[rustdoc search] allow setting crates to search in before showing results

[lolbinarycat]

specifically, this would mean moving the #crate-search dropdown so it is available outside of the results screen.

on desktops, it could be moved into the <rustdoc-search> search form, and displayed in a single line. on mobile, it could be displayed directly underneath the search bar line.

this would significantly speed up searching through the nightly-rustc docs when you already know what crate you want to search, although it would require some small refactorings of rustdoc's search.js.

[notriddle]

It used to work that way. There are two reasons why it doesn't any more:

1. less clutter Controls and search in Rustdoc header are visually distracting #59840
2. less stuff that needs to load Improve loading of crates.js and sidebar-items.js #98124

[lolbinarycat]

hmm... it might be possible to offset both of those...

what if it was prerendered, and we hid it behind a setting?

[notriddle]

A better possibility might be to show it when you focus the search box, without requiring you to type anything into it first.

[lolbinarycat]

yeah, that's actually what i was originally going to suggest for mobile.

i think that should be doable with just css, actually.