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
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
1692
1693
1694
1695
1696
1697
1698
1699
1700
1701
1702
1703
1704
1705
1706
1707
1708
1709
1710
1711
1712
1713
1714
1715
1716
1717
1718
1719
1720
1721
1722
1723
1724
1725
1726
1727
1728
1729
1730
1731
1732
1733
1734
1735
1736
1737
1738
1739
1740
1741
1742
1743
1744
1745
1746
1747
1748
1749
1750
1751
1752
1753
1754
1755
1756
1757
1758
1759
1760
1761
1762
1763
1764
1765
1766
1767
1768
1769
1770
1771
1772
1773
1774
1775
1776
1777
1778
1779
1780
1781
1782
1783
1784
1785
1786
1787
1788
1789
1790
1791
1792
1793
1794
1795
1796
1797
1798
1799
1800
1801
1802
1803
1804
1805
1806
1807
1808
1809
1810
1811
1812
1813
1814
1815
1816
1817
1818
1819
1820
1821
1822
1823
1824
1825
1826
1827
1828
1829
1830
1831
1832
1833
1834
1835
1836
1837
1838
1839
1840
1841
1842
1843
1844
1845
1846
1847
1848
1849
1850
1851
1852
1853
1854
1855
1856
1857
1858
1859
1860
1861
1862
1863
1864
1865
1866
1867
1868
1869
1870
1871
1872
1873
1874
1875
1876
1877
1878
1879
1880
1881
1882
1883
1884
1885
1886
1887
1888
1889
1890
1891
1892
1893
1894
1895
1896
1897
1898
1899
1900
1901
1902
1903
1904
1905
1906
1907
1908
1909
1910
1911
1912
1913
1914
1915
1916
1917
1918
1919
1920
1921
1922
1923
1924
1925
1926
1927
1928
1929
1930
1931
1932
1933
1934
1935
1936
1937
1938
1939
1940
1941
1942
1943
1944
1945
1946
1947
1948
1949
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020
2021
2022
2023
2024
2025
2026
2027
2028
2029
2030
2031
2032
2033
2034
2035
2036
2037
2038
2039
2040
2041
2042
2043
2044
2045
2046
2047
2048
2049
2050
2051
2052
2053
2054
2055
2056
2057
2058
2059
2060
2061
2062
2063
2064
2065
2066
2067
2068
2069
2070
2071
2072
2073
2074
2075
2076
2077
2078
2079
2080
2081
2082
2083
2084
2085
2086
2087
2088
2089
2090
2091
2092
2093
2094
2095
2096
2097
2098
2099
2100
2101
2102
2103
2104
2105
2106
2107
2108
2109
2110
2111
2112
2113
2114
2115
2116
2117
2118
2119
2120
2121
2122
2123
2124
2125
2126
2127
2128
2129
2130
2131
2132
2133
2134
2135
2136
2137
2138
2139
2140
2141
2142
2143
2144
2145
2146
2147
2148
2149
2150
2151
2152
2153
2154
2155
2156
2157
2158
2159
2160
2161
2162
2163
2164
2165
2166
2167
2168
2169
2170
2171
2172
2173
2174
2175
2176
2177
2178
2179
2180
2181
2182
2183
2184
2185
2186
2187
2188
2189
2190
2191
2192
2193
2194
2195
2196
2197
2198
2199
2200
2201
2202
2203
2204
2205
2206
2207
2208
2209
2210
2211
2212
2213
2214
2215
2216
2217
2218
2219
2220
2221
2222
2223
2224
2225
2226
2227
2228
2229
2230
2231
2232
2233
2234
2235
2236
2237
2238
2239
2240
2241
2242
2243
2244
2245
2246
2247
2248
2249
2250
2251
2252
2253
2254
2255
2256
2257
2258
2259
2260
2261
2262
2263
2264
2265
2266
2267
2268
2269
2270
2271
2272
2273
2274
2275
2276
2277
2278
2279
2280
2281
2282
2283
2284
2285
2286
2287
2288
2289
2290
2291
2292
2293
2294
2295
2296
2297
2298
2299
2300
2301
2302
2303
2304
2305
2306
2307
2308
2309
2310
2311
2312
2313
2314
2315
2316
2317
2318
2319
2320
2321
2322
2323
2324
2325
2326
2327
2328
2329
2330
2331
2332
2333
2334
2335
2336
2337
2338
2339
2340
2341
2342
2343
2344
2345
2346
2347
2348
2349
2350
2351
2352
2353
2354
2355
2356
2357
2358
2359
2360
2361
2362
2363
2364
2365
2366
2367
2368
2369
2370
2371
2372
2373
2374
2375
2376
2377
2378
2379
2380
2381
2382
2383
2384
2385
2386
2387
2388
2389
2390
2391
2392
2393
2394
2395
2396
2397
2398
2399
2400
2401
2402
2403
2404
2405
2406
2407
2408
2409
2410
2411
2412
2413
2414
2415
2416
2417
2418
2419
2420
2421
2422
2423
2424
2425
2426
2427
2428
2429
2430
2431
2432
2433
2434
2435
2436
2437
2438
2439
2440
2441
2442
2443
2444
2445
2446
2447
2448
2449
2450
2451
2452
2453
2454
2455
2456
2457
2458
2459
2460
2461
2462
2463
2464
2465
2466
2467
2468
2469
2470
2471
2472
2473
2474
2475
2476
2477
2478
2479
2480
2481
2482
2483
2484
2485
2486
2487
2488
2489
2490
2491
2492
2493
2494
2495
2496
2497
use crate::component::func::{Func, LiftContext, LowerContext, Options};
use crate::component::matching::InstanceType;
use crate::component::storage::{storage_as_slice, storage_as_slice_mut};
use crate::prelude::*;
use crate::runtime::vm::component::ComponentInstance;
use crate::runtime::vm::SendSyncPtr;
use crate::{AsContextMut, StoreContext, StoreContextMut, ValRaw};
use alloc::borrow::Cow;
use alloc::sync::Arc;
use core::fmt;
use core::marker;
use core::mem::{self, MaybeUninit};
use core::ptr::NonNull;
use core::str;
use wasmtime_environ::component::{
    CanonicalAbiInfo, ComponentTypes, InterfaceType, StringEncoding, VariantInfo, MAX_FLAT_PARAMS,
    MAX_FLAT_RESULTS,
};

/// A statically-typed version of [`Func`] which takes `Params` as input and
/// returns `Return`.
///
/// This is an efficient way to invoke a WebAssembly component where if the
/// inputs and output are statically known this can eschew the vast majority of
/// machinery and checks when calling WebAssembly. This is the most optimized
/// way to call a WebAssembly component.
///
/// Note that like [`Func`] this is a pointer within a [`Store`](crate::Store)
/// and usage will panic if used with the wrong store.
///
/// This type is primarily created with the [`Func::typed`] API.
///
/// See [`ComponentType`] for more information about supported types.
pub struct TypedFunc<Params, Return> {
    func: Func,

    // The definition of this field is somewhat subtle and may be surprising.
    // Naively one might expect something like
    //
    //      _marker: marker::PhantomData<fn(Params) -> Return>,
    //
    // Since this is a function pointer after all. The problem with this
    // definition though is that it imposes the wrong variance on `Params` from
    // what we want. Abstractly a `fn(Params)` is able to store `Params` within
    // it meaning you can only give it `Params` that live longer than the
    // function pointer.
    //
    // With a component model function, however, we're always copying data from
    // the host into the guest, so we are never storing pointers to `Params`
    // into the guest outside the duration of a `call`, meaning we can actually
    // accept values in `TypedFunc::call` which live for a shorter duration
    // than the `Params` argument on the struct.
    //
    // This all means that we don't use a phantom function pointer, but instead
    // feign phantom storage here to get the variance desired.
    _marker: marker::PhantomData<(Params, Return)>,
}

impl<Params, Return> Copy for TypedFunc<Params, Return> {}

impl<Params, Return> Clone for TypedFunc<Params, Return> {
    fn clone(&self) -> TypedFunc<Params, Return> {
        *self
    }
}

impl<Params, Return> TypedFunc<Params, Return>
where
    Params: ComponentNamedList + Lower,
    Return: ComponentNamedList + Lift,
{
    /// Creates a new [`TypedFunc`] from the provided component [`Func`],
    /// unsafely asserting that the underlying function takes `Params` as
    /// input and returns `Return`.
    ///
    /// # Unsafety
    ///
    /// This is an unsafe function because it does not verify that the [`Func`]
    /// provided actually implements this signature. It's up to the caller to
    /// have performed some other sort of check to ensure that the signature is
    /// correct.
    pub unsafe fn new_unchecked(func: Func) -> TypedFunc<Params, Return> {
        TypedFunc {
            _marker: marker::PhantomData,
            func,
        }
    }

    /// Returns the underlying un-typed [`Func`] that this [`TypedFunc`]
    /// references.
    pub fn func(&self) -> &Func {
        &self.func
    }

    /// Calls the underlying WebAssembly component function using the provided
    /// `params` as input.
    ///
    /// This method is used to enter into a component. Execution happens within
    /// the `store` provided. The `params` are copied into WebAssembly memory
    /// as appropriate and a core wasm function is invoked.
    ///
    /// # Post-return
    ///
    /// In the component model each function can have a "post return" specified
    /// which allows cleaning up the arguments returned to the host. For example
    /// if WebAssembly returns a string to the host then it might be a uniquely
    /// allocated string which, after the host finishes processing it, needs to
    /// be deallocated in the wasm instance's own linear memory to prevent
    /// memory leaks in wasm itself. The `post-return` canonical abi option is
    /// used to configured this.
    ///
    /// To accommodate this feature of the component model after invoking a
    /// function via [`TypedFunc::call`] you must next invoke
    /// [`TypedFunc::post_return`]. Note that the return value of the function
    /// should be processed between these two function calls. The return value
    /// continues to be usable from an embedder's perspective after
    /// `post_return` is called, but after `post_return` is invoked it may no
    /// longer retain the same value that the wasm module originally returned.
    ///
    /// Also note that [`TypedFunc::post_return`] must be invoked irrespective
    /// of whether the canonical ABI option `post-return` was configured or not.
    /// This means that embedders must unconditionally call
    /// [`TypedFunc::post_return`] when a function returns. If this function
    /// call returns an error, however, then [`TypedFunc::post_return`] is not
    /// required.
    ///
    /// # Errors
    ///
    /// This function can return an error for a number of reasons:
    ///
    /// * If the wasm itself traps during execution.
    /// * If the wasm traps while copying arguments into memory.
    /// * If the wasm provides bad allocation pointers when copying arguments
    ///   into memory.
    /// * If the wasm returns a value which violates the canonical ABI.
    /// * If this function's instances cannot be entered, for example if the
    ///   instance is currently calling a host function.
    /// * If a previous function call occurred and the corresponding
    ///   `post_return` hasn't been invoked yet.
    ///
    /// In general there are many ways that things could go wrong when copying
    /// types in and out of a wasm module with the canonical ABI, and certain
    /// error conditions are specific to certain types. For example a
    /// WebAssembly module can't return an invalid `char`. When allocating space
    /// for this host to copy a string into the returned pointer must be
    /// in-bounds in memory.
    ///
    /// If an error happens then the error should contain detailed enough
    /// information to understand which part of the canonical ABI went wrong
    /// and what to inspect.
    ///
    /// # Panics
    ///
    /// Panics if this is called on a function in an asynchronous store. This
    /// only works with functions defined within a synchronous store. Also
    /// panics if `store` does not own this function.
    pub fn call(&self, store: impl AsContextMut, params: Params) -> Result<Return> {
        assert!(
            !store.as_context().async_support(),
            "must use `call_async` when async support is enabled on the config"
        );
        self.call_impl(store, params)
    }

    /// Exactly like [`Self::call`], except for use on asynchronous stores.
    ///
    /// # Panics
    ///
    /// Panics if this is called on a function in a synchronous store. This
    /// only works with functions defined within an asynchronous store. Also
    /// panics if `store` does not own this function.
    #[cfg(feature = "async")]
    pub async fn call_async<T>(
        &self,
        mut store: impl AsContextMut<Data = T>,
        params: Params,
    ) -> Result<Return>
    where
        T: Send,
        Params: Send + Sync,
        Return: Send + Sync,
    {
        let mut store = store.as_context_mut();
        assert!(
            store.0.async_support(),
            "cannot use `call_async` when async support is not enabled on the config"
        );
        store
            .on_fiber(|store| self.call_impl(store, params))
            .await?
    }

    fn call_impl(&self, mut store: impl AsContextMut, params: Params) -> Result<Return> {
        let store = &mut store.as_context_mut();
        // Note that this is in theory simpler than it might read at this time.
        // Here we're doing a runtime dispatch on the `flatten_count` for the
        // params/results to see whether they're inbounds. This creates 4 cases
        // to handle. In reality this is a highly optimizable branch where LLVM
        // will easily figure out that only one branch here is taken.
        //
        // Otherwise this current construction is done to ensure that the stack
        // space reserved for the params/results is always of the appropriate
        // size (as the params/results needed differ depending on the "flatten"
        // count)
        if Params::flatten_count() <= MAX_FLAT_PARAMS {
            if Return::flatten_count() <= MAX_FLAT_RESULTS {
                self.func.call_raw(
                    store,
                    &params,
                    Self::lower_stack_args,
                    Self::lift_stack_result,
                )
            } else {
                self.func.call_raw(
                    store,
                    &params,
                    Self::lower_stack_args,
                    Self::lift_heap_result,
                )
            }
        } else {
            if Return::flatten_count() <= MAX_FLAT_RESULTS {
                self.func.call_raw(
                    store,
                    &params,
                    Self::lower_heap_args,
                    Self::lift_stack_result,
                )
            } else {
                self.func.call_raw(
                    store,
                    &params,
                    Self::lower_heap_args,
                    Self::lift_heap_result,
                )
            }
        }
    }

    /// Lower parameters directly onto the stack specified by the `dst`
    /// location.
    ///
    /// This is only valid to call when the "flatten count" is small enough, or
    /// when the canonical ABI says arguments go through the stack rather than
    /// the heap.
    fn lower_stack_args<T>(
        cx: &mut LowerContext<'_, T>,
        params: &Params,
        ty: InterfaceType,
        dst: &mut MaybeUninit<Params::Lower>,
    ) -> Result<()> {
        assert!(Params::flatten_count() <= MAX_FLAT_PARAMS);
        params.lower(cx, ty, dst)?;
        Ok(())
    }

    /// Lower parameters onto a heap-allocated location.
    ///
    /// This is used when the stack space to be used for the arguments is above
    /// the `MAX_FLAT_PARAMS` threshold. Here the wasm's `realloc` function is
    /// invoked to allocate space and then parameters are stored at that heap
    /// pointer location.
    fn lower_heap_args<T>(
        cx: &mut LowerContext<'_, T>,
        params: &Params,
        ty: InterfaceType,
        dst: &mut MaybeUninit<ValRaw>,
    ) -> Result<()> {
        assert!(Params::flatten_count() > MAX_FLAT_PARAMS);

        // Memory must exist via validation if the arguments are stored on the
        // heap, so we can create a `MemoryMut` at this point. Afterwards
        // `realloc` is used to allocate space for all the arguments and then
        // they're all stored in linear memory.
        //
        // Note that `realloc` will bake in a check that the returned pointer is
        // in-bounds.
        let ptr = cx.realloc(0, 0, Params::ALIGN32, Params::SIZE32)?;
        params.store(cx, ty, ptr)?;

        // Note that the pointer here is stored as a 64-bit integer. This allows
        // this to work with either 32 or 64-bit memories. For a 32-bit memory
        // it'll just ignore the upper 32 zero bits, and for 64-bit memories
        // this'll have the full 64-bits. Note that for 32-bit memories the call
        // to `realloc` above guarantees that the `ptr` is in-bounds meaning
        // that we will know that the zero-extended upper bits of `ptr` are
        // guaranteed to be zero.
        //
        // This comment about 64-bit integers is also referred to below with
        // "WRITEPTR64".
        dst.write(ValRaw::i64(ptr as i64));

        Ok(())
    }

    /// Lift the result of a function directly from the stack result.
    ///
    /// This is only used when the result fits in the maximum number of stack
    /// slots.
    fn lift_stack_result(
        cx: &mut LiftContext<'_>,
        ty: InterfaceType,
        dst: &Return::Lower,
    ) -> Result<Return> {
        assert!(Return::flatten_count() <= MAX_FLAT_RESULTS);
        Return::lift(cx, ty, dst)
    }

    /// Lift the result of a function where the result is stored indirectly on
    /// the heap.
    fn lift_heap_result(
        cx: &mut LiftContext<'_>,
        ty: InterfaceType,
        dst: &ValRaw,
    ) -> Result<Return> {
        assert!(Return::flatten_count() > MAX_FLAT_RESULTS);
        // FIXME: needs to read an i64 for memory64
        let ptr = usize::try_from(dst.get_u32()).err2anyhow()?;
        if ptr % usize::try_from(Return::ALIGN32).err2anyhow()? != 0 {
            bail!("return pointer not aligned");
        }

        let bytes = cx
            .memory()
            .get(ptr..)
            .and_then(|b| b.get(..Return::SIZE32))
            .ok_or_else(|| anyhow::anyhow!("pointer out of bounds of memory"))?;
        Return::load(cx, ty, bytes)
    }

    /// See [`Func::post_return`]
    pub fn post_return(&self, store: impl AsContextMut) -> Result<()> {
        self.func.post_return(store)
    }

    /// See [`Func::post_return_async`]
    #[cfg(feature = "async")]
    pub async fn post_return_async<T: Send>(
        &self,
        store: impl AsContextMut<Data = T>,
    ) -> Result<()> {
        self.func.post_return_async(store).await
    }
}

/// A trait representing a static list of named types that can be passed to or
/// returned from a [`TypedFunc`].
///
/// This trait is implemented for a number of tuple types and is not expected
/// to be implemented externally. The contents of this trait are hidden as it's
/// intended to be an implementation detail of Wasmtime. The contents of this
/// trait are not covered by Wasmtime's stability guarantees.
///
/// For more information about this trait see [`Func::typed`] and
/// [`TypedFunc`].
//
// Note that this is an `unsafe` trait, and the unsafety means that
// implementations of this trait must be correct or otherwise [`TypedFunc`]
// would not be memory safe. The main reason this is `unsafe` is the
// `typecheck` function which must operate correctly relative to the `AsTuple`
// interpretation of the implementor.
pub unsafe trait ComponentNamedList: ComponentType {}

/// A trait representing types which can be passed to and read from components
/// with the canonical ABI.
///
/// This trait is implemented for Rust types which can be communicated to
/// components. The [`Func::typed`] and [`TypedFunc`] Rust items are the main
/// consumers of this trait.
///
/// Supported Rust types include:
///
/// | Component Model Type              | Rust Type                            |
/// |-----------------------------------|--------------------------------------|
/// | `{s,u}{8,16,32,64}`               | `{i,u}{8,16,32,64}`                  |
/// | `f{32,64}`                        | `f{32,64}`                           |
/// | `bool`                            | `bool`                               |
/// | `char`                            | `char`                               |
/// | `tuple<A, B>`                     | `(A, B)`                             |
/// | `option<T>`                       | `Option<T>`                          |
/// | `result`                          | `Result<(), ()>`                     |
/// | `result<T>`                       | `Result<T, ()>`                      |
/// | `result<_, E>`                    | `Result<(), E>`                      |
/// | `result<T, E>`                    | `Result<T, E>`                       |
/// | `string`                          | `String`, `&str`, or [`WasmStr`]     |
/// | `list<T>`                         | `Vec<T>`, `&[T]`, or [`WasmList`]    |
/// | `own<T>`, `borrow<T>`             | [`Resource<T>`] or [`ResourceAny`]   |
/// | `record`                          | [`#[derive(ComponentType)]`][d-cm]   |
/// | `variant`                         | [`#[derive(ComponentType)]`][d-cm]   |
/// | `enum`                            | [`#[derive(ComponentType)]`][d-cm]   |
/// | `flags`                           | [`flags!`][f-m]                      |
///
/// [`Resource<T>`]: crate::component::Resource
/// [`ResourceAny`]: crate::component::ResourceAny
/// [d-cm]: macro@crate::component::ComponentType
/// [f-m]: crate::component::flags
///
/// Rust standard library pointers such as `&T`, `Box<T>`, `Rc<T>`, and `Arc<T>`
/// additionally represent whatever type `T` represents in the component model.
/// Note that types such as `record`, `variant`, `enum`, and `flags` are
/// generated by the embedder at compile time. These macros derive
/// implementation of this trait for custom types to map to custom types in the
/// component model. Note that for `record`, `variant`, `enum`, and `flags`
/// those types are often generated by the
/// [`bindgen!`](crate::component::bindgen) macro from WIT definitions.
///
/// Types that implement [`ComponentType`] are used for `Params` and `Return`
/// in [`TypedFunc`] and [`Func::typed`].
///
/// The contents of this trait are hidden as it's intended to be an
/// implementation detail of Wasmtime. The contents of this trait are not
/// covered by Wasmtime's stability guarantees.
//
// Note that this is an `unsafe` trait as `TypedFunc`'s safety heavily relies on
// the correctness of the implementations of this trait. Some ways in which this
// trait must be correct to be safe are:
//
// * The `Lower` associated type must be a `ValRaw` sequence. It doesn't have to
//   literally be `[ValRaw; N]` but when laid out in memory it must be adjacent
//   `ValRaw` values and have a multiple of the size of `ValRaw` and the same
//   alignment.
//
// * The `lower` function must initialize the bits within `Lower` that are going
//   to be read by the trampoline that's used to enter core wasm. A trampoline
//   is passed `*mut Lower` and will read the canonical abi arguments in
//   sequence, so all of the bits must be correctly initialized.
//
// * The `size` and `align` functions must be correct for this value stored in
//   the canonical ABI. The `Cursor<T>` iteration of these bytes rely on this
//   for correctness as they otherwise eschew bounds-checking.
//
// There are likely some other correctness issues which aren't documented as
// well, this isn't intended to be an exhaustive list. It suffices to say,
// though, that correctness bugs in this trait implementation are highly likely
// to lead to security bugs, which again leads to the `unsafe` in the trait.
//
// Also note that this trait specifically is not sealed because we have a proc
// macro that generates implementations of this trait for external types in a
// `#[derive]`-like fashion.
pub unsafe trait ComponentType {
    /// Representation of the "lowered" form of this component value.
    ///
    /// Lowerings lower into core wasm values which are represented by `ValRaw`.
    /// This `Lower` type must be a list of `ValRaw` as either a literal array
    /// or a struct where every field is a `ValRaw`. This must be `Copy` (as
    /// `ValRaw` is `Copy`) and support all byte patterns. This being correct is
    /// one reason why the trait is unsafe.
    #[doc(hidden)]
    type Lower: Copy;

    /// The information about this type's canonical ABI (size/align/etc).
    #[doc(hidden)]
    const ABI: CanonicalAbiInfo;

    #[doc(hidden)]
    const SIZE32: usize = Self::ABI.size32 as usize;
    #[doc(hidden)]
    const ALIGN32: u32 = Self::ABI.align32;

    #[doc(hidden)]
    const IS_RUST_UNIT_TYPE: bool = false;

    /// Returns the number of core wasm abi values will be used to represent
    /// this type in its lowered form.
    ///
    /// This divides the size of `Self::Lower` by the size of `ValRaw`.
    #[doc(hidden)]
    fn flatten_count() -> usize {
        assert!(mem::size_of::<Self::Lower>() % mem::size_of::<ValRaw>() == 0);
        assert!(mem::align_of::<Self::Lower>() == mem::align_of::<ValRaw>());
        mem::size_of::<Self::Lower>() / mem::size_of::<ValRaw>()
    }

    /// Performs a type-check to see whether this component value type matches
    /// the interface type `ty` provided.
    #[doc(hidden)]
    fn typecheck(ty: &InterfaceType, types: &InstanceType<'_>) -> Result<()>;
}

#[doc(hidden)]
pub unsafe trait ComponentVariant: ComponentType {
    const CASES: &'static [Option<CanonicalAbiInfo>];
    const INFO: VariantInfo = VariantInfo::new_static(Self::CASES);
    const PAYLOAD_OFFSET32: usize = Self::INFO.payload_offset32 as usize;
}

/// Host types which can be passed to WebAssembly components.
///
/// This trait is implemented for all types that can be passed to components
/// either as parameters of component exports or returns of component imports.
/// This trait represents the ability to convert from the native host
/// representation to the canonical ABI.
///
/// Built-in types to Rust such as `Option<T>` implement this trait as
/// appropriate. For a mapping of component model to Rust types see
/// [`ComponentType`].
///
/// For user-defined types, for example `record` types mapped to Rust `struct`s,
/// this crate additionally has
/// [`#[derive(Lower)]`](macro@crate::component::Lower).
///
/// Note that like [`ComponentType`] the definition of this trait is intended to
/// be an internal implementation detail of Wasmtime at this time. It's
/// recommended to use the `#[derive(Lower)]` implementation instead.
pub unsafe trait Lower: ComponentType {
    /// Performs the "lower" function in the canonical ABI.
    ///
    /// This method will lower the current value into a component. The `lower`
    /// function performs a "flat" lowering into the `dst` specified which is
    /// allowed to be uninitialized entering this method but is guaranteed to be
    /// fully initialized if the method returns `Ok(())`.
    ///
    /// The `cx` context provided is the context within which this lowering is
    /// happening. This contains information such as canonical options specified
    /// (e.g. string encodings, memories, etc), the store itself, along with
    /// type information.
    ///
    /// The `ty` parameter is the destination type that is being lowered into.
    /// For example this is the component's "view" of the type that is being
    /// lowered. This is guaranteed to have passed a `typecheck` earlier.
    ///
    /// This will only be called if `typecheck` passes for `Op::Lower`.
    #[doc(hidden)]
    fn lower<T>(
        &self,
        cx: &mut LowerContext<'_, T>,
        ty: InterfaceType,
        dst: &mut MaybeUninit<Self::Lower>,
    ) -> Result<()>;

    /// Performs the "store" operation in the canonical ABI.
    ///
    /// This function will store `self` into the linear memory described by
    /// `cx` at the `offset` provided.
    ///
    /// It is expected that `offset` is a valid offset in memory for
    /// `Self::SIZE32` bytes. At this time that's not an unsafe contract as it's
    /// always re-checked on all stores, but this is something that will need to
    /// be improved in the future to remove extra bounds checks. For now this
    /// function will panic if there's a bug and `offset` isn't valid within
    /// memory.
    ///
    /// The `ty` type information passed here is the same as the type
    /// information passed to `lower` above, and is the component's own view of
    /// what the resulting type should be.
    ///
    /// This will only be called if `typecheck` passes for `Op::Lower`.
    #[doc(hidden)]
    fn store<T>(
        &self,
        cx: &mut LowerContext<'_, T>,
        ty: InterfaceType,
        offset: usize,
    ) -> Result<()>;

    /// Provided method to lower a list of `Self` into memory.
    ///
    /// Requires that `offset` has already been checked for alignment and
    /// validity in terms of being in-bounds, otherwise this may panic.
    ///
    /// This is primarily here to get overridden for implementations of integers
    /// which can avoid some extra fluff and use a pattern that's more easily
    /// optimizable by LLVM.
    #[doc(hidden)]
    fn store_list<T>(
        cx: &mut LowerContext<'_, T>,
        ty: InterfaceType,
        mut offset: usize,
        items: &[Self],
    ) -> Result<()>
    where
        Self: Sized,
    {
        for item in items {
            item.store(cx, ty, offset)?;
            offset += Self::SIZE32;
        }
        Ok(())
    }
}

/// Host types which can be created from the canonical ABI.
///
/// This is the mirror of the [`Lower`] trait where it represents the capability
/// of acquiring items from WebAssembly and passing them to the host.
///
/// Built-in types to Rust such as `Option<T>` implement this trait as
/// appropriate. For a mapping of component model to Rust types see
/// [`ComponentType`].
///
/// For user-defined types, for example `record` types mapped to Rust `struct`s,
/// this crate additionally has
/// [`#[derive(Lift)]`](macro@crate::component::Lift).
///
/// Note that like [`ComponentType`] the definition of this trait is intended to
/// be an internal implementation detail of Wasmtime at this time. It's
/// recommended to use the `#[derive(Lift)]` implementation instead.
pub unsafe trait Lift: Sized + ComponentType {
    /// Performs the "lift" operation in the canonical ABI.
    ///
    /// This function performs a "flat" lift operation from the `src` specified
    /// which is a sequence of core wasm values. The lifting operation will
    /// validate core wasm values and produce a `Self` on success.
    ///
    /// The `cx` provided contains contextual information such as the store
    /// that's being loaded from, canonical options, and type information.
    ///
    /// The `ty` parameter is the origin component's specification for what the
    /// type that is being lifted is. For example this is the record type or the
    /// resource type that is being lifted.
    ///
    /// Note that this has a default implementation but if `typecheck` passes
    /// for `Op::Lift` this needs to be overridden.
    #[doc(hidden)]
    fn lift(cx: &mut LiftContext<'_>, ty: InterfaceType, src: &Self::Lower) -> Result<Self>;

    /// Performs the "load" operation in the canonical ABI.
    ///
    /// This will read the `bytes` provided, which are a sub-slice into the
    /// linear memory described by `cx`. The `bytes` array provided is
    /// guaranteed to be `Self::SIZE32` bytes large. All of memory is then also
    /// available through `cx` for bounds-checks and such as necessary for
    /// strings/lists.
    ///
    /// The `ty` argument is the type that's being loaded, as described by the
    /// original component.
    ///
    /// Note that this has a default implementation but if `typecheck` passes
    /// for `Op::Lift` this needs to be overridden.
    #[doc(hidden)]
    fn load(cx: &mut LiftContext<'_>, ty: InterfaceType, bytes: &[u8]) -> Result<Self>;

    /// Converts `list` into a `Vec<T>`, used in `Lift for Vec<T>`.
    ///
    /// This is primarily here to get overridden for implementations of integers
    /// which can avoid some extra fluff and use a pattern that's more easily
    /// optimizable by LLVM.
    #[doc(hidden)]
    fn load_list(cx: &mut LiftContext<'_>, list: &WasmList<Self>) -> Result<Vec<Self>>
    where
        Self: Sized,
    {
        (0..list.len)
            .map(|index| list.get_from_store(cx, index).unwrap())
            .collect()
    }
}

// Macro to help generate "forwarding implementations" of `ComponentType` to
// another type, used for wrappers in Rust like `&T`, `Box<T>`, etc. Note that
// these wrappers only implement lowering because lifting native Rust types
// cannot be done.
macro_rules! forward_type_impls {
    ($(($($generics:tt)*) $a:ty => $b:ty,)*) => ($(
        unsafe impl <$($generics)*> ComponentType for $a {
            type Lower = <$b as ComponentType>::Lower;

            const ABI: CanonicalAbiInfo = <$b as ComponentType>::ABI;

            #[inline]
            fn typecheck(ty: &InterfaceType, types: &InstanceType<'_>) -> Result<()> {
                <$b as ComponentType>::typecheck(ty, types)
            }
        }
    )*)
}

forward_type_impls! {
    (T: ComponentType + ?Sized) &'_ T => T,
    (T: ComponentType + ?Sized) Box<T> => T,
    (T: ComponentType + ?Sized) alloc::rc::Rc<T> => T,
    (T: ComponentType + ?Sized) alloc::sync::Arc<T> => T,
    () String => str,
    (T: ComponentType) Vec<T> => [T],
}

macro_rules! forward_lowers {
    ($(($($generics:tt)*) $a:ty => $b:ty,)*) => ($(
        unsafe impl <$($generics)*> Lower for $a {
            fn lower<U>(
                &self,
                cx: &mut LowerContext<'_, U>,
                ty: InterfaceType,
                dst: &mut MaybeUninit<Self::Lower>,
            ) -> Result<()> {
                <$b as Lower>::lower(self, cx, ty, dst)
            }

            fn store<U>(
                &self,
                cx: &mut LowerContext<'_, U>,
                ty: InterfaceType,
                offset: usize,
            ) -> Result<()> {
                <$b as Lower>::store(self, cx, ty, offset)
            }
        }
    )*)
}

forward_lowers! {
    (T: Lower + ?Sized) &'_ T => T,
    (T: Lower + ?Sized) Box<T> => T,
    (T: Lower + ?Sized) alloc::rc::Rc<T> => T,
    (T: Lower + ?Sized) alloc::sync::Arc<T> => T,
    () String => str,
    (T: Lower) Vec<T> => [T],
}

macro_rules! forward_string_lifts {
    ($($a:ty,)*) => ($(
        unsafe impl Lift for $a {
            #[inline]
            fn lift(cx: &mut LiftContext<'_>, ty: InterfaceType, src: &Self::Lower) -> Result<Self> {
                Ok(<WasmStr as Lift>::lift(cx, ty, src)?.to_str_from_memory(cx.memory())?.into())
            }

            #[inline]
            fn load(cx: &mut LiftContext<'_>, ty: InterfaceType, bytes: &[u8]) -> Result<Self> {
                Ok(<WasmStr as Lift>::load(cx, ty, bytes)?.to_str_from_memory(cx.memory())?.into())
            }
        }
    )*)
}

forward_string_lifts! {
    Box<str>,
    alloc::rc::Rc<str>,
    alloc::sync::Arc<str>,
    String,
}

macro_rules! forward_list_lifts {
    ($($a:ty,)*) => ($(
        unsafe impl <T: Lift> Lift for $a {
            fn lift(cx: &mut LiftContext<'_>, ty: InterfaceType, src: &Self::Lower) -> Result<Self> {
                let list = <WasmList::<T> as Lift>::lift(cx, ty, src)?;
                Ok(T::load_list(cx, &list)?.into())
            }

            fn load(cx: &mut LiftContext<'_>, ty: InterfaceType, bytes: &[u8]) -> Result<Self> {
                let list = <WasmList::<T> as Lift>::load(cx, ty, bytes)?;
                Ok(T::load_list(cx, &list)?.into())
            }
        }
    )*)
}

forward_list_lifts! {
    Box<[T]>,
    alloc::rc::Rc<[T]>,
    alloc::sync::Arc<[T]>,
    Vec<T>,
}

// Macro to help generate `ComponentType` implementations for primitive types
// such as integers, char, bool, etc.
macro_rules! integers {
    ($($primitive:ident = $ty:ident in $field:ident/$get:ident with abi:$abi:ident,)*) => ($(
        unsafe impl ComponentType for $primitive {
            type Lower = ValRaw;

            const ABI: CanonicalAbiInfo = CanonicalAbiInfo::$abi;

            fn typecheck(ty: &InterfaceType, _types: &InstanceType<'_>) -> Result<()> {
                match ty {
                    InterfaceType::$ty => Ok(()),
                    other => bail!("expected `{}` found `{}`", desc(&InterfaceType::$ty), desc(other))
                }
            }
        }

        unsafe impl Lower for $primitive {
            #[inline]
            #[allow(trivial_numeric_casts)]
            fn lower<T>(
                &self,
                _cx: &mut LowerContext<'_, T>,
                ty: InterfaceType,
                dst: &mut MaybeUninit<Self::Lower>,
            ) -> Result<()> {
                debug_assert!(matches!(ty, InterfaceType::$ty));
                dst.write(ValRaw::$field(*self as $field));
                Ok(())
            }

            #[inline]
            fn store<T>(
                &self,
                cx: &mut LowerContext<'_, T>,
                ty: InterfaceType,
                offset: usize,
            ) -> Result<()> {
                debug_assert!(matches!(ty, InterfaceType::$ty));
                debug_assert!(offset % Self::SIZE32 == 0);
                *cx.get(offset) = self.to_le_bytes();
                Ok(())
            }

            fn store_list<T>(
                cx: &mut LowerContext<'_, T>,
                ty: InterfaceType,
                offset: usize,
                items: &[Self],
            ) -> Result<()> {
                debug_assert!(matches!(ty, InterfaceType::$ty));

                // Double-check that the CM alignment is at least the host's
                // alignment for this type which should be true for all
                // platforms.
                assert!((Self::ALIGN32 as usize) >= mem::align_of::<Self>());

                // Slice `cx`'s memory to the window that we'll be modifying.
                // This should all have already been verified in terms of
                // alignment and sizing meaning that these assertions here are
                // not truly necessary but are instead double-checks.
                //
                // Note that we're casting a `[u8]` slice to `[Self]` with
                // `align_to_mut` which is not safe in general but is safe in
                // our specific case as all `u8` patterns are valid `Self`
                // patterns since `Self` is an integral type.
                let dst = &mut cx.as_slice_mut()[offset..][..items.len() * Self::SIZE32];
                let (before, middle, end) = unsafe { dst.align_to_mut::<Self>() };
                assert!(before.is_empty() && end.is_empty());
                assert_eq!(middle.len(), items.len());

                // And with all that out of the way perform the copying loop.
                // This is not a `copy_from_slice` because endianness needs to
                // be handled here, but LLVM should pretty easily transform this
                // into a memcpy on little-endian platforms.
                for (dst, src) in middle.iter_mut().zip(items) {
                    *dst = src.to_le();
                }
                Ok(())
            }
        }

        unsafe impl Lift for $primitive {
            #[inline]
            #[allow(trivial_numeric_casts)]
            fn lift(_cx: &mut LiftContext<'_>, ty: InterfaceType, src: &Self::Lower) -> Result<Self> {
                debug_assert!(matches!(ty, InterfaceType::$ty));
                Ok(src.$get() as $primitive)
            }

            #[inline]
            fn load(_cx: &mut LiftContext<'_>, ty: InterfaceType, bytes: &[u8]) -> Result<Self> {
                debug_assert!(matches!(ty, InterfaceType::$ty));
                debug_assert!((bytes.as_ptr() as usize) % Self::SIZE32 == 0);
                Ok($primitive::from_le_bytes(bytes.try_into().unwrap()))
            }

            fn load_list(cx: &mut LiftContext<'_>, list: &WasmList<Self>) -> Result<Vec<Self>> {
                Ok(
                    list._as_le_slice(cx.memory())
                        .iter()
                        .map(|i| Self::from_le(*i))
                        .collect(),
                )
            }
        }
    )*)
}

integers! {
    i8 = S8 in i32/get_i32 with abi:SCALAR1,
    u8 = U8 in u32/get_u32 with abi:SCALAR1,
    i16 = S16 in i32/get_i32 with abi:SCALAR2,
    u16 = U16 in u32/get_u32 with abi:SCALAR2,
    i32 = S32 in i32/get_i32 with abi:SCALAR4,
    u32 = U32 in u32/get_u32 with abi:SCALAR4,
    i64 = S64 in i64/get_i64 with abi:SCALAR8,
    u64 = U64 in u64/get_u64 with abi:SCALAR8,
}

macro_rules! floats {
    ($($float:ident/$get_float:ident = $ty:ident with abi:$abi:ident)*) => ($(const _: () = {
        /// All floats in-and-out of the canonical abi always have their nan
        /// payloads canonicalized. conveniently the `NAN` constant in rust has
        /// the same representation as canonical nan, so we can use that for the
        /// nan value.
        #[inline]
        fn canonicalize(float: $float) -> $float {
            if float.is_nan() {
                $float::NAN
            } else {
                float
            }
        }

        unsafe impl ComponentType for $float {
            type Lower = ValRaw;

            const ABI: CanonicalAbiInfo = CanonicalAbiInfo::$abi;

            fn typecheck(ty: &InterfaceType, _types: &InstanceType<'_>) -> Result<()> {
                match ty {
                    InterfaceType::$ty => Ok(()),
                    other => bail!("expected `{}` found `{}`", desc(&InterfaceType::$ty), desc(other))
                }
            }
        }

        unsafe impl Lower for $float {
            #[inline]
            fn lower<T>(
                &self,
                _cx: &mut LowerContext<'_, T>,
                ty: InterfaceType,
                dst: &mut MaybeUninit<Self::Lower>,
            ) -> Result<()> {
                debug_assert!(matches!(ty, InterfaceType::$ty));
                dst.write(ValRaw::$float(canonicalize(*self).to_bits()));
                Ok(())
            }

            #[inline]
            fn store<T>(
                &self,
                cx: &mut LowerContext<'_, T>,
                ty: InterfaceType,
                offset: usize,
            ) -> Result<()> {
                debug_assert!(matches!(ty, InterfaceType::$ty));
                debug_assert!(offset % Self::SIZE32 == 0);
                let ptr = cx.get(offset);
                *ptr = canonicalize(*self).to_bits().to_le_bytes();
                Ok(())
            }
        }

        unsafe impl Lift for $float {
            #[inline]
            fn lift(_cx: &mut LiftContext<'_>, ty: InterfaceType, src: &Self::Lower) -> Result<Self> {
                debug_assert!(matches!(ty, InterfaceType::$ty));
                Ok(canonicalize($float::from_bits(src.$get_float())))
            }

            #[inline]
            fn load(_cx: &mut LiftContext<'_>, ty: InterfaceType, bytes: &[u8]) -> Result<Self> {
                debug_assert!(matches!(ty, InterfaceType::$ty));
                debug_assert!((bytes.as_ptr() as usize) % Self::SIZE32 == 0);
                Ok(canonicalize($float::from_le_bytes(bytes.try_into().unwrap())))
            }
        }
    };)*)
}

floats! {
    f32/get_f32 = Float32 with abi:SCALAR4
    f64/get_f64 = Float64 with abi:SCALAR8
}

unsafe impl ComponentType for bool {
    type Lower = ValRaw;

    const ABI: CanonicalAbiInfo = CanonicalAbiInfo::SCALAR1;

    fn typecheck(ty: &InterfaceType, _types: &InstanceType<'_>) -> Result<()> {
        match ty {
            InterfaceType::Bool => Ok(()),
            other => bail!("expected `bool` found `{}`", desc(other)),
        }
    }
}

unsafe impl Lower for bool {
    fn lower<T>(
        &self,
        _cx: &mut LowerContext<'_, T>,
        ty: InterfaceType,
        dst: &mut MaybeUninit<Self::Lower>,
    ) -> Result<()> {
        debug_assert!(matches!(ty, InterfaceType::Bool));
        dst.write(ValRaw::i32(*self as i32));
        Ok(())
    }

    fn store<T>(
        &self,
        cx: &mut LowerContext<'_, T>,
        ty: InterfaceType,
        offset: usize,
    ) -> Result<()> {
        debug_assert!(matches!(ty, InterfaceType::Bool));
        debug_assert!(offset % Self::SIZE32 == 0);
        cx.get::<1>(offset)[0] = *self as u8;
        Ok(())
    }
}

unsafe impl Lift for bool {
    #[inline]
    fn lift(_cx: &mut LiftContext<'_>, ty: InterfaceType, src: &Self::Lower) -> Result<Self> {
        debug_assert!(matches!(ty, InterfaceType::Bool));
        match src.get_i32() {
            0 => Ok(false),
            _ => Ok(true),
        }
    }

    #[inline]
    fn load(_cx: &mut LiftContext<'_>, ty: InterfaceType, bytes: &[u8]) -> Result<Self> {
        debug_assert!(matches!(ty, InterfaceType::Bool));
        match bytes[0] {
            0 => Ok(false),
            _ => Ok(true),
        }
    }
}

unsafe impl ComponentType for char {
    type Lower = ValRaw;

    const ABI: CanonicalAbiInfo = CanonicalAbiInfo::SCALAR4;

    fn typecheck(ty: &InterfaceType, _types: &InstanceType<'_>) -> Result<()> {
        match ty {
            InterfaceType::Char => Ok(()),
            other => bail!("expected `char` found `{}`", desc(other)),
        }
    }
}

unsafe impl Lower for char {
    #[inline]
    fn lower<T>(
        &self,
        _cx: &mut LowerContext<'_, T>,
        ty: InterfaceType,
        dst: &mut MaybeUninit<Self::Lower>,
    ) -> Result<()> {
        debug_assert!(matches!(ty, InterfaceType::Char));
        dst.write(ValRaw::u32(u32::from(*self)));
        Ok(())
    }

    #[inline]
    fn store<T>(
        &self,
        cx: &mut LowerContext<'_, T>,
        ty: InterfaceType,
        offset: usize,
    ) -> Result<()> {
        debug_assert!(matches!(ty, InterfaceType::Char));
        debug_assert!(offset % Self::SIZE32 == 0);
        *cx.get::<4>(offset) = u32::from(*self).to_le_bytes();
        Ok(())
    }
}

unsafe impl Lift for char {
    #[inline]
    fn lift(_cx: &mut LiftContext<'_>, ty: InterfaceType, src: &Self::Lower) -> Result<Self> {
        debug_assert!(matches!(ty, InterfaceType::Char));
        Ok(char::try_from(src.get_u32()).err2anyhow()?)
    }

    #[inline]
    fn load(_cx: &mut LiftContext<'_>, ty: InterfaceType, bytes: &[u8]) -> Result<Self> {
        debug_assert!(matches!(ty, InterfaceType::Char));
        debug_assert!((bytes.as_ptr() as usize) % Self::SIZE32 == 0);
        let bits = u32::from_le_bytes(bytes.try_into().unwrap());
        Ok(char::try_from(bits).err2anyhow()?)
    }
}

// TODO: these probably need different constants for memory64
const UTF16_TAG: usize = 1 << 31;
const MAX_STRING_BYTE_LENGTH: usize = (1 << 31) - 1;

// Note that this is similar to `ComponentType for WasmStr` except it can only
// be used for lowering, not lifting.
unsafe impl ComponentType for str {
    type Lower = [ValRaw; 2];

    const ABI: CanonicalAbiInfo = CanonicalAbiInfo::POINTER_PAIR;

    fn typecheck(ty: &InterfaceType, _types: &InstanceType<'_>) -> Result<()> {
        match ty {
            InterfaceType::String => Ok(()),
            other => bail!("expected `string` found `{}`", desc(other)),
        }
    }
}

unsafe impl Lower for str {
    fn lower<T>(
        &self,
        cx: &mut LowerContext<'_, T>,
        ty: InterfaceType,
        dst: &mut MaybeUninit<[ValRaw; 2]>,
    ) -> Result<()> {
        debug_assert!(matches!(ty, InterfaceType::String));
        let (ptr, len) = lower_string(cx, self)?;
        // See "WRITEPTR64" above for why this is always storing a 64-bit
        // integer.
        map_maybe_uninit!(dst[0]).write(ValRaw::i64(ptr as i64));
        map_maybe_uninit!(dst[1]).write(ValRaw::i64(len as i64));
        Ok(())
    }

    fn store<T>(
        &self,
        cx: &mut LowerContext<'_, T>,
        ty: InterfaceType,
        offset: usize,
    ) -> Result<()> {
        debug_assert!(matches!(ty, InterfaceType::String));
        debug_assert!(offset % (Self::ALIGN32 as usize) == 0);
        let (ptr, len) = lower_string(cx, self)?;
        // FIXME: needs memory64 handling
        *cx.get(offset + 0) = (ptr as i32).to_le_bytes();
        *cx.get(offset + 4) = (len as i32).to_le_bytes();
        Ok(())
    }
}

fn lower_string<T>(cx: &mut LowerContext<'_, T>, string: &str) -> Result<(usize, usize)> {
    // Note that in general the wasm module can't assume anything about what the
    // host strings are encoded as. Additionally hosts are allowed to have
    // differently-encoded strings at runtime. Finally when copying a string
    // into wasm it's somewhat strict in the sense that the various patterns of
    // allocation and such are already dictated for us.
    //
    // In general what this means is that when copying a string from the host
    // into the destination we need to follow one of the cases of copying into
    // WebAssembly. It doesn't particularly matter which case as long as it ends
    // up in the right encoding. For example a destination encoding of
    // latin1+utf16 has a number of ways to get copied into and we do something
    // here that isn't the default "utf8 to latin1+utf16" since we have access
    // to simd-accelerated helpers in the `encoding_rs` crate. This is ok though
    // because we can fake that the host string was already stored in latin1
    // format and follow that copy pattern instead.
    match cx.options.string_encoding() {
        // This corresponds to `store_string_copy` in the canonical ABI where
        // the host's representation is utf-8 and the wasm module wants utf-8 so
        // a copy is all that's needed (and the `realloc` can be precise for the
        // initial memory allocation).
        StringEncoding::Utf8 => {
            if string.len() > MAX_STRING_BYTE_LENGTH {
                bail!(
                    "string length of {} too large to copy into wasm",
                    string.len()
                );
            }
            let ptr = cx.realloc(0, 0, 1, string.len())?;
            cx.as_slice_mut()[ptr..][..string.len()].copy_from_slice(string.as_bytes());
            Ok((ptr, string.len()))
        }

        // This corresponds to `store_utf8_to_utf16` in the canonical ABI. Here
        // an over-large allocation is performed and then shrunk afterwards if
        // necessary.
        StringEncoding::Utf16 => {
            let size = string.len() * 2;
            if size > MAX_STRING_BYTE_LENGTH {
                bail!(
                    "string length of {} too large to copy into wasm",
                    string.len()
                );
            }
            let mut ptr = cx.realloc(0, 0, 2, size)?;
            let mut copied = 0;
            let bytes = &mut cx.as_slice_mut()[ptr..][..size];
            for (u, bytes) in string.encode_utf16().zip(bytes.chunks_mut(2)) {
                let u_bytes = u.to_le_bytes();
                bytes[0] = u_bytes[0];
                bytes[1] = u_bytes[1];
                copied += 1;
            }
            if (copied * 2) < size {
                ptr = cx.realloc(ptr, size, 2, copied * 2)?;
            }
            Ok((ptr, copied))
        }

        StringEncoding::CompactUtf16 => {
            // This corresponds to `store_string_to_latin1_or_utf16`
            let bytes = string.as_bytes();
            let mut iter = string.char_indices();
            let mut ptr = cx.realloc(0, 0, 2, bytes.len())?;
            let mut dst = &mut cx.as_slice_mut()[ptr..][..bytes.len()];
            let mut result = 0;
            while let Some((i, ch)) = iter.next() {
                // Test if this `char` fits into the latin1 encoding.
                if let Ok(byte) = u8::try_from(u32::from(ch)) {
                    dst[result] = byte;
                    result += 1;
                    continue;
                }

                // .. if utf16 is forced to be used then the allocation is
                // bumped up to the maximum size.
                let worst_case = bytes
                    .len()
                    .checked_mul(2)
                    .ok_or_else(|| anyhow!("byte length overflow"))?;
                if worst_case > MAX_STRING_BYTE_LENGTH {
                    bail!("byte length too large");
                }
                ptr = cx.realloc(ptr, bytes.len(), 2, worst_case)?;
                dst = &mut cx.as_slice_mut()[ptr..][..worst_case];

                // Previously encoded latin1 bytes are inflated to their 16-bit
                // size for utf16
                for i in (0..result).rev() {
                    dst[2 * i] = dst[i];
                    dst[2 * i + 1] = 0;
                }

                // and then the remainder of the string is encoded.
                for (u, bytes) in string[i..]
                    .encode_utf16()
                    .zip(dst[2 * result..].chunks_mut(2))
                {
                    let u_bytes = u.to_le_bytes();
                    bytes[0] = u_bytes[0];
                    bytes[1] = u_bytes[1];
                    result += 1;
                }
                if worst_case > 2 * result {
                    ptr = cx.realloc(ptr, worst_case, 2, 2 * result)?;
                }
                return Ok((ptr, result | UTF16_TAG));
            }
            if result < bytes.len() {
                ptr = cx.realloc(ptr, bytes.len(), 2, result)?;
            }
            Ok((ptr, result))
        }
    }
}

/// Representation of a string located in linear memory in a WebAssembly
/// instance.
///
/// This type can be used in place of `String` and `str` for string-taking APIs
/// in some situations. The purpose of this type is to represent a range of
/// validated bytes within a component but does not actually copy the bytes. The
/// primary method, [`WasmStr::to_str`], attempts to return a reference to the
/// string directly located in the component's memory, avoiding a copy into the
/// host if possible.
///
/// The downside of this type, however, is that accessing a string requires a
/// [`Store`](crate::Store) pointer (via [`StoreContext`]). Bindings generated
/// by [`bindgen!`](crate::component::bindgen), for example, do not have access
/// to [`StoreContext`] and thus can't use this type.
///
/// This is intended for more advanced use cases such as defining functions
/// directly in a [`Linker`](crate::component::Linker). It's expected that in
/// the future [`bindgen!`](crate::component::bindgen) will also have a way to
/// use this type.
///
/// This type is used with [`TypedFunc`], for example, when WebAssembly returns
/// a string. This type cannot be used to give a string to WebAssembly, instead
/// `&str` should be used for that (since it's coming from the host).
///
/// Note that this type represents an in-bounds string in linear memory, but it
/// does not represent a valid string (e.g. valid utf-8). Validation happens
/// when [`WasmStr::to_str`] is called.
///
/// Also note that this type does not implement [`Lower`], it only implements
/// [`Lift`].
pub struct WasmStr {
    ptr: usize,
    len: usize,
    options: Options,
}

impl WasmStr {
    fn new(ptr: usize, len: usize, cx: &mut LiftContext<'_>) -> Result<WasmStr> {
        let byte_len = match cx.options.string_encoding() {
            StringEncoding::Utf8 => Some(len),
            StringEncoding::Utf16 => len.checked_mul(2),
            StringEncoding::CompactUtf16 => {
                if len & UTF16_TAG == 0 {
                    Some(len)
                } else {
                    (len ^ UTF16_TAG).checked_mul(2)
                }
            }
        };
        match byte_len.and_then(|len| ptr.checked_add(len)) {
            Some(n) if n <= cx.memory().len() => {}
            _ => bail!("string pointer/length out of bounds of memory"),
        }
        Ok(WasmStr {
            ptr,
            len,
            options: *cx.options,
        })
    }

    /// Returns the underlying string that this cursor points to.
    ///
    /// Note that this will internally decode the string from the wasm's
    /// encoding to utf-8 and additionally perform validation.
    ///
    /// The `store` provided must be the store where this string lives to
    /// access the correct memory.
    ///
    /// # Errors
    ///
    /// Returns an error if the string wasn't encoded correctly (e.g. invalid
    /// utf-8).
    ///
    /// # Panics
    ///
    /// Panics if this string is not owned by `store`.
    //
    // TODO: should add accessors for specifically utf-8 and utf-16 that perhaps
    // in an opt-in basis don't do validation. Additionally there should be some
    // method that returns `[u16]` after validating to avoid the utf16-to-utf8
    // transcode.
    pub fn to_str<'a, T: 'a>(&self, store: impl Into<StoreContext<'a, T>>) -> Result<Cow<'a, str>> {
        let store = store.into().0;
        let memory = self.options.memory(store);
        self.to_str_from_memory(memory)
    }

    fn to_str_from_memory<'a>(&self, memory: &'a [u8]) -> Result<Cow<'a, str>> {
        match self.options.string_encoding() {
            StringEncoding::Utf8 => self.decode_utf8(memory),
            StringEncoding::Utf16 => self.decode_utf16(memory, self.len),
            StringEncoding::CompactUtf16 => {
                if self.len & UTF16_TAG == 0 {
                    self.decode_latin1(memory)
                } else {
                    self.decode_utf16(memory, self.len ^ UTF16_TAG)
                }
            }
        }
    }

    fn decode_utf8<'a>(&self, memory: &'a [u8]) -> Result<Cow<'a, str>> {
        // Note that bounds-checking already happen in construction of `WasmStr`
        // so this is never expected to panic. This could theoretically be
        // unchecked indexing if we're feeling wild enough.
        Ok(str::from_utf8(&memory[self.ptr..][..self.len])
            .err2anyhow()?
            .into())
    }

    fn decode_utf16<'a>(&self, memory: &'a [u8], len: usize) -> Result<Cow<'a, str>> {
        // See notes in `decode_utf8` for why this is panicking indexing.
        let memory = &memory[self.ptr..][..len * 2];
        Ok(core::char::decode_utf16(
            memory
                .chunks(2)
                .map(|chunk| u16::from_le_bytes(chunk.try_into().unwrap())),
        )
        .collect::<Result<String, _>>()
        .err2anyhow()?
        .into())
    }

    fn decode_latin1<'a>(&self, memory: &'a [u8]) -> Result<Cow<'a, str>> {
        // See notes in `decode_utf8` for why this is panicking indexing.
        Ok(encoding_rs::mem::decode_latin1(
            &memory[self.ptr..][..self.len],
        ))
    }
}

// Note that this is similar to `ComponentType for str` except it can only be
// used for lifting, not lowering.
unsafe impl ComponentType for WasmStr {
    type Lower = <str as ComponentType>::Lower;

    const ABI: CanonicalAbiInfo = CanonicalAbiInfo::POINTER_PAIR;

    fn typecheck(ty: &InterfaceType, _types: &InstanceType<'_>) -> Result<()> {
        match ty {
            InterfaceType::String => Ok(()),
            other => bail!("expected `string` found `{}`", desc(other)),
        }
    }
}

unsafe impl Lift for WasmStr {
    #[inline]
    fn lift(cx: &mut LiftContext<'_>, ty: InterfaceType, src: &Self::Lower) -> Result<Self> {
        debug_assert!(matches!(ty, InterfaceType::String));
        // FIXME: needs memory64 treatment
        let ptr = src[0].get_u32();
        let len = src[1].get_u32();
        let (ptr, len) = (
            usize::try_from(ptr).err2anyhow()?,
            usize::try_from(len).err2anyhow()?,
        );
        WasmStr::new(ptr, len, cx)
    }

    #[inline]
    fn load(cx: &mut LiftContext<'_>, ty: InterfaceType, bytes: &[u8]) -> Result<Self> {
        debug_assert!(matches!(ty, InterfaceType::String));
        debug_assert!((bytes.as_ptr() as usize) % (Self::ALIGN32 as usize) == 0);
        // FIXME: needs memory64 treatment
        let ptr = u32::from_le_bytes(bytes[..4].try_into().unwrap());
        let len = u32::from_le_bytes(bytes[4..].try_into().unwrap());
        let (ptr, len) = (
            usize::try_from(ptr).err2anyhow()?,
            usize::try_from(len).err2anyhow()?,
        );
        WasmStr::new(ptr, len, cx)
    }
}

unsafe impl<T> ComponentType for [T]
where
    T: ComponentType,
{
    type Lower = [ValRaw; 2];

    const ABI: CanonicalAbiInfo = CanonicalAbiInfo::POINTER_PAIR;

    fn typecheck(ty: &InterfaceType, types: &InstanceType<'_>) -> Result<()> {
        match ty {
            InterfaceType::List(t) => T::typecheck(&types.types[*t].element, types),
            other => bail!("expected `list` found `{}`", desc(other)),
        }
    }
}

unsafe impl<T> Lower for [T]
where
    T: Lower,
{
    fn lower<U>(
        &self,
        cx: &mut LowerContext<'_, U>,
        ty: InterfaceType,
        dst: &mut MaybeUninit<[ValRaw; 2]>,
    ) -> Result<()> {
        let elem = match ty {
            InterfaceType::List(i) => cx.types[i].element,
            _ => bad_type_info(),
        };
        let (ptr, len) = lower_list(cx, elem, self)?;
        // See "WRITEPTR64" above for why this is always storing a 64-bit
        // integer.
        map_maybe_uninit!(dst[0]).write(ValRaw::i64(ptr as i64));
        map_maybe_uninit!(dst[1]).write(ValRaw::i64(len as i64));
        Ok(())
    }

    fn store<U>(
        &self,
        cx: &mut LowerContext<'_, U>,
        ty: InterfaceType,
        offset: usize,
    ) -> Result<()> {
        let elem = match ty {
            InterfaceType::List(i) => cx.types[i].element,
            _ => bad_type_info(),
        };
        debug_assert!(offset % (Self::ALIGN32 as usize) == 0);
        let (ptr, len) = lower_list(cx, elem, self)?;
        *cx.get(offset + 0) = (ptr as i32).to_le_bytes();
        *cx.get(offset + 4) = (len as i32).to_le_bytes();
        Ok(())
    }
}

// FIXME: this is not a memcpy for `T` where `T` is something like `u8`.
//
// Some attempts to fix this have proved not fruitful. In isolation an attempt
// was made where:
//
// * `MemoryMut` stored a `*mut [u8]` as its "last view" of memory to avoid
//   reloading the base pointer constantly. This view is reset on `realloc`.
// * The bounds-checks in `MemoryMut::get` were removed (replaced with unsafe
//   indexing)
//
// Even then though this didn't correctly vectorized for `Vec<u8>`. It's not
// entirely clear why but it appeared that it's related to reloading the base
// pointer to memory (I guess from `MemoryMut` itself?). Overall I'm not really
// clear on what's happening there, but this is surely going to be a performance
// bottleneck in the future.
fn lower_list<T, U>(
    cx: &mut LowerContext<'_, U>,
    ty: InterfaceType,
    list: &[T],
) -> Result<(usize, usize)>
where
    T: Lower,
{
    let elem_size = T::SIZE32;
    let size = list
        .len()
        .checked_mul(elem_size)
        .ok_or_else(|| anyhow!("size overflow copying a list"))?;
    let ptr = cx.realloc(0, 0, T::ALIGN32, size)?;
    T::store_list(cx, ty, ptr, list)?;
    Ok((ptr, list.len()))
}

/// Representation of a list of values that are owned by a WebAssembly instance.
///
/// For some more commentary about the rationale for this type see the
/// documentation of [`WasmStr`]. In summary this type can avoid a copy when
/// passing data to the host in some situations but is additionally more
/// cumbersome to use by requiring a [`Store`](crate::Store) to be provided.
///
/// This type is used whenever a `(list T)` is returned from a [`TypedFunc`],
/// for example. This type represents a list of values that are stored in linear
/// memory which are waiting to be read.
///
/// Note that this type represents only a valid range of bytes for the list
/// itself, it does not represent validity of the elements themselves and that's
/// performed when they're iterated.
///
/// Note that this type does not implement the [`Lower`] trait, only [`Lift`].
pub struct WasmList<T> {
    ptr: usize,
    len: usize,
    options: Options,
    elem: InterfaceType,
    // NB: it would probably be more efficient to store a non-atomic index-style
    // reference to something inside a `StoreOpaque`, but that's not easily
    // available at this time, so it's left as a future exercise.
    types: Arc<ComponentTypes>,
    instance: SendSyncPtr<ComponentInstance>,
    _marker: marker::PhantomData<T>,
}

impl<T: Lift> WasmList<T> {
    fn new(
        ptr: usize,
        len: usize,
        cx: &mut LiftContext<'_>,
        elem: InterfaceType,
    ) -> Result<WasmList<T>> {
        match len
            .checked_mul(T::SIZE32)
            .and_then(|len| ptr.checked_add(len))
        {
            Some(n) if n <= cx.memory().len() => {}
            _ => bail!("list pointer/length out of bounds of memory"),
        }
        if ptr % usize::try_from(T::ALIGN32).err2anyhow()? != 0 {
            bail!("list pointer is not aligned")
        }
        Ok(WasmList {
            ptr,
            len,
            options: *cx.options,
            elem,
            types: cx.types.clone(),
            instance: SendSyncPtr::new(NonNull::new(cx.instance_ptr()).unwrap()),
            _marker: marker::PhantomData,
        })
    }

    /// Returns the item length of this vector
    #[inline]
    pub fn len(&self) -> usize {
        self.len
    }

    /// Gets the `n`th element of this list.
    ///
    /// Returns `None` if `index` is out of bounds. Returns `Some(Err(..))` if
    /// the value couldn't be decoded (it was invalid). Returns `Some(Ok(..))`
    /// if the value is valid.
    ///
    /// # Panics
    ///
    /// This function will panic if the string did not originally come from the
    /// `store` specified.
    //
    // TODO: given that interface values are intended to be consumed in one go
    // should we even expose a random access iteration API? In theory all
    // consumers should be validating through the iterator.
    pub fn get(&self, mut store: impl AsContextMut, index: usize) -> Option<Result<T>> {
        let store = store.as_context_mut().0;
        self.options.store_id().assert_belongs_to(store.id());
        // This should be safe because the unsafety lies in the `self.instance`
        // pointer passed in has previously been validated by the lifting
        // context this was originally created within and with the check above
        // this is guaranteed to be the same store. This means that this should
        // be carrying over the original assertion from the original creation of
        // the lifting context that created this type.
        let mut cx =
            unsafe { LiftContext::new(store, &self.options, &self.types, self.instance.as_ptr()) };
        self.get_from_store(&mut cx, index)
    }

    fn get_from_store(&self, cx: &mut LiftContext<'_>, index: usize) -> Option<Result<T>> {
        if index >= self.len {
            return None;
        }
        // Note that this is using panicking indexing and this is expected to
        // never fail. The bounds-checking here happened during the construction
        // of the `WasmList` itself which means these should always be in-bounds
        // (and wasm memory can only grow). This could theoretically be
        // unchecked indexing if we're confident enough and it's actually a perf
        // issue one day.
        let bytes = &cx.memory()[self.ptr + index * T::SIZE32..][..T::SIZE32];
        Some(T::load(cx, self.elem, bytes))
    }

    /// Returns an iterator over the elements of this list.
    ///
    /// Each item of the list may fail to decode and is represented through the
    /// `Result` value of the iterator.
    pub fn iter<'a, U: 'a>(
        &'a self,
        store: impl Into<StoreContextMut<'a, U>>,
    ) -> impl ExactSizeIterator<Item = Result<T>> + 'a {
        let store = store.into().0;
        self.options.store_id().assert_belongs_to(store.id());
        // See comments about unsafety in the `get` method.
        let mut cx =
            unsafe { LiftContext::new(store, &self.options, &self.types, self.instance.as_ptr()) };
        (0..self.len).map(move |i| self.get_from_store(&mut cx, i).unwrap())
    }
}

macro_rules! raw_wasm_list_accessors {
    ($($i:ident)*) => ($(
        impl WasmList<$i> {
            /// Get access to the raw underlying memory for this list.
            ///
            /// This method will return a direct slice into the original wasm
            /// module's linear memory where the data for this slice is stored.
            /// This allows the embedder to have efficient access to the
            /// underlying memory if needed and avoid copies and such if
            /// desired.
            ///
            /// Note that multi-byte integers are stored in little-endian format
            /// so portable processing of this slice must be aware of the host's
            /// byte-endianness. The `from_le` constructors in the Rust standard
            /// library should be suitable for converting from little-endian.
            ///
            /// # Panics
            ///
            /// Panics if the `store` provided is not the one from which this
            /// slice originated.
            pub fn as_le_slice<'a, T: 'a>(&self, store: impl Into<StoreContext<'a, T>>) -> &'a [$i] {
                let memory = self.options.memory(store.into().0);
                self._as_le_slice(memory)
            }

            fn _as_le_slice<'a>(&self, all_of_memory: &'a [u8]) -> &'a [$i] {
                // See comments in `WasmList::get` for the panicking indexing
                let byte_size = self.len * mem::size_of::<$i>();
                let bytes = &all_of_memory[self.ptr..][..byte_size];

                // The canonical ABI requires that everything is aligned to its
                // own size, so this should be an aligned array. Furthermore the
                // alignment of primitive integers for hosts should be smaller
                // than or equal to the size of the primitive itself, meaning
                // that a wasm canonical-abi-aligned list is also aligned for
                // the host. That should mean that the head/tail slices here are
                // empty.
                //
                // Also note that the `unsafe` here is needed since the type
                // we're aligning to isn't guaranteed to be valid, but in our
                // case it's just integers and bytes so this should be safe.
                unsafe {
                    let (head, body, tail) = bytes.align_to::<$i>();
                    assert!(head.is_empty() && tail.is_empty());
                    body
                }
            }
        }
    )*)
}

raw_wasm_list_accessors! {
    i8 i16 i32 i64
    u8 u16 u32 u64
}

// Note that this is similar to `ComponentType for str` except it can only be
// used for lifting, not lowering.
unsafe impl<T: ComponentType> ComponentType for WasmList<T> {
    type Lower = <[T] as ComponentType>::Lower;

    const ABI: CanonicalAbiInfo = CanonicalAbiInfo::POINTER_PAIR;

    fn typecheck(ty: &InterfaceType, types: &InstanceType<'_>) -> Result<()> {
        <[T] as ComponentType>::typecheck(ty, types)
    }
}

unsafe impl<T: Lift> Lift for WasmList<T> {
    fn lift(cx: &mut LiftContext<'_>, ty: InterfaceType, src: &Self::Lower) -> Result<Self> {
        let elem = match ty {
            InterfaceType::List(i) => cx.types[i].element,
            _ => bad_type_info(),
        };
        // FIXME: needs memory64 treatment
        let ptr = src[0].get_u32();
        let len = src[1].get_u32();
        let (ptr, len) = (
            usize::try_from(ptr).err2anyhow()?,
            usize::try_from(len).err2anyhow()?,
        );
        WasmList::new(ptr, len, cx, elem)
    }

    fn load(cx: &mut LiftContext<'_>, ty: InterfaceType, bytes: &[u8]) -> Result<Self> {
        let elem = match ty {
            InterfaceType::List(i) => cx.types[i].element,
            _ => bad_type_info(),
        };
        debug_assert!((bytes.as_ptr() as usize) % (Self::ALIGN32 as usize) == 0);
        // FIXME: needs memory64 treatment
        let ptr = u32::from_le_bytes(bytes[..4].try_into().unwrap());
        let len = u32::from_le_bytes(bytes[4..].try_into().unwrap());
        let (ptr, len) = (
            usize::try_from(ptr).err2anyhow()?,
            usize::try_from(len).err2anyhow()?,
        );
        WasmList::new(ptr, len, cx, elem)
    }
}

/// Verify that the given wasm type is a tuple with the expected fields in the right order.
fn typecheck_tuple(
    ty: &InterfaceType,
    types: &InstanceType<'_>,
    expected: &[fn(&InterfaceType, &InstanceType<'_>) -> Result<()>],
) -> Result<()> {
    match ty {
        InterfaceType::Tuple(t) => {
            let tuple = &types.types[*t];
            if tuple.types.len() != expected.len() {
                bail!(
                    "expected {}-tuple, found {}-tuple",
                    expected.len(),
                    tuple.types.len()
                );
            }
            for (ty, check) in tuple.types.iter().zip(expected) {
                check(ty, types)?;
            }
            Ok(())
        }
        other => bail!("expected `tuple` found `{}`", desc(other)),
    }
}

/// Verify that the given wasm type is a record with the expected fields in the right order and with the right
/// names.
pub fn typecheck_record(
    ty: &InterfaceType,
    types: &InstanceType<'_>,
    expected: &[(&str, fn(&InterfaceType, &InstanceType<'_>) -> Result<()>)],
) -> Result<()> {
    match ty {
        InterfaceType::Record(index) => {
            let fields = &types.types[*index].fields;

            if fields.len() != expected.len() {
                bail!(
                    "expected record of {} fields, found {} fields",
                    expected.len(),
                    fields.len()
                );
            }

            for (field, &(name, check)) in fields.iter().zip(expected) {
                check(&field.ty, types)
                    .with_context(|| format!("type mismatch for field {name}"))?;

                if field.name != name {
                    bail!("expected record field named {}, found {}", name, field.name);
                }
            }

            Ok(())
        }
        other => bail!("expected `record` found `{}`", desc(other)),
    }
}

/// Verify that the given wasm type is a variant with the expected cases in the right order and with the right
/// names.
pub fn typecheck_variant(
    ty: &InterfaceType,
    types: &InstanceType<'_>,
    expected: &[(
        &str,
        Option<fn(&InterfaceType, &InstanceType<'_>) -> Result<()>>,
    )],
) -> Result<()> {
    match ty {
        InterfaceType::Variant(index) => {
            let cases = &types.types[*index].cases;

            if cases.len() != expected.len() {
                bail!(
                    "expected variant of {} cases, found {} cases",
                    expected.len(),
                    cases.len()
                );
            }

            for ((case_name, case_ty), &(name, check)) in cases.iter().zip(expected) {
                if *case_name != name {
                    bail!("expected variant case named {name}, found {case_name}");
                }

                match (check, case_ty) {
                    (Some(check), Some(ty)) => check(ty, types)
                        .with_context(|| format!("type mismatch for case {name}"))?,
                    (None, None) => {}
                    (Some(_), None) => {
                        bail!("case `{name}` has no type but one was expected")
                    }
                    (None, Some(_)) => {
                        bail!("case `{name}` has a type but none was expected")
                    }
                }
            }

            Ok(())
        }
        other => bail!("expected `variant` found `{}`", desc(other)),
    }
}

/// Verify that the given wasm type is a enum with the expected cases in the right order and with the right
/// names.
pub fn typecheck_enum(
    ty: &InterfaceType,
    types: &InstanceType<'_>,
    expected: &[&str],
) -> Result<()> {
    match ty {
        InterfaceType::Enum(index) => {
            let names = &types.types[*index].names;

            if names.len() != expected.len() {
                bail!(
                    "expected enum of {} names, found {} names",
                    expected.len(),
                    names.len()
                );
            }

            for (name, expected) in names.iter().zip(expected) {
                if name != expected {
                    bail!("expected enum case named {}, found {}", expected, name);
                }
            }

            Ok(())
        }
        other => bail!("expected `enum` found `{}`", desc(other)),
    }
}

/// Verify that the given wasm type is a flags type with the expected flags in the right order and with the right
/// names.
pub fn typecheck_flags(
    ty: &InterfaceType,
    types: &InstanceType<'_>,
    expected: &[&str],
) -> Result<()> {
    match ty {
        InterfaceType::Flags(index) => {
            let names = &types.types[*index].names;

            if names.len() != expected.len() {
                bail!(
                    "expected flags type with {} names, found {} names",
                    expected.len(),
                    names.len()
                );
            }

            for (name, expected) in names.iter().zip(expected) {
                if name != expected {
                    bail!("expected flag named {}, found {}", expected, name);
                }
            }

            Ok(())
        }
        other => bail!("expected `flags` found `{}`", desc(other)),
    }
}

/// Format the specified bitflags using the specified names for debugging
pub fn format_flags(bits: &[u32], names: &[&str], f: &mut fmt::Formatter) -> fmt::Result {
    f.write_str("(")?;
    let mut wrote = false;
    for (index, name) in names.iter().enumerate() {
        if ((bits[index / 32] >> (index % 32)) & 1) != 0 {
            if wrote {
                f.write_str("|")?;
            } else {
                wrote = true;
            }

            f.write_str(name)?;
        }
    }
    f.write_str(")")
}

unsafe impl<T> ComponentType for Option<T>
where
    T: ComponentType,
{
    type Lower = TupleLower2<<u32 as ComponentType>::Lower, T::Lower>;

    const ABI: CanonicalAbiInfo = CanonicalAbiInfo::variant_static(&[None, Some(T::ABI)]);

    fn typecheck(ty: &InterfaceType, types: &InstanceType<'_>) -> Result<()> {
        match ty {
            InterfaceType::Option(t) => T::typecheck(&types.types[*t].ty, types),
            other => bail!("expected `option` found `{}`", desc(other)),
        }
    }
}

unsafe impl<T> ComponentVariant for Option<T>
where
    T: ComponentType,
{
    const CASES: &'static [Option<CanonicalAbiInfo>] = &[None, Some(T::ABI)];
}

unsafe impl<T> Lower for Option<T>
where
    T: Lower,
{
    fn lower<U>(
        &self,
        cx: &mut LowerContext<'_, U>,
        ty: InterfaceType,
        dst: &mut MaybeUninit<Self::Lower>,
    ) -> Result<()> {
        let payload = match ty {
            InterfaceType::Option(ty) => cx.types[ty].ty,
            _ => bad_type_info(),
        };
        match self {
            None => {
                map_maybe_uninit!(dst.A1).write(ValRaw::i32(0));
                // Note that this is unsafe as we're writing an arbitrary
                // bit-pattern to an arbitrary type, but part of the unsafe
                // contract of the `ComponentType` trait is that we can assign
                // any bit-pattern. By writing all zeros here we're ensuring
                // that the core wasm arguments this translates to will all be
                // zeros (as the canonical ABI requires).
                unsafe {
                    map_maybe_uninit!(dst.A2).as_mut_ptr().write_bytes(0u8, 1);
                }
            }
            Some(val) => {
                map_maybe_uninit!(dst.A1).write(ValRaw::i32(1));
                val.lower(cx, payload, map_maybe_uninit!(dst.A2))?;
            }
        }
        Ok(())
    }

    fn store<U>(
        &self,
        cx: &mut LowerContext<'_, U>,
        ty: InterfaceType,
        offset: usize,
    ) -> Result<()> {
        debug_assert!(offset % (Self::ALIGN32 as usize) == 0);
        let payload = match ty {
            InterfaceType::Option(ty) => cx.types[ty].ty,
            _ => bad_type_info(),
        };
        match self {
            None => {
                cx.get::<1>(offset)[0] = 0;
            }
            Some(val) => {
                cx.get::<1>(offset)[0] = 1;
                val.store(cx, payload, offset + (Self::INFO.payload_offset32 as usize))?;
            }
        }
        Ok(())
    }
}

unsafe impl<T> Lift for Option<T>
where
    T: Lift,
{
    fn lift(cx: &mut LiftContext<'_>, ty: InterfaceType, src: &Self::Lower) -> Result<Self> {
        let payload = match ty {
            InterfaceType::Option(ty) => cx.types[ty].ty,
            _ => bad_type_info(),
        };
        Ok(match src.A1.get_i32() {
            0 => None,
            1 => Some(T::lift(cx, payload, &src.A2)?),
            _ => bail!("invalid option discriminant"),
        })
    }

    fn load(cx: &mut LiftContext<'_>, ty: InterfaceType, bytes: &[u8]) -> Result<Self> {
        debug_assert!((bytes.as_ptr() as usize) % (Self::ALIGN32 as usize) == 0);
        let payload_ty = match ty {
            InterfaceType::Option(ty) => cx.types[ty].ty,
            _ => bad_type_info(),
        };
        let discrim = bytes[0];
        let payload = &bytes[Self::INFO.payload_offset32 as usize..];
        match discrim {
            0 => Ok(None),
            1 => Ok(Some(T::load(cx, payload_ty, payload)?)),
            _ => bail!("invalid option discriminant"),
        }
    }
}

#[derive(Clone, Copy)]
#[repr(C)]
pub struct ResultLower<T: Copy, E: Copy> {
    tag: ValRaw,
    payload: ResultLowerPayload<T, E>,
}

#[derive(Clone, Copy)]
#[repr(C)]
union ResultLowerPayload<T: Copy, E: Copy> {
    ok: T,
    err: E,
}

unsafe impl<T, E> ComponentType for Result<T, E>
where
    T: ComponentType,
    E: ComponentType,
{
    type Lower = ResultLower<T::Lower, E::Lower>;

    const ABI: CanonicalAbiInfo = CanonicalAbiInfo::variant_static(&[Some(T::ABI), Some(E::ABI)]);

    fn typecheck(ty: &InterfaceType, types: &InstanceType<'_>) -> Result<()> {
        match ty {
            InterfaceType::Result(r) => {
                let result = &types.types[*r];
                match &result.ok {
                    Some(ty) => T::typecheck(ty, types)?,
                    None if T::IS_RUST_UNIT_TYPE => {}
                    None => bail!("expected no `ok` type"),
                }
                match &result.err {
                    Some(ty) => E::typecheck(ty, types)?,
                    None if E::IS_RUST_UNIT_TYPE => {}
                    None => bail!("expected no `err` type"),
                }
                Ok(())
            }
            other => bail!("expected `result` found `{}`", desc(other)),
        }
    }
}

/// Lowers the payload of a variant into the storage for the entire payload,
/// handling writing zeros at the end of the representation if this payload is
/// smaller than the entire flat representation.
///
/// * `payload` - the flat storage space for the entire payload of the variant
/// * `typed_payload` - projection from the payload storage space to the
///   individual storage space for this variant.
/// * `lower` - lowering operation used to initialize the `typed_payload` return
///   value.
///
/// For more information on this se the comments in the `Lower for Result`
/// implementation below.
pub unsafe fn lower_payload<P, T>(
    payload: &mut MaybeUninit<P>,
    typed_payload: impl FnOnce(&mut MaybeUninit<P>) -> &mut MaybeUninit<T>,
    lower: impl FnOnce(&mut MaybeUninit<T>) -> Result<()>,
) -> Result<()> {
    let typed = typed_payload(payload);
    lower(typed)?;

    let typed_len = storage_as_slice(typed).len();
    let payload = storage_as_slice_mut(payload);
    for slot in payload[typed_len..].iter_mut() {
        *slot = ValRaw::u64(0);
    }
    Ok(())
}

unsafe impl<T, E> ComponentVariant for Result<T, E>
where
    T: ComponentType,
    E: ComponentType,
{
    const CASES: &'static [Option<CanonicalAbiInfo>] = &[Some(T::ABI), Some(E::ABI)];
}

unsafe impl<T, E> Lower for Result<T, E>
where
    T: Lower,
    E: Lower,
{
    fn lower<U>(
        &self,
        cx: &mut LowerContext<'_, U>,
        ty: InterfaceType,
        dst: &mut MaybeUninit<Self::Lower>,
    ) -> Result<()> {
        let (ok, err) = match ty {
            InterfaceType::Result(ty) => {
                let ty = &cx.types[ty];
                (ty.ok, ty.err)
            }
            _ => bad_type_info(),
        };

        // This implementation of `Lower::lower`, if you're reading these from
        // the top of this file, is the first location that the "join" logic of
        // the component model's canonical ABI encountered. The rough problem is
        // that let's say we have a component model type of the form:
        //
        //      (result u64 (error (tuple f32 u16)))
        //
        // The flat representation of this is actually pretty tricky. Currently
        // it is:
        //
        //      i32 i64 i32
        //
        // The first `i32` is the discriminant for the `result`, and the payload
        // is represented by `i64 i32`. The "ok" variant will only use the `i64`
        // and the "err" variant will use both `i64` and `i32`.
        //
        // In the "ok" variant the first issue is encountered. The size of one
        // variant may not match the size of the other variants. All variants
        // start at the "front" but when lowering a type we need to be sure to
        // initialize the later variants (lest we leak random host memory into
        // the guest module). Due to how the `Lower` type is represented as a
        // `union` of all the variants what ends up happening here is that
        // internally within the `lower_payload` after the typed payload is
        // lowered the remaining bits of the payload that weren't initialized
        // are all set to zero. This will guarantee that we'll write to all the
        // slots for each variant.
        //
        // The "err" variant encounters the second issue, however, which is that
        // the flat representation for each type may differ between payloads. In
        // the "ok" arm an `i64` is written, but the `lower` implementation for
        // the "err" arm will write an `f32` and then an `i32`. For this
        // implementation of `lower` to be valid the `f32` needs to get inflated
        // to an `i64` with zero-padding in the upper bits. What may be
        // surprising, however, is that none of this is handled in this file.
        // This implementation looks like it's blindly deferring to `E::lower`
        // and hoping it does the right thing.
        //
        // In reality, however, the correctness of variant lowering relies on
        // two subtle details of the `ValRaw` implementation in Wasmtime:
        //
        // 1. First the `ValRaw` value always contains little-endian values.
        //    This means that if a `u32` is written, a `u64` is read, and then
        //    the `u64` has its upper bits truncated the original value will
        //    always be retained. This is primarily here for big-endian
        //    platforms where if it weren't little endian then the opposite
        //    would occur and the wrong value would be read.
        //
        // 2. Second, and perhaps even more subtly, the `ValRaw` constructors
        //    for 32-bit types actually always initialize 64-bits of the
        //    `ValRaw`. In the component model flat ABI only 32 and 64-bit types
        //    are used so 64-bits is big enough to contain everything. This
        //    means that when a `ValRaw` is written into the destination it will
        //    always, whether it's needed or not, be "ready" to get extended up
        //    to 64-bits.
        //
        // Put together these two subtle guarantees means that all `Lower`
        // implementations can be written "naturally" as one might naively
        // expect. Variants will, on each arm, zero out remaining fields and all
        // writes to the flat representation will automatically be 64-bit writes
        // meaning that if the value is read as a 64-bit value, which isn't
        // known at the time of the write, it'll still be correct.
        match self {
            Ok(e) => {
                map_maybe_uninit!(dst.tag).write(ValRaw::i32(0));
                unsafe {
                    lower_payload(
                        map_maybe_uninit!(dst.payload),
                        |payload| map_maybe_uninit!(payload.ok),
                        |dst| match ok {
                            Some(ok) => e.lower(cx, ok, dst),
                            None => Ok(()),
                        },
                    )
                }
            }
            Err(e) => {
                map_maybe_uninit!(dst.tag).write(ValRaw::i32(1));
                unsafe {
                    lower_payload(
                        map_maybe_uninit!(dst.payload),
                        |payload| map_maybe_uninit!(payload.err),
                        |dst| match err {
                            Some(err) => e.lower(cx, err, dst),
                            None => Ok(()),
                        },
                    )
                }
            }
        }
    }

    fn store<U>(
        &self,
        cx: &mut LowerContext<'_, U>,
        ty: InterfaceType,
        offset: usize,
    ) -> Result<()> {
        let (ok, err) = match ty {
            InterfaceType::Result(ty) => {
                let ty = &cx.types[ty];
                (ty.ok, ty.err)
            }
            _ => bad_type_info(),
        };
        debug_assert!(offset % (Self::ALIGN32 as usize) == 0);
        let payload_offset = Self::INFO.payload_offset32 as usize;
        match self {
            Ok(e) => {
                cx.get::<1>(offset)[0] = 0;
                if let Some(ok) = ok {
                    e.store(cx, ok, offset + payload_offset)?;
                }
            }
            Err(e) => {
                cx.get::<1>(offset)[0] = 1;
                if let Some(err) = err {
                    e.store(cx, err, offset + payload_offset)?;
                }
            }
        }
        Ok(())
    }
}

unsafe impl<T, E> Lift for Result<T, E>
where
    T: Lift,
    E: Lift,
{
    #[inline]
    fn lift(cx: &mut LiftContext<'_>, ty: InterfaceType, src: &Self::Lower) -> Result<Self> {
        let (ok, err) = match ty {
            InterfaceType::Result(ty) => {
                let ty = &cx.types[ty];
                (ty.ok, ty.err)
            }
            _ => bad_type_info(),
        };
        // Note that this implementation specifically isn't trying to actually
        // reinterpret or alter the bits of `lower` depending on which variant
        // we're lifting. This ends up all working out because the value is
        // stored in little-endian format.
        //
        // When stored in little-endian format the `{T,E}::Lower`, when each
        // individual `ValRaw` is read, means that if an i64 value, extended
        // from an i32 value, was stored then when the i32 value is read it'll
        // automatically ignore the upper bits.
        //
        // This "trick" allows us to seamlessly pass through the `Self::Lower`
        // representation into the lifting/lowering without trying to handle
        // "join"ed types as per the canonical ABI. It just so happens that i64
        // bits will naturally be reinterpreted as f64. Additionally if the
        // joined type is i64 but only the lower bits are read that's ok and we
        // don't need to validate the upper bits.
        //
        // This is largely enabled by WebAssembly/component-model#35 where no
        // validation needs to be performed for ignored bits and bytes here.
        Ok(match src.tag.get_i32() {
            0 => Ok(unsafe { lift_option(cx, ok, &src.payload.ok)? }),
            1 => Err(unsafe { lift_option(cx, err, &src.payload.err)? }),
            _ => bail!("invalid expected discriminant"),
        })
    }

    #[inline]
    fn load(cx: &mut LiftContext<'_>, ty: InterfaceType, bytes: &[u8]) -> Result<Self> {
        debug_assert!((bytes.as_ptr() as usize) % (Self::ALIGN32 as usize) == 0);
        let discrim = bytes[0];
        let payload = &bytes[Self::INFO.payload_offset32 as usize..];
        let (ok, err) = match ty {
            InterfaceType::Result(ty) => {
                let ty = &cx.types[ty];
                (ty.ok, ty.err)
            }
            _ => bad_type_info(),
        };
        match discrim {
            0 => Ok(Ok(load_option(cx, ok, &payload[..T::SIZE32])?)),
            1 => Ok(Err(load_option(cx, err, &payload[..E::SIZE32])?)),
            _ => bail!("invalid expected discriminant"),
        }
    }
}

fn lift_option<T>(cx: &mut LiftContext<'_>, ty: Option<InterfaceType>, src: &T::Lower) -> Result<T>
where
    T: Lift,
{
    match ty {
        Some(ty) => T::lift(cx, ty, src),
        None => Ok(empty_lift()),
    }
}

fn load_option<T>(cx: &mut LiftContext<'_>, ty: Option<InterfaceType>, bytes: &[u8]) -> Result<T>
where
    T: Lift,
{
    match ty {
        Some(ty) => T::load(cx, ty, bytes),
        None => Ok(empty_lift()),
    }
}

fn empty_lift<T>() -> T
where
    T: Lift,
{
    assert!(T::IS_RUST_UNIT_TYPE);
    assert_eq!(mem::size_of::<T>(), 0);
    unsafe { MaybeUninit::uninit().assume_init() }
}

macro_rules! impl_component_ty_for_tuples {
    ($n:tt $($t:ident)*) => {paste::paste!{
        #[allow(non_snake_case)]
        #[doc(hidden)]
        #[derive(Clone, Copy)]
        #[repr(C)]
        pub struct [<TupleLower$n>]<$($t),*> {
            $($t: $t,)*
            _align_tuple_lower0_correctly: [ValRaw; 0],
        }

        #[allow(non_snake_case)]
        unsafe impl<$($t,)*> ComponentType for ($($t,)*)
            where $($t: ComponentType),*
        {
            type Lower = [<TupleLower$n>]<$($t::Lower),*>;

            const ABI: CanonicalAbiInfo = CanonicalAbiInfo::record_static(&[
                $($t::ABI),*
            ]);

            const IS_RUST_UNIT_TYPE: bool = {
                let mut _is_unit = true;
                $(
                    let _anything_to_bind_the_macro_variable = $t::IS_RUST_UNIT_TYPE;
                    _is_unit = false;
                )*
                _is_unit
            };

            fn typecheck(
                ty: &InterfaceType,
                types: &InstanceType<'_>,
            ) -> Result<()> {
                typecheck_tuple(ty, types, &[$($t::typecheck),*])
            }
        }

        #[allow(non_snake_case)]
        unsafe impl<$($t,)*> Lower for ($($t,)*)
            where $($t: Lower),*
        {
            fn lower<U>(
                &self,
                cx: &mut LowerContext<'_, U>,
                ty: InterfaceType,
                _dst: &mut MaybeUninit<Self::Lower>,
            ) -> Result<()> {
                let types = match ty {
                    InterfaceType::Tuple(t) => &cx.types[t].types,
                    _ => bad_type_info(),
                };
                let ($($t,)*) = self;
                let mut _types = types.iter();
                $(
                    let ty = *_types.next().unwrap_or_else(bad_type_info);
                    $t.lower(cx, ty, map_maybe_uninit!(_dst.$t))?;
                )*
                Ok(())
            }

            fn store<U>(
                &self,
                cx: &mut LowerContext<'_, U>,
                ty: InterfaceType,
                mut _offset: usize,
            ) -> Result<()> {
                debug_assert!(_offset % (Self::ALIGN32 as usize) == 0);
                let types = match ty {
                    InterfaceType::Tuple(t) => &cx.types[t].types,
                    _ => bad_type_info(),
                };
                let ($($t,)*) = self;
                let mut _types = types.iter();
                $(
                    let ty = *_types.next().unwrap_or_else(bad_type_info);
                    $t.store(cx, ty, $t::ABI.next_field32_size(&mut _offset))?;
                )*
                Ok(())
            }
        }

        #[allow(non_snake_case)]
        unsafe impl<$($t,)*> Lift for ($($t,)*)
            where $($t: Lift),*
        {
            #[inline]
            fn lift(cx: &mut LiftContext<'_>, ty: InterfaceType, _src: &Self::Lower) -> Result<Self> {
                let types = match ty {
                    InterfaceType::Tuple(t) => &cx.types[t].types,
                    _ => bad_type_info(),
                };
                let mut _types = types.iter();
                Ok(($(
                    $t::lift(
                        cx,
                        *_types.next().unwrap_or_else(bad_type_info),
                        &_src.$t,
                    )?,
                )*))
            }

            #[inline]
            fn load(cx: &mut LiftContext<'_>, ty: InterfaceType, bytes: &[u8]) -> Result<Self> {
                debug_assert!((bytes.as_ptr() as usize) % (Self::ALIGN32 as usize) == 0);
                let types = match ty {
                    InterfaceType::Tuple(t) => &cx.types[t].types,
                    _ => bad_type_info(),
                };
                let mut _types = types.iter();
                let mut _offset = 0;
                $(
                    let ty = *_types.next().unwrap_or_else(bad_type_info);
                    let $t = $t::load(cx, ty, &bytes[$t::ABI.next_field32_size(&mut _offset)..][..$t::SIZE32])?;
                )*
                Ok(($($t,)*))
            }
        }

        #[allow(non_snake_case)]
        unsafe impl<$($t,)*> ComponentNamedList for ($($t,)*)
            where $($t: ComponentType),*
        {}
    }};
}

for_each_function_signature!(impl_component_ty_for_tuples);

pub fn desc(ty: &InterfaceType) -> &'static str {
    match ty {
        InterfaceType::U8 => "u8",
        InterfaceType::S8 => "s8",
        InterfaceType::U16 => "u16",
        InterfaceType::S16 => "s16",
        InterfaceType::U32 => "u32",
        InterfaceType::S32 => "s32",
        InterfaceType::U64 => "u64",
        InterfaceType::S64 => "s64",
        InterfaceType::Float32 => "f32",
        InterfaceType::Float64 => "f64",
        InterfaceType::Bool => "bool",
        InterfaceType::Char => "char",
        InterfaceType::String => "string",
        InterfaceType::List(_) => "list",
        InterfaceType::Tuple(_) => "tuple",
        InterfaceType::Option(_) => "option",
        InterfaceType::Result(_) => "result",

        InterfaceType::Record(_) => "record",
        InterfaceType::Variant(_) => "variant",
        InterfaceType::Flags(_) => "flags",
        InterfaceType::Enum(_) => "enum",
        InterfaceType::Own(_) => "owned resource",
        InterfaceType::Borrow(_) => "borrowed resource",
    }
}

#[cold]
#[doc(hidden)]
pub fn bad_type_info<T>() -> T {
    // NB: should consider something like `unreachable_unchecked` here if this
    // becomes a performance bottleneck at some point, but that also comes with
    // a tradeoff of propagating a lot of unsafety, so it may not be worth it.
    panic!("bad type information detected");
}