env reporting environment variables starting with exclamation mark (possible path problems)?

[sdbbs]

To keep the long story short: I am generally a happy user of MINGW64 and MSYS2, and try to regularly keep updated. Eventually, I wanted to refresh a project I'm building using CMake - and suddenly, it started failing. It appeared to me that it is trying to build in some wrong directories, so I decided to check env - and I saw some variables I've never seen before:

$ env
ProgramFiles(x86)=C:\Program Files (x86)
!::=::\
CommonProgramFiles(x86)=C:\Program Files (x86)\Common Files
!C:=C:\Users\user\Desktop
...

I really don't understand how can a variable name be named !:: or !C: - the !C: one seems to be related to the folder where the MSYS2/MINGW64 was first started. I cannot even echo the content of these variables, and I cannot unset them; interestingly export -p does not report them.

I tried asking in https://stackoverflow.com/questions/76355596/env-reporting-environment-variables-starting-with-exclamation-mark-msys2-mingw6 but I doubt I'll get an answer there. In any case, I cannot recall ever seeing these "exclamation mark"-named environment variables ever, so I guess this is something recent. And the only reports I found, that I thought might be somewhat related, are:

• Unexpected/Incorrect Path Conversion when Exporting Environment Variables #152
• Unexpected change in path conversion behaviour after #143 #150
• Strange behavior of MSYS shell version 5.1.8 MSYS2-packages#3605

Anyways - does anyone have any idea what do the !:: and !C: environment variables mean, and whether I could somehow unset them?

Btw, in my CMake project that is failing, if I call my "Windows" CMake install, like this:

$ /c/Program\ Files/CMake/bin/cmake.exe --version | head -n1
cmake version 3.14.3

$ /c/Program\ Files/CMake/bin/cmake.exe  ../ -DCMAKE_BUILD_TYPE=Debug -G "MSYS Makefiles"

... then most of my project builds, apart from one of the final targets (and it used to build fine in this setup a couple of months ago).

If I instead use the MINGW64 cmake:

$ cmake --version | head -n1
cmake version 3.26.4

$ cmake.exe  ../ -DCMAKE_BUILD_TYPE=Debug -G "MSYS Makefiles"

... then make fails almost immediately with the following (and it used to build fine also in this setup a couple of months ago):

/C/msys64/usr/bin/python3.11.exe: can't open file '/c/src/my_project/build/pico-sdk/src/rp2_common/boot_stage2/C:/src/rp2040_pico/pico-sdk/src/rp2_common/boot_stage2/pad_checksum': [Errno 2] No such file or directory

At least, I can tell that /C/msys64/.../C:/src/... is under no circumstance a valid path.

Is it possible, that these "exclamation-mark-named" environment variables !:: and !C: mess up path calculation as used the bash and/or CMake ...? Any ideas how do I get my build system working again?

[dg0yt]

Anyways - does anyone have any idea what do the !:: and !C: environment variables mean, and whether I could somehow unset them?

I happened to stumble over this today:

msys2-runtime/winsup/cygwin/environ.cc

Lines 1373 to 1379 in d52da78

	/* See if environment variable is "special" in a Windows sense.
	Under NT, the current directories for visited drives are stored
	as =C:=\bar. Cygwin converts the '=' to '!' for hopefully obvious
	reasons. We need to convert it back when building the envblock */
	if (s[0] == L'!' && (iswdrive (s + 1) || (s[1] == L':' && s[2] == L':'))
	&& s[3] == L'=')
	*s = L'=';