Run button for playground missing from books

[ollie27]

Since the move to mdBook (part of #39588) the "Run" button to run code example in the playground is missing from the book, nomicon and reference.

before after

[steveklabnik]

Yes, this requires PRs to fix. I'm working on it.

Please file these issues against the respective repositories for them, thanks! (Or, in this case, don't worry about it, as I'm already doing it.)

[ollie27]

This is a regression in rustbook which is in this repository...

[carols10cents]

@ollie27 Rustbook is now a thin layer on top of mdBook, and mdbook has not implemented playground integration yet.

[steveklabnik]

... it is partially implemented, and we can make it work.

I already had a PR in the works until I trashed my linux partition. :(

Regardless, it involves changing the text of the actual books. It's gonna be really nice! I'm hoping to knock it all out monday.