Add test for typedef deref

src/test/rustdoc/deref-typedef.rs

@@ -0,0 +1,19 @@
+#![crate_name = "foo"]
+
+// @has 'foo/struct.Bar.html'
+// @has '-' '//*[@id="deref-methods"]' 'Methods from Deref<Target = FooB>'
+// @has '-' '//*[@class="impl-items"]//*[@id="method.happy"]' 'pub fn happy(&self)'
+// @has '-' '//*[@class="sidebar-title"]' 'Methods from Deref<Target=FooB>'
+// @has '-' '//*[@class="sidebar-links"]/a[@href="#method.happy"]' 'happy'
+pub struct FooA;
+pub type FooB = FooA;
+
+impl FooA {
+    pub fn happy(&self) {}
+}
+
+pub struct Bar;
+impl std::ops::Deref for Bar {
+    type Target = FooB;
+    fn deref(&self) -> &FooB { unimplemented!() }
+}