CMake: Fix unnecessary rebuilds in Visual Studio projects

User projects that use qt_add_qml_module faced unnecessary rebuilds when
using the Unix Makefiles or Visual Studio project generators.

The bug was in qt6_extract_metatypes. For the mentioned generators (and
CMake 3.16 in general), we detect that CMake's AUTOMOC does not generate
a dep file, and we use a custom target to trigger cmake_automoc_parser.
That custom target runs always.

Later, we call moc to generate meta_types/foo_metatypes.json.gen. To not
trigger a rebuild of its dependents on every build, we use
copy_if_different to generate meta_types/foo_metatypes.json only if the
content changed.

The custom command for that had two outputs:
     add_custom_command(
         OUTPUT
             ${metatypes_file_gen}
             ${metatypes_file}
         ...
     )

The Unix Makefile generator and the Visual Studio generator (and
potentiall others) interpret this as "all outputs are always changed by
this command". The Unix Makefile generator even ensures that by creating
an additional foo_metatypes.json target that touches the file. In the
generated Visual Studio project, the dependents of foo_metatypes.json
are out of date even if the actual file has an old timestamp.

In projects with QML modules, the QML type registration depends on this
file. This caused a near full rebuild of the affected QML modules.

We must mark foo_metatypes.json as byproduct of the command to specify
that it is a file "whose modification time may or may not be newer than
the dependencies".

Unfortunately, there's CMake upstream issue #19005. There's a dependency
missing between the consuming target and the byproduct. We must work
around this issue with an additional custom command with OUTPUT
${metatypes_file}.

Pick-to: 6.5
Fixes: QTBUG-106683
Change-Id: Icc2f7227c3e7b523631c1f361e29e26ae4efc46c
Reviewed-by: Alexey Edelev <alexey.edelev@qt.io>

0 files changed, 0 insertions, 0 deletions