parsing limitations.
Sundown parses
```
~~~
as a valid codeblock (i.e. mismatching delimiters), which made using
rustdoc on its own documentation impossible (since it used nested
codeblocks to demonstrate how testable codesnippets worked).
This modifies those snippets so that they're delimited by indentation,
but this then means they're tested by `rustdoc --test` & rendered as
Rust code (because there's no way to add `notrust` to
indentation-delimited code blocks). A comment is added to stop the
compiler reading the text too closely, but this unfortunately has to be
visible in the final docs, since that's the text on which the
highlighting happens.
This theoretically gives rustdoc the ability to render our guides,
tutorial and manual (not in practice, since the files themselves need to
be adjusted slightly to use Sundown-compatible functionality).
Fixes#11392.
It's too easy to forget the `rust` tag to have a code example tested, and it's
far more common to have testable code than untestable code.
This alters rustdoc to have only two directives, `ignore` and `should_fail`. The
`ignore` directive ignores the code block entirely, and the `should_fail`
directive has been fixed to only fail the test if the code execution fails, not
also compilation.
