Add examples/documentation for closures

Cargo.toml

@@ -32,6 +32,7 @@ members = [
   "examples/math",
   "examples/performance",
   "examples/wasm-in-wasm",
+  "examples/closures",
 ]
 
 [profile.release]

DESIGN.md

@@ -872,6 +872,7 @@ Under the hood this generates shims that do a bunch of translation, but it
 suffices to say that a call in wasm to `foo` should always return
 appropriately.
 
+
 ## Customizing import behavior
 
 The `#[wasm_bindgen]` macro supports a good amount of configuration for
@@ -1037,6 +1038,49 @@ possibilities!
   All of these functions will call `console.log` in Rust, but each identifier
   will have only one signature in Rust.
 
+## Closures
+
+Closures are a particularly tricky topic in wasm-bindgen right now. They use
+somewhat advanced language features to currently be implemented and *still* the
+amount of functionality you can use is quite limiting.
+
+Most of the implementation details of closures can be found in `src/convert.rs`
+and `src/closure.rs`, effectively the `ToRefWasmBoundary` implementations for
+closure types. Stack closures are pretty straightforward in that they pass
+a function pointer and a data pointer to JS. This function pointer is accessed
+via the exported `WebAssembly.Table` in JS, and the data pointer is passed along
+eventually when the JS closure is invoked.
+
+Stack closures currently only support `Fn` because there's no great location to
+insert a `RefCell` for types like `FnMut`. This restriction may be lift-able
+though in the future...
+
+Long-lived closures are a bit more complicated. The general idea there is:
+
+* First you create a `Closure`. This manufactures a JS callback and "passes it"
+  to Rust so Rust can store it.
+* Next you later pass it as `&Closure<...>` to JS. This extracts the callback
+  from Rust and passes it to JS.
+* Finally you eventually drop the Rust `Closure` which invalidates the JS
+  closure.
+
+Creation of the initial JS function is done with a bunch of
+`__wbindgen_cb_arityN` functions. These functions create a JS closure with the
+given arity (number of arguments). This isn't really that scalable unfortunately
+and also means that it's very difficult to support richer types one day. Unsure
+how to solve this.
+
+The `ToRefWasmBoundary` is quite straightforward for `Closure` as it just plucks
+out the JS closure and passes it along. The real meat comes down to the
+`WasmShim` internal trait. This is implemented for all the *unsized* closure
+types to avoid running afoul with coherence. Each trait impl defines a shim
+function to be invokeable from JS as well as the ability to wrap up the sized
+verion (aka transition from `F: FnMut()` to `FnMut()`). Impls for `FnMut` also
+embed the `RefCell` internally.
+
+The `WasmShim` design is basically the first thing that got working today. It's
+not great and will likely change in the future to hopefully be more flexible!
+
 ## Wrapping up
 
 That's currently at least what `wasm-bindgen` has to offer! If you've got more

README.md

@@ -24,8 +24,8 @@ Notable features of this project includes:
 * Importing JS functionality in to Rust such as [DOM manipulation][dom-ex],
   [console logging][console-log], or [performance monitoring][perf-ex].
 * [Exporting Rust functionality][smorg-ex] to JS such as classes, functions, etc.
-* Working with rich types like strings, numbers, classes, and objects rather
-  than simply `u32` and floats.
+* Working with rich types like strings, numbers, classes, closures, and objects
+  rather than simply `u32` and floats.
 
 This project is still relatively new but feedback is of course always
 welcome! If you're curious about the design plus even more information about
@@ -397,6 +397,79 @@ export class Awesome {
 booted.then(main);
 ```
 
+## Closures
+
+The `#[wasm_bindgen]` attribute supports a limited subset of Rust closures being
+passed to JS at this time. There are plans to expand this support currently but
+it's not clear how to proceed unfortunately. In any case some examples of what
+you can do are:
+
+```rust
+#[wasm_bindgen]
+extern {
+    fn foo(a: &Fn()); // must be `Fn`, not `FnMut`
+}
+```
+
+Here a function `foo` is imported from JS where the first argument is a *stack
+closure*. You can call this function with a `&Fn()` argument and JS will receive
+a JS function. When the `foo` function returns, however, the JS function will be
+invalidated and any future usage of it will raise an exception.
+
+Closures also support arguments and return values native to the wasm type
+system, aka f32/u32:
+
+```rust
+#[wasm_bindgen]
+extern {
+    fn bar(a: &Fn(u32, f32) -> f64);
+}
+```
+
+At this time [types like strings aren't supported][cbstr] unfortunately.
+
+[cbstr]: https://github.com/rustwasm/wasm-bindgen/issues/104
+
+Sometimes the stack behavior of these closures is not desired. For example you'd
+like to schedule a closure to be run on the next turn of the event loop in JS
+through `setTimeout`. For this you want the imported function to return but the
+JS closure still needs to be valid!
+
+To support this use case you can also do:
+
+```rust
+use wasm_bindgen::prelude::*;
+
+#[wasm_bindgen]
+extern {
+    fn baz(a: &Closure<Fn()>);
+}
+```
+
+The `Closure` type is defined in the `wasm_bindgen` crate and represents a "long
+lived" closure. The JS closure passed to `baz` is still valid after `baz`
+returns, and the validity of the JS closure is tied to the lifetime of the
+`Closure` in Rust. Once `Closure` is dropped it will deallocate its internal
+memory and invalidate the corresponding JS function.
+
+Unlike stack closures a `Closure` supports `FnMut`:
+
+```rust
+use wasm_bindgen::prelude::*;
+
+#[wasm_bindgen]
+extern {
+    fn another(a: &Closure<FnMut() -> u32>);
+}
+```
+
+Like stack closures, however, only wasm types like u32/f32 are supported today.
+
+At this time you cannot [pass a JS closure to Rust][cbjs], you can only pass a
+Rust closure to JS in limited circumstances.
+
+[cbjs]: https://github.com/rustwasm/wasm-bindgen/issues/103
+
 ## Feature reference
 
 Here this section will attempt to be a reference for the various features

crates/cli-support/src/js.rs

@@ -255,6 +255,12 @@ impl<'a> Context<'a> {
                     dropRef(i);
                 }")
             });
+            bind("__wbindgen_cb_forget", &|me| {
+                me.expose_drop_ref();
+                String::from("function(i) {
+                    dropRef(i);
+                }")
+            });
         }
 
         self.rewrite_imports(module_name);

examples/README.md

@@ -22,3 +22,5 @@ The examples here are:
   operations in Rust
 * `wasm-in-wasm` - how to interact with namespaced APIs like
   `WebAssembly.Module` and shows off creation of a WebAssembly module from Rust
+* `closures` - an example of how to invoke functions like `setInterval` or use
+  the `onclick` property in conjunction with closures.

examples/closures/.gitignore

@@ -0,0 +1,4 @@
+package-lock.json
+closures.js
+closures_bg.js
+closures_bg.wasm

examples/closures/Cargo.toml

@@ -0,0 +1,10 @@
+[package]
+name = "closures"
+version = "0.1.0"
+authors = ["Alex Crichton <alex@alexcrichton.com>"]
+
+[lib]
+crate-type = ["cdylib"]
+
+[dependencies]
+wasm-bindgen = { path = "../.." }

examples/closures/README.md

@@ -0,0 +1,20 @@
+# Closure examples
+
+This directory is an example of using the `#[wasm_bindgen]` macro with closures
+to interact with the DOM.
+
+You can build the example with:
+
+```
+$ ./build.sh
+```
+
+(or running the commands on Windows manually)
+
+and then opening up `index.html` in a web browser should show a hello message on
+the web page generated by the wasm.
+
+For more information about this example be sure to check out
+[`hello_world`][hello] which also has more comments about caveats and such.
+
+[hello]: https://github.com/alexcrichton/wasm-bindgen/tree/master/examples/hello_world

examples/closures/build.sh

@@ -0,0 +1,12 @@
+#!/bin/sh
+
+# For more coments about what's going on here, see the `hello_world` example
+
+set -ex
+
+cargo +nightly build --target wasm32-unknown-unknown
+cargo +nightly run --manifest-path ../../crates/cli/Cargo.toml \
+  --bin wasm-bindgen -- \
+  ../../target/wasm32-unknown-unknown/debug/closures.wasm --out-dir .
+#npm install
+npm run serve

examples/closures/index.html

@@ -0,0 +1,40 @@
+<html>
+  <head>
+    <meta content="text/html;charset=utf-8" http-equiv="Content-Type"/>
+    <style>
+      #green-square {
+        background: green;
+        width: 200px;
+        height: 200px;
+        text-align: center;
+        line-height: 200px;
+        color: white;
+      }
+      #green-square span {
+        display: inline-block;
+        vertical-align: middle;
+      }
+    </style>
+  </head>
+  <body>
+    <div id='loading'>
+      Loading...
+    </div>
+    <div id='script' style='display:none'>
+      <p>
+        The current time is:
+        <span id='current-time'>...</span>
+      </p>
+
+      <div id='green-square'>
+        <span>Click me!</span>
+      </div>
+      <p>
+        You've clicked the green square
+        <span id='num-clicks'>0</span>
+        times
+      </p>
+    </div>
+    <script src='./index.js'></script>
+  </body>
+</html>

examples/closures/index.js

@@ -0,0 +1,4 @@
+// For more comments about what's going on here, check out the `hello_world`
+// example
+const rust = import("./closures");
+rust.then(m => m.run());

examples/closures/package.json

@@ -0,0 +1,10 @@
+{
+  "scripts": {
+    "serve": "webpack-dev-server"
+  },
+  "devDependencies": {
+    "webpack": "^4.0.1",
+    "webpack-cli": "^2.0.10",
+    "webpack-dev-server": "^3.1.0"
+  }
+}

examples/closures/src/lib.rs

@@ -0,0 +1,94 @@
+#![feature(proc_macro, wasm_custom_section, wasm_import_module)]
+
+extern crate wasm_bindgen;
+
+use wasm_bindgen::prelude::*;
+
+#[wasm_bindgen]
+extern {
+    // Binding for the `setInverval` method in JS. This function takes a "long
+    // lived" closure as the first argument so we use `Closure` instead of
+    // a bare `&Fn()` which only surives for that one stack frame.
+    //
+    // The second argument is then the interval and the return value is how we
+    // clear this interval. We're not going to clear our interval in this
+    // example though so the return value is ignored.
+    #[wasm_bindgen(js_name = setInterval)]
+    fn set_interval(cb: &Closure<FnMut()>, delay: u32) -> f64;
+
+    // Bindings for JS `Date` so we can update our local timer
+    type Date;
+    #[wasm_bindgen(constructor)]
+    fn new() -> Date;
+    #[wasm_bindgen(method, js_name = toLocaleString)]
+    fn to_locale_string(this: &Date) -> String;
+
+    // Bindings for `document` and various methods of updating HTML elements.
+    // Like with the `dom` example these'll ideally be upstream in a generated
+    // crate one day but for now we manually define them.
+    type HTMLDocument;
+    static document: HTMLDocument;
+    #[wasm_bindgen(method, js_name = getElementById)]
+    fn get_element_by_id(this: &HTMLDocument, id: &str) -> Element;
+    #[wasm_bindgen(method, js_name = getElementById)]
+    fn get_html_element_by_id(this: &HTMLDocument, id: &str) -> HTMLElement;
+
+    type Element;
+    #[wasm_bindgen(method, setter = innerHTML)]
+    fn set_inner_html(this: &Element, html: &str);
+
+    type HTMLElement;
+    #[wasm_bindgen(method, setter)]
+    fn set_onclick(this: &HTMLElement, cb: &Closure<FnMut()>);
+    #[wasm_bindgen(method, getter)]
+    fn style(this: &HTMLElement) -> CSS2Properties;
+
+    type CSS2Properties;
+    #[wasm_bindgen(method, setter)]
+    fn set_display(this: &CSS2Properties, display: &str);
+}
+
+#[wasm_bindgen]
+pub fn run() {
+    // Set up a clock on our page and update it each second to ensure it's got
+    // an accurate date.
+    let a = Closure::new(update_time);
+    set_interval(&a, 1000);
+    update_time();
+    fn update_time() {
+        document.get_element_by_id("current-time")
+            .set_inner_html(&Date::new().to_locale_string());
+    }
+
+    // We also want to count the number of times that our green square has been
+    // clicked. Our callback will update the `#num-clicks` div
+    let square = document.get_html_element_by_id("green-square");
+    let mut clicks = 0;
+    let b = Closure::new(move || {
+        clicks += 1;
+        document.get_element_by_id("num-clicks")
+            .set_inner_html(&clicks.to_string());
+    });
+    square.set_onclick(&b);
+
+    // The instances of `Closure` that we created will invalidate their
+    // corresponding JS callback whenever they're dropped, so if we were to
+    // normally return from `run` then both of our registered closures will
+    // raise exceptions when invoked.
+    //
+    // Normally we'd store these handles to later get dropped at an appropriate
+    // time but for now we want these to be global handlers so we use the
+    // `forget` method to drop them without invalidating the closure. Note that
+    // this is leaking memory in Rust, so this should be done judiciously!
+    a.forget();
+    b.forget();
+
+    // And finally now that our demo is ready to go let's switch things up so
+    // everything is displayed and our loading prompt is hidden.
+    document.get_html_element_by_id("loading")
+        .style()
+        .set_display("none");
+    document.get_html_element_by_id("script")
+        .style()
+        .set_display("block");
+}

examples/closures/webpack.config.js

@@ -0,0 +1,10 @@
+const path = require('path');
+
+module.exports = {
+  entry: "./index.js",
+  output: {
+    path: path.resolve(__dirname, "dist"),
+    filename: "index.js",
+  },
+  mode: "development"
+};

src/closure.rs

@@ -1,11 +1,155 @@
-use std::mem::ManuallyDrop;
-use std::marker::{self, Unsize};
+//! Support for long-lived closures in `wasm-bindgen`
+//!
+//! This module defines the `Closure` type which is used to pass "owned
+//! closures" from Rust to JS. Some more details can be found on the `Closure`
+//! type itself.
+
+use std::mem::{self, ManuallyDrop};
+use std::marker::Unsize;
 
 use {throw, JsValue};
 use convert::*;
 use __rt::WasmRefCell;
 
-pub unsafe trait WasmShim<'a> {
+/// A handle to both a closure in Rust as well as JS closure which will invoke
+/// the Rust closure.
+///
+/// A `Closure` is the primary way that a `'static` lifetime closure is
+/// transferred from Rust to JS. `Closure` currently requires that the closures
+/// it's created with have the `'static` lifetime in Rust for soundness reasons.
+///
+/// This type is a "handle" in the sense that whenever it is dropped it will
+/// invalidate the JS closure that it refers to. Any usage of the closure in JS
+/// after the `Closure` has been dropped will raise an exception. It's then up
+/// to you to arrange for `Closure` to be properly deallocate at an appropriate
+/// location in your program.
+///
+/// The type parameter on `Closure` is the type of closure that this represents.
+/// Currently this can only be the `Fn` and `FnMut` traits with up to 7
+/// arguments (and an optional return value). The arguments/return value of the
+/// trait must be numbers like `u32` for now, although this restriction may be
+/// lifted in the future!
+///
+/// # Example
+///
+/// ```rust,no_run
+/// #[wasm_bindgen]
+/// extern {
+///     fn setTimeout(closure: &Closure<FnMut()>, time: u32);
+///
+///     #[wasm_bindgen(js_namespace = console)]
+///     fn log(s: &str);
+/// }
+///
+/// #[wasm_bindgen]
+/// pub struct ClosureHandle(Closure<FnMut()>);
+///
+/// #[wasm_bindgen]
+/// pub fn run() -> ClosureHandle {
+///     // First up we use `Closure::new` to wrap up a Rust closure and create
+///     a JS closure.
+///     let cb = Closure::new(|| {
+///         log("timeout elapsed!");
+///     });
+///
+///     // Next we pass this via reference to the `setTimeout` function, and
+///     // `setTimeout` gets a handle to the corresponding JS closure.
+///     setTimeout(&cb, 1_000);
+///
+///     // If we were to drop `cb` here it would cause an exception to be raised
+///     // when the timeout elapses. Instead we *return* our handle back to JS
+///     // so JS can tell us later when it would like to deallocate this handle.
+///     ClosureHandle(cb)
+/// }
+/// ```
+pub struct Closure<T: WasmShim + ?Sized> {
+    _inner: T::Wrapper,
+    js: ManuallyDrop<JsValue>,
+}
+
+impl<T> Closure<T>
+    where T: WasmShim + ?Sized,
+{
+    /// Creates a new instance of `Closure` from the provided Rust closure.
+    ///
+    /// Note that the closure provided here, `F`, has a few requirements
+    /// associated with it:
+    ///
+    /// * It must implement `Fn` or `FnMut`
+    /// * It must be `'static`, aka no stack references (use the `move` keyword)
+    /// * It can have at most 7 arguments
+    /// * Its arguments and return values are all wasm types like u32/f64.
+    ///
+    /// This is unfortunately pretty restrictive for now but hopefully some of
+    /// these restrictions can be lifted in the future!
+    pub fn new<F>(t: F) -> Closure<T>
+        where F: Unsize<T> + 'static
+    {
+        Closure::wrap(T::wrap(t))
+    }
+
+    /// A mostly internal function to wrap a boxed closure inside a `Closure`
+    /// type.
+    ///
+    /// This is the function where the JS closure is manufactured.
+    pub fn wrap(t: T::Wrapper) -> Closure<T> {
+        unsafe {
+            let data = T::data(&t);
+            let js = T::factory()(T::shim(), data[0], data[1]);
+            Closure {
+                _inner: t,
+                js: ManuallyDrop::new(JsValue::from_abi(js, &mut GlobalStack::new())),
+            }
+        }
+    }
+
+    /// Leaks this `Closure` to ensure it remains valid for the duration of the
+    /// entire program.
+    ///
+    /// > **Note**: this function will leak memory. It should be used sparingly
+    /// > to ensure the memory leak doesn't affect the program too much.
+    ///
+    /// When a `Closure` is dropped it will invalidate the associated JS
+    /// closure, but this isn't always desired. Some callbacks are alive for
+    /// the entire duration of the program, so this can be used to conveniently
+    /// leak this instance of `Closure` while performing as much internal
+    /// cleanup as it can.
+    pub fn forget(self) {
+        unsafe {
+            super::__wbindgen_cb_forget(self.js.to_abi_ref(&mut GlobalStack::new()));
+            mem::forget(self);
+        }
+    }
+}
+
+// `Closure` can only be passed by reference to imports.
+impl<T> ToRefWasmBoundary for Closure<T>
+    where T: WasmShim + ?Sized,
+{
+    type Abi = u32;
+    const DESCRIPTOR: Descriptor = T::DESCRIPTOR;
+
+    fn to_abi_ref(&self, extra: &mut Stack) -> u32 {
+        self.js.to_abi_ref(extra)
+    }
+}
+
+impl<T> Drop for Closure<T>
+    where T: WasmShim + ?Sized,
+{
+    fn drop(&mut self) {
+        unsafe {
+            let idx = self.js.to_abi_ref(&mut GlobalStack::new());
+            super::__wbindgen_cb_drop(idx);
+        }
+    }
+}
+
+/// An internal trait for the `Closure` type.
+///
+/// This trait is not stable and it's not recommended to use this in bounds or
+/// implement yourself.
+pub unsafe trait WasmShim {
     #[doc(hidden)]
     const DESCRIPTOR: Descriptor;
     #[doc(hidden)]
@@ -15,7 +159,7 @@ pub unsafe trait WasmShim<'a> {
     #[doc(hidden)]
     fn factory() -> unsafe extern fn(u32, u32, u32) -> u32;
     #[doc(hidden)]
-    fn wrap<U>(u: U) -> Self::Wrapper where U: Unsize<Self> + 'a;
+    fn wrap<U>(u: U) -> Self::Wrapper where U: Unsize<Self> + 'static;
     #[doc(hidden)]
     fn data(t: &Self::Wrapper) -> [u32; 2];
 }
@@ -30,9 +174,9 @@ macro_rules! doit {
         ($($var:ident)*) => $arity:ident
     )*) => ($(
         // Fn with no return
-        unsafe impl<'a, $($var: WasmAbi),*> WasmShim<'a> for Fn($($var),*) + 'a {
+        unsafe impl<$($var: WasmAbi),*> WasmShim for Fn($($var),*) {
             const DESCRIPTOR: Descriptor = DESCRIPTOR_FUNC;
-            type Wrapper = Box<Fn($($var),*) + 'a>;
+            type Wrapper = Box<Fn($($var),*)>;
 
             fn shim() -> u32 {
                 #[allow(non_snake_case)]
@@ -53,7 +197,7 @@ macro_rules! doit {
                 super::$arity
             }
 
-            fn wrap<U>(u: U) -> Self::Wrapper where U: Unsize<Self> + 'a {
+            fn wrap<U>(u: U) -> Self::Wrapper where U: Unsize<Self> + 'static {
                 Box::new(u) as Box<Self>
             }
 
@@ -65,9 +209,9 @@ macro_rules! doit {
         }
 
         // Fn with a return
-        unsafe impl<'a, $($var: WasmAbi,)* R: WasmAbi> WasmShim<'a> for Fn($($var),*) -> R + 'a {
+        unsafe impl<$($var: WasmAbi,)* R: WasmAbi> WasmShim for Fn($($var),*) -> R {
             const DESCRIPTOR: Descriptor = DESCRIPTOR_FUNC;
-            type Wrapper = Box<Fn($($var),*) -> R + 'a>;
+            type Wrapper = Box<Fn($($var),*) -> R>;
 
             fn shim() -> u32 {
                 #[allow(non_snake_case)]
@@ -88,7 +232,7 @@ macro_rules! doit {
                 super::$arity
             }
 
-            fn wrap<U>(u: U) -> Self::Wrapper where U: Unsize<Self> + 'a {
+            fn wrap<U>(u: U) -> Self::Wrapper where U: Unsize<Self> + 'static {
                 Box::new(u) as Box<Self>
             }
 
@@ -100,9 +244,9 @@ macro_rules! doit {
         }
 
         // FnMut with no return
-        unsafe impl<'a, $($var: WasmAbi),*> WasmShim<'a> for FnMut($($var),*) + 'a {
+        unsafe impl<$($var: WasmAbi),*> WasmShim for FnMut($($var),*) {
             const DESCRIPTOR: Descriptor = DESCRIPTOR_FUNC;
-            type Wrapper = Box<WasmRefCell<FnMut($($var),*) + 'a>>;
+            type Wrapper = Box<WasmRefCell<FnMut($($var),*)>>;
 
             fn shim() -> u32 {
                 #[allow(non_snake_case)]
@@ -127,7 +271,7 @@ macro_rules! doit {
                 super::$arity
             }
 
-            fn wrap<U>(u: U) -> Self::Wrapper where U: Unsize<Self> + 'a {
+            fn wrap<U>(u: U) -> Self::Wrapper where U: Unsize<Self> + 'static {
                 Box::new(WasmRefCell::new(u)) as Box<_>
             }
 
@@ -139,9 +283,9 @@ macro_rules! doit {
         }
 
         // FnMut with a return
-        unsafe impl<'a, $($var: WasmAbi,)* R: WasmAbi> WasmShim<'a> for FnMut($($var),*) -> R + 'a {
+        unsafe impl<$($var: WasmAbi,)* R: WasmAbi> WasmShim for FnMut($($var),*) -> R {
             const DESCRIPTOR: Descriptor = DESCRIPTOR_FUNC;
-            type Wrapper = Box<WasmRefCell<FnMut($($var),*) -> R + 'a>>;
+            type Wrapper = Box<WasmRefCell<FnMut($($var),*) -> R>>;
 
             fn shim() -> u32 {
                 #[allow(non_snake_case)]
@@ -166,7 +310,7 @@ macro_rules! doit {
                 super::$arity
             }
 
-            fn wrap<U>(u: U) -> Self::Wrapper where U: Unsize<Self> + 'a {
+            fn wrap<U>(u: U) -> Self::Wrapper where U: Unsize<Self> + 'static {
                 Box::new(WasmRefCell::new(u)) as Box<_>
             }
 
@@ -189,53 +333,3 @@ doit! {
     (A B C D E F) => __wbindgen_cb_arity6
     (A B C D E F G) => __wbindgen_cb_arity7
 }
-
-pub struct Closure<'a, T: WasmShim<'a> + ?Sized + 'a> {
-    _inner: T::Wrapper,
-    js: ManuallyDrop<JsValue>,
-    _marker: marker::PhantomData<&'a ()>,
-}
-
-impl<'a, T> Closure<'a, T>
-    where T: WasmShim<'a> + ?Sized,
-{
-    pub fn new<U>(t: U) -> Closure<'a, T>
-        where U: Unsize<T> + 'a
-    {
-        Closure::wrap(T::wrap(t))
-    }
-
-    pub fn wrap(t: T::Wrapper) -> Closure<'a, T> {
-        unsafe {
-            let data = T::data(&t);
-            let js = T::factory()(T::shim(), data[0], data[1]);
-            Closure {
-                _inner: t,
-                js: ManuallyDrop::new(JsValue::from_abi(js, &mut GlobalStack::new())),
-                _marker: marker::PhantomData,
-            }
-        }
-    }
-}
-
-impl<'a, T> ToRefWasmBoundary for Closure<'a, T>
-    where T: WasmShim<'a> + ?Sized,
-{
-    type Abi = u32;
-    const DESCRIPTOR: Descriptor = T::DESCRIPTOR;
-
-    fn to_abi_ref(&self, extra: &mut Stack) -> u32 {
-        self.js.to_abi_ref(extra)
-    }
-}
-
-impl<'a, T> Drop for Closure<'a, T>
-    where T: WasmShim<'a> + ?Sized,
-{
-    fn drop(&mut self) {
-        unsafe {
-            let idx = self.js.to_abi_ref(&mut GlobalStack::new());
-            super::__wbindgen_cb_drop(idx);
-        }
-    }
-}

src/lib.rs

@@ -242,6 +242,7 @@ extern {
     fn __wbindgen_cb_arity6(a: u32, b: u32, c: u32) -> u32;
     fn __wbindgen_cb_arity7(a: u32, b: u32, c: u32) -> u32;
     fn __wbindgen_cb_drop(idx: u32);
+    fn __wbindgen_cb_forget(idx: u32);
 }
 
 impl Clone for JsValue {