Support .should_fail doc tests

[brson]

Right now all the doc examples that are demonstrating failure are xfailed.

[bstrie]

Which doc examples? I'm not seeing anything xfailed in the tutorial.

[toddaaro]

Sounds like a useful thing ... but I'm not sure what this thing is. @brson, could you elaborate a bit more on this?

[huonw]

Triage visit: I had a brief look at what would be required for this, and most of it is fairly mechanical, although I don't know enough about make to be able to set up the makefiles correctly; specifically, I believe these lines require being able to set --mode run-fail for specific files (and might as well add --mode compile-fail).

[huonw]

Still not supported. One way to do this would make rustdoc run the manual/tutorial/guide examples, rather than have a separate driver for it.

[alexcrichton]

Closing, we have this with the should_fail directive on code blocks.