Windows: Qt Debug dlls conflict with CXX-Qt

[LeonMatthesKDAB]

On Windows, due to Rust always linking to the non-Debug runtime, Corrosion users need to do the same with all linked in C++ dependencies.
This includes Qt!

On Windows, Qt ships debug dlls that are built in debug mode and link to the debug runtime.
This is not compatible with CXX-Qt, and we must ensure that our users always link to the right Qt binaries (e.g. the non-Debug ones, even in a Debug build).

We may be able to fix this in cxx-qt-cmake for our users, otherwise we should emit a warning/error, like we do if you have set the incorrect MSVC runtime.

Otherwise, we may run into UB, as the debug and non-debug runtimes on Windows are not ABI-compatible!

Encountered by @narnaud in yesterdays training.

[LeonMatthesKDAB]

The solution for this is outlined here:

rust-lang/rust#39016 (comment)

We can include this in cxx-qt-cmake with something like this:

target_link_options(<crate>-static INTERFACE /NODEFAULTLIB:msvcrt /DEFAULTLIB:msvcrtd)

Open Question: In a pure Cargo build, do we also link to the debug dlls automatically? If so, we need to emit the right linker flags in the build script as well.

[LeonMatthesKDAB]

It turns out this unfortunately doesn't work as easily.
My example was still compiling with the "MultiThreadedDLL" option set and just linking to the debug runtime.
For some reason this worked, but seems very problematic.

The other approach would be to just always use the release runtime.
I finally figured out how to to do this.
You can tell CMake to map to the Release config of the Qt components with:

set_target_properties(Qt6::Core PROPERTIES MAP_IMPORTED_CONFIG_DEBUG "RELEASE")
set_target_properties(Qt6::Quick PROPERTIES MAP_IMPORTED_CONFIG_DEBUG "RELEASE")
set_target_properties(Qt6::Gui PROPERTIES MAP_IMPORTED_CONFIG_DEBUG "RELEASE")
set_target_properties(Qt6::Qml PROPERTIES MAP_IMPORTED_CONFIG_DEBUG "RELEASE")
set_target_properties(Qt6::QuickControls2 PROPERTIES MAP_IMPORTED_CONFIG_DEBUG "RELEASE")

Unfortunately this has to be done for all Qt packages that link into the final binary...
We should seriously consider improving the Rust ecosystems support for the debug runtime so that we can finally fix this for good.