Indentation of # hidden code

[dtolnay]

In https://github.com/brson/rust-cookbook/pull/107 we discovered that mdBook is indenting hidden code by one space when expanded.

a
# b

... turns into:

a
 b

This is not the behavior of rustdoc. For example look at this standard library example and then click through on the "Run" button here. The line is not indented by one space.

[steveklabnik]

Interesting; I thought I copied rustdoc's behavior here exactly.