Add test for DerefMut methods

src/test/rustdoc/deref-mut-methods.rs

@@ -0,0 +1,29 @@
+#![crate_name = "foo"]
+
+use std::ops;
+
+pub struct Foo;
+
+impl Foo {
+    pub fn foo(&mut self) {}
+}
+
+// @has foo/struct.Bar.html
+// @has - '//div[@class="sidebar-links"]/a[@href="#method.foo"]' 'foo'
+pub struct Bar {
+    foo: Foo,
+}
+
+impl ops::Deref for Bar {
+    type Target = Foo;
+
+    fn deref(&self) -> &Foo {
+        &self.foo
+    }
+}
+
+impl ops::DerefMut for Bar {
+    fn deref_mut(&mut self) -> &mut Foo {
+        &mut self.foo
+    }
+}