Add rust::Box member type aliases
diff --git a/gen/write.rs b/gen/write.rs
index e0396c9..e6fb4e9 100644
--- a/gen/write.rs
+++ b/gen/write.rs
@@ -79,6 +79,7 @@
 
 fn write_includes(out: &mut OutFile, types: &Types) {
     let mut has_int = false;
+    let mut has_box = false;
     let mut has_unique_ptr = false;
     let mut has_string = false;
 
@@ -90,6 +91,7 @@
                 Some(CxxString) => has_string = true,
                 Some(Bool) | Some(RustString) | None => {}
             },
+            Type::RustBox(_) => has_box = true,
             Type::UniquePtr(_) => has_unique_ptr = true,
             _ => {}
         }
@@ -104,6 +106,9 @@
     if has_string {
         writeln!(out, "#include <string>");
     }
+    if has_box {
+        writeln!(out, "#include <type_traits>");
+    }
 }
 
 fn write_include_cxxbridge(out: &mut OutFile, types: &Types) {