Add an option to allow rustdoc to list modules by appearance

[varkor]

The --sort-modules-by-appearance option will list modules in the
order that they appear in the source, rather than sorting them
alphabetically (as is the default). This resolves #8552.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @QuietMisdreavus (or someone else) soon.

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[QuietMisdreavus]

Thanks for the PR! This looks pretty good, but there's a couple things i want to poke at:

First, the tidy check failed because the optflag line was too long. This is what failed the travis check:

[00:04:12] tidy error: /checkout/src/librustdoc/lib.rs:257: line longer than 100 chars
[00:04:13] some tidy checks failed

(If you want to test this yourself before uploading any new commits, you can run x.py test src/tools/tidy which doesn't take long to run. If you've been building rustdoc locally and adding --stage 1 to your builds, you can add that here as well.)

Next, i'd like to see a test for this. How well do you know XPath? I think the ordering could be possible to express in the XPath that the rustdoc tests use, and i know that adding the flag for rustdoc is possible. The tests for rustdoc output currently live in src/test/rustdoc. and are just single-file libraries you stick in there with special comments that are read by src/etc/htmldocck.py. There's a big comment at the front of htmldocck.py that explains the test directives available.

[varkor]

I couldn't quite work out how to use XPath effectively here, but at least for the current page generation, simply using a regex worked sufficiently. I've added the test — thanks, I should have considered adding one earlier!

[QuietMisdreavus]

Nice! This looks great now. Thanks for making the flag an unstable one - that'll help if we need to mess with it later. If travis checks out, i'll go ahead and r+ it!

[QuietMisdreavus]

@bors r+

[bors]

📌 Commit b3c6102 has been approved by QuietMisdreavus