I get the following output:

% cargo watch -- cargo run -- -h
Error: I/O error: Permission denied (os error 13)

there's indeed a folder I can't access:

% find > /dev/null 
find: ‘./svntogit-community/sh4d0wup/trunk/pkg’: Permission denied

I assume this error is from cargo-watch (but possibly in the watchexec codebase), I think this should either not be fatal, or the path should be in the error message.