Just an update.
I was lead to believe that a problem with the UUID in the file:
$ cat /etc/initramfs-tools/conf.d/resume
caused the error.
For some reason, this UUID was not reflecting the real UUID of the swap file.
I was about to mark this as "solved" by that. But for my surprise, look at this events (reported as an image here in imagebin, sorry about that):
After gcc fatal error saying the file (that is open in another terminal, with vim, which I just compiled a few lines above successfully,) didn't exist, I went to the other terminal, typed ":w" in vim, so to save it again, and back to the compile terminal it found the file.
How come!?? It is amazing. This machine is doomed, or the system, not sure. Something is really wrong here. I can lose files just like that!?
Thanks for any input.
PS. Maybe I should start a new installation from scratch, or maybe just to be sure, start using the dual boot I've installed as Devuan, which I'm still not fully using. It is just there, just in case. I don't know. Maybe it is a KDE thing, because I tried to TEST all hardware (disk and memory).
PS. Ok, I was fast in screenshot, afraid to lose the error. But now that I wrote this email easily, here it is, the same info in the image above, as text. No need to see the image anyway.
[20181010.010115, !5088]$ gcc mequine.c -o mequine.x -Wall -Wextra -g -O0
mequine.c: In function ‘main’:
mequine.c:21:17: warning: implicit declaration of function ‘printcscc’ [-Wimplicit-function-declaration]
[20181010.010304, !5088]$ gcc mequine.c -o mequine.x -Wall -Wextra -g -O0
gcc: error: mequine.c: No such file or directory
gcc: fatal error: no input files
[20181010.010328, !5088]$ gcc mequine.c -o mequine.x -Wall -Wextra -g -O0
[20181010.010345, !5088]$ springe ~/tmp/nofileScreenshot_20181010_010612.png
>>>> It was a simple cycle: compile, error, fix, save, compile again (AND THE FILE DISAPPEARED), save again, compile again (ALL GOOD).