Unit-like structs doc: Improve code sample

[MajorBreakfast]

r? @steveklabnik

BTW it seems that

let p = Proton{};

compiles without an error. That's why I didn't add it to the example. It's about consistency anyway.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @steveklabnik (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

[steveklabnik]

@bors: r+ rollup

thanks! (not sure what's up with travis)

[bors]

📌 Commit 3c5001f has been approved by steveklabnik

[frewsxcv]

@bors r-

This results in a doctest error:

https://ci.appveyor.com/project/rust-lang/rust/build/1.0.2166/job/mh1vjrnly4lrqgty

[durka]

Can examples in the book be marked compile-fail? Otherwise that code block will need to be ignored, or the error demonstration line commented out.

[frewsxcv]

Can examples in the book be marked compile-fail?

I think so:

https://github.com/rust-lang/rust/blob/master/src/librustdoc/html/markdown.rs#L541-L562

[MajorBreakfast]

@durka@frewsxcv Thx for the hint. I've added the tag. Now it should work. Let's wait for the tests to confirm it. :)

[frewsxcv]

@bors r+ rollup

[bors]

📌 Commit 8f1a0af has been approved by frewsxcv