Rollup merge of #34536 - ollie27:rustdoc_module_impls, r=alexcrichton

rustdoc: Fix empty Implementations section on some module pages

These are caused by `DefaultImpl`s.

For example [`core::marker`](https://doc.rust-lang.org/nightly/core/marker/#impls).

src/librustdoc/html/render.rs

@@ -1696,6 +1696,9 @@ fn item_module(w: &mut fmt::Formatter, cx: &Context,
     document(w, cx, item)?;
 
     let mut indices = (0..items.len()).filter(|i| {
+        if let clean::DefaultImplItem(..) = items[*i].inner {
+            return false;
+        }
         !cx.maybe_ignore_item(&items[*i])
     }).collect::<Vec<usize>>();
 

src/test/rustdoc/module-impls.rs

@@ -0,0 +1,15 @@
+// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![crate_name = "foo"]
+
+pub use std::marker::Send;
+
+// @!has foo/index.html 'Implementations'