indexing_getter, indexing_setter, and indexing_deleter

These three attributes indicate that a method is an dynamically intercepted getter, setter, or deleter on the receiver object itself, rather than a direct access of the receiver's properties. It is equivalent calling the Proxy handler for the obj[prop] operation with some dynamic prop variable in JavaScript, rather than a normal static property access like obj.prop on a normal JavaScript Object.

This is useful for binding to Proxys and some builtin DOM types that dynamically intercept property accesses.

• indexing_getter corresponds to obj[prop] operation in JavaScript. The function annotated must have a this receiver parameter, a single parameter that is used for indexing into the receiver (prop), and a return type.

• indexing_setter corresponds to the obj[prop] = val operation in JavaScript. The function annotated must have a this receiver parameter, a parameter for indexing into the receiver (prop), and a value parameter (val).

• indexing_deleter corresponds to delete obj[prop] operation in JavaScript. The function annotated must have a this receiver and a single parameter for indexing into the receiver (prop).

These must always be used in conjunction with the structural and method flags.

For example, consider this JavaScript snippet that uses Proxy:

const foo = new Proxy({}, {
    get(obj, prop) {
        return prop in obj ? obj[prop] : prop.length;
    },
    set(obj, prop, value) {
        obj[prop] = value;
    },
    deleteProperty(obj, prop) {
        delete obj[prop];
    },
});

foo.ten;
// 3

foo.ten = 10;
foo.ten;
// 10

delete foo.ten;
foo.ten;
// 3

To bind that in wasm-bindgen in Rust, we would use the indexing_* attributes on methods:

#[wasm_bindgen]
extern "C" {
    type Foo;
    #[wasm_bindgen(thread_local_v2)]
    static FOO: Foo;

    #[wasm_bindgen(method, structural, indexing_getter)]
    fn get(this: &Foo, prop: &str) -> u32;

    #[wasm_bindgen(method, structural, indexing_setter)]
    fn set(this: &Foo, prop: &str, val: u32);

    #[wasm_bindgen(method, structural, indexing_deleter)]
    fn delete(this: &Foo, prop: &str);
}

FOO.with(|foo| {
    assert_eq!(foo.get("ten"), 3);

    foo.set("ten", 10);
    assert_eq!(foo.get("ten"), 10);

    foo.delete("ten");
    assert_eq!(foo.get("ten"), 3);
});