This is a new feature in Rebar3 3.7.0, to write custom compilers to be used with it. It is useful whenever you have files of a different language that you want to build alongside Erlang resources. Starting with Rebar3 3.14.0, the interface was enhanced to allow better tracking of files in a directed acyclic graph (DAG), which lets you annotate build artifacts and allows Rebar3 to track dependencies across applications. This interface is currently used internally for .xrl, .yrl, and .mib files...
