SyntaxCheckPlugin was not able to find cargo, but doesn't report not being able to find it, because the actual error message is /bin/sh: cargo: command not found, whereas the error checker looks for "error".
I installed Rust through rustup, so cargo is in $HOME/.cargo/bin. But the path of SyntaxCheckPlugin is the default: /usr/bin:/bin:/usr/sbin:/sbin. #92 did not address this plugin.