I read https://www.slicer.org/wiki/Documentation/Nightly/Developers/Tutorials/QtDesigner,and set the environment variable QT_PLUGIN_PATH to the directory Slicer-build/bin,but widgets do not appear in Qt Designer
Does running
<cmake-build-dir>\Slicer-build\Slicer.exe --designer
not provide those widgets?
Also make sure Slicer and QtDesigner is built using the same build mode. If you use a precompiled Qt version then you need Slicer Qt designer plugins built in Release mode.