There are quite a few alt-JS language implementations available today - Objective-J (like Objective-C, but for JS), EGL (4GL from IBM), Clamato (Smalltalk), Schemes, Pythons, and Rubys. It would be nice to be able to support these somehow in Web Inspector.
The additional information, or pointers to it, would be added to the generated JS files, perhaps in the same kind of comment/directive format used for //@sourceURL= that allows naming your evals.
An initial implementation which just supported an alternate source representation and line number map could be used for setting breakpoints and stepping through the code. Dealing with mapping locals and other variables/properties, as well as stack manipulation, would be a lot more work. As a point of reference, some alt-JVM languages make use of existing Java bytecode facilities to handle just the source code/line numbers, and the resulting debugger experience is acceptable (eg, Groovy).
Given the alternative source and line number mappings, we can translate requests to/from the the JS engine to render the original source in the debugger views, supporting setting breakpoints, stepping, etc.
Tom Robinson noted that Caja seems to be generating debug info like this:
Should also check with FireBug folks to see if they're doing anything.
There was some chit-chat on stuff like this today on es-discuss:
Suggest we close this bug as a DUP of bug 63940 ; better yet, since there may be more bugs on this subject, we could have an "umbrella" bug and tie them together with Depends on (or however it's done). I suppose this one could be the umbrella.
Inspector supports source maps.