Unresolved imports QGuiApplication, QQmlApplicationEngine when building with Cargo on Windows

[Artalus]

I am following the Book and copypasting the code from there as I go. I am on Windows, so initially I ran into #1120 , but after switching to an Admin shell, I now get this:

PS D:\git\qtrs> cargo build
   Compiling link-cplusplus v1.0.9
   Compiling cxx v1.0.135
   Compiling cxx-qt v0.7.0
   Compiling cxx-qt-lib v0.7.0
   Compiling qtrs v0.1.0 (D:\git\qtrs)
error[E0432]: unresolved imports `cxx_qt_lib::QGuiApplication`, `cxx_qt_lib::QQmlApplicationEngine`
 --> src/main.rs:3:18
  |
3 | use cxx_qt_lib::{QGuiApplication, QQmlApplicationEngine, QUrl};
  |                  ^^^^^^^^^^^^^^^  ^^^^^^^^^^^^^^^^^^^^^ no `QQmlApplicationEngine` in the root
  |                  |
  |                  no `QGuiApplication` in the root
  |                  help: a similar name exists in the module: `QCoreApplication`

For more information about this error, try `rustc --explain E0432`.
error: could not compile `qtrs` (bin "qtrs") due to 1 previous error

My Cargo.toml:

[package]
name = "qtrs"
version = "0.1.0"
edition = "2021"

[dependencies]
cxx = "1.0.135"
cxx-qt = "0.7.0"
cxx-qt-lib = "0.7.0"

[build-dependencies]
# The link_qt_object_files feature is required for statically linking Qt 6.
cxx-qt-build = { version = "0.7.0", features = [ "link_qt_object_files" ] }

The only difference from https://kdab.github.io/cxx-qt/book/getting-started/4-cargo-executable.html is that cxx is 1.0.135 instead of 1.0.95, but downgrading it does not change anything, as cargo keeps downloading 1.0.135 for some reason.

The copypasted contents from Book:

src\cxxqt_object.rs

/// The bridge definition for our QObject
#[cxx_qt::bridge]
pub mod qobject {

    unsafe extern "C++" {
        include!("cxx-qt-lib/qstring.h");
        /// An alias to the QString type
        type QString = cxx_qt_lib::QString;
    }

    unsafe extern "RustQt" {
        // The QObject definition
        // We tell CXX-Qt that we want a QObject class with the name MyObject
        // based on the Rust struct MyObjectRust.
        #[qobject]
        #[qml_element]
        #[qproperty(i32, number)]
        #[qproperty(QString, string)]
        #[namespace = "my_object"]
        type MyObject = super::MyObjectRust;
    }

    unsafe extern "RustQt" {
        // Declare the invokable methods we want to expose on the QObject
        #[qinvokable]
        #[cxx_name = "incrementNumber"]
        fn increment_number(self: Pin<&mut MyObject>);

        #[qinvokable]
        #[cxx_name = "sayHi"]
        fn say_hi(self: &MyObject, string: &QString, number: i32);
    }
}

use core::pin::Pin;
use cxx_qt_lib::QString;

/// The Rust struct for the QObject
#[derive(Default)]
pub struct MyObjectRust {
    number: i32,
    string: QString,
}

impl qobject::MyObject {
    /// Increment the number Q_PROPERTY
    pub fn increment_number(self: Pin<&mut Self>) {
        let previous = *self.number();
        self.set_number(previous + 1);
    }

    /// Print a log message with the given string and number
    pub fn say_hi(&self, string: &QString, number: i32) {
        println!("Hi from Rust! String is '{string}' and number is {number}");
    }
}

src\main.rs

pub mod cxxqt_object;

use cxx_qt_lib::{QGuiApplication, QQmlApplicationEngine, QUrl};

fn main() {
    // Create the application and engine
    let mut app = QGuiApplication::new();
    let mut engine = QQmlApplicationEngine::new();

    // Load the QML path into the engine
    if let Some(engine) = engine.as_mut() {
        engine.load(&QUrl::from("qrc:/qt/qml/com/kdab/cxx_qt/demo/qml/main.qml"));
    }

    if let Some(engine) = engine.as_mut() {
        // Listen to a signal from the QML Engine
        engine
            .as_qqmlengine()
            .on_quit(|_| {
                println!("QML Quit!");
            })
            .release();
    }

    // Start the app
    if let Some(app) = app.as_mut() {
        app.exec();
    }
}

build.rs

use cxx_qt_build::{CxxQtBuilder, QmlModule};

fn main() {
    CxxQtBuilder::new()
        // Link Qt's Network library
        // - Qt Core is always linked
        // - Qt Gui is linked by enabling the qt_gui Cargo feature of cxx-qt-lib.
        // - Qt Qml is linked by enabling the qt_qml Cargo feature of cxx-qt-lib.
        // - Qt Qml requires linking Qt Network on macOS
        .qt_module("Network")
        .qml_module(QmlModule {
            uri: "com.kdab.cxx_qt.demo",
            rust_files: &["src/cxxqt_object.rs"],
            qml_files: &["qml/main.qml"],
            ..Default::default()
        })
        .build();
}

[ubas-of-the-bush]

@Artalus

I just ran into this problem as well!

To solve it, you need to specify that you want all features of cxx-qt-lib: cxx-qt-lib = {version = "0.7.0", features = [ "full" ]}

Once I added that to my Cargo.toml, rust-analyser recognised those imports you mentioned, and it compiled fine.

The book also specifies the wrong feature flag in the reference Cargo.toml; it specifies cxx-qt-lib = { version="0.7", features = ["qt_full"] }, but cargo couldn't reconcile "qt_full" with version=^0.7.0 - perhaps the name was changed?

[redstrate]

@Artalus

I just ran into this problem as well!

To solve it, you need to specify that you want all features of cxx-qt-lib: cxx-qt-lib = {version = "0.7.0", features = [ "full" ]}

Once I added that to my Cargo.toml, rust-analyser recognised those imports you mentioned, and it compiled fine.

The book also specifies the wrong feature flag in the reference Cargo.toml; it specifies cxx-qt-lib = { version="0.7", features = ["qt_full"] }, but cargo couldn't reconcile "qt_full" with version=^0.7.0 - perhaps the name was changed?

You probably only need qt_qml and qt_gui, not the rest of the features. We still have the qt_full feature, what's the error you get when using it?

[ubas-of-the-bush]

Just tried and failed to reproduce the error I allegedly got when using the "qt_full" feature flag; my code, which is essentially the example in the book, compiled just fine with ["full"], ["qt_full"], and ["qt_qml", "qt_gui"] feature flags. Copy-and-pasting the Cargo.toml in the book also compiles fine.

(My bad, sorry - maybe I made a typo in the Cargo.toml and didn't notice?)

[LeonMatthesKDAB]

Thank you @ubas-of-the-bush for following up on this.

This should be solved by enabling qt_full or another feature set that enables qt_qml and qt_gui, as you describe.
We may need to point this out more clearly in the documentation.

I will close the issue for now.

@Artalus if you still encounter the issue even after enabling the required features, please reopen the issue.