While that is a good point, it is quite unsightly to panic from unwrapping. Instead it should propagate that error back to the top level where a nicer error message can be generated.
More importantly cp should continue copying other inputs even if one of the source files fails to be readable. Handling errors by panic doesn't suggest that will be the case.
I don’t know if that is _more_ important, but I agree that it is also important :)
The reason I say this is that while users will expect this behavior, doing it well (so that it can be parallelized, for example) requires careful thinking about how errors should flow, how lines are printed to stdout, etc.
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