rustdoc: Fixed an asynchronous loading of rustdoc sidebars.

[lifthrasiir]

We require the deferred loading, not just an opportunistic asynchronous loading. (Yes, that was my oversight, as I only checked it locally...) I think <script defer> is safe to use, according to http://caniuse.com/#feat=script-defer.

[rust-highfive]

r? @steveklabnik

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[Manishearth]

@bors: r+ rollup

[bors]

@bors r=Manishearth a3e4a16

[bors]

⌛ Testing commit a3e4a16 with merge b8c6eb1...

[killercup]

@lifthrasiir, why don't you insert the script tag for sidebar-items.js below the one for main.js?

If I read your code correctly, the position of the script tag is completely irrelevant to the rendering (you are not doing any document.write). The way you are doing it now depends on the fact that main.js was executed first, though. Thus, you need to use defer which would otherwise be unnecessary.

[lifthrasiir]

@killercup Simply because we had no existing machinary to put such "deferred" scripts into the bottom. This commit is intended as the quickest fix; if we had several deferring scripts then we should really have to consider that though. (And multiple uses of <script defer> in IE 9 or earlier have some problem, IIRC.)

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-32-nopt-t, auto-win-32-opt, auto-win-64-nopt-t, auto-win-64-opt