guide: add indexing_{getter,setter,deleter} attribute page

guide/src/SUMMARY.md

@@ -21,6 +21,7 @@
       - [`catch`](./reference/attributes/on-js-imports/catch.md)
       - [`constructor`](./reference/attributes/on-js-imports/constructor.md)
       - [`getter` and `setter`](./reference/attributes/on-js-imports/getter-and-setter.md)
+      - [`indexing_getter`, `indexing_setter`, and `indexing_deleter`](./reference/attributes/on-js-imports/indexing-getter-setter-deleter.md)
       - [`js_class = "Blah"`](./reference/attributes/on-js-imports/js_class.md)
       - [`js_name`](./reference/attributes/on-js-imports/js_name.md)
       - [`js_namespace`](./reference/attributes/on-js-imports/js_namespace.md)

guide/src/reference/attributes/on-js-imports/indexing-getter-setter-deleter.md

@@ -0,0 +1,82 @@
+# `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 `Proxy`s 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`:
+
+```js
+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:
+
+```rust
+#[wasm_bindgen]
+extern {
+    type Foo;
+    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);
+}
+
+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);
+```