Easy "stdout contains" assertions

[killercup]

I'm trying to use this for a project instead of the published version of assert_cli. I'm super impressed by what you've accomplished here in the short time!

It seems that currently, there is no easy way to check that the output of a command contains some text. Or did I miss something?

Predicates only has a str::contains but stdout is a Vec<u8>. It seems that it could work with predicate::function, but it is quite cumbersome. I'm basically looking for good way of writing .stdout(&predicate::function(|x: &Vec<u8>| String::from_utf8_lossy(x).contains("Done!"))) -- but ideally without the need to use from_utf8_lossy. This last requirement is also why I opened this issue here, and not on the predicates, I expect we may need to talk about how to best deal with the Vec<u8> representation.

[epage]

Any Predicate<str> has an extension method from_utf8 to turn it into one that should be usable for stdout... I hope. Can't remember if that has an explicit test. Check the tests folder for possible examples.

[killercup]

Ah, very cool. Couldn't find it because it's not yet in a released version. Sadly, it also doesn't seem to like Vec<u8> because "the trait predicates::core::Predicate<std::vec::Vec<u8>> is not implemented for predicates::str::Utf8Predicate<predicates::str::ContainsPredicate>".

[epage]

Didn't realize it wasn't published. I'll get it out and update the tests, fixing ergonomics along the way.

[epage]

Need to come up with solutions for assert-rs/predicates-rs#20

Specifically for this case, the best I'm coming up with is adding a deref() via extension traits and having IntoOutputPredicate call it on Predicate<&str>