Document direct implementations on type aliases.

This improves #32077, but is not a complete fix. For a type alias `type
NewType = AliasedType`, it will include any `impl NewType` and `impl
Trait for NewType` blocks in the documentation for `NewType`.

A complete fix would include the implementations from the aliased type
in the type alias' documentation, so that users have a complete
picture of methods that are available on the alias. However, to do this
properly would require a fix for #14072, as the alias may affect the
type parameters of the type alias, making the documentation difficult to
understand. (That is, for `type Result = std::result::Result<(), ()>` we
would ideally show documentation for `impl Result<(), ()>`, rather than
generic documentation for `impl<T, E> Result<T, E>`).

I think this improvement is worthwhile, as it exposes implementations
which are not currently documented by rustdoc. The documentation for the
implementations on the aliased type are still accessible by clicking
through to the docs for that type. (Although perhaps it's now less
obvious to the user that they should click-through to get there).

src/librustdoc/html/render.rs

@@ -3082,7 +3082,13 @@ fn item_typedef(w: &mut fmt::Formatter, cx: &Context, it: &clean::Item,
            where_clause = WhereClause { gens: &t.generics, indent: 0, end_newline: true },
            type_ = t.type_)?;
 
-    document(w, cx, it)
+    document(w, cx, it)?;
+
+    // Render any items associated directly to this alias, as otherwise they
+    // won't be visible anywhere in the docs. It would be nice to also show
+    // associated items from the aliased type (see discussion in #32077), but
+    // we need #14072 to make sense of the generics.
+    render_assoc_items(w, cx, it, it.def_id, AssocItemRender::All)
 }
 
 impl<'a> fmt::Display for Sidebar<'a> {
@@ -3092,7 +3098,7 @@ impl<'a> fmt::Display for Sidebar<'a> {
         let parentlen = cx.current.len() - if it.is_mod() {1} else {0};
 
         if it.is_struct() || it.is_trait() || it.is_primitive() || it.is_union()
-            || it.is_enum() || it.is_mod()
+            || it.is_enum() || it.is_mod() || it.is_typedef()
         {
             write!(fmt, "<p class='location'>")?;
             match it.inner {
@@ -3101,6 +3107,7 @@ impl<'a> fmt::Display for Sidebar<'a> {
                 clean::PrimitiveItem(..) => write!(fmt, "Primitive Type ")?,
                 clean::UnionItem(..) => write!(fmt, "Union ")?,
                 clean::EnumItem(..) => write!(fmt, "Enum ")?,
+                clean::TypedefItem(..) => write!(fmt, "Type Definition ")?,
                 clean::ModuleItem(..) => if it.is_crate() {
                     write!(fmt, "Crate ")?;
                 } else {
@@ -3117,6 +3124,7 @@ impl<'a> fmt::Display for Sidebar<'a> {
                 clean::PrimitiveItem(ref p) => sidebar_primitive(fmt, it, p)?,
                 clean::UnionItem(ref u) => sidebar_union(fmt, it, u)?,
                 clean::EnumItem(ref e) => sidebar_enum(fmt, it, e)?,
+                clean::TypedefItem(ref t, _) => sidebar_typedef(fmt, it, t)?,
                 clean::ModuleItem(ref m) => sidebar_module(fmt, it, &m.items)?,
                 _ => (),
             }
@@ -3259,6 +3267,16 @@ fn sidebar_primitive(fmt: &mut fmt::Formatter, it: &clean::Item,
     Ok(())
 }
 
+fn sidebar_typedef(fmt: &mut fmt::Formatter, it: &clean::Item,
+                   _t: &clean::Typedef) -> fmt::Result {
+    let sidebar = sidebar_assoc_items(it);
+
+    if !sidebar.is_empty() {
+        write!(fmt, "<div class=\"block items\"><ul>{}</ul></div>", sidebar)?;
+    }
+    Ok(())
+}
+
 fn sidebar_union(fmt: &mut fmt::Formatter, it: &clean::Item,
                  u: &clean::Union) -> fmt::Result {
     let mut sidebar = String::new();