One of the least visible features, yet one I find among the most interesting aspects of the Acceleo code generator, is the traceability between the generated code, the model used as input of the generation, and the generator itself. Basically, you always know where that esoteric line of code came from : which part of the generation template generated it, and which model element triggered its generation.
This feature has known a series of improvements as we used it intensively with the UML to java generator. We are now confident that we can record and display accurate information even for some of the most complex use cases. This generator isn't the most complex generator we could write with Acceleo, but it is quite complete nonetheless. Here are some examples of what the Traceability can provide to architects developping their code generators :
Determine which model element triggered the generation of a line of code
On the left-hand side, a file that has been generated. On the right-hand side, an editor opened as result of using the action "open input" : the model is opened and the exact element that was used to generate the part of code is selected.
Find the part of a generator that created a given part of the code
On the left-hand side, the same generated file as above. On the right-hand side, the result of using the "open generator" action : the Acceleo generator which generated that selected part of the code is opened, with the exact source expression selected.
These are but a few of the features that can be derived from the synchronization between code, model, and generators. Some more examples include : previewing the result of a re-generation, incremental generation, round-trip (updating the input model according to manual changes in the output generated code)...
Most of these features are better seen in video to get an idea. If you want to see some of them in action, some more flash videos of what traceability can do for you are available on the Obeo network (though a free registration is required).