Minification makes JS parsing slower

[GuillaumeGomez]

This bug has been spotted by @est31:

Rustdoc search-index.js regression

There seems to be a regression in how quickly rustdoc pages load.

Just do the following with Firefox 64.0:

1. visit https://doc.rust-lang.org/1.32.0/core/convert/trait.TryFrom.html
2. open network tab in developer console
3. reload the page with cold caches (ctrl+shift+R)
4. At the bottom there is a number load: XXX in pink

Now do the same procedure for the nightly docs https://doc.rust-lang.org/1.32.0/core/convert/trait.TryFrom.html

What I see is a difference in the pink load time. For nightly, it's 466 ms and for stable, it's 1.6 seconds.

Now let's dig a little bit deeper. There is a "performance analysis" tab in the Firefox developer tools.
Here you can see what the browser is doing to load a page.

If you visit the nightly page, there is a clear yellow bar with 597 ms wait time for me. If you click this bar you can see that it's spending in search-index.js. If you visit the stable page, you have to search a bit for the yellow bar of search-index.js but if you find it, it's much much smaller. Only 52 ms!

So I'd claim that the regression in load time is due to a parsing time regression for search-index.js.

I can experience the bug on a personal project that uses nightly as well, and here it's far more grave (multiple seconds to wait), most probably because it's a bigger project.

I experience this bug only since a few days, thus I believe this to be a regression of this PR: #56869

One idea I had is to use an array instead of creating a lot of variables. It'll certainly take a bit more space but I think it'll allow to remove the parsing performance issue.

[SimonSapin]

Can the loading of this large data be delayed until search is actually used? Could it be JSON loaded over XHR instead of a script?

[jethrogb]

Related: https://internals.rust-lang.org/t/something-wrong-with-local-cargo-doc-documents/9282 . If your search-index.js gets big enough, it can take several seconds to load during which the entire page hangs. I think this should be a high-priority issue.

[jethrogb]

https://users.rust-lang.org/t/firefox-is-much-slower-than-chrome-or-edge-on-some-rust-document-pages/25455/5