Rollup merge of rust-lang#61505 - ebarnard:doc-shrink, r=GuillaumeGomez

Only show methods that appear in `impl` blocks in the Implementors sections of trait doc pages

In the "Implementors" and "Implementations on Foreign Types" sections, only show methods that appear in the `impl` block for that type. This has the benefit of
- Reducing the size of the Iterator page, and other large trait documentation pages.
- Retaining documentation on the `impl` blocks and functions in the `impl` blocks.
- Indicating which provided methods are overridden.
- Making the documentation match the structure of the code being documented.
- Being a small change that can be easily backed out if issues arise.

A set of Rust stdlib docs build with this change are [available here](https://ebarnard.github.io/2019-06-03-rust-smaller-trait-implementers-docs/).

The size of the [`Iterator` doc page](https://ebarnard.github.io/2019-06-03-rust-smaller-trait-implementers-docs/std/iter/trait.Iterator.html) is reduced from 14.4MB (latest nightly) to 724kB.

Before:
<img width="1411" alt="Screenshot 2019-06-03 at 23 12 17" src="https://user-images.githubusercontent.com/1059683/58837971-1722a780-8655-11e9-8d81-51e48130951d.png">

After:
<img width="1428" alt="Screenshot 2019-06-03 at 16 41 27" src="https://user-images.githubusercontent.com/1059683/58814907-84ffac80-861e-11e9-8692-79be473a5299.png">

cc rust-lang#55900

src/librustdoc/html/render.rs

@@ -3066,7 +3066,7 @@ fn render_implementor(cx: &Context, implementor: &Impl, w: &mut fmt::Formatter<'
         _ => false,
     };
     render_impl(w, cx, implementor, AssocItemLink::Anchor(None), RenderMode::Normal,
-                implementor.impl_item.stable_since(), false, Some(use_absolute), false)?;
+                implementor.impl_item.stable_since(), false, Some(use_absolute), false, false)?;
     Ok(())
 }
 
@@ -3077,7 +3077,7 @@ fn render_impls(cx: &Context, w: &mut fmt::Formatter<'_>,
         let did = i.trait_did().unwrap();
         let assoc_link = AssocItemLink::GotoSource(did, &i.inner_impl().provided_trait_methods);
         render_impl(w, cx, i, assoc_link,
-                    RenderMode::Normal, containing_item.stable_since(), true, None, false)?;
+                    RenderMode::Normal, containing_item.stable_since(), true, None, false, true)?;
     }
     Ok(())
 }
@@ -3307,7 +3307,7 @@ fn item_trait(
                 );
                 render_impl(w, cx, &implementor, assoc_link,
                             RenderMode::Normal, implementor.impl_item.stable_since(), false,
-                            None, true)?;
+                            None, true, false)?;
             }
             write_loading_content(w, "")?;
         }
@@ -3979,7 +3979,7 @@ fn render_assoc_items(w: &mut fmt::Formatter<'_>,
         };
         for i in &non_trait {
             render_impl(w, cx, i, AssocItemLink::Anchor(None), render_mode,
-                        containing_item.stable_since(), true, None, false)?;
+                        containing_item.stable_since(), true, None, false, true)?;
         }
     }
     if let AssocItemRender::DerefFor { .. } = what {
@@ -4161,7 +4161,8 @@ fn spotlight_decl(decl: &clean::FnDecl) -> Result<String, fmt::Error> {
 
 fn render_impl(w: &mut fmt::Formatter<'_>, cx: &Context, i: &Impl, link: AssocItemLink<'_>,
                render_mode: RenderMode, outer_version: Option<&str>, show_def_docs: bool,
-               use_absolute: Option<bool>, is_on_foreign_type: bool) -> fmt::Result {
+               use_absolute: Option<bool>, is_on_foreign_type: bool,
+               show_default_items: bool) -> fmt::Result {
     if render_mode == RenderMode::Normal {
         let id = cx.derive_id(match i.inner_impl().trait_ {
             Some(ref t) => if is_on_foreign_type {
@@ -4345,9 +4346,13 @@ fn render_impl(w: &mut fmt::Formatter<'_>, cx: &Context, i: &Impl, link: AssocIt
 
     // If we've implemented a trait, then also emit documentation for all
     // default items which weren't overridden in the implementation block.
-    if let Some(t) = trait_ {
-        render_default_items(w, cx, t, &i.inner_impl(),
-                             render_mode, outer_version, show_def_docs)?;
+    // We don't emit documentation for default items if they appear in the
+    // Implementations on Foreign Types or Implementors sections.
+    if show_default_items {
+        if let Some(t) = trait_ {
+            render_default_items(w, cx, t, &i.inner_impl(),
+                                render_mode, outer_version, show_def_docs)?;
+        }
     }
     write!(w, "</div>")?;