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If you're going to do that then please be considerate enough to add links to the other places you cross posted. This will let us avoid wasting time due to duplicate effort and also help others who have the same questions and find your post to discover all the relevant information. When you post links please always use the chain links icon on the toolbar to make them clickable.
flounder:
See a listing of the source and corresponding generated code
See the preprocessed source
See a link map
The Arduino IDE is intended to be very beginner friendly. For this reason, adding advanced features like these is not really desirable. It's easy enough to do any of these things via external tools so the IDE is not limiting advanced users from achieving any of these things.
flounder:
See a listing of the source and corresponding generated code
Despite the above, I do kind of like the idea of being able to preview the code generated by the sketch. A common complaint about the Arduino IDE from detractors is that it does things to your code behind the scenes so people aren't required to write completely valid C++ code. It would be educational for people to be able to easily see just what changes have been made. In rare cases the sketch preprocessor does screw up and so this would also be useful for troubleshooting but in that case it's easy enough to just open the generated file. The sketch preprocessor has recently been completely rewritten for release in Arduino IDE 1.9.0. The new behavior is to add the contents of all included files in the generated file, which means the file is thousands of lines long. So the actual file would no longer be something an average user would want to look at.
flounder:
Predefine symbols, e.g., -D_DEBUG
This sort of thing has definitely been requested before. It has been rejected by Arduino because they feel it could lead to less user-friendly library interfaces. You can get a lot more information on related proposals by following the network of links that spread out from this pull request: