Pass -fapplication_extension to modules

[keith]

Currently when you build for an application extension rules_apple (or rules_swift) don't pass -fapplication_extension or -application_extension to the compiler. This is passed to the linker to create the final application extension binary but I assume based on the ld manpage that this just validates dynamic libraries being used, and not the limited API set (for example some API from UIKit are valid, but some like UIApplication are not).

In the case of shared transitive dependencies between applications and their extensions, Xcode always passes these arguments and uses the same build artifact for both final binaries. I think that case is ideal (as opposed to building them twice similar to #319).

[thomasvl]

Yea, on the linker side it just sets a flag, the compiler needs the flag to change the warnings/error during compilation.

We've had some discussions internally about this and the problem is avoiding the double build; so it currently isn't passed.

Slight aside - Xcode doesn't always use the same artifacts, it all depends on how you set thing up. If you put sources/dependencies directly into your extension and app targets, they are compiled twice with the differing flags. When you use a framework and mark it for extension safe, then it can generate the warnings/errors and both the app and extension(s) can link against the framework with the App being able to call apis tagged as not extension safe while the extension can't call those apis.

[keith]

Ah yea, to clarify I meant that we would have a shared module, not just shared sources, so the module could be compiled once.