Testing error messages from procedural macros

[sphynx]

As noted here, it would be nice to add tests for checking error messages got from procedural macros during compilation time. Since they are visible to users, it's important to keep them readable, predictable and stable between versions.

An example of an error message from a macro in compile time:

error: proc-macro derive panicked
  --> src/main.rs:11:10
   |
11 | #[derive(StructOpt, Debug)]
   |          ^^^^^^^^^
   |
   = help: message: cannot parse structopt attributes: ( parse ( 2 + 2 ) ): Error("expected identifier")

error: aborting due to previous error

There is no easy way to run tests in compile time using conventional [test] attributes. However, I've found a nice package called trybuild from @dtolnay, author of syn and quote. The package tries to compile test programs and matches the output against predefined .stderr files, which looks like exactly what is needed here.

I can take a closer look on that package and then incorporate it in structopt testing infrastructure.

[TeXitoi]

trybuild seems the way to go, I'll accept such a PR.