Flag: configuration flags

[steveklabnik]

This issue is part of https://github.com/steveklabnik/rustdoc/issues/125

--cfg: passing configuration flags

Using this flag looks like this:

$ rustdoc src/lib.rs --cfg feature="foo"

This flag accepts the same values as rustc --cfg, and uses it to configure compilation. The example above uses feature, but any of the cfg values are acceptable.

---

Should we keep this flag or not?

[steveklabnik]

This seems pretty important.

[hjr3]

Per https://doc.rust-lang.org/book/first-edition/conditional-compilation.html, there does not seem to be a way to pass arbitrary configuration flags from cargo. We are limited to only toggling which features we want to be turned on.