Code samples in demo

[emilk]

I thought something like this would be very nice to show to beginners:

However, the clever macro tricks I use to get the source code excerpts have some problems: file!() gives us a relative path when running cargo check, but an absolute path when running cargo test.

I may have to look for another way to accomplish this.

[ryo33]

I've opened a topic about the file!() behavior at
https://rust-lang.zulipchat.com/#narrow/stream/122651-general/topic/file!.28.29.20output.20varies.20in.20condition/near/313877507

@emilk If you have any additions or additional discussion, I would be happy if you would join.