Docs: Visualizing Coverage

[kpcyrd]

It seems it's possible to visualize corpus coverage with libfuzzer and clang coverage:

https://github.com/google/fuzzer-test-suite/blob/master/tutorial/libFuzzerTutorial.md#visualizing-coverage

It would be interesting to have instructions on how to do this with cargo-fuzz so I can see if I'm currently missing any important branches.

[yshui]

I tried using cargo fuzz with the -Zinstrument-coverage rustc option, but I got "symbols defined in discarded section" error during linking.