Add tests to make sure the output of "rustc --expand pretty" can compile

[erickt]

I ran into an issue in #5578 where error!("...") was getting expanded into log(1, "...") instead of __log(1, "..."), which resulted in a compile error. We should add some tests that make sure we can still compile code after macros have been expanded.

[jbclements]

To clarify: if I understand you correctly, you're saying that the expansion of error!("...") was being rendered by pprust as log(1,"..."), not that the expansion was actually incorrect. Is that correct?

First off, that would obviously be a bug in the pretty-printer, which should be fixed. The larger issue of making sure that pretty-printed code compiles I would nominate for well-covered.

[erickt]

@jbclements: Yep, that was the problem, it's fixed now though. The larger issue still stands.

[graydon]

just a bug, removing milestone/nomination.

[klutzy]

Fixed by #14106!

[huonw]

Yay!