Emacs is a lisp interpreter disguised as an editor. The obscure lisp references are modules for that editor (think CPAN for Emacs). The rcov-overlay module calls ruby's coverage tool, brings the results back to emacs as a JSON data set, and highlights the resulting lines in the code that are covered or not. Linum is a module that shows line numbers in an emacs buffer, but allows you to customize what those line numbers look like.

My goals are:

• First, get the rcov-overlay port in place, but calling Devel::Cover tools instead. This would include returning the data in JSON format from Devel::Cover (currently does not exist). This would be (I think, I have only read the source) similar to Devel::Cover::Report::Vim. This would be in two parts - the rcov emacs component and the Perl module to generate the data.
• Second, make use of the customization features of the linum module to be able to indicate (say with differently colored pipe symbols) what type of coverage is failed for a given line number.

The first is to get the JSON reporting module in place, the second is to be able to get it identifying what is actually being reported. As an alternative for the second point, perhaps tooltips (hovertext, IntelliSense, whatever your editor's term of choice happens to be) might be the path to take. Not sure yet. Perhaps both will be selectable.

Hope that clarifies it a bit.