Line hiding in Rust code blocks is inconsistent with `rustdoc`

[max-heller]

Problem

Lines in Rust code blocks beginning with a # and followed immediately by a character other than !, [, #, or are hidden by mdbook but shown by rustdoc.

For example, the following code block is rendered as empty by mdbook:

```rust
#fn main() {}
```

Meanwhile, the following code block in a doc comment:

/// ```
/// #fn main() {}
/// ```
pub struct Foo;

is rendered as

Steps

1. mdbook init
2. echo '```rust\n#fn main() {}\n```' > src/chapter_1.md
3. mdbook serve
4. Observe an empty code block. When the "Show hidden lines" button is clicked, the code block contains fn main() {}

Possible Solution(s)

mdbook should implement rustdoc's behavior since the docs claim it does:

For the Rust language, you can use the # character as a prefix which will hide lines like you would with Rustdoc.

Notes

rustdoc's rules are:

• If a line begins with # (note the space following the #), the line is hidden
• Two consecutive hashes ## can be used to prevent the hiding of a line that would otherwise be hidden
  • To write a non-hidden line that starts with # , write ## which will render as #

Is mdbook willing to change this in a non-breaking release since it seems like a bug according to the docs, or would you rather wait until 0.5?

Version

mdbook v0.4.43

[ehuss]

Yea, I'd like to see a PR synchronizing with rustdoc's rules.