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
use super::invoke_wasm_and_catch_traps;
use crate::prelude::*;
use crate::runtime::vm::{VMFuncRef, VMOpaqueContext};
use crate::store::{AutoAssertNoGc, StoreOpaque};
use crate::{
    AsContext, AsContextMut, Engine, Func, FuncType, HeapType, NoFunc, RefType, StoreContextMut,
    ValRaw, ValType,
};
use core::ffi::c_void;
use core::marker;
use core::mem::{self, MaybeUninit};
use core::num::NonZeroUsize;
use core::ptr::{self};
use wasmtime_environ::VMSharedTypeIndex;

/// A statically typed WebAssembly function.
///
/// Values of this type represent statically type-checked WebAssembly functions.
/// The function within a [`TypedFunc`] is statically known to have `Params` as its
/// parameters and `Results` as its results.
///
/// This structure is created via [`Func::typed`] or [`TypedFunc::new_unchecked`].
/// For more documentation about this see those methods.
pub struct TypedFunc<Params, Results> {
    _a: marker::PhantomData<fn(Params) -> Results>,
    ty: FuncType,
    func: Func,
}

impl<Params, Results> Clone for TypedFunc<Params, Results> {
    fn clone(&self) -> TypedFunc<Params, Results> {
        Self {
            _a: marker::PhantomData,
            ty: self.ty.clone(),
            func: self.func,
        }
    }
}

impl<Params, Results> TypedFunc<Params, Results>
where
    Params: WasmParams,
    Results: WasmResults,
{
    /// An unchecked version of [`Func::typed`] which does not perform a
    /// typecheck and simply assumes that the type declared here matches the
    /// type of this function.
    ///
    /// The semantics of this function are the same as [`Func::typed`] except
    /// that no error is returned because no typechecking is done.
    ///
    /// # Unsafety
    ///
    /// This function only safe to call if `typed` would otherwise return `Ok`
    /// for the same `Params` and `Results` specified. If `typed` would return
    /// an error then the returned `TypedFunc` is memory unsafe to invoke.
    pub unsafe fn new_unchecked(store: impl AsContext, func: Func) -> TypedFunc<Params, Results> {
        let store = store.as_context().0;
        Self::_new_unchecked(store, func)
    }

    pub(crate) unsafe fn _new_unchecked(
        store: &StoreOpaque,
        func: Func,
    ) -> TypedFunc<Params, Results> {
        let ty = func.load_ty(store);
        TypedFunc {
            _a: marker::PhantomData,
            ty,
            func,
        }
    }

    /// Returns the underlying [`Func`] that this is wrapping, losing the static
    /// type information in the process.
    pub fn func(&self) -> &Func {
        &self.func
    }

    /// Invokes this WebAssembly function with the specified parameters.
    ///
    /// Returns either the results of the call, or a [`Trap`] if one happened.
    ///
    /// For more information, see the [`Func::typed`] and [`Func::call`]
    /// documentation.
    ///
    /// # Errors
    ///
    /// For more information on errors see the documentation on [`Func::call`].
    ///
    /// # Panics
    ///
    /// This function will panic if it is called when the underlying [`Func`] is
    /// connected to an asynchronous store.
    ///
    /// [`Trap`]: crate::Trap
    pub fn call(&self, mut store: impl AsContextMut, params: Params) -> Result<Results> {
        let mut store = store.as_context_mut();
        assert!(
            !store.0.async_support(),
            "must use `call_async` with async stores"
        );
        if Self::need_gc_before_call_raw(store.0, &params) {
            store.0.gc();
        }
        let func = self.func.vm_func_ref(store.0);
        unsafe { Self::call_raw(&mut store, &self.ty, func, params) }
    }

    /// Invokes this WebAssembly function with the specified parameters.
    ///
    /// Returns either the results of the call, or a [`Trap`] if one happened.
    ///
    /// For more information, see the [`Func::typed`] and [`Func::call_async`]
    /// documentation.
    ///
    /// # Errors
    ///
    /// For more information on errors see the documentation on [`Func::call`].
    ///
    /// # Panics
    ///
    /// This function will panic if it is called when the underlying [`Func`] is
    /// connected to a synchronous store.
    ///
    /// [`Trap`]: crate::Trap
    #[cfg(feature = "async")]
    pub async fn call_async<T>(
        &self,
        mut store: impl AsContextMut<Data = T>,
        params: Params,
    ) -> Result<Results>
    where
        T: Send,
    {
        let mut store = store.as_context_mut();
        assert!(
            store.0.async_support(),
            "must use `call` with non-async stores"
        );
        if Self::need_gc_before_call_raw(store.0, &params) {
            store.0.gc_async().await;
        }
        store
            .on_fiber(|store| {
                let func = self.func.vm_func_ref(store.0);
                unsafe { Self::call_raw(store, &self.ty, func, params) }
            })
            .await?
    }

    #[inline]
    pub(crate) fn need_gc_before_call_raw(_store: &StoreOpaque, _params: &Params) -> bool {
        #[cfg(feature = "gc")]
        {
            // See the comment in `Func::call_impl_check_args`.
            let num_gc_refs = _params.vmgcref_pointing_to_object_count();
            if let Some(num_gc_refs) = NonZeroUsize::new(num_gc_refs) {
                return _store
                    .unwrap_gc_store()
                    .gc_heap
                    .need_gc_before_entering_wasm(num_gc_refs);
            }
        }

        false
    }

    /// Do a raw call of a typed function.
    ///
    /// # Safety
    ///
    /// `func` must be of the given type.
    ///
    /// If `Self::need_gc_before_call_raw`, then the caller must have done a GC
    /// just before calling this method.
    pub(crate) unsafe fn call_raw<T>(
        store: &mut StoreContextMut<'_, T>,
        ty: &FuncType,
        func: ptr::NonNull<VMFuncRef>,
        params: Params,
    ) -> Result<Results> {
        // double-check that params/results match for this function's type in
        // debug mode.
        if cfg!(debug_assertions) {
            Self::debug_typecheck(store.0, func.as_ref().type_index);
        }

        // Validate that all runtime values flowing into this store indeed
        // belong within this store, otherwise it would be unsafe for store
        // values to cross each other.

        union Storage<T: Copy, U: Copy> {
            params: MaybeUninit<T>,
            results: U,
        }

        let mut storage = Storage::<Params::ValRawStorage, Results::ValRawStorage> {
            params: MaybeUninit::uninit(),
        };

        {
            let mut store = AutoAssertNoGc::new(store.0);
            params.store(&mut store, ty, &mut storage.params)?;
        }

        // Try to capture only a single variable (a tuple) in the closure below.
        // This means the size of the closure is one pointer and is much more
        // efficient to move in memory. This closure is actually invoked on the
        // other side of a C++ shim, so it can never be inlined enough to make
        // the memory go away, so the size matters here for performance.
        let mut captures = (func, storage);

        let result = invoke_wasm_and_catch_traps(store, |caller| {
            let (func_ref, storage) = &mut captures;
            let func_ref = func_ref.as_ref();
            (func_ref.array_call)(
                func_ref.vmctx,
                VMOpaqueContext::from_vmcontext(caller),
                (storage as *mut Storage<_, _>) as *mut ValRaw,
                mem::size_of_val::<Storage<_, _>>(storage) / mem::size_of::<ValRaw>(),
            );
        });

        let (_, storage) = captures;
        result?;

        let mut store = AutoAssertNoGc::new(store.0);
        Ok(Results::load(&mut store, &storage.results))
    }

    /// Purely a debug-mode assertion, not actually used in release builds.
    fn debug_typecheck(store: &StoreOpaque, func: VMSharedTypeIndex) {
        let ty = FuncType::from_shared_type_index(store.engine(), func);
        Params::typecheck(store.engine(), ty.params(), TypeCheckPosition::Param)
            .expect("params should match");
        Results::typecheck(store.engine(), ty.results(), TypeCheckPosition::Result)
            .expect("results should match");
    }
}

#[doc(hidden)]
#[derive(Copy, Clone)]
pub enum TypeCheckPosition {
    Param,
    Result,
}

/// A trait implemented for types which can be arguments and results for
/// closures passed to [`Func::wrap`] as well as parameters to [`Func::typed`].
///
/// This trait should not be implemented by user types. This trait may change at
/// any time internally. The types which implement this trait, however, are
/// stable over time.
///
/// For more information see [`Func::wrap`] and [`Func::typed`]
pub unsafe trait WasmTy: Send {
    // Do a "static" (aka at time of `func.typed::<P, R>()`) ahead-of-time type
    // check for this type at the given position. You probably don't need to
    // override this trait method.
    #[doc(hidden)]
    #[inline]
    fn typecheck(engine: &Engine, actual: ValType, position: TypeCheckPosition) -> Result<()> {
        let expected = Self::valtype();
        debug_assert!(expected.comes_from_same_engine(engine));
        debug_assert!(actual.comes_from_same_engine(engine));
        match position {
            // The caller is expecting to receive a `T` and the callee is
            // actually returning a `U`, so ensure that `U <: T`.
            TypeCheckPosition::Result => actual.ensure_matches(engine, &expected),
            // The caller is expecting to pass a `T` and the callee is expecting
            // to receive a `U`, so ensure that `T <: U`.
            TypeCheckPosition::Param => match (expected.as_ref(), actual.as_ref()) {
                // ... except that this technically-correct check would overly
                // restrict the usefulness of our typed function APIs for the
                // specific case of concrete reference types. Let's work through
                // an example.
                //
                // Consider functions that take a `(ref param $some_func_type)`
                // parameter:
                //
                // * We cannot have a static `wasmtime::SomeFuncTypeRef` type
                //   that implements `WasmTy` specifically for `(ref null
                //   $some_func_type)` because Wasm modules, and their types,
                //   are loaded dynamically at runtime.
                //
                // * Therefore the embedder's only option for `T <: (ref null
                //   $some_func_type)` is `T = (ref null nofunc)` aka
                //   `Option<wasmtime::NoFunc>`.
                //
                // * But that static type means they can *only* pass in the null
                //   function reference as an argument to the typed function.
                //   This is way too restrictive! For ergonomics, we want them
                //   to be able to pass in a `wasmtime::Func` whose type is
                //   `$some_func_type`!
                //
                // To lift this constraint and enable better ergonomics for
                // embedders, we allow `top(T) <: top(U)` -- i.e. they are part
                // of the same type hierarchy and a dynamic cast could possibly
                // succeed -- for the specific case of concrete heap type
                // parameters, and fall back to dynamic type checks on the
                // arguments passed to each invocation, as necessary.
                (Some(expected_ref), Some(actual_ref)) if actual_ref.heap_type().is_concrete() => {
                    expected_ref
                        .heap_type()
                        .top()
                        .ensure_matches(engine, &actual_ref.heap_type().top())
                }
                _ => expected.ensure_matches(engine, &actual),
            },
        }
    }

    // The value type that this Type represents.
    #[doc(hidden)]
    fn valtype() -> ValType;

    #[doc(hidden)]
    fn may_gc() -> bool {
        match Self::valtype() {
            ValType::Ref(_) => true,
            ValType::I32 | ValType::I64 | ValType::F32 | ValType::F64 | ValType::V128 => false,
        }
    }

    // Dynamic checks that this value is being used with the correct store
    // context.
    #[doc(hidden)]
    fn compatible_with_store(&self, store: &StoreOpaque) -> bool;

    // Dynamic checks that `self <: actual` for concrete type arguments. See the
    // comment above in `WasmTy::typecheck`.
    //
    // Only ever called for concrete reference type arguments, so any type which
    // is not in a type hierarchy with concrete reference types can implement
    // this with `unreachable!()`.
    #[doc(hidden)]
    fn dynamic_concrete_type_check(
        &self,
        store: &StoreOpaque,
        nullable: bool,
        actual: &HeapType,
    ) -> Result<()>;

    // Is this a GC-managed reference that actually points to a GC object? That
    // is, `self` is *not* an `i31`, null reference, or uninhabited type.
    //
    // Note that it is okay if this returns false positives (i.e. `true` for
    // `Rooted<AnyRef>` without actually looking up the rooted `anyref` in the
    // store and reflecting on it to determine whether it is actually an
    // `i31`). However, it is not okay if this returns false negatives.
    #[doc(hidden)]
    #[inline]
    fn is_vmgcref_and_points_to_object(&self) -> bool {
        Self::valtype().is_vmgcref_type_and_points_to_object()
    }

    // Store `self` into `ptr`.
    //
    // NB: We _must not_ trigger a GC when passing refs from host code into Wasm
    // (e.g. returned from a host function or passed as arguments to a Wasm
    // function). After insertion into the activations table, the reference is
    // no longer rooted. If multiple references are being sent from the host
    // into Wasm and we allowed GCs during insertion, then the following events
    // could happen:
    //
    // * Reference A is inserted into the activations table. This does not
    //   trigger a GC, but does fill the table to capacity.
    //
    // * The caller's reference to A is removed. Now the only reference to A is
    //   from the activations table.
    //
    // * Reference B is inserted into the activations table. Because the table
    //   is at capacity, a GC is triggered.
    //
    // * A is reclaimed because the only reference keeping it alive was the
    //   activation table's reference (it isn't inside any Wasm frames on the
    //   stack yet, so stack scanning and stack maps don't increment its
    //   reference count).
    //
    // * We transfer control to Wasm, giving it A and B. Wasm uses A. That's a
    //   use-after-free bug.
    //
    // In conclusion, to prevent uses-after-free bugs, we cannot GC while
    // converting types into their raw ABI forms.
    #[doc(hidden)]
    fn store(self, store: &mut AutoAssertNoGc<'_>, ptr: &mut MaybeUninit<ValRaw>) -> Result<()>;

    // Load a version of `Self` from the `ptr` provided.
    //
    // # Safety
    //
    // This function is unsafe as it's up to the caller to ensure that `ptr` is
    // valid for this given type.
    #[doc(hidden)]
    unsafe fn load(store: &mut AutoAssertNoGc<'_>, ptr: &ValRaw) -> Self;
}

macro_rules! integers {
    ($($primitive:ident/$get_primitive:ident => $ty:ident)*) => ($(
        unsafe impl WasmTy for $primitive {
            #[inline]
            fn valtype() -> ValType {
                ValType::$ty
            }
            #[inline]
            fn compatible_with_store(&self, _: &StoreOpaque) -> bool {
                true
            }
            #[inline]
            fn dynamic_concrete_type_check(&self, _: &StoreOpaque, _: bool, _: &HeapType) -> Result<()> {
                unreachable!()
            }
            #[inline]
            fn store(self, _store: &mut AutoAssertNoGc<'_>, ptr: &mut MaybeUninit<ValRaw>) -> Result<()> {
                ptr.write(ValRaw::$primitive(self));
                Ok(())
            }
            #[inline]
            unsafe fn load(_store: &mut AutoAssertNoGc<'_>, ptr: &ValRaw) -> Self {
                ptr.$get_primitive()
            }
        }
    )*)
}

integers! {
    i32/get_i32 => I32
    i64/get_i64 => I64
    u32/get_u32 => I32
    u64/get_u64 => I64
}

macro_rules! floats {
    ($($float:ident/$int:ident/$get_float:ident => $ty:ident)*) => ($(
        unsafe impl WasmTy for $float {
            #[inline]
            fn valtype() -> ValType {
                ValType::$ty
            }
            #[inline]
            fn compatible_with_store(&self, _: &StoreOpaque) -> bool {
                true
            }
            #[inline]
            fn dynamic_concrete_type_check(&self, _: &StoreOpaque, _: bool, _: &HeapType) -> Result<()> {
                unreachable!()
            }
            #[inline]
            fn store(self, _store: &mut AutoAssertNoGc<'_>, ptr: &mut MaybeUninit<ValRaw>) -> Result<()> {
                ptr.write(ValRaw::$float(self.to_bits()));
                Ok(())
            }
            #[inline]
            unsafe fn load(_store: &mut AutoAssertNoGc<'_>, ptr: &ValRaw) -> Self {
                $float::from_bits(ptr.$get_float())
            }
        }
    )*)
}

floats! {
    f32/u32/get_f32 => F32
    f64/u64/get_f64 => F64
}

unsafe impl WasmTy for NoFunc {
    #[inline]
    fn valtype() -> ValType {
        ValType::Ref(RefType::new(false, HeapType::NoFunc))
    }

    #[inline]
    fn compatible_with_store(&self, _store: &StoreOpaque) -> bool {
        match self._inner {}
    }

    #[inline]
    fn dynamic_concrete_type_check(&self, _: &StoreOpaque, _: bool, _: &HeapType) -> Result<()> {
        match self._inner {}
    }

    #[inline]
    fn is_vmgcref_and_points_to_object(&self) -> bool {
        match self._inner {}
    }

    #[inline]
    fn store(self, _store: &mut AutoAssertNoGc<'_>, _ptr: &mut MaybeUninit<ValRaw>) -> Result<()> {
        match self._inner {}
    }

    #[inline]
    unsafe fn load(_store: &mut AutoAssertNoGc<'_>, _ptr: &ValRaw) -> Self {
        unreachable!("NoFunc is uninhabited")
    }
}

unsafe impl WasmTy for Option<NoFunc> {
    #[inline]
    fn valtype() -> ValType {
        ValType::Ref(RefType::new(true, HeapType::NoFunc))
    }

    #[inline]
    fn compatible_with_store(&self, _store: &StoreOpaque) -> bool {
        true
    }

    #[inline]
    fn dynamic_concrete_type_check(
        &self,
        _: &StoreOpaque,
        nullable: bool,
        ty: &HeapType,
    ) -> Result<()> {
        if nullable {
            // `(ref null nofunc) <: (ref null $f)` for all function types `$f`.
            Ok(())
        } else {
            bail!("argument type mismatch: expected non-nullable (ref {ty}), found null reference")
        }
    }

    #[inline]
    fn store(self, _store: &mut AutoAssertNoGc<'_>, ptr: &mut MaybeUninit<ValRaw>) -> Result<()> {
        ptr.write(ValRaw::funcref(ptr::null_mut()));
        Ok(())
    }

    #[inline]
    unsafe fn load(_store: &mut AutoAssertNoGc<'_>, _ptr: &ValRaw) -> Self {
        None
    }
}

unsafe impl WasmTy for Func {
    #[inline]
    fn valtype() -> ValType {
        ValType::Ref(RefType::new(false, HeapType::Func))
    }

    #[inline]
    fn compatible_with_store(&self, store: &StoreOpaque) -> bool {
        store.store_data().contains(self.0)
    }

    #[inline]
    fn dynamic_concrete_type_check(
        &self,
        store: &StoreOpaque,
        _nullable: bool,
        expected: &HeapType,
    ) -> Result<()> {
        let expected = expected.unwrap_concrete_func();
        self.ensure_matches_ty(store, expected)
            .context("argument type mismatch for reference to concrete type")
    }

    #[inline]
    fn store(self, store: &mut AutoAssertNoGc<'_>, ptr: &mut MaybeUninit<ValRaw>) -> Result<()> {
        let abi = self.vm_func_ref(store);
        ptr.write(ValRaw::funcref(abi.cast::<c_void>().as_ptr()));
        Ok(())
    }

    #[inline]
    unsafe fn load(store: &mut AutoAssertNoGc<'_>, ptr: &ValRaw) -> Self {
        let p = ptr.get_funcref();
        debug_assert!(!p.is_null());
        Func::from_vm_func_ref(store, p.cast()).unwrap()
    }
}

unsafe impl WasmTy for Option<Func> {
    #[inline]
    fn valtype() -> ValType {
        ValType::FUNCREF
    }

    #[inline]
    fn compatible_with_store(&self, store: &StoreOpaque) -> bool {
        if let Some(f) = self {
            store.store_data().contains(f.0)
        } else {
            true
        }
    }

    fn dynamic_concrete_type_check(
        &self,
        store: &StoreOpaque,
        nullable: bool,
        expected: &HeapType,
    ) -> Result<()> {
        if let Some(f) = self {
            let expected = expected.unwrap_concrete_func();
            f.ensure_matches_ty(store, expected)
                .context("argument type mismatch for reference to concrete type")
        } else if nullable {
            Ok(())
        } else {
            bail!("argument type mismatch: expected non-nullable (ref {expected}), found null reference")
        }
    }

    #[inline]
    fn store(self, store: &mut AutoAssertNoGc<'_>, ptr: &mut MaybeUninit<ValRaw>) -> Result<()> {
        let raw = if let Some(f) = self {
            f.vm_func_ref(store).as_ptr()
        } else {
            ptr::null_mut()
        };
        ptr.write(ValRaw::funcref(raw.cast::<c_void>()));
        Ok(())
    }

    #[inline]
    unsafe fn load(store: &mut AutoAssertNoGc<'_>, ptr: &ValRaw) -> Self {
        Func::from_vm_func_ref(store, ptr.get_funcref().cast())
    }
}

/// A trait used for [`Func::typed`] and with [`TypedFunc`] to represent the set of
/// parameters for wasm functions.
///
/// This is implemented for bare types that can be passed to wasm as well as
/// tuples of those types.
pub unsafe trait WasmParams: Send {
    #[doc(hidden)]
    type ValRawStorage: Copy;

    #[doc(hidden)]
    fn typecheck(
        engine: &Engine,
        params: impl ExactSizeIterator<Item = crate::ValType>,
        position: TypeCheckPosition,
    ) -> Result<()>;

    #[doc(hidden)]
    fn vmgcref_pointing_to_object_count(&self) -> usize;

    #[doc(hidden)]
    fn store(
        self,
        store: &mut AutoAssertNoGc<'_>,
        func_ty: &FuncType,
        dst: &mut MaybeUninit<Self::ValRawStorage>,
    ) -> Result<()>;
}

// Forward an impl from `T` to `(T,)` for convenience if there's only one
// parameter.
unsafe impl<T> WasmParams for T
where
    T: WasmTy,
{
    type ValRawStorage = <(T,) as WasmParams>::ValRawStorage;

    fn typecheck(
        engine: &Engine,
        params: impl ExactSizeIterator<Item = crate::ValType>,
        position: TypeCheckPosition,
    ) -> Result<()> {
        <(T,) as WasmParams>::typecheck(engine, params, position)
    }

    #[inline]
    fn vmgcref_pointing_to_object_count(&self) -> usize {
        T::is_vmgcref_and_points_to_object(self) as usize
    }

    #[inline]
    fn store(
        self,
        store: &mut AutoAssertNoGc<'_>,
        func_ty: &FuncType,
        dst: &mut MaybeUninit<Self::ValRawStorage>,
    ) -> Result<()> {
        <(T,) as WasmParams>::store((self,), store, func_ty, dst)
    }
}

macro_rules! impl_wasm_params {
    ($n:tt $($t:ident)*) => {
        #[allow(non_snake_case)]
        unsafe impl<$($t: WasmTy,)*> WasmParams for ($($t,)*) {
            type ValRawStorage = [ValRaw; $n];

            fn typecheck(
                _engine: &Engine,
                mut params: impl ExactSizeIterator<Item = crate::ValType>,
                _position: TypeCheckPosition,
            ) -> Result<()> {
                let mut _n = 0;

                $(
                    match params.next() {
                        Some(t) => {
                            _n += 1;
                            $t::typecheck(_engine, t, _position)?
                        },
                        None => bail!("expected {} types, found {}", $n, params.len() + _n),
                    }
                )*

                match params.next() {
                    None => Ok(()),
                    Some(_) => {
                        _n += 1;
                        bail!("expected {} types, found {}", $n, params.len() + _n)
                    },
                }
            }

            #[inline]
            fn vmgcref_pointing_to_object_count(&self) -> usize {
                let ($(ref $t,)*) = self;
                0 $(
                    + $t.is_vmgcref_and_points_to_object() as usize
                )*
            }


            #[inline]
            fn store(
                self,
                _store: &mut AutoAssertNoGc<'_>,
                _func_ty: &FuncType,
                _ptr: &mut MaybeUninit<Self::ValRawStorage>,
            ) -> Result<()> {
                let ($($t,)*) = self;

                let mut _i = 0;
                $(
                    if !$t.compatible_with_store(_store) {
                        bail!("attempt to pass cross-`Store` value to Wasm as function argument");
                    }

                    if $t::valtype().is_ref() {
                        let param_ty = _func_ty.param(_i).unwrap();
                        let ref_ty = param_ty.unwrap_ref();
                        let heap_ty = ref_ty.heap_type();
                        if heap_ty.is_concrete() {
                            $t.dynamic_concrete_type_check(_store, ref_ty.is_nullable(), heap_ty)?;
                        }
                    }

                    let dst = map_maybe_uninit!(_ptr[_i]);
                    $t.store(_store, dst)?;

                    _i += 1;
                )*
                Ok(())
            }
        }
    };
}

for_each_function_signature!(impl_wasm_params);

/// A trait used for [`Func::typed`] and with [`TypedFunc`] to represent the set of
/// results for wasm functions.
pub unsafe trait WasmResults: WasmParams {
    #[doc(hidden)]
    unsafe fn load(store: &mut AutoAssertNoGc<'_>, abi: &Self::ValRawStorage) -> Self;
}

// Forwards from a bare type `T` to the 1-tuple type `(T,)`
unsafe impl<T: WasmTy> WasmResults for T {
    unsafe fn load(store: &mut AutoAssertNoGc<'_>, abi: &Self::ValRawStorage) -> Self {
        <(T,) as WasmResults>::load(store, abi).0
    }
}

macro_rules! impl_wasm_results {
    ($n:tt $($t:ident)*) => {
        #[allow(non_snake_case, unused_variables)]
        unsafe impl<$($t: WasmTy,)*> WasmResults for ($($t,)*) {
            unsafe fn load(store: &mut AutoAssertNoGc<'_>, abi: &Self::ValRawStorage) -> Self {
                let [$($t,)*] = abi;
                ($($t::load(store, $t),)*)
            }
        }
    };
}

for_each_function_signature!(impl_wasm_results);