Document proc-macros despite bugs in rustdoc

[jyn514]

Use documentation in /target/doc if /target/$target/doc doesn't exist.
In that case, also don't pass --target to rustdoc.

Closes #422.

I also moved build_target into a separate function since build_package was getting kind of big.

r? @QuietMisdreavus

[QuietMisdreavus]

Is there an issue on Cargo or rustdoc tracking the problem upstream?

I remember we talked about there sometimes being a problem with it not respecting --static-root-path. Does not passing --target on the "test build" solve that?

I also see that you don't build any extra targets for proc-macros. This gets around the immediate issue, but i can't shake the feeling that there's going to be a proc-macro somewhere that does something different per-platform. This might work for now, but we should really be reporting this to Cargo so that this can be fixed properly.

[jyn514]

Is there an issue on Cargo or rustdoc tracking the problem upstream?

Yes, rust-lang/rust#66796.

Does not passing --target on the "test build" solve that?

Yes.

there's going to be a proc-macro somewhere that does something different per-platform

Probably, and it might make sense come back to this once the upstream issue is fixed, but I think this hits the vast majority of use cases without being terribly hard to change in the future.

[jyn514]

Just for help reviewing - you can build an example proc macro with docker-compose run web build crate rstest 0.4.0, the compiled docs should look something like this:

[QuietMisdreavus]

This should work for now as a stopgap solution. I want to ask the Cargo team about this issue, and see if it's intended behavior like it is for build scripts. In the meantime, this should allow proc-macro crates to actually display documentation. I'm not happy about not having anything in the "Platforms" menu, but that can possibly be fixed separately. For now, let's just get this in.