rustdoc: add a list of headings to the sidebar

[QuietMisdreavus]

It's another misdreavus rustdoc PR, which means it's time for Bikeshed City once again! :3

In an effort to aid navigation in long documentation pages, this PR adds a listing of headings to the sidebars of pages where such headings exist. For example, for structs, links to their fields, inherent methods, and trait implementations are available where applicable.

Examples:

• Modules/Crate roots
  
• Enums
  
• Primitives
  
• Traits
  
• Structs
  

Open questions:

• Right now, these kinds of pages (and also unions) are the only pages that will receive the name header - pages for functions, constants, macros, etc, won't have the corresponding name in their sidebar. Should I print the name regardless and only add table-of-contents links for pages that have them? This would make them match, for example, a struct with no public fields, no methods, and no trait implementations. The latter would still have a "Struct MyStruct" line above the module contents, with no header links to speak of, whereas a function wouldn't even have "Function my_function".
• This is only a header listing, but there has been requests to include a more-complete listing of fields/methods/traits/etc, for example in Include a list of struct methods in reference sidebar on struct pages #41123.

[rust-highfive]

r? @steveklabnik

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[GuillaumeGomez]

Nice catch!

[QuietMisdreavus]

I was so confused why enums weren't getting the sidebar stuff, until I decided to trace back through what is_enum was doing. Apparently it's been like this for a while, but there's also no other users for this method, so it's not harming anything.

[GuillaumeGomez]

But now it's working correctly, which is great.

[GuillaumeGomez]

👍

[GuillaumeGomez]

Oh and please clean your git history.

[QuietMisdreavus]

Can do. I'll squash when this is ready to roll, in case there's some aesthetic commentary.

[GuillaumeGomez]

Well, please put a whitespace after // please. (since we're at it...)

[QuietMisdreavus]

Huh, the earlier commit failed Travis because the link-checker failed me for broken TOC links. Good job, link-checker! (^^)b

[carols10cents]

Hm is this waiting on a squash by @QuietMisdreavus or a review from @steveklabnik?

[QuietMisdreavus]

Probably mainly on me, but I was wanting to see if there were any remarks on the "open questions" I had on the bottom of the lead comment. If @steveklabnik and @GuillaumeGomez are fine with this as-is, though, I'll squash and carry on.

[GuillaumeGomez]

Good for me.

[steveklabnik]

Let's do it!

[GuillaumeGomez]

@bors: r+

[bors]

📌 Commit 27bfbd5 has been approved by GuillaumeGomez