Tobias Rittweiler (SLIME hacker and genrally froopy hood) outlined the following in an email conversation about directions to improve source location information in ABCL:

Low hanging fruits are the following:

 * Instead of just storing pathname & file-position also store:

        - a dspec that it what kind of the symbol is, e.g.

              (DEFUN FOO)
              (DEFMETHOD FOO (..specializers))
              (DEFVAR *FOO*)
          etc.

       - a snippet of the source code.

       - You can also look into SBCL's compiler how it constructs
         source paths (indices paths into the AST), it's pretty easy to
         do but requires some work. It's called GET-SOURCE-CODE, iirc.

         (Midterm goal)

  * At the moment, only one source location is stored. That doesn't mix
    well with CL's nature that a symbol can represent many things, and
    in particular not with GFs and methods. E.g. at the moment, ABCL
    only remember the source location of the last-recently loaded
    method.

       - Long term goal: Store source location information in method /
         function objects themselves. Make it extensible, so users can
         add source-locations for their own objects.

and

Another thing:

SWANK-COMPILE-STRING must be implemented on top of COMPILE-FILE, the
current implementation is wrong.

However, if you use COMPILE-FILE you'll go over a temporary file,
messing up EXT:SOURCE-PATHNAME and -FILE-POSITION. SBCL does that by a
:PLIST argument for WITH-COMPILATION-UNIT, and accessors from
debug-infos. CCL provides special keyword arguments to
COMPILE-FILE. Look into their swank backends.