The error indicates that tokio isn't configured right. tokio has to be built with the unstable flag. There is a file ".cargo/config" which sets this flag. If you set RUSTFLAGS you will clobber this setting, so you have to add "--cfg tokio_unstable" to RUSTFLAGS if you override it. Gossip has a Cargo.lock which fixes in place the versions of all the dependenices so you can't be running some new tokio that breaks.