Aspect to generate workspace-wide rustdoc

[acmcarther]

This issue is motivated by several observations

• rust_doc rules don't really have tunable parameters
• It should be possible to generate rust_doc decls for all rust_library rules
• It should be possible to link generated rust_doc together in terms of exact dependencies
• It should be possible to index workspace wide rustdoc by BUILD path

I think aspects are a good fit for this, and a simple packaged rules_rust implementation could enable generating lots of immediately useful docs (including for external deps!). As a stretch goal, it could also probably supplement the generated information with details from the workspace, and perhaps even provide a simple binary to serve the unified documentation.

[mfarrugi]

Sounds good to me, since it removes the noise of declaring extra redundant targets.

Would this treatment make sense for doctests too? I imagine regular tests use dev dependencies a little too often for that to make sense.

How does rustdoc let us organize docs by build paths?

[acmcarther]

My understanding is that rustdoc provides lots of niceties for single crates, but basically nothing for multiple crates. Projects that expose documentation for multiple crates do so in a custom way.

I figure that if sufficiently motivated, we could probably supply something similar.... a generated static site that proides an aggregation of the complete set of documentation. I was thinking that such an aggregation probably ought to be indexed by the BUILD path to each crate, and could probably also provide cross-links to dependencies and dependents.

I haven't set aside a lot of time to work on this yet, but I thought about it out of the blue and I figure the minimal example is actually not too difficult to set up.