1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
//! 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.

#![allow(clippy::fn_to_numeric_cast)]

use std::fmt;
use std::mem::{self, ManuallyDrop};
use std::prelude::v1::*;

use crate::convert::*;
use crate::describe::*;
use crate::throw_str;
use crate::JsValue;
use crate::UnwrapThrowExt;

/// 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).
///
/// # Examples
///
/// Here are a number of examples of using `Closure`.
///
/// ## Using the `setInterval` API
///
/// Sample usage of `Closure` to invoke the `setInterval` API.
///
/// ```rust,no_run
/// use wasm_bindgen::prelude::*;
///
/// #[wasm_bindgen]
/// extern "C" {
///     fn setInterval(closure: &Closure<dyn FnMut()>, time: u32) -> i32;
///     fn clearInterval(id: i32);
///
///     #[wasm_bindgen(js_namespace = console)]
///     fn log(s: &str);
/// }
///
/// #[wasm_bindgen]
/// pub struct IntervalHandle {
///     interval_id: i32,
///     _closure: Closure<dyn FnMut()>,
/// }
///
/// impl Drop for IntervalHandle {
///     fn drop(&mut self) {
///         clearInterval(self.interval_id);
///     }
/// }
///
/// #[wasm_bindgen]
/// pub fn run() -> IntervalHandle {
///     // First up we use `Closure::new` to wrap up a Rust closure and create
///     // a JS closure.
///     let cb = Closure::new(|| {
///         log("interval elapsed!");
///     });
///
///     // Next we pass this via reference to the `setInterval` function, and
///     // `setInterval` gets a handle to the corresponding JS closure.
///     let interval_id = setInterval(&cb, 1_000);
///
///     // If we were to drop `cb` here it would cause an exception to be raised
///     // whenever the interval elapses. Instead we *return* our handle back to JS
///     // so JS can decide when to cancel the interval and deallocate the closure.
///     IntervalHandle {
///         interval_id,
///         _closure: cb,
///     }
/// }
/// ```
///
/// ## Casting a `Closure` to a `js_sys::Function`
///
/// This is the same `setInterval` example as above, except it is using
/// `web_sys` (which uses `js_sys::Function` for callbacks) instead of manually
/// writing bindings to `setInterval` and other Web APIs.
///
/// ```rust,ignore
/// use wasm_bindgen::JsCast;
///
/// #[wasm_bindgen]
/// pub struct IntervalHandle {
///     interval_id: i32,
///     _closure: Closure<dyn FnMut()>,
/// }
///
/// impl Drop for IntervalHandle {
///     fn drop(&mut self) {
///         let window = web_sys::window().unwrap();
///         window.clear_interval_with_handle(self.interval_id);
///     }
/// }
///
/// #[wasm_bindgen]
/// pub fn run() -> Result<IntervalHandle, JsValue> {
///     let cb = Closure::new(|| {
///         web_sys::console::log_1(&"interval elapsed!".into());
///     });
///
///     let window = web_sys::window().unwrap();
///     let interval_id = window.set_interval_with_callback_and_timeout_and_arguments_0(
///         // Note this method call, which uses `as_ref()` to get a `JsValue`
///         // from our `Closure` which is then converted to a `&Function`
///         // using the `JsCast::unchecked_ref` function.
///         cb.as_ref().unchecked_ref(),
///         1_000,
///     )?;
///
///     // Same as above.
///     Ok(IntervalHandle {
///         interval_id,
///         _closure: cb,
///     })
/// }
/// ```
///
/// ## Using `FnOnce` and `Closure::once` with `requestAnimationFrame`
///
/// Because `requestAnimationFrame` only calls its callback once, we can use
/// `FnOnce` and `Closure::once` with it.
///
/// ```rust,no_run
/// use wasm_bindgen::prelude::*;
///
/// #[wasm_bindgen]
/// extern "C" {
///     fn requestAnimationFrame(closure: &Closure<dyn FnMut()>) -> u32;
///     fn cancelAnimationFrame(id: u32);
///
///     #[wasm_bindgen(js_namespace = console)]
///     fn log(s: &str);
/// }
///
/// #[wasm_bindgen]
/// pub struct AnimationFrameHandle {
///     animation_id: u32,
///     _closure: Closure<dyn FnMut()>,
/// }
///
/// impl Drop for AnimationFrameHandle {
///     fn drop(&mut self) {
///         cancelAnimationFrame(self.animation_id);
///     }
/// }
///
/// // A type that will log a message when it is dropped.
/// struct LogOnDrop(&'static str);
/// impl Drop for LogOnDrop {
///     fn drop(&mut self) {
///         log(self.0);
///     }
/// }
///
/// #[wasm_bindgen]
/// pub fn run() -> AnimationFrameHandle {
///     // We are using `Closure::once` which takes a `FnOnce`, so the function
///     // can drop and/or move things that it closes over.
///     let fired = LogOnDrop("animation frame fired or canceled");
///     let cb = Closure::once(move || drop(fired));
///
///     // Schedule the animation frame!
///     let animation_id = requestAnimationFrame(&cb);
///
///     // Again, return a handle to JS, so that the closure is not dropped
///     // immediately and JS can decide whether to cancel the animation frame.
///     AnimationFrameHandle {
///         animation_id,
///         _closure: cb,
///     }
/// }
/// ```
///
/// ## Converting `FnOnce`s directly into JavaScript Functions with `Closure::once_into_js`
///
/// If we don't want to allow a `FnOnce` to be eagerly dropped (maybe because we
/// just want it to drop after it is called and don't care about cancellation)
/// then we can use the `Closure::once_into_js` function.
///
/// This is the same `requestAnimationFrame` example as above, but without
/// supporting early cancellation.
///
/// ```
/// use wasm_bindgen::prelude::*;
///
/// #[wasm_bindgen]
/// extern "C" {
///     // We modify the binding to take an untyped `JsValue` since that is what
///     // is returned by `Closure::once_into_js`.
///     //
///     // If we were using the `web_sys` binding for `requestAnimationFrame`,
///     // then the call sites would cast the `JsValue` into a `&js_sys::Function`
///     // using `f.unchecked_ref::<js_sys::Function>()`. See the `web_sys`
///     // example above for details.
///     fn requestAnimationFrame(callback: JsValue);
///
///     #[wasm_bindgen(js_namespace = console)]
///     fn log(s: &str);
/// }
///
/// // A type that will log a message when it is dropped.
/// struct LogOnDrop(&'static str);
/// impl Drop for LogOnDrop {
///     fn drop(&mut self) {
///         log(self.0);
///     }
/// }
///
/// #[wasm_bindgen]
/// pub fn run() {
///     // We are using `Closure::once_into_js` which takes a `FnOnce` and
///     // converts it into a JavaScript function, which is returned as a
///     // `JsValue`.
///     let fired = LogOnDrop("animation frame fired");
///     let cb = Closure::once_into_js(move || drop(fired));
///
///     // Schedule the animation frame!
///     requestAnimationFrame(cb);
///
///     // No need to worry about whether or not we drop a `Closure`
///     // here or return some sort of handle to JS!
/// }
/// ```
pub struct Closure<T: ?Sized> {
    js: ManuallyDrop<JsValue>,
    data: ManuallyDrop<Box<T>>,
}

union FatPtr<T: ?Sized> {
    ptr: *mut T,
    fields: (usize, usize),
}

impl<T> Closure<T>
where
    T: ?Sized + WasmClosure,
{
    /// Creates a new instance of `Closure` from the provided Rust function.
    ///
    /// Note that the closure provided here, `F`, has a few requirements
    /// associated with it:
    ///
    /// * It must implement `Fn` or `FnMut` (for `FnOnce` functions see
    ///   `Closure::once` and `Closure::once_into_js`).
    ///
    /// * 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 types that can be shared with
    ///   JS (i.e. have `#[wasm_bindgen]` annotations or are simple numbers,
    ///   etc.)
    pub fn new<F>(t: F) -> Closure<T>
    where
        F: IntoWasmClosure<T> + 'static,
    {
        Closure::wrap(Box::new(t).unsize())
    }

    /// A more direct version of `Closure::new` which creates a `Closure` from
    /// a `Box<dyn Fn>`/`Box<dyn FnMut>`, which is how it's kept internally.
    pub fn wrap(mut data: Box<T>) -> Closure<T> {
        assert_eq!(mem::size_of::<*const T>(), mem::size_of::<FatPtr<T>>());
        let (a, b) = unsafe {
            FatPtr {
                ptr: &mut *data as *mut T,
            }
            .fields
        };

        // Here we need to create a `JsValue` with the data and `T::invoke()`
        // function pointer. To do that we... take a few unconventional turns.
        // In essence what happens here is this:
        //
        // 1. First up, below we call a function, `breaks_if_inlined`. This
        //    function, as the name implies, does not work if it's inlined.
        //    More on that in a moment.
        // 2. This function internally calls a special import recognized by the
        //    `wasm-bindgen` CLI tool, `__wbindgen_describe_closure`. This
        //    imported symbol is similar to `__wbindgen_describe` in that it's
        //    not intended to show up in the final binary but it's an
        //    intermediate state for a `wasm-bindgen` binary.
        // 3. The `__wbindgen_describe_closure` import is namely passed a
        //    descriptor function, monomorphized for each invocation.
        //
        // Most of this doesn't actually make sense to happen at runtime! The
        // real magic happens when `wasm-bindgen` comes along and updates our
        // generated code. When `wasm-bindgen` runs it performs a few tasks:
        //
        // * First, it finds all functions that call
        //   `__wbindgen_describe_closure`. These are all `breaks_if_inlined`
        //   defined below as the symbol isn't called anywhere else.
        // * Next, `wasm-bindgen` executes the `breaks_if_inlined`
        //   monomorphized functions, passing it dummy arguments. This will
        //   execute the function just enough to invoke the special import,
        //   namely telling us about the function pointer that is the describe
        //   shim.
        // * This knowledge is then used to actually find the descriptor in the
        //   function table which is then executed to figure out the signature
        //   of the closure.
        // * Finally, and probably most heinously, the call to
        //   `breaks_if_inlined` is rewritten to call an otherwise globally
        //   imported function. This globally imported function will generate
        //   the `JsValue` for this closure specialized for the signature in
        //   question.
        //
        // Later on `wasm-gc` will clean up all the dead code and ensure that
        // we don't actually call `__wbindgen_describe_closure` at runtime. This
        // means we will end up not actually calling `breaks_if_inlined` in the
        // final binary, all calls to that function should be pruned.
        //
        // See crates/cli-support/src/js/closures.rs for a more information
        // about what's going on here.

        extern "C" fn describe<T: WasmClosure + ?Sized>() {
            inform(CLOSURE);
            T::describe()
        }

        #[inline(never)]
        unsafe fn breaks_if_inlined<T: WasmClosure + ?Sized>(a: usize, b: usize) -> u32 {
            super::__wbindgen_describe_closure(a as u32, b as u32, describe::<T> as u32)
        }

        let idx = unsafe { breaks_if_inlined::<T>(a, b) };

        Closure {
            js: ManuallyDrop::new(JsValue::_new(idx)),
            data: ManuallyDrop::new(data),
        }
    }

    /// Release memory management of this closure from Rust to the JS GC.
    ///
    /// When a `Closure` is dropped it will release the Rust memory and
    /// invalidate the associated JS closure, but this isn't always desired.
    /// Some callbacks are alive for the entire duration of the program or for a
    /// lifetime dynamically managed by the JS GC. This function can be used
    /// to drop this `Closure` while keeping the associated JS function still
    /// valid.
    ///
    /// By default this function will leak memory. This can be dangerous if this
    /// function is called many times in an application because the memory leak
    /// will overwhelm the page quickly and crash the wasm.
    ///
    /// If the browser, however, supports weak references, then this function
    /// will not leak memory. Instead the Rust memory will be reclaimed when the
    /// JS closure is GC'd. Weak references are not enabled by default since
    /// they're still a proposal for the JS standard. They can be enabled with
    /// `WASM_BINDGEN_WEAKREF=1` when running `wasm-bindgen`, however.
    pub fn into_js_value(self) -> JsValue {
        let idx = self.js.idx;
        mem::forget(self);
        JsValue::_new(idx)
    }

    /// Same as `into_js_value`, but doesn't return a value.
    pub fn forget(self) {
        drop(self.into_js_value());
    }
}

// NB: we use a specific `T` for this `Closure<T>` impl block to avoid every
// call site having to provide an explicit, turbo-fished type like
// `Closure::<dyn FnOnce()>::once(...)`.
impl Closure<dyn FnOnce()> {
    /// Create a `Closure` from a function that can only be called once.
    ///
    /// Since we have no way of enforcing that JS cannot attempt to call this
    /// `FnOne(A...) -> R` more than once, this produces a `Closure<dyn FnMut(A...)
    /// -> R>` that will dynamically throw a JavaScript error if called more
    /// than once.
    ///
    /// # Example
    ///
    /// ```rust,no_run
    /// use wasm_bindgen::prelude::*;
    ///
    /// // Create an non-`Copy`, owned `String`.
    /// let mut s = String::from("Hello");
    ///
    /// // Close over `s`. Since `f` returns `s`, it is `FnOnce` and can only be
    /// // called once. If it was called a second time, it wouldn't have any `s`
    /// // to work with anymore!
    /// let f = move || {
    ///     s += ", World!";
    ///     s
    /// };
    ///
    /// // Create a `Closure` from `f`. Note that the `Closure`'s type parameter
    /// // is `FnMut`, even though `f` is `FnOnce`.
    /// let closure: Closure<dyn FnMut() -> String> = Closure::once(f);
    /// ```
    pub fn once<F, A, R>(fn_once: F) -> Closure<F::FnMut>
    where
        F: 'static + WasmClosureFnOnce<A, R>,
    {
        Closure::wrap(fn_once.into_fn_mut())
    }

    /// Convert a `FnOnce(A...) -> R` into a JavaScript `Function` object.
    ///
    /// If the JavaScript function is invoked more than once, it will throw an
    /// exception.
    ///
    /// Unlike `Closure::once`, this does *not* return a `Closure` that can be
    /// dropped before the function is invoked to deallocate the closure. The
    /// only way the `FnOnce` is deallocated is by calling the JavaScript
    /// function. If the JavaScript function is never called then the `FnOnce`
    /// and everything it closes over will leak.
    ///
    /// ```rust,ignore
    /// use wasm_bindgen::{prelude::*, JsCast};
    ///
    /// let f = Closure::once_into_js(move || {
    ///     // ...
    /// });
    ///
    /// assert!(f.is_instance_of::<js_sys::Function>());
    /// ```
    pub fn once_into_js<F, A, R>(fn_once: F) -> JsValue
    where
        F: 'static + WasmClosureFnOnce<A, R>,
    {
        fn_once.into_js_function()
    }
}

/// A trait for converting an `FnOnce(A...) -> R` into a `FnMut(A...) -> R` that
/// will throw if ever called more than once.
#[doc(hidden)]
pub trait WasmClosureFnOnce<A, R>: 'static {
    type FnMut: ?Sized + 'static + WasmClosure;

    fn into_fn_mut(self) -> Box<Self::FnMut>;

    fn into_js_function(self) -> JsValue;
}

impl<T: ?Sized> AsRef<JsValue> for Closure<T> {
    fn as_ref(&self) -> &JsValue {
        &self.js
    }
}

impl<T> WasmDescribe for Closure<T>
where
    T: WasmClosure + ?Sized,
{
    fn describe() {
        inform(EXTERNREF);
    }
}

// `Closure` can only be passed by reference to imports.
impl<'a, T> IntoWasmAbi for &'a Closure<T>
where
    T: WasmClosure + ?Sized,
{
    type Abi = u32;

    fn into_abi(self) -> u32 {
        (&*self.js).into_abi()
    }
}

impl<'a, T> OptionIntoWasmAbi for &'a Closure<T>
where
    T: WasmClosure + ?Sized,
{
    fn none() -> Self::Abi {
        0
    }
}

fn _check() {
    fn _assert<T: IntoWasmAbi>() {}
    _assert::<&Closure<dyn Fn()>>();
    _assert::<&Closure<dyn Fn(String)>>();
    _assert::<&Closure<dyn Fn() -> String>>();
    _assert::<&Closure<dyn FnMut()>>();
    _assert::<&Closure<dyn FnMut(String)>>();
    _assert::<&Closure<dyn FnMut() -> String>>();
}

impl<T> fmt::Debug for Closure<T>
where
    T: ?Sized,
{
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(f, "Closure {{ ... }}")
    }
}

impl<T> Drop for Closure<T>
where
    T: ?Sized,
{
    fn drop(&mut self) {
        unsafe {
            // this will implicitly drop our strong reference in addition to
            // invalidating all future invocations of the closure
            if super::__wbindgen_cb_drop(self.js.idx) != 0 {
                ManuallyDrop::drop(&mut self.data);
            }
        }
    }
}

/// 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.
#[doc(hidden)]
pub unsafe trait WasmClosure {
    fn describe();
}

/// 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.
#[doc(hidden)]
pub trait IntoWasmClosure<T: ?Sized> {
    fn unsize(self: Box<Self>) -> Box<T>;
}

// The memory safety here in these implementations below is a bit tricky. We
// want to be able to drop the `Closure` object from within the invocation of a
// `Closure` for cases like promises. That means that while it's running we
// might drop the `Closure`, but that shouldn't invalidate the environment yet.
//
// Instead what we do is to wrap closures in `Rc` variables. The main `Closure`
// has a strong reference count which keeps the trait object alive. Each
// invocation of a closure then *also* clones this and gets a new reference
// count. When the closure returns it will release the reference count.
//
// This means that if the main `Closure` is dropped while it's being invoked
// then destruction is deferred until execution returns. Otherwise it'll
// deallocate data immediately.

macro_rules! doit {
    ($(
        ($($var:ident)*)
    )*) => ($(
        unsafe impl<$($var,)* R> WasmClosure for dyn Fn($($var),*) -> R + 'static
            where $($var: FromWasmAbi + 'static,)*
                  R: ReturnWasmAbi + 'static,
        {
            fn describe() {
                #[allow(non_snake_case)]
                unsafe extern "C" fn invoke<$($var: FromWasmAbi,)* R: ReturnWasmAbi>(
                    a: usize,
                    b: usize,
                    $($var: <$var as FromWasmAbi>::Abi),*
                ) -> <R as ReturnWasmAbi>::Abi {
                    if a == 0 {
                        throw_str("closure invoked after being dropped");
                    }
                    // Make sure all stack variables are converted before we
                    // convert `ret` as it may throw (for `Result`, for
                    // example)
                    let ret = {
                        let f: *const dyn Fn($($var),*) -> R =
                            FatPtr { fields: (a, b) }.ptr;
                        $(
                            let $var = <$var as FromWasmAbi>::from_abi($var);
                        )*
                        (*f)($($var),*)
                    };
                    ret.return_abi()
                }

                inform(invoke::<$($var,)* R> as u32);

                unsafe extern fn destroy<$($var: FromWasmAbi,)* R: ReturnWasmAbi>(
                    a: usize,
                    b: usize,
                ) {
                    // This can be called by the JS glue in erroneous situations
                    // such as when the closure has already been destroyed. If
                    // that's the case let's not make things worse by
                    // segfaulting and/or asserting, so just ignore null
                    // pointers.
                    if a == 0 {
                        return;
                    }
                    drop(Box::from_raw(FatPtr::<dyn Fn($($var,)*) -> R> {
                        fields: (a, b)
                    }.ptr));
                }
                inform(destroy::<$($var,)* R> as u32);

                <&Self>::describe();
            }
        }

        unsafe impl<$($var,)* R> WasmClosure for dyn FnMut($($var),*) -> R + 'static
            where $($var: FromWasmAbi + 'static,)*
                  R: ReturnWasmAbi + 'static,
        {
            fn describe() {
                #[allow(non_snake_case)]
                unsafe extern "C" fn invoke<$($var: FromWasmAbi,)* R: ReturnWasmAbi>(
                    a: usize,
                    b: usize,
                    $($var: <$var as FromWasmAbi>::Abi),*
                ) -> <R as ReturnWasmAbi>::Abi {
                    if a == 0 {
                        throw_str("closure invoked recursively or after being dropped");
                    }
                    // Make sure all stack variables are converted before we
                    // convert `ret` as it may throw (for `Result`, for
                    // example)
                    let ret = {
                        let f: *const dyn FnMut($($var),*) -> R =
                            FatPtr { fields: (a, b) }.ptr;
                        let f = f as *mut dyn FnMut($($var),*) -> R;
                        $(
                            let $var = <$var as FromWasmAbi>::from_abi($var);
                        )*
                        (*f)($($var),*)
                    };
                    ret.return_abi()
                }

                inform(invoke::<$($var,)* R> as u32);

                unsafe extern fn destroy<$($var: FromWasmAbi,)* R: ReturnWasmAbi>(
                    a: usize,
                    b: usize,
                ) {
                    // See `Fn()` above for why we simply return
                    if a == 0 {
                        return;
                    }
                    drop(Box::from_raw(FatPtr::<dyn FnMut($($var,)*) -> R> {
                        fields: (a, b)
                    }.ptr));
                }
                inform(destroy::<$($var,)* R> as u32);

                <&mut Self>::describe();
            }
        }

        #[allow(non_snake_case, unused_parens)]
        impl<T, $($var,)* R> WasmClosureFnOnce<($($var),*), R> for T
            where T: 'static + FnOnce($($var),*) -> R,
                  $($var: FromWasmAbi + 'static,)*
                  R: ReturnWasmAbi + 'static
        {
            type FnMut = dyn FnMut($($var),*) -> R;

            fn into_fn_mut(self) -> Box<Self::FnMut> {
                let mut me = Some(self);
                Box::new(move |$($var),*| {
                    let me = me.take().expect_throw("FnOnce called more than once");
                    me($($var),*)
                })
            }

            fn into_js_function(self) -> JsValue {
                use std::rc::Rc;
                use crate::__rt::WasmRefCell;

                let mut me = Some(self);

                let rc1 = Rc::new(WasmRefCell::new(None));
                let rc2 = rc1.clone();

                let closure = Closure::wrap(Box::new(move |$($var),*| {
                    // Invoke ourself and get the result.
                    let me = me.take().expect_throw("FnOnce called more than once");
                    let result = me($($var),*);

                    // And then drop the `Rc` holding this function's `Closure`
                    // alive.
                    debug_assert_eq!(Rc::strong_count(&rc2), 1);
                    let option_closure = rc2.borrow_mut().take();
                    debug_assert!(option_closure.is_some());
                    drop(option_closure);

                    result
                }) as Box<dyn FnMut($($var),*) -> R>);

                let js_val = closure.as_ref().clone();

                *rc1.borrow_mut() = Some(closure);
                debug_assert_eq!(Rc::strong_count(&rc1), 2);
                drop(rc1);

                js_val
            }
        }

        impl<T, $($var,)* R> IntoWasmClosure<dyn FnMut($($var),*) -> R> for T
            where T: 'static + FnMut($($var),*) -> R,
                  $($var: FromWasmAbi + 'static,)*
                  R: ReturnWasmAbi + 'static,
        {
            fn unsize(self: Box<Self>) -> Box<dyn FnMut($($var),*) -> R> { self }
        }

        impl<T, $($var,)* R> IntoWasmClosure<dyn Fn($($var),*) -> R> for T
            where T: 'static + Fn($($var),*) -> R,
                  $($var: FromWasmAbi + 'static,)*
                  R: ReturnWasmAbi + 'static,
        {
            fn unsize(self: Box<Self>) -> Box<dyn Fn($($var),*) -> R> { self }
        }
    )*)
}

doit! {
    ()
    (A)
    (A B)
    (A B C)
    (A B C D)
    (A B C D E)
    (A B C D E F)
    (A B C D E F G)
    (A B C D E F G H)
}

// Copy the above impls down here for where there's only one argument and it's a
// reference. We could add more impls for more kinds of references, but it
// becomes a combinatorial explosion quickly. Let's see how far we can get with
// just this one! Maybe someone else can figure out voodoo so we don't have to
// duplicate.

unsafe impl<A, R> WasmClosure for dyn Fn(&A) -> R
where
    A: RefFromWasmAbi,
    R: ReturnWasmAbi + 'static,
{
    fn describe() {
        #[allow(non_snake_case)]
        unsafe extern "C" fn invoke<A: RefFromWasmAbi, R: ReturnWasmAbi>(
            a: usize,
            b: usize,
            arg: <A as RefFromWasmAbi>::Abi,
        ) -> <R as ReturnWasmAbi>::Abi {
            if a == 0 {
                throw_str("closure invoked after being dropped");
            }
            // Make sure all stack variables are converted before we
            // convert `ret` as it may throw (for `Result`, for
            // example)
            let ret = {
                let f: *const dyn Fn(&A) -> R = FatPtr { fields: (a, b) }.ptr;
                let arg = <A as RefFromWasmAbi>::ref_from_abi(arg);
                (*f)(&*arg)
            };
            ret.return_abi()
        }

        inform(invoke::<A, R> as u32);

        unsafe extern "C" fn destroy<A: RefFromWasmAbi, R: ReturnWasmAbi>(a: usize, b: usize) {
            // See `Fn()` above for why we simply return
            if a == 0 {
                return;
            }
            drop(Box::from_raw(
                FatPtr::<dyn Fn(&A) -> R> { fields: (a, b) }.ptr,
            ));
        }
        inform(destroy::<A, R> as u32);

        <&Self>::describe();
    }
}

unsafe impl<A, R> WasmClosure for dyn FnMut(&A) -> R
where
    A: RefFromWasmAbi,
    R: ReturnWasmAbi + 'static,
{
    fn describe() {
        #[allow(non_snake_case)]
        unsafe extern "C" fn invoke<A: RefFromWasmAbi, R: ReturnWasmAbi>(
            a: usize,
            b: usize,
            arg: <A as RefFromWasmAbi>::Abi,
        ) -> <R as ReturnWasmAbi>::Abi {
            if a == 0 {
                throw_str("closure invoked recursively or after being dropped");
            }
            // Make sure all stack variables are converted before we
            // convert `ret` as it may throw (for `Result`, for
            // example)
            let ret = {
                let f: *const dyn FnMut(&A) -> R = FatPtr { fields: (a, b) }.ptr;
                let f = f as *mut dyn FnMut(&A) -> R;
                let arg = <A as RefFromWasmAbi>::ref_from_abi(arg);
                (*f)(&*arg)
            };
            ret.return_abi()
        }

        inform(invoke::<A, R> as u32);

        unsafe extern "C" fn destroy<A: RefFromWasmAbi, R: ReturnWasmAbi>(a: usize, b: usize) {
            // See `Fn()` above for why we simply return
            if a == 0 {
                return;
            }
            drop(Box::from_raw(
                FatPtr::<dyn FnMut(&A) -> R> { fields: (a, b) }.ptr,
            ));
        }
        inform(destroy::<A, R> as u32);

        <&mut Self>::describe();
    }
}

#[allow(non_snake_case)]
impl<T, A, R> WasmClosureFnOnce<(&A,), R> for T
where
    T: 'static + FnOnce(&A) -> R,
    A: RefFromWasmAbi + 'static,
    R: ReturnWasmAbi + 'static,
{
    type FnMut = dyn FnMut(&A) -> R;

    fn into_fn_mut(self) -> Box<Self::FnMut> {
        let mut me = Some(self);
        Box::new(move |arg| {
            let me = me.take().expect_throw("FnOnce called more than once");
            me(arg)
        })
    }

    fn into_js_function(self) -> JsValue {
        use crate::__rt::WasmRefCell;
        use std::rc::Rc;

        let mut me = Some(self);

        let rc1 = Rc::new(WasmRefCell::new(None));
        let rc2 = rc1.clone();

        let closure = Closure::wrap(Box::new(move |arg: &A| {
            // Invoke ourself and get the result.
            let me = me.take().expect_throw("FnOnce called more than once");
            let result = me(arg);

            // And then drop the `Rc` holding this function's `Closure`
            // alive.
            debug_assert_eq!(Rc::strong_count(&rc2), 1);
            let option_closure = rc2.borrow_mut().take();
            debug_assert!(option_closure.is_some());
            drop(option_closure);

            result
        }) as Box<dyn FnMut(&A) -> R>);

        let js_val = closure.as_ref().clone();

        *rc1.borrow_mut() = Some(closure);
        debug_assert_eq!(Rc::strong_count(&rc1), 2);
        drop(rc1);

        js_val
    }
}