Teach moc to output a Make-style depfile

If moc is invoked with the --output-dep-file option, it will generate
a "moc_<source_file_name>.d" dep file which contains dependency
entries that can be consumed by a Makefile / Ninja build system.

This is useful for build tools (like CMake) to know when moc should be
re-ran.
In the future, it might also be useful for ccache (teach ccache not to
re-run moc when not necessary).

The dependency list contains: the original source file, the passed
--include files (like moc_predefs.h), the include files that
were discovered while preprocessing the source file, and the plugin
metadata json files listed in Q_PLUGIN_METADATA macros.

The file paths are encoded using QFile::encodeName, so using the local
8-bit encoding.

The paths are also escaped (so ' ' replaced by '\ ', '$' by '$$',
etc) according to the Make-style rules as described in
clang's dep file generator
https://github.com/llvm/llvm-project/blob/release/9.x/clang/lib/Frontend/DependencyFile.cpp#L233

For reference, the equivalent Ninja depfile parser source code can be
found at
https://github.com/ninja-build/ninja/blob/v1.9.0/src/depfile_parser.in.cc#L37

Additional options that can be passed:
--dep-file-path - to change the location where the dep file should be
generated.
--dep-file-rule-name - to change the rule name (first line) of the
dep file (useful when no -o option is specified, so output goes to
stdout).

Encoding story.
Note that moc doesn't handle non-local-8-bit characters properly when
processing include directives at the preprocessor step. Specifically
the content of the main input file is read as a raw byte array (which
can be UTF-8 encoded) and then each include directive is resolved via
Preprocessor::resolveInclude(), which calls QString::fromLocal8Bit().

Because moc uses the QtBootstrap library, only a limited set of codecs
are available: various UTF 8 / 16 / 32 codecs and
QLatin1Codec (ISO-8859-15).

This means that on Windows, if the source input file is UTF-8 encoded,
and contains include names with UTF-8 characters (like an emoji or any
character >= 127 that is not in the QLatin1 codec), moc will fail to
resolve and process that include, and thus no dep file entry will be
created either.

On macOS / QNX / WASM the main locale is UTF-8, so file content
and paths will be processed correctly (hardcoded via QT_LOCALE_IS_UTF8
in src/corelib/codecs/qtextcodec_p.h).

On Linux it will depend on the current locale / encoding set,
and if that encoding is one of the ones supported above. UTF-8 should
work fine.

[ChangeLog][QtCore][moc] moc can now output a ".d" dep file that can
be consumed by other build systems.

Task-number: QTBUG-74521
Task-number: QTBUG-76598
Change-Id: I5585631ff1bbbae4e2875cade9cb6c20ed018c0a
Reviewed-by: Leander Beernaert <leander.beernaert@qt.io>
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

1 files changed, 1 insertions, 0 deletions

diff --git a/src/tools/moc/moc.h b/src/tools/moc/moc.h
index 687ea2552f..5d1ae0ad6d 100644
--- a/src/tools/moc/moc.h
+++ b/src/tools/moc/moc.h
@@ -223,6 +223,7 @@ public:
     QHash<QByteArray, QByteArray> knownQObjectClasses;
     QHash<QByteArray, QByteArray> knownGadgets;
     QMap<QString, QJsonArray> metaArgs;
+    QVector<QString> parsedPluginMetadataFiles;
 
     void parse();
     void generate(FILE *out, FILE *jsonOutput);