Windows is perfectly okay with environment variables of the form "MESON_SUBDIR=", at least I can set such a variable in e.g. a python script and communicate it to another process. Although cmd.exe's set IIUC will delete it? But msys2 is not a cmd.exe emulator...

However, trying to run this fails:

"env" "MY_ENV=1" "MESON_SOURCE_ROOT=D:\a\meson\meson\test cases\common\51 run target" \
    "MESON_BUILD_ROOT=D:\a\meson\meson\tmp59june4u" "MESON_SUBDIR=" \
    "MESONINTROSPECT=D:\a\_temp\msys64\mingw32\bin\python3.exe D:\a\meson\meson\meson.py introspect" \
    "D:\a\_temp\msys64\mingw32\bin\python3.exe" "D:/a/meson/meson/test cases/common/51 run target/check-env.py" \
    "../test cases/common/51 run target" "." "../test cases/common/51 run target/"

Traceback (most recent call last):
  File "D:\a\meson\meson\test cases\common\51 run target\check-env.py", line 8, in <module>
    assert 'MESON_SUBDIR' in os.environ
AssertionError

This was brought up e.g. in msys2/MINGW-packages#1599 in the past.

...

It seems very wrong for the msys2 runtime to do this. If I run python3.exe basically-env.py "MESON_SUBDIR=" <script> it works flawlessly, but if I run env.exe "MESON_SUBDIR=" <script> it refuses to work because it's trying to emulate cmd.exe instead of Unix?

Unfortunately, Meson needs this functionality, because "the concept of being empty" is crucial information that needs to be passed from one process to another. I would like to pass this information via the most efficient program for setting such information, but it looks like I might need to restrict that to "real unixes".

Thoughts? Is it possible this restriction can be lifted?