Can you run :call gutentags#toggletrace() so we can see what Gutentags is doing behind the scenes? When you open a file in a Rust project, it should tell you if it detected the project type correctly. Then it will tell you the command line it's using.
I'm wondering if a ctags executable path that contains && is maybe tripping the script that wants to run it (see this line).
Once you have the debug output and the log file, you can try the generated command line yourself to check what's maybe wrong with it?
Ah didn't know the cmd was being executed in a shell script. The issue then is that source gutentags.sh drops the ctags function into the current shell. But it is unavailable to any child shells. Like the one running in gutentags shell script. I would have to do an export -f ctags in the script where I define a custom ctags function.
There are few other considerations and I would have to look at them when I have time, like possibly needing to use bash -c for my own script. Anyway, this is what I'll just do right now and it works.
/home/.vim/shims/gutentags.sh with no function just a cmd.
rusty-tags vi && mv rusty-tags.vi tags.temp
let g:gutentags_ctags_executable_rust = '/home/.vim/shims/gutentags.sh'