Pull request 1252 in include-what-you-use: iwyu test util: add support for EXTRA IWYU ARGS env variable

[notifi...@include-what-you-use.org]

New pull request 1252 by Flowdalic: iwyu_test_util: add support for EXTRA_IWYU_ARGS env variable
https://github.com/include-what-you-use/include-what-you-use/pull/1252

When running the tests as part of Gentoo's emerge process (in src_test()) the tests fail due clang's include directory missing.

The emerge phase that runs the tests is performed with the application under test still in the build directory. And this is what breaks the IWYU tests there.

The include-what-you-use binary seems to have, amongst other, the following default include path (as per
"include-what-you-use-0.19_build/bin/include-what-you-use -E -x c++ - -v < /dev/null"):

ignoring nonexistent directory
"/var/tmp/portage/dev-util/include-what-you-use-0.19/work/include-what-you-use-0.19_build/bin/../../../../lib/clang/15.0.7/include"

Here,
/var/tmp/portage/dev-util/include-what-you-use-0.19/work/include-what-you-use-0.19_build/ is the build directory. Unfortunately, the whole path to the clang include is invalid (non-existent). It will become correct once IWYU is merged into the live filesystem.

As workaround I added EXTRA_IWYU_ARGS to the run_iwyu_tests.py script, to be able to provide the correct include path by invoking run_iwyu_test.py basically like:

EXTRA_IWYU_ARGS="-I /usr/lib/clang/15.0.7/include" \
run_iwyu_tests.py \
-- ${BUILD_DIR}/bin/include-what-you-use

Related Gentoo bug: https://bugs.gentoo.org/905214