rustdoc: Render methods/impls for bare traits

This renders a "Methods" and "Trait Implementations" section for each item
implemented for a bare trait itself.

Closes #19055

src/test/rustdoc/issue-19055.rs

@@ -0,0 +1,30 @@
+// Copyright 2015 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.
+
+// @has issue_19055/trait.Any.html
+pub trait Any {}
+
+impl<'any> Any + 'any {
+    // @has - '//*[@id="method.is"]' 'fn is'
+    pub fn is<T: 'static>(&self) -> bool { loop {} }
+
+    // @has - '//*[@id="method.downcast_ref"]' 'fn downcast_ref'
+    pub fn downcast_ref<T: 'static>(&self) -> Option<&T> { loop {} }
+
+    // @has - '//*[@id="method.downcast_mut"]' 'fn downcast_mut'
+    pub fn downcast_mut<T: 'static>(&mut self) -> Option<&mut T> { loop {} }
+}
+
+pub trait Foo {
+    fn foo(&self) {}
+}
+
+// @has - '//*[@id="method.foo"]' 'fn foo'
+impl Foo for Any {}