Build fails when using only QML files in a module (no rust bridges)

[dphaldes]

When trying to build applications with only QML and no rust bridges, the build fails with

cargo:warning=cc1plus: fatal error: /home/chonky/build/rust/kono/target/debug/build/kono-c626df56a33c2707/out/qt-build-utils/qml_plugin/kono_qmltyperegistration.cpp: No such file or directory
cargo:warning=compilation terminated.

Buildscript:

use cxx_qt_build::{CxxQtBuilder, QmlModule};

fn main() {
    CxxQtBuilder::new()
        .qml_module(QmlModule::<&str, &str> {
            uri: "kono",
            qml_files: &["src/qml/main.qml"],
            ..Default::default()
        })
        .build();
}

While this is not a big issue, QML-only applications are a possibility and a meaningful error message would be appreciated.
I stumbled upon this when trying to incrementally add qt to an existing app and this error confused me a lot.

[ahayzen-kdab]

Thanks for reporting this! We've had a similar situation before and probably missed a case.

For 0.8 we plan on refactoring the build API #1125, where one of the ideas is that one crate could become one QML module and export to CMake or a library crate etc. So this will become increasingly important to support having parts of the input empty.