cxx-qt multi-module problem

Given this cxx-qt crate, observe the following:

• cargo build succeeds. Static library is generated.
• cargo test fails with several multiple definition errors and several undefined symbol errors.

Now, comment out one of the modules, both in build.rs (the .rs file) and in lib.rs (the library file). Example:

lib.rs

pub mod module_a;
// pub mod module_b;

build.rs

use cxx_qt_build::{CxxQtBuilder, QmlModule};

fn main() {
    CxxQtBuilder::new()
        .cc_builder(|cc| {
            cc.include("src/cxx");
        })
        .qt_module("Network")
        .qml_module(QmlModule {
            uri: "qml_module",
            rust_files: &[
                "src/module_a.rs",
                // "src/module_b.rs",
            ],
            qml_files: &[] as &[&'static str],
            ..Default::default()
        })
        .build();
}

Now, cargo test succeeds.

Question

How to modify this structure such that both modules can be compiled into the same static library and cargo test can run all of the tests together?