debug_reprs taking up 41% of test runtime

[davidrusu]

It seems like we're formatting the test case arguments even on passing test runs, e.g. here is the flamegraph of a test run where the test passed, but we see 41% of runtime spent building up debug_reprs.

Does this make sense to you? do you see a way of modifying the test runner to only render the debug representation on failure?

[BurntSushi]

I'm not sure off the top of my head and this isn't high on my priority list. I would encourage you to investigate.

[davidrusu]

I think I have a solution in #304