Linking bindgen-generated libraries against other bindgen-generated libraries

[asajeffrey]

If there are two libraries a.h:

struct Foo { ... }

and b.h:

#include "a.h"
Foo bar() { ... }

and both of these have sys crates generated by bindgen, there should be a way to get b_sys to use the Foo from a_sys. We can almost do this by putting the following into the build.rs of b_sys:

   builder.raw_line("use a_sys::*");

but this produces:

use a_sys::*;
pub mod root {
   ...
   fn bar() -> root::Foo { ... }
}

That is the raw_line is outside the root crate, not inside it. If we could put raw lines inside the root, this would be pretty easy.

[asajeffrey]

IRC conversation with @emilio: https://mozilla.logbot.info/servo/20180503#c14702783