The no_deref attribute can be used to say that no Deref impl should be generated for an imported type. If this attribute is not present, a Deref impl will be generated with a Target of the type's first extends attribute, or Target = JsValue if there are no extends attributes.

fn main() {
extern "C" {
    type Foo;

    fn baz(this: &Foo)

    #[wasm_bindgen(extends = Foo, no_deref)]
    type Bar;

fn do_stuff(bar: &Bar) {
    bar.baz() // Does not compile