It had been a source of huge bloat in rustdoc outputs. Of course,
we can simply disable compiler docs (as `rustc` generates over 90M
of HTML) but this approach fares better even after such decision.
Each directory now has `sidebar-items.js`, which immediately calls
`initSidebarItems` with a JSON sidebar data. This file is shared
throughout every item in the sidebar. The current item is
highlighted via a separate JS snippet (`window.sidebarCurrent`).
The JS file is designed to be loaded asynchronously, as the sidebar
is rendered before the content and slow sidebar loading blocks
the entire rendering. For the minimal accessibility without JS,
links to the parent items are left in HTML.
In the future, it might also be possible to integrate crates data
with the same fashion: `sidebar-items.js` at the root path will do
that. (Currently rustdoc skips writing JS in that case.)
This has a huge impact on the size of rustdoc outputs. Originally
it was 326MB uncompressed (37.7MB gzipped, 6.1MB xz compressed);
it is 169MB uncompressed (11.9MB gzipped, 5.9MB xz compressed) now.
The sidebar JS only takes 10MB uncompressed & 0.3MB gzipped.
9e28156551