How to use Qt backend on Linux?

[DavidVentura]

I'd like to use the Qt backend & renderer (dynamically linked?) so I can ship my app to Qt environments.

Initially I tried

slint = { version = "1.13.1", default-features = false, features = ["backend-qt", "std", "compat-1-2", "renderer-femtovg"] }

but this fails to build due to a missing opengl feature on i-slint-renderer-femtovg
I manually added it

i-slint-renderer-femtovg = { version = "1.13.1", features = ["opengl"] }

but on launch I just get

Error: No default Slint platform was selected, and no Slint platform was initialized

This page says "enable one of the renderer-* features) but none of the renderers in that list mention "Qt".

How do I enable only the Qt backend?

[tronical]

I think you're doing everything correctly. The OpenGL build issue is a bug, but it should be okay to leave out "renderer-femtovg" from the features. "backend-qt" comes with a backend and a renderer.

The runtime error message suggests that perhaps Qt could not be found at build time. If you run cargo clean -p qttypes and build again, do you see any warnings?

[DavidVentura]

Didn't work

$ cargo clean -p qttypes
    Updating crates.io index
     Locking 9 packages to latest Rust 1.90.0 compatible versions
      Adding core-text v21.0.0
      Adding dwrote v0.11.5
      Adding femtovg v0.17.0
      Adding fnv v1.0.7
      Adding gbm v0.18.0
      Adding gbm-sys v0.4.0
      Adding i-slint-renderer-femtovg v1.13.1
      Adding lru v0.16.1
      Adding wio v0.2.2
     Removed 149 files, 193.9MiB total

doesn't link to qt:

$ ldd target/debug/offline-translator-linux
        linux-vdso.so.1 (0x00007ffda03bd000)
        libstdc++.so.6 => /lib/x86_64-linux-gnu/libstdc++.so.6 (0x000079ec31a00000)
        libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x000079ec33390000)
        libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x000079ec31d17000)
        libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x000079ec31600000)
        /lib64/ld-linux-x86-64.so.2 (0x000079ec333df000)

no platform:

$ ./target/debug/offline-translator-linux 
Error: No default Slint platform was selected, and no Slint platform was initialized

Cargo.toml:

slint = { version = "1.13.1", default-features = false, features = ["backend-qt", "std", "compat-1-2", "renderer-femtovg"] }
i-slint-renderer-femtovg = { version = "1.13.1", features = ["opengl"] }

main.rs

use slint::{self, Model, ModelExt, VecModel};

slint::include_modules!();

fn main() -> Result<(), Box<dyn Error>> {
    let ui = AppWindow::new()?;
    // ...
}

[DavidVentura]

$ cargo clean -p qttypes && cargo clean -p i-slint-backend-qt
     Removed 26 files, 28.0MiB total
     Removed 26 files, 26.5MiB total
$ cargo build
   Compiling qttypes v0.2.12
   Compiling i-slint-backend-qt v1.13.1
   Compiling i-slint-backend-selector v1.13.1
   Compiling slint v1.13.1
   Compiling offline-translator-linux v0.1.0 (/home/david/git/offline-translator-linux)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 1.54s
$ ldd target/debug/offline-translator-linux
        linux-vdso.so.1 (0x00007ffcfbdca000)
        libstdc++.so.6 => /lib/x86_64-linux-gnu/libstdc++.so.6 (0x00007931fbe00000)
        libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x00007931fd76a000)
        libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007931fc117000)
        libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007931fba00000)
        /lib64/ld-linux-x86-64.so.2 (0x00007931fd7b9000)

I was trying to avoid doing a full clean because one of my deps takes a while to build; deleted the whole target dir and got exactly the same outcome; qt is not linked and startup gives

Error: No default Slint platform was selected, and no Slint platform was initialized

I assume there's no shenanigans with debug build == interpreter == no qt?

[tronical]

Hm looks all good. What does qmake —version print?

I’m a bit puzzled. At this point I can only defer to the docs you’ve probably already seen:

https://github.com/slint-ui/slint/blob/master/docs/install_qt.md

perhaps @ogoffart has an idea?

[DavidVentura]

$ qmake --version
QMake version 3.1
Using Qt version 5.15.13 in /usr/lib/x86_64-linux-gnu
$ qmake6 --version
QMake version 3.1
Using Qt version 6.4.2 in /usr/lib/x86_64-linux-gnu

and no warnings at all 😞
I'm on a 24.04 KDE desktop,

[ogoffart]

Can you check the log of the qttypes build script?
They should be in target/debug/build/qttypes-xxxxx/output or stderr (where xxxxx is some random hash)

[DavidVentura]

after a cargo clean -p qttypes and a build I get two build dirs:

$ ls target/debug/build/qttypes-
qttypes-2f580a7594dc7b3e/ qttypes-4825e2c789773ac0/ 

output is

cargo:ERROR_MESSAGE=Cannot open `\"/usr/include/x86_64-linux-gnu/qt6/QtCore/qconfig.h\"`, please make sure that the Qt headers are installed.\nNo such file or directory (os error 2)

installed qt6-base-dev and now I get a file that properly links to Qt!

The docs mention On many distributions, you also need the -dev packages. but it wasn't clear for me what was missing (qt6-base-dev)

maybe the build could print a warning on this case? even if it's opt-in via a feature, it would be nice

[ogoffart]

maybe the build could print a warning on this case? even if it's opt-in via a feature, it would be nice

How?
We do output it in a warning. But cargo doesn't show warning for dependencies.

I think cargo build --verbose would also show the warning.

[DavidVentura]

In my opinion: qt mode should be a feature, non-default, and the build should fail in this case, not produce a warning.
Generating a non-working binary is not useful.

I understand it's probably a breaking change, so it'd be hard to do.