Support ignoring lints in editable playground blocks

[djmitche]

Problem

In a short Rust snippet, it's common to include code which has warnings like dead-code or unused-variables.

In a non-editable snippet, these can be hidden with the hidelines support. However, in a runnable snippet, they always appear.

Proposed Solution

Add a new config:

[output.html.playground]
prefix = "#![allow(unused)]"

and inject this string as a prefix to every playground run.

Notes

No response

[djmitche]

(This should probably be disabled by a class on the code block, e.g., noprefix)

[max-heller]

Would it make sense for mdbook to mirror rustdoc and automatically allow a set of lints?

[djmitche]

That's not a bad idea, as long as it's customizable!