GHC already collects all RdrNames for imported things, for use when reporting unused imports. But it doesn’t collect the SrcSpan of the occurrences, nor does it collect occurrences of locally-bound things.

I found the spot where the collected RdrNames are used to generate the unused import warnings, but I don't quite understand where they are gathered. Is there an AST traversal function somewhere that gathers these RdrNames? If so, I could use it as a blue print to write my own traversal.

I suggest you write a general traversal looking like

data Gather var res

= Gather { g_empty :: res

                  , g_union :: res -> res -> res

                  , g_occ :: Located var -> res

                 , g_del :: Located var -> res -> res }

getExpr :: Gather v res -> HsExpr v -> res

.. and similarly for each other data type...

Could you expand a little bit on this design? Is the idea that the Gather data type carries functions to apply in different parts of the AST? What's "occ" short for, OccName? What about "del"?

There are different kind of ASTs (e.g. after renaming, after type checking, etc), which one should I use if I want to gather all qualified names?

Thanks!

-- Johan