Beginning with the Eclipse version Photon there is a new feature available to show inline information within the source code. I used this new feature to start developing an Open source plugin as addon for ABAP in Eclipse.
The plugin shows mainly two types of information within the code:
Showing information about ABAP code as inline text
For each method as first information the number of referencing ABAP objects are shown. Additionally to the overall number of references also the number of references from test classes are shown if this option is activated in the preferences.
The second part shows the method header information in the following form:
- Accessor (public, protected, private),
- Returning Parameter between the two 
- Importing, Exporting or Changing parameters between the squared brackets
But maybe its easier to see the plugin in action
The plugin is still in development but can already be downloaded over the Eclipse marketplace and the source code is available on GitHub. The only prerequisite is to use the Eclipse version Photon, 2018-09 or 2018-12 and of course ABAP in Eclipse has to be installed as the plugin reuses some ADT features.
Have fun while trying it out and let me know your experiences and impressions.