there's no binaries in $bin even when not cross-compiling

so these files aren't created when cross-compiling

$ tree ./result-bin
./result-bin
└── lib
    └── qt-5.15.8
        └── qml
            └── QtWebChannel
                ├── libdeclarative_webchannel.so
                ├── plugins.qmltypes
                └── qmldir

Let me clarify: QtWebChannel does not produce anything at all in $bin when cross-compiling.

This causes Nix to reject the build because the builder failed to create an expected output.

Let me clarify: QtWebChannel does not produce anything at all in $bin when cross-compiling.

i understood that but why are qml files not generated when cross-compiling

plugins.qmltypes has this comment

// This file was auto-generated by:
// 'qmlplugindump -nonrelocatable -dependencies dependencies.json QtWebChannel 1.15'

qmlplugindump is in qtdeclarative which is in qtInputs which is propagatedBuildInputs in qtModule so the issue may be fixed by adding qtdeclarative to nativeBuildInputs

If there are no files in $out/bin why do we even have the bin output? The files seem to be wrong there in the first place.

I will investigate.

It might be a week or two before I have time to return to this qt5-cross stuff, but I will get to it. Native-compiling stuff on my laptop is too painful.