Add rustdoc test for Deref to primitive types

Author: GuillaumeGomez

src/test/rustdoc/deref-to-primitive.rs

@@ -0,0 +1,15 @@
+#![crate_name = "foo"]
+
+// @has 'foo/struct.Foo.html'
+// @has - '//*[@id="deref-methods-i32"]' 'Methods from Deref<Target = i32>'
+// @has - '//*[@id="deref-methods-i32-1"]//*[@id="associatedconstant.BITS"]/h4' \
+//        'pub const BITS: u32 = 32u32'
+pub struct Foo(i32);
+
+impl std::ops::Deref for Foo {
+    type Target = i32;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}