How to read lambdas in android / R8 stack trace de-obfuscated with retrace.jar?

Question

Specifically, lines similar to:

<my-package>.-$$Lambda$<my-class>$TBpBoCodhQrLRrW-jmx762QZ0VY.run(Unknown Source:2)

<my-package> and <my-class> were obviously replaced.

Perhaps there are some options to R8 to preserve pertinent information?

Answer 1

The classes with names starting with -$$Lambda$ are generated by R8 (and D8) when desugaring Java lambdas. These are synthetic classes, and they have no corresponding item in the input program. Normally a -$$Lambda$ will not be on the top of stack, as they will always call the lambda$xxx method generated by javac which has references back to the original source.

If you are using retrace.jar from ProGuard, please make sure to use a recent version which supports inline frames in the mapping file. Version 6+ should work.