Don't build default target twice

[jyn514]

This is the non-breaking changes from #532. I also kept the additional documentation for existing metadata.

Copy/pasted from that PR for convenience:

Architecture Changes

This removes the duplicated 'default' build which is run twice.

• The default build no longer passes --target to Cargo for the common case where default-target is not set. This will fix Cfg missing during build script build #440. As a side effect, it removes most of the hacking around I had to do to get proc macros to work.
• The target for the default build is now stored in the database. As before, the default target is x86_64-unknown-linux-gnu and can be overriden with default-target = ... in Cargo.toml
• The target for the default build is no longer run twice
• The web server redirects requests to /:crate/:version/:default-target to /:crate/:version/, keeping the subpage if it exists.

This has been tested both by building with the new code and by building with the old code and running cratesfyi database migrate. Note that if cratesfyi database migrate is not run, the web server will panic whenever viewing a crate built with the old code.

r? @pietroalbini

[pietroalbini]

I still need to test building crates locally, but this looks great!

The only change I'd like to see is on the dropdown menu allowing you to choose the target: with this PR, if you click the default target you'll be sent first to /:crate/:version/:default-target, and then redirected to /:crate/:version. I'd be nice to remove that redirect.

[pietroalbini]

Build code also seem to work fine!

[jyn514]

if you click the default target you'll be sent first to /:crate/:version/:default-target, and then redirected to /:crate/:version. I'd be nice to remove that redirect.

I'm not sure how to fix this. Currently we set those links like this in templates/navigation_rustdoc.hbs:

                {{#each doc_targets}}
                <li class="pure-menu-item"><a href="/{{../../content.crate_details.name}}/{{../../content.crate_details.version}}/{{this}}/{{../../content.crate_details.target_name}}/" class="pure-menu-link">{{this}}</a></li>
                {{/each}}

We'd have to compare each doc_target to the default_target, but I don't know how to do that in handlebars. The API reference says that #if has to be passed a single value, it can't handle == or anything like that.

[pietroalbini]

I'm not sure how to fix this.

I'd generate a Map<TargetName, TargetUrl> from Rust and pass it to the template, sidestepping the handlebars limitations.

[jyn514]

I don't see a way to send a data structure to a Page, we'd have to make it part of CrateDetails.

Is it ok if we add this at a later point? It's not hurting usability, you still get sent to the right page.

[pietroalbini]

I don't see a way to send a data structure to a Page, we'd have to make it part of CrateDetails.

Yep.

Is it ok if we add this at a later point? It's not hurting usability, you still get sent to the right page.

Ok, can you open an issue about that?

[jyn514]

Opened: #549