Meaning of "No repo mapping" on version 0.45.1

[d-r-jenkins]

I am stuck using 0.45.1, and I keep getting "No repo mapping found!" when I run bazel run //:gen_rust_project -- //path/to/anything (an alias to @rules_rust//tools/rust_analyzer:gen_rust_project) and I am struggling to work out what exactly this means... I know it is an older version, but can someone explain exactly what this means and and tips to debug it?