Auto merge of #37728 - QuietMisdreavus:rustdoc-enum-struct, r=GuillaumeGomez

rustdoc: fold fields for enum struct variants into a docblock

Per discussion in #37500, this PR updates the enum rendering code to wrap variants with named struct fields in a `docblock` span that is hidden automatically upon load of the page. This gives struct variant fields a clean separation from other enum variants, giving a boost to the readability of such documentation. Preview output is available [on the issue page](https://github.com/rust-lang/rust/issues/37500#issuecomment-260069269), but for the sake of completeness I'll include the images here again.

![rustdoc struct enum variant 4 part 1](https://cloud.githubusercontent.com/assets/5217170/20231925/96160b7e-a82a-11e6-945b-bbba95c5e4bc.PNG)

When you initially load the page, there's a line under variants with struct fields letting you know you can click to expand the listing.

![rustdoc struct enum variant 4 part 2](https://cloud.githubusercontent.com/assets/3050060/20232067/1dc63266-a866-11e6-9555-8fb1c8afdcec.png)

If you click to expand, the header and table unfold into a nicely-indented listing.

If you want to take a look in your own browser and screen size, [I've got this version hosted on my server](https://shiva.icesoldier.me/doctest/doctest/enum.OldTopicRemoval.html).

Fixes #37500

r? @GuillaumeGomez

src/librustdoc/html/render.rs

@@ -2488,8 +2488,13 @@ fn item_enum(w: &mut fmt::Formatter, cx: &Context, it: &clean::Item,
             if let clean::VariantItem(Variant {
                 kind: VariantKind::Struct(ref s)
             }) = variant.inner {
-                write!(w, "<h3 class='fields'>Fields</h3>\n
-                           <table>")?;
+                let variant_id = derive_id(format!("{}.{}.fields",
+                                                   ItemType::Variant,
+                                                   variant.name.as_ref().unwrap()));
+                write!(w, "<span class='docblock autohide sub-variant' id='{id}'>",
+                       id = variant_id)?;
+                write!(w, "<h3 class='fields'>Fields of <code>{name}</code></h3>\n
+                           <table>", name = variant.name.as_ref().unwrap())?;
                 for field in &s.fields {
                     use clean::StructFieldItem;
                     if let StructFieldItem(ref ty) = field.inner {
@@ -2513,7 +2518,7 @@ fn item_enum(w: &mut fmt::Formatter, cx: &Context, it: &clean::Item,
                         write!(w, "</td></tr>")?;
                     }
                 }
-                write!(w, "</table>")?;
+                write!(w, "</table></span>")?;
             }
             render_stability_since(w, variant, it)?;
         }