message: rust: more builder lifetimes logic
diff: @@ -352,8 +352,7 @@ class RustGenerator : public BaseGenerator {
  case ftFloat:
  case ftBool:
  case ftEnumKey:
  - case ftUnionKey:
  - case ftStruct: { return false; }
  + case ftUnionKey: { return false; }
  default: { return true; }
  }
  }
  