      // where #[derive(Default)] fails on older Rust versions.
+      code_ += "impl Default for {{STRUCT_OTY}} {";
+      code_ += "  fn default() -> Self {";
+      code_ += "    Self {";
+      ForAllStructFields(struct_def, [&](const FieldDef& field) {
+        const auto full_type = GetFullType(field.value.type);
+        switch (full_type) {
+          case ftArrayOfBuiltin: {
+            // e.g. [0; 64] for [u8; 64]
+            code_ += "    {{FIELD}}: [0; " +
+                     NumToString(field.value.type.fixed_length) + "],";
+            break;
+          }
+          case ftArrayOfEnum:
+          case ftArrayOfStruct: {
+            // e.g. Default::default() works for arrays of structs/enums
+            // since their size is bounded by the schema, but use
+            // array_init for safety with large arrays.
+            code_ +=
+                "    {{FIELD}}: ::flatbuffers::array_init(|_| "
+                "Default::default()),";
+            break;
+          }
+          default: {
+            std::string default_value =
+                GetDefaultValue(field, kObject);
+            code_ += "    {{FIELD}}: " + default_value + ",";
+            break;
+          }
+        }
+      });
+      code_ += "    }";
+      code_ += "  }";
+      code_ += "}";
       // The `pack` method that turns the native struct into its Flatbuffers
       // counterpart.
       code_ += "impl {{STRUCT_OTY}} {";

tests/arrays_test.fbs

@@ -18,6 +18,10 @@ struct ArrayStruct{
   f:[int64:2];
 }
 
+struct LargeArrayStruct {
+  d:[ubyte:64];
+}
+
 table ArrayTable{
   a:ArrayStruct;
 }