core/cell.rs
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
//! Shareable mutable containers.
//!
//! Rust memory safety is based on this rule: Given an object `T`, it is only possible to
//! have one of the following:
//!
//! - Having several immutable references (`&T`) to the object (also known as **aliasing**).
//! - Having one mutable reference (`&mut T`) to the object (also known as **mutability**).
//!
//! This is enforced by the Rust compiler. However, there are situations where this rule is not
//! flexible enough. Sometimes it is required to have multiple references to an object and yet
//! mutate it.
//!
//! Shareable mutable containers exist to permit mutability in a controlled manner, even in the
//! presence of aliasing. [`Cell<T>`], [`RefCell<T>`], and [`OnceCell<T>`] allow doing this in
//! a single-threaded way—they do not implement [`Sync`]. (If you need to do aliasing and
//! mutation among multiple threads, [`Mutex<T>`], [`RwLock<T>`], [`OnceLock<T>`] or [`atomic`]
//! types are the correct data structures to do so).
//!
//! Values of the `Cell<T>`, `RefCell<T>`, and `OnceCell<T>` types may be mutated through shared
//! references (i.e. the common `&T` type), whereas most Rust types can only be mutated through
//! unique (`&mut T`) references. We say these cell types provide 'interior mutability'
//! (mutable via `&T`), in contrast with typical Rust types that exhibit 'inherited mutability'
//! (mutable only via `&mut T`).
//!
//! Cell types come in three flavors: `Cell<T>`, `RefCell<T>`, and `OnceCell<T>`. Each provides
//! a different way of providing safe interior mutability.
//!
//! ## `Cell<T>`
//!
//! [`Cell<T>`] implements interior mutability by moving values in and out of the cell. That is, an
//! `&mut T` to the inner value can never be obtained, and the value itself cannot be directly
//! obtained without replacing it with something else. Both of these rules ensure that there is
//! never more than one reference pointing to the inner value. This type provides the following
//! methods:
//!
//! - For types that implement [`Copy`], the [`get`](Cell::get) method retrieves the current
//! interior value by duplicating it.
//! - For types that implement [`Default`], the [`take`](Cell::take) method replaces the current
//! interior value with [`Default::default()`] and returns the replaced value.
//! - All types have:
//! - [`replace`](Cell::replace): replaces the current interior value and returns the replaced
//! value.
//! - [`into_inner`](Cell::into_inner): this method consumes the `Cell<T>` and returns the
//! interior value.
//! - [`set`](Cell::set): this method replaces the interior value, dropping the replaced value.
//!
//! `Cell<T>` is typically used for more simple types where copying or moving values isn't too
//! resource intensive (e.g. numbers), and should usually be preferred over other cell types when
//! possible. For larger and non-copy types, `RefCell` provides some advantages.
//!
//! ## `RefCell<T>`
//!
//! [`RefCell<T>`] uses Rust's lifetimes to implement "dynamic borrowing", a process whereby one can
//! claim temporary, exclusive, mutable access to the inner value. Borrows for `RefCell<T>`s are
//! tracked at _runtime_, unlike Rust's native reference types which are entirely tracked
//! statically, at compile time.
//!
//! An immutable reference to a `RefCell`'s inner value (`&T`) can be obtained with
//! [`borrow`](`RefCell::borrow`), and a mutable borrow (`&mut T`) can be obtained with
//! [`borrow_mut`](`RefCell::borrow_mut`). When these functions are called, they first verify that
//! Rust's borrow rules will be satisfied: any number of immutable borrows are allowed or a
//! single mutable borrow is allowed, but never both. If a borrow is attempted that would violate
//! these rules, the thread will panic.
//!
//! The corresponding [`Sync`] version of `RefCell<T>` is [`RwLock<T>`].
//!
//! ## `OnceCell<T>`
//!
//! [`OnceCell<T>`] is somewhat of a hybrid of `Cell` and `RefCell` that works for values that
//! typically only need to be set once. This means that a reference `&T` can be obtained without
//! moving or copying the inner value (unlike `Cell`) but also without runtime checks (unlike
//! `RefCell`). However, its value can also not be updated once set unless you have a mutable
//! reference to the `OnceCell`.
//!
//! `OnceCell` provides the following methods:
//!
//! - [`get`](OnceCell::get): obtain a reference to the inner value
//! - [`set`](OnceCell::set): set the inner value if it is unset (returns a `Result`)
//! - [`get_or_init`](OnceCell::get_or_init): return the inner value, initializing it if needed
//! - [`get_mut`](OnceCell::get_mut): provide a mutable reference to the inner value, only available
//! if you have a mutable reference to the cell itself.
//!
//! The corresponding [`Sync`] version of `OnceCell<T>` is [`OnceLock<T>`].
//!
//! ## `LazyCell<T, F>`
//!
//! A common pattern with OnceCell is, for a given OnceCell, to use the same function on every
//! call to [`OnceCell::get_or_init`] with that cell. This is what is offered by [`LazyCell`],
//! which pairs cells of `T` with functions of `F`, and always calls `F` before it yields `&T`.
//! This happens implicitly by simply attempting to dereference the LazyCell to get its contents,
//! so its use is much more transparent with a place which has been initialized by a constant.
//!
//! More complicated patterns that don't fit this description can be built on `OnceCell<T>` instead.
//!
//! `LazyCell` works by providing an implementation of `impl Deref` that calls the function,
//! so you can just use it by dereference (e.g. `*lazy_cell` or `lazy_cell.deref()`).
//!
//! The corresponding [`Sync`] version of `LazyCell<T, F>` is [`LazyLock<T, F>`].
//!
//! # When to choose interior mutability
//!
//! The more common inherited mutability, where one must have unique access to mutate a value, is
//! one of the key language elements that enables Rust to reason strongly about pointer aliasing,
//! statically preventing crash bugs. Because of that, inherited mutability is preferred, and
//! interior mutability is something of a last resort. Since cell types enable mutation where it
//! would otherwise be disallowed though, there are occasions when interior mutability might be
//! appropriate, or even *must* be used, e.g.
//!
//! * Introducing mutability 'inside' of something immutable
//! * Implementation details of logically-immutable methods.
//! * Mutating implementations of [`Clone`].
//!
//! ## Introducing mutability 'inside' of something immutable
//!
//! Many shared smart pointer types, including [`Rc<T>`] and [`Arc<T>`], provide containers that can
//! be cloned and shared between multiple parties. Because the contained values may be
//! multiply-aliased, they can only be borrowed with `&`, not `&mut`. Without cells it would be
//! impossible to mutate data inside of these smart pointers at all.
//!
//! It's very common then to put a `RefCell<T>` inside shared pointer types to reintroduce
//! mutability:
//!
//! ```
//! use std::cell::{RefCell, RefMut};
//! use std::collections::HashMap;
//! use std::rc::Rc;
//!
//! fn main() {
//! let shared_map: Rc<RefCell<_>> = Rc::new(RefCell::new(HashMap::new()));
//! // Create a new block to limit the scope of the dynamic borrow
//! {
//! let mut map: RefMut<'_, _> = shared_map.borrow_mut();
//! map.insert("africa", 92388);
//! map.insert("kyoto", 11837);
//! map.insert("piccadilly", 11826);
//! map.insert("marbles", 38);
//! }
//!
//! // Note that if we had not let the previous borrow of the cache fall out
//! // of scope then the subsequent borrow would cause a dynamic thread panic.
//! // This is the major hazard of using `RefCell`.
//! let total: i32 = shared_map.borrow().values().sum();
//! println!("{total}");
//! }
//! ```
//!
//! Note that this example uses `Rc<T>` and not `Arc<T>`. `RefCell<T>`s are for single-threaded
//! scenarios. Consider using [`RwLock<T>`] or [`Mutex<T>`] if you need shared mutability in a
//! multi-threaded situation.
//!
//! ## Implementation details of logically-immutable methods
//!
//! Occasionally it may be desirable not to expose in an API that there is mutation happening
//! "under the hood". This may be because logically the operation is immutable, but e.g., caching
//! forces the implementation to perform mutation; or because you must employ mutation to implement
//! a trait method that was originally defined to take `&self`.
//!
//! ```
//! # #![allow(dead_code)]
//! use std::cell::OnceCell;
//!
//! struct Graph {
//! edges: Vec<(i32, i32)>,
//! span_tree_cache: OnceCell<Vec<(i32, i32)>>
//! }
//!
//! impl Graph {
//! fn minimum_spanning_tree(&self) -> Vec<(i32, i32)> {
//! self.span_tree_cache
//! .get_or_init(|| self.calc_span_tree())
//! .clone()
//! }
//!
//! fn calc_span_tree(&self) -> Vec<(i32, i32)> {
//! // Expensive computation goes here
//! vec![]
//! }
//! }
//! ```
//!
//! ## Mutating implementations of `Clone`
//!
//! This is simply a special - but common - case of the previous: hiding mutability for operations
//! that appear to be immutable. The [`clone`](Clone::clone) method is expected to not change the
//! source value, and is declared to take `&self`, not `&mut self`. Therefore, any mutation that
//! happens in the `clone` method must use cell types. For example, [`Rc<T>`] maintains its
//! reference counts within a `Cell<T>`.
//!
//! ```
//! use std::cell::Cell;
//! use std::ptr::NonNull;
//! use std::process::abort;
//! use std::marker::PhantomData;
//!
//! struct Rc<T: ?Sized> {
//! ptr: NonNull<RcBox<T>>,
//! phantom: PhantomData<RcBox<T>>,
//! }
//!
//! struct RcBox<T: ?Sized> {
//! strong: Cell<usize>,
//! refcount: Cell<usize>,
//! value: T,
//! }
//!
//! impl<T: ?Sized> Clone for Rc<T> {
//! fn clone(&self) -> Rc<T> {
//! self.inc_strong();
//! Rc {
//! ptr: self.ptr,
//! phantom: PhantomData,
//! }
//! }
//! }
//!
//! trait RcBoxPtr<T: ?Sized> {
//!
//! fn inner(&self) -> &RcBox<T>;
//!
//! fn strong(&self) -> usize {
//! self.inner().strong.get()
//! }
//!
//! fn inc_strong(&self) {
//! self.inner()
//! .strong
//! .set(self.strong()
//! .checked_add(1)
//! .unwrap_or_else(|| abort() ));
//! }
//! }
//!
//! impl<T: ?Sized> RcBoxPtr<T> for Rc<T> {
//! fn inner(&self) -> &RcBox<T> {
//! unsafe {
//! self.ptr.as_ref()
//! }
//! }
//! }
//! ```
//!
//! [`Arc<T>`]: ../../std/sync/struct.Arc.html
//! [`Rc<T>`]: ../../std/rc/struct.Rc.html
//! [`RwLock<T>`]: ../../std/sync/struct.RwLock.html
//! [`Mutex<T>`]: ../../std/sync/struct.Mutex.html
//! [`OnceLock<T>`]: ../../std/sync/struct.OnceLock.html
//! [`LazyLock<T, F>`]: ../../std/sync/struct.LazyLock.html
//! [`Sync`]: ../../std/marker/trait.Sync.html
//! [`atomic`]: crate::sync::atomic
#![stable(feature = "rust1", since = "1.0.0")]
use crate::cmp::Ordering;
use crate::fmt::{self, Debug, Display};
use crate::marker::{PhantomData, Unsize};
use crate::mem;
use crate::ops::{CoerceUnsized, Deref, DerefMut, DerefPure, DispatchFromDyn};
use crate::pin::PinCoerceUnsized;
use crate::ptr::{self, NonNull};
mod lazy;
mod once;
#[stable(feature = "lazy_cell", since = "1.80.0")]
pub use lazy::LazyCell;
#[stable(feature = "once_cell", since = "1.70.0")]
pub use once::OnceCell;
/// A mutable memory location.
///
/// # Memory layout
///
/// `Cell<T>` has the same [memory layout and caveats as
/// `UnsafeCell<T>`](UnsafeCell#memory-layout). In particular, this means that
/// `Cell<T>` has the same in-memory representation as its inner type `T`.
///
/// # Examples
///
/// In this example, you can see that `Cell<T>` enables mutation inside an
/// immutable struct. In other words, it enables "interior mutability".
///
/// ```
/// use std::cell::Cell;
///
/// struct SomeStruct {
/// regular_field: u8,
/// special_field: Cell<u8>,
/// }
///
/// let my_struct = SomeStruct {
/// regular_field: 0,
/// special_field: Cell::new(1),
/// };
///
/// let new_value = 100;
///
/// // ERROR: `my_struct` is immutable
/// // my_struct.regular_field = new_value;
///
/// // WORKS: although `my_struct` is immutable, `special_field` is a `Cell`,
/// // which can always be mutated
/// my_struct.special_field.set(new_value);
/// assert_eq!(my_struct.special_field.get(), new_value);
/// ```
///
/// See the [module-level documentation](self) for more.
#[stable(feature = "rust1", since = "1.0.0")]
#[repr(transparent)]
#[rustc_pub_transparent]
pub struct Cell<T: ?Sized> {
value: UnsafeCell<T>,
}
#[stable(feature = "rust1", since = "1.0.0")]
unsafe impl<T: ?Sized> Send for Cell<T> where T: Send {}
// Note that this negative impl isn't strictly necessary for correctness,
// as `Cell` wraps `UnsafeCell`, which is itself `!Sync`.
// However, given how important `Cell`'s `!Sync`-ness is,
// having an explicit negative impl is nice for documentation purposes
// and results in nicer error messages.
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> !Sync for Cell<T> {}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: Copy> Clone for Cell<T> {
#[inline]
fn clone(&self) -> Cell<T> {
Cell::new(self.get())
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: Default> Default for Cell<T> {
/// Creates a `Cell<T>`, with the `Default` value for T.
#[inline]
fn default() -> Cell<T> {
Cell::new(Default::default())
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: PartialEq + Copy> PartialEq for Cell<T> {
#[inline]
fn eq(&self, other: &Cell<T>) -> bool {
self.get() == other.get()
}
}
#[stable(feature = "cell_eq", since = "1.2.0")]
impl<T: Eq + Copy> Eq for Cell<T> {}
#[stable(feature = "cell_ord", since = "1.10.0")]
impl<T: PartialOrd + Copy> PartialOrd for Cell<T> {
#[inline]
fn partial_cmp(&self, other: &Cell<T>) -> Option<Ordering> {
self.get().partial_cmp(&other.get())
}
#[inline]
fn lt(&self, other: &Cell<T>) -> bool {
self.get() < other.get()
}
#[inline]
fn le(&self, other: &Cell<T>) -> bool {
self.get() <= other.get()
}
#[inline]
fn gt(&self, other: &Cell<T>) -> bool {
self.get() > other.get()
}
#[inline]
fn ge(&self, other: &Cell<T>) -> bool {
self.get() >= other.get()
}
}
#[stable(feature = "cell_ord", since = "1.10.0")]
impl<T: Ord + Copy> Ord for Cell<T> {
#[inline]
fn cmp(&self, other: &Cell<T>) -> Ordering {
self.get().cmp(&other.get())
}
}
#[stable(feature = "cell_from", since = "1.12.0")]
impl<T> From<T> for Cell<T> {
/// Creates a new `Cell<T>` containing the given value.
fn from(t: T) -> Cell<T> {
Cell::new(t)
}
}
impl<T> Cell<T> {
/// Creates a new `Cell` containing the given value.
///
/// # Examples
///
/// ```
/// use std::cell::Cell;
///
/// let c = Cell::new(5);
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_const_stable(feature = "const_cell_new", since = "1.24.0")]
#[inline]
pub const fn new(value: T) -> Cell<T> {
Cell { value: UnsafeCell::new(value) }
}
/// Sets the contained value.
///
/// # Examples
///
/// ```
/// use std::cell::Cell;
///
/// let c = Cell::new(5);
///
/// c.set(10);
/// ```
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
pub fn set(&self, val: T) {
self.replace(val);
}
/// Swaps the values of two `Cell`s.
///
/// The difference with `std::mem::swap` is that this function doesn't
/// require a `&mut` reference.
///
/// # Panics
///
/// This function will panic if `self` and `other` are different `Cell`s that partially overlap.
/// (Using just standard library methods, it is impossible to create such partially overlapping `Cell`s.
/// However, unsafe code is allowed to e.g. create two `&Cell<[i32; 2]>` that partially overlap.)
///
/// # Examples
///
/// ```
/// use std::cell::Cell;
///
/// let c1 = Cell::new(5i32);
/// let c2 = Cell::new(10i32);
/// c1.swap(&c2);
/// assert_eq!(10, c1.get());
/// assert_eq!(5, c2.get());
/// ```
#[inline]
#[stable(feature = "move_cell", since = "1.17.0")]
pub fn swap(&self, other: &Self) {
// This function documents that it *will* panic, and intrinsics::is_nonoverlapping doesn't
// do the check in const, so trying to use it here would be inviting unnecessary fragility.
fn is_nonoverlapping<T>(src: *const T, dst: *const T) -> bool {
let src_usize = src.addr();
let dst_usize = dst.addr();
let diff = src_usize.abs_diff(dst_usize);
diff >= size_of::<T>()
}
if ptr::eq(self, other) {
// Swapping wouldn't change anything.
return;
}
if !is_nonoverlapping(self, other) {
// See <https://github.com/rust-lang/rust/issues/80778> for why we need to stop here.
panic!("`Cell::swap` on overlapping non-identical `Cell`s");
}
// SAFETY: This can be risky if called from separate threads, but `Cell`
// is `!Sync` so this won't happen. This also won't invalidate any
// pointers since `Cell` makes sure nothing else will be pointing into
// either of these `Cell`s. We also excluded shenanigans like partially overlapping `Cell`s,
// so `swap` will just properly copy two full values of type `T` back and forth.
unsafe {
mem::swap(&mut *self.value.get(), &mut *other.value.get());
}
}
/// Replaces the contained value with `val`, and returns the old contained value.
///
/// # Examples
///
/// ```
/// use std::cell::Cell;
///
/// let cell = Cell::new(5);
/// assert_eq!(cell.get(), 5);
/// assert_eq!(cell.replace(10), 5);
/// assert_eq!(cell.get(), 10);
/// ```
#[inline]
#[stable(feature = "move_cell", since = "1.17.0")]
#[rustc_const_unstable(feature = "const_cell", issue = "131283")]
#[rustc_confusables("swap")]
pub const fn replace(&self, val: T) -> T {
// SAFETY: This can cause data races if called from a separate thread,
// but `Cell` is `!Sync` so this won't happen.
mem::replace(unsafe { &mut *self.value.get() }, val)
}
/// Unwraps the value, consuming the cell.
///
/// # Examples
///
/// ```
/// use std::cell::Cell;
///
/// let c = Cell::new(5);
/// let five = c.into_inner();
///
/// assert_eq!(five, 5);
/// ```
#[stable(feature = "move_cell", since = "1.17.0")]
#[rustc_const_stable(feature = "const_cell_into_inner", since = "CURRENT_RUSTC_VERSION")]
#[rustc_allow_const_fn_unstable(const_precise_live_drops)]
pub const fn into_inner(self) -> T {
self.value.into_inner()
}
}
impl<T: Copy> Cell<T> {
/// Returns a copy of the contained value.
///
/// # Examples
///
/// ```
/// use std::cell::Cell;
///
/// let c = Cell::new(5);
///
/// let five = c.get();
/// ```
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_const_unstable(feature = "const_cell", issue = "131283")]
pub const fn get(&self) -> T {
// SAFETY: This can cause data races if called from a separate thread,
// but `Cell` is `!Sync` so this won't happen.
unsafe { *self.value.get() }
}
/// Updates the contained value using a function and returns the new value.
///
/// # Examples
///
/// ```
/// #![feature(cell_update)]
///
/// use std::cell::Cell;
///
/// let c = Cell::new(5);
/// let new = c.update(|x| x + 1);
///
/// assert_eq!(new, 6);
/// assert_eq!(c.get(), 6);
/// ```
#[inline]
#[unstable(feature = "cell_update", issue = "50186")]
pub fn update<F>(&self, f: F) -> T
where
F: FnOnce(T) -> T,
{
let old = self.get();
let new = f(old);
self.set(new);
new
}
}
impl<T: ?Sized> Cell<T> {
/// Returns a raw pointer to the underlying data in this cell.
///
/// # Examples
///
/// ```
/// use std::cell::Cell;
///
/// let c = Cell::new(5);
///
/// let ptr = c.as_ptr();
/// ```
#[inline]
#[stable(feature = "cell_as_ptr", since = "1.12.0")]
#[rustc_const_stable(feature = "const_cell_as_ptr", since = "1.32.0")]
#[rustc_never_returns_null_ptr]
pub const fn as_ptr(&self) -> *mut T {
self.value.get()
}
/// Returns a mutable reference to the underlying data.
///
/// This call borrows `Cell` mutably (at compile-time) which guarantees
/// that we possess the only reference.
///
/// However be cautious: this method expects `self` to be mutable, which is
/// generally not the case when using a `Cell`. If you require interior
/// mutability by reference, consider using `RefCell` which provides
/// run-time checked mutable borrows through its [`borrow_mut`] method.
///
/// [`borrow_mut`]: RefCell::borrow_mut()
///
/// # Examples
///
/// ```
/// use std::cell::Cell;
///
/// let mut c = Cell::new(5);
/// *c.get_mut() += 1;
///
/// assert_eq!(c.get(), 6);
/// ```
#[inline]
#[stable(feature = "cell_get_mut", since = "1.11.0")]
#[rustc_const_unstable(feature = "const_cell", issue = "131283")]
pub const fn get_mut(&mut self) -> &mut T {
self.value.get_mut()
}
/// Returns a `&Cell<T>` from a `&mut T`
///
/// # Examples
///
/// ```
/// use std::cell::Cell;
///
/// let slice: &mut [i32] = &mut [1, 2, 3];
/// let cell_slice: &Cell<[i32]> = Cell::from_mut(slice);
/// let slice_cell: &[Cell<i32>] = cell_slice.as_slice_of_cells();
///
/// assert_eq!(slice_cell.len(), 3);
/// ```
#[inline]
#[stable(feature = "as_cell", since = "1.37.0")]
#[rustc_const_unstable(feature = "const_cell", issue = "131283")]
pub const fn from_mut(t: &mut T) -> &Cell<T> {
// SAFETY: `&mut` ensures unique access.
unsafe { &*(t as *mut T as *const Cell<T>) }
}
}
impl<T: Default> Cell<T> {
/// Takes the value of the cell, leaving `Default::default()` in its place.
///
/// # Examples
///
/// ```
/// use std::cell::Cell;
///
/// let c = Cell::new(5);
/// let five = c.take();
///
/// assert_eq!(five, 5);
/// assert_eq!(c.into_inner(), 0);
/// ```
#[stable(feature = "move_cell", since = "1.17.0")]
pub fn take(&self) -> T {
self.replace(Default::default())
}
}
#[unstable(feature = "coerce_unsized", issue = "18598")]
impl<T: CoerceUnsized<U>, U> CoerceUnsized<Cell<U>> for Cell<T> {}
// Allow types that wrap `Cell` to also implement `DispatchFromDyn`
// and become dyn-compatible method receivers.
// Note that currently `Cell` itself cannot be a method receiver
// because it does not implement Deref.
// In other words:
// `self: Cell<&Self>` won't work
// `self: CellWrapper<Self>` becomes possible
#[unstable(feature = "dispatch_from_dyn", issue = "none")]
impl<T: DispatchFromDyn<U>, U> DispatchFromDyn<Cell<U>> for Cell<T> {}
impl<T> Cell<[T]> {
/// Returns a `&[Cell<T>]` from a `&Cell<[T]>`
///
/// # Examples
///
/// ```
/// use std::cell::Cell;
///
/// let slice: &mut [i32] = &mut [1, 2, 3];
/// let cell_slice: &Cell<[i32]> = Cell::from_mut(slice);
/// let slice_cell: &[Cell<i32>] = cell_slice.as_slice_of_cells();
///
/// assert_eq!(slice_cell.len(), 3);
/// ```
#[stable(feature = "as_cell", since = "1.37.0")]
#[rustc_const_unstable(feature = "const_cell", issue = "131283")]
pub const fn as_slice_of_cells(&self) -> &[Cell<T>] {
// SAFETY: `Cell<T>` has the same memory layout as `T`.
unsafe { &*(self as *const Cell<[T]> as *const [Cell<T>]) }
}
}
impl<T, const N: usize> Cell<[T; N]> {
/// Returns a `&[Cell<T>; N]` from a `&Cell<[T; N]>`
///
/// # Examples
///
/// ```
/// #![feature(as_array_of_cells)]
/// use std::cell::Cell;
///
/// let mut array: [i32; 3] = [1, 2, 3];
/// let cell_array: &Cell<[i32; 3]> = Cell::from_mut(&mut array);
/// let array_cell: &[Cell<i32>; 3] = cell_array.as_array_of_cells();
/// ```
#[unstable(feature = "as_array_of_cells", issue = "88248")]
#[rustc_const_unstable(feature = "as_array_of_cells", issue = "88248")]
pub const fn as_array_of_cells(&self) -> &[Cell<T>; N] {
// SAFETY: `Cell<T>` has the same memory layout as `T`.
unsafe { &*(self as *const Cell<[T; N]> as *const [Cell<T>; N]) }
}
}
/// A mutable memory location with dynamically checked borrow rules
///
/// See the [module-level documentation](self) for more.
#[cfg_attr(not(test), rustc_diagnostic_item = "RefCell")]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct RefCell<T: ?Sized> {
borrow: Cell<BorrowFlag>,
// Stores the location of the earliest currently active borrow.
// This gets updated whenever we go from having zero borrows
// to having a single borrow. When a borrow occurs, this gets included
// in the generated `BorrowError`/`BorrowMutError`
#[cfg(feature = "debug_refcell")]
borrowed_at: Cell<Option<&'static crate::panic::Location<'static>>>,
value: UnsafeCell<T>,
}
/// An error returned by [`RefCell::try_borrow`].
#[stable(feature = "try_borrow", since = "1.13.0")]
#[non_exhaustive]
pub struct BorrowError {
#[cfg(feature = "debug_refcell")]
location: &'static crate::panic::Location<'static>,
}
#[stable(feature = "try_borrow", since = "1.13.0")]
impl Debug for BorrowError {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
let mut builder = f.debug_struct("BorrowError");
#[cfg(feature = "debug_refcell")]
builder.field("location", self.location);
builder.finish()
}
}
#[stable(feature = "try_borrow", since = "1.13.0")]
impl Display for BorrowError {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
Display::fmt("already mutably borrowed", f)
}
}
/// An error returned by [`RefCell::try_borrow_mut`].
#[stable(feature = "try_borrow", since = "1.13.0")]
#[non_exhaustive]
pub struct BorrowMutError {
#[cfg(feature = "debug_refcell")]
location: &'static crate::panic::Location<'static>,
}
#[stable(feature = "try_borrow", since = "1.13.0")]
impl Debug for BorrowMutError {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
let mut builder = f.debug_struct("BorrowMutError");
#[cfg(feature = "debug_refcell")]
builder.field("location", self.location);
builder.finish()
}
}
#[stable(feature = "try_borrow", since = "1.13.0")]
impl Display for BorrowMutError {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
Display::fmt("already borrowed", f)
}
}
// This ensures the panicking code is outlined from `borrow_mut` for `RefCell`.
#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
#[track_caller]
#[cold]
fn panic_already_borrowed(err: BorrowMutError) -> ! {
panic!("already borrowed: {:?}", err)
}
// This ensures the panicking code is outlined from `borrow` for `RefCell`.
#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
#[track_caller]
#[cold]
fn panic_already_mutably_borrowed(err: BorrowError) -> ! {
panic!("already mutably borrowed: {:?}", err)
}
// Positive values represent the number of `Ref` active. Negative values
// represent the number of `RefMut` active. Multiple `RefMut`s can only be
// active at a time if they refer to distinct, nonoverlapping components of a
// `RefCell` (e.g., different ranges of a slice).
//
// `Ref` and `RefMut` are both two words in size, and so there will likely never
// be enough `Ref`s or `RefMut`s in existence to overflow half of the `usize`
// range. Thus, a `BorrowFlag` will probably never overflow or underflow.
// However, this is not a guarantee, as a pathological program could repeatedly
// create and then mem::forget `Ref`s or `RefMut`s. Thus, all code must
// explicitly check for overflow and underflow in order to avoid unsafety, or at
// least behave correctly in the event that overflow or underflow happens (e.g.,
// see BorrowRef::new).
type BorrowFlag = isize;
const UNUSED: BorrowFlag = 0;
#[inline(always)]
fn is_writing(x: BorrowFlag) -> bool {
x < UNUSED
}
#[inline(always)]
fn is_reading(x: BorrowFlag) -> bool {
x > UNUSED
}
impl<T> RefCell<T> {
/// Creates a new `RefCell` containing `value`.
///
/// # Examples
///
/// ```
/// use std::cell::RefCell;
///
/// let c = RefCell::new(5);
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_const_stable(feature = "const_refcell_new", since = "1.24.0")]
#[inline]
pub const fn new(value: T) -> RefCell<T> {
RefCell {
value: UnsafeCell::new(value),
borrow: Cell::new(UNUSED),
#[cfg(feature = "debug_refcell")]
borrowed_at: Cell::new(None),
}
}
/// Consumes the `RefCell`, returning the wrapped value.
///
/// # Examples
///
/// ```
/// use std::cell::RefCell;
///
/// let c = RefCell::new(5);
///
/// let five = c.into_inner();
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_const_stable(feature = "const_cell_into_inner", since = "CURRENT_RUSTC_VERSION")]
#[rustc_allow_const_fn_unstable(const_precise_live_drops)]
#[inline]
pub const fn into_inner(self) -> T {
// Since this function takes `self` (the `RefCell`) by value, the
// compiler statically verifies that it is not currently borrowed.
self.value.into_inner()
}
/// Replaces the wrapped value with a new one, returning the old value,
/// without deinitializing either one.
///
/// This function corresponds to [`std::mem::replace`](../mem/fn.replace.html).
///
/// # Panics
///
/// Panics if the value is currently borrowed.
///
/// # Examples
///
/// ```
/// use std::cell::RefCell;
/// let cell = RefCell::new(5);
/// let old_value = cell.replace(6);
/// assert_eq!(old_value, 5);
/// assert_eq!(cell, RefCell::new(6));
/// ```
#[inline]
#[stable(feature = "refcell_replace", since = "1.24.0")]
#[track_caller]
#[rustc_confusables("swap")]
pub fn replace(&self, t: T) -> T {
mem::replace(&mut *self.borrow_mut(), t)
}
/// Replaces the wrapped value with a new one computed from `f`, returning
/// the old value, without deinitializing either one.
///
/// # Panics
///
/// Panics if the value is currently borrowed.
///
/// # Examples
///
/// ```
/// use std::cell::RefCell;
/// let cell = RefCell::new(5);
/// let old_value = cell.replace_with(|&mut old| old + 1);
/// assert_eq!(old_value, 5);
/// assert_eq!(cell, RefCell::new(6));
/// ```
#[inline]
#[stable(feature = "refcell_replace_swap", since = "1.35.0")]
#[track_caller]
pub fn replace_with<F: FnOnce(&mut T) -> T>(&self, f: F) -> T {
let mut_borrow = &mut *self.borrow_mut();
let replacement = f(mut_borrow);
mem::replace(mut_borrow, replacement)
}
/// Swaps the wrapped value of `self` with the wrapped value of `other`,
/// without deinitializing either one.
///
/// This function corresponds to [`std::mem::swap`](../mem/fn.swap.html).
///
/// # Panics
///
/// Panics if the value in either `RefCell` is currently borrowed, or
/// if `self` and `other` point to the same `RefCell`.
///
/// # Examples
///
/// ```
/// use std::cell::RefCell;
/// let c = RefCell::new(5);
/// let d = RefCell::new(6);
/// c.swap(&d);
/// assert_eq!(c, RefCell::new(6));
/// assert_eq!(d, RefCell::new(5));
/// ```
#[inline]
#[stable(feature = "refcell_swap", since = "1.24.0")]
pub fn swap(&self, other: &Self) {
mem::swap(&mut *self.borrow_mut(), &mut *other.borrow_mut())
}
}
impl<T: ?Sized> RefCell<T> {
/// Immutably borrows the wrapped value.
///
/// The borrow lasts until the returned `Ref` exits scope. Multiple
/// immutable borrows can be taken out at the same time.
///
/// # Panics
///
/// Panics if the value is currently mutably borrowed. For a non-panicking variant, use
/// [`try_borrow`](#method.try_borrow).
///
/// # Examples
///
/// ```
/// use std::cell::RefCell;
///
/// let c = RefCell::new(5);
///
/// let borrowed_five = c.borrow();
/// let borrowed_five2 = c.borrow();
/// ```
///
/// An example of panic:
///
/// ```should_panic
/// use std::cell::RefCell;
///
/// let c = RefCell::new(5);
///
/// let m = c.borrow_mut();
/// let b = c.borrow(); // this causes a panic
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
#[inline]
#[track_caller]
pub fn borrow(&self) -> Ref<'_, T> {
match self.try_borrow() {
Ok(b) => b,
Err(err) => panic_already_mutably_borrowed(err),
}
}
/// Immutably borrows the wrapped value, returning an error if the value is currently mutably
/// borrowed.
///
/// The borrow lasts until the returned `Ref` exits scope. Multiple immutable borrows can be
/// taken out at the same time.
///
/// This is the non-panicking variant of [`borrow`](#method.borrow).
///
/// # Examples
///
/// ```
/// use std::cell::RefCell;
///
/// let c = RefCell::new(5);
///
/// {
/// let m = c.borrow_mut();
/// assert!(c.try_borrow().is_err());
/// }
///
/// {
/// let m = c.borrow();
/// assert!(c.try_borrow().is_ok());
/// }
/// ```
#[stable(feature = "try_borrow", since = "1.13.0")]
#[inline]
#[cfg_attr(feature = "debug_refcell", track_caller)]
pub fn try_borrow(&self) -> Result<Ref<'_, T>, BorrowError> {
match BorrowRef::new(&self.borrow) {
Some(b) => {
#[cfg(feature = "debug_refcell")]
{
// `borrowed_at` is always the *first* active borrow
if b.borrow.get() == 1 {
self.borrowed_at.set(Some(crate::panic::Location::caller()));
}
}
// SAFETY: `BorrowRef` ensures that there is only immutable access
// to the value while borrowed.
let value = unsafe { NonNull::new_unchecked(self.value.get()) };
Ok(Ref { value, borrow: b })
}
None => Err(BorrowError {
// If a borrow occurred, then we must already have an outstanding borrow,
// so `borrowed_at` will be `Some`
#[cfg(feature = "debug_refcell")]
location: self.borrowed_at.get().unwrap(),
}),
}
}
/// Mutably borrows the wrapped value.
///
/// The borrow lasts until the returned `RefMut` or all `RefMut`s derived
/// from it exit scope. The value cannot be borrowed while this borrow is
/// active.
///
/// # Panics
///
/// Panics if the value is currently borrowed. For a non-panicking variant, use
/// [`try_borrow_mut`](#method.try_borrow_mut).
///
/// # Examples
///
/// ```
/// use std::cell::RefCell;
///
/// let c = RefCell::new("hello".to_owned());
///
/// *c.borrow_mut() = "bonjour".to_owned();
///
/// assert_eq!(&*c.borrow(), "bonjour");
/// ```
///
/// An example of panic:
///
/// ```should_panic
/// use std::cell::RefCell;
///
/// let c = RefCell::new(5);
/// let m = c.borrow();
///
/// let b = c.borrow_mut(); // this causes a panic
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
#[inline]
#[track_caller]
pub fn borrow_mut(&self) -> RefMut<'_, T> {
match self.try_borrow_mut() {
Ok(b) => b,
Err(err) => panic_already_borrowed(err),
}
}
/// Mutably borrows the wrapped value, returning an error if the value is currently borrowed.
///
/// The borrow lasts until the returned `RefMut` or all `RefMut`s derived
/// from it exit scope. The value cannot be borrowed while this borrow is
/// active.
///
/// This is the non-panicking variant of [`borrow_mut`](#method.borrow_mut).
///
/// # Examples
///
/// ```
/// use std::cell::RefCell;
///
/// let c = RefCell::new(5);
///
/// {
/// let m = c.borrow();
/// assert!(c.try_borrow_mut().is_err());
/// }
///
/// assert!(c.try_borrow_mut().is_ok());
/// ```
#[stable(feature = "try_borrow", since = "1.13.0")]
#[inline]
#[cfg_attr(feature = "debug_refcell", track_caller)]
pub fn try_borrow_mut(&self) -> Result<RefMut<'_, T>, BorrowMutError> {
match BorrowRefMut::new(&self.borrow) {
Some(b) => {
#[cfg(feature = "debug_refcell")]
{
self.borrowed_at.set(Some(crate::panic::Location::caller()));
}
// SAFETY: `BorrowRefMut` guarantees unique access.
let value = unsafe { NonNull::new_unchecked(self.value.get()) };
Ok(RefMut { value, borrow: b, marker: PhantomData })
}
None => Err(BorrowMutError {
// If a borrow occurred, then we must already have an outstanding borrow,
// so `borrowed_at` will be `Some`
#[cfg(feature = "debug_refcell")]
location: self.borrowed_at.get().unwrap(),
}),
}
}
/// Returns a raw pointer to the underlying data in this cell.
///
/// # Examples
///
/// ```
/// use std::cell::RefCell;
///
/// let c = RefCell::new(5);
///
/// let ptr = c.as_ptr();
/// ```
#[inline]
#[stable(feature = "cell_as_ptr", since = "1.12.0")]
#[rustc_never_returns_null_ptr]
pub fn as_ptr(&self) -> *mut T {
self.value.get()
}
/// Returns a mutable reference to the underlying data.
///
/// Since this method borrows `RefCell` mutably, it is statically guaranteed
/// that no borrows to the underlying data exist. The dynamic checks inherent
/// in [`borrow_mut`] and most other methods of `RefCell` are therefore
/// unnecessary.
///
/// This method can only be called if `RefCell` can be mutably borrowed,
/// which in general is only the case directly after the `RefCell` has
/// been created. In these situations, skipping the aforementioned dynamic
/// borrowing checks may yield better ergonomics and runtime-performance.
///
/// In most situations where `RefCell` is used, it can't be borrowed mutably.
/// Use [`borrow_mut`] to get mutable access to the underlying data then.
///
/// [`borrow_mut`]: RefCell::borrow_mut()
///
/// # Examples
///
/// ```
/// use std::cell::RefCell;
///
/// let mut c = RefCell::new(5);
/// *c.get_mut() += 1;
///
/// assert_eq!(c, RefCell::new(6));
/// ```
#[inline]
#[stable(feature = "cell_get_mut", since = "1.11.0")]
pub fn get_mut(&mut self) -> &mut T {
self.value.get_mut()
}
/// Undo the effect of leaked guards on the borrow state of the `RefCell`.
///
/// This call is similar to [`get_mut`] but more specialized. It borrows `RefCell` mutably to
/// ensure no borrows exist and then resets the state tracking shared borrows. This is relevant
/// if some `Ref` or `RefMut` borrows have been leaked.
///
/// [`get_mut`]: RefCell::get_mut()
///
/// # Examples
///
/// ```
/// #![feature(cell_leak)]
/// use std::cell::RefCell;
///
/// let mut c = RefCell::new(0);
/// std::mem::forget(c.borrow_mut());
///
/// assert!(c.try_borrow().is_err());
/// c.undo_leak();
/// assert!(c.try_borrow().is_ok());
/// ```
#[unstable(feature = "cell_leak", issue = "69099")]
pub fn undo_leak(&mut self) -> &mut T {
*self.borrow.get_mut() = UNUSED;
self.get_mut()
}
/// Immutably borrows the wrapped value, returning an error if the value is
/// currently mutably borrowed.
///
/// # Safety
///
/// Unlike `RefCell::borrow`, this method is unsafe because it does not
/// return a `Ref`, thus leaving the borrow flag untouched. Mutably
/// borrowing the `RefCell` while the reference returned by this method
/// is alive is undefined behaviour.
///
/// # Examples
///
/// ```
/// use std::cell::RefCell;
///
/// let c = RefCell::new(5);
///
/// {
/// let m = c.borrow_mut();
/// assert!(unsafe { c.try_borrow_unguarded() }.is_err());
/// }
///
/// {
/// let m = c.borrow();
/// assert!(unsafe { c.try_borrow_unguarded() }.is_ok());
/// }
/// ```
#[stable(feature = "borrow_state", since = "1.37.0")]
#[inline]
pub unsafe fn try_borrow_unguarded(&self) -> Result<&T, BorrowError> {
if !is_writing(self.borrow.get()) {
// SAFETY: We check that nobody is actively writing now, but it is
// the caller's responsibility to ensure that nobody writes until
// the returned reference is no longer in use.
// Also, `self.value.get()` refers to the value owned by `self`
// and is thus guaranteed to be valid for the lifetime of `self`.
Ok(unsafe { &*self.value.get() })
} else {
Err(BorrowError {
// If a borrow occurred, then we must already have an outstanding borrow,
// so `borrowed_at` will be `Some`
#[cfg(feature = "debug_refcell")]
location: self.borrowed_at.get().unwrap(),
})
}
}
}
impl<T: Default> RefCell<T> {
/// Takes the wrapped value, leaving `Default::default()` in its place.
///
/// # Panics
///
/// Panics if the value is currently borrowed.
///
/// # Examples
///
/// ```
/// use std::cell::RefCell;
///
/// let c = RefCell::new(5);
/// let five = c.take();
///
/// assert_eq!(five, 5);
/// assert_eq!(c.into_inner(), 0);
/// ```
#[stable(feature = "refcell_take", since = "1.50.0")]
pub fn take(&self) -> T {
self.replace(Default::default())
}
}
#[stable(feature = "rust1", since = "1.0.0")]
unsafe impl<T: ?Sized> Send for RefCell<T> where T: Send {}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> !Sync for RefCell<T> {}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: Clone> Clone for RefCell<T> {
/// # Panics
///
/// Panics if the value is currently mutably borrowed.
#[inline]
#[track_caller]
fn clone(&self) -> RefCell<T> {
RefCell::new(self.borrow().clone())
}
/// # Panics
///
/// Panics if `source` is currently mutably borrowed.
#[inline]
#[track_caller]
fn clone_from(&mut self, source: &Self) {
self.get_mut().clone_from(&source.borrow())
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: Default> Default for RefCell<T> {
/// Creates a `RefCell<T>`, with the `Default` value for T.
#[inline]
fn default() -> RefCell<T> {
RefCell::new(Default::default())
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized + PartialEq> PartialEq for RefCell<T> {
/// # Panics
///
/// Panics if the value in either `RefCell` is currently mutably borrowed.
#[inline]
fn eq(&self, other: &RefCell<T>) -> bool {
*self.borrow() == *other.borrow()
}
}
#[stable(feature = "cell_eq", since = "1.2.0")]
impl<T: ?Sized + Eq> Eq for RefCell<T> {}
#[stable(feature = "cell_ord", since = "1.10.0")]
impl<T: ?Sized + PartialOrd> PartialOrd for RefCell<T> {
/// # Panics
///
/// Panics if the value in either `RefCell` is currently mutably borrowed.
#[inline]
fn partial_cmp(&self, other: &RefCell<T>) -> Option<Ordering> {
self.borrow().partial_cmp(&*other.borrow())
}
/// # Panics
///
/// Panics if the value in either `RefCell` is currently mutably borrowed.
#[inline]
fn lt(&self, other: &RefCell<T>) -> bool {
*self.borrow() < *other.borrow()
}
/// # Panics
///
/// Panics if the value in either `RefCell` is currently mutably borrowed.
#[inline]
fn le(&self, other: &RefCell<T>) -> bool {
*self.borrow() <= *other.borrow()
}
/// # Panics
///
/// Panics if the value in either `RefCell` is currently mutably borrowed.
#[inline]
fn gt(&self, other: &RefCell<T>) -> bool {
*self.borrow() > *other.borrow()
}
/// # Panics
///
/// Panics if the value in either `RefCell` is currently mutably borrowed.
#[inline]
fn ge(&self, other: &RefCell<T>) -> bool {
*self.borrow() >= *other.borrow()
}
}
#[stable(feature = "cell_ord", since = "1.10.0")]
impl<T: ?Sized + Ord> Ord for RefCell<T> {
/// # Panics
///
/// Panics if the value in either `RefCell` is currently mutably borrowed.
#[inline]
fn cmp(&self, other: &RefCell<T>) -> Ordering {
self.borrow().cmp(&*other.borrow())
}
}
#[stable(feature = "cell_from", since = "1.12.0")]
impl<T> From<T> for RefCell<T> {
/// Creates a new `RefCell<T>` containing the given value.
fn from(t: T) -> RefCell<T> {
RefCell::new(t)
}
}
#[unstable(feature = "coerce_unsized", issue = "18598")]
impl<T: CoerceUnsized<U>, U> CoerceUnsized<RefCell<U>> for RefCell<T> {}
struct BorrowRef<'b> {
borrow: &'b Cell<BorrowFlag>,
}
impl<'b> BorrowRef<'b> {
#[inline]
fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRef<'b>> {
let b = borrow.get().wrapping_add(1);
if !is_reading(b) {
// Incrementing borrow can result in a non-reading value (<= 0) in these cases:
// 1. It was < 0, i.e. there are writing borrows, so we can't allow a read borrow
// due to Rust's reference aliasing rules
// 2. It was isize::MAX (the max amount of reading borrows) and it overflowed
// into isize::MIN (the max amount of writing borrows) so we can't allow
// an additional read borrow because isize can't represent so many read borrows
// (this can only happen if you mem::forget more than a small constant amount of
// `Ref`s, which is not good practice)
None
} else {
// Incrementing borrow can result in a reading value (> 0) in these cases:
// 1. It was = 0, i.e. it wasn't borrowed, and we are taking the first read borrow
// 2. It was > 0 and < isize::MAX, i.e. there were read borrows, and isize
// is large enough to represent having one more read borrow
borrow.set(b);
Some(BorrowRef { borrow })
}
}
}
impl Drop for BorrowRef<'_> {
#[inline]
fn drop(&mut self) {
let borrow = self.borrow.get();
debug_assert!(is_reading(borrow));
self.borrow.set(borrow - 1);
}
}
impl Clone for BorrowRef<'_> {
#[inline]
fn clone(&self) -> Self {
// Since this Ref exists, we know the borrow flag
// is a reading borrow.
let borrow = self.borrow.get();
debug_assert!(is_reading(borrow));
// Prevent the borrow counter from overflowing into
// a writing borrow.
assert!(borrow != BorrowFlag::MAX);
self.borrow.set(borrow + 1);
BorrowRef { borrow: self.borrow }
}
}
/// Wraps a borrowed reference to a value in a `RefCell` box.
/// A wrapper type for an immutably borrowed value from a `RefCell<T>`.
///
/// See the [module-level documentation](self) for more.
#[stable(feature = "rust1", since = "1.0.0")]
#[must_not_suspend = "holding a Ref across suspend points can cause BorrowErrors"]
#[rustc_diagnostic_item = "RefCellRef"]
pub struct Ref<'b, T: ?Sized + 'b> {
// NB: we use a pointer instead of `&'b T` to avoid `noalias` violations, because a
// `Ref` argument doesn't hold immutability for its whole scope, only until it drops.
// `NonNull` is also covariant over `T`, just like we would have with `&T`.
value: NonNull<T>,
borrow: BorrowRef<'b>,
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> Deref for Ref<'_, T> {
type Target = T;
#[inline]
fn deref(&self) -> &T {
// SAFETY: the value is accessible as long as we hold our borrow.
unsafe { self.value.as_ref() }
}
}
#[unstable(feature = "deref_pure_trait", issue = "87121")]
unsafe impl<T: ?Sized> DerefPure for Ref<'_, T> {}
impl<'b, T: ?Sized> Ref<'b, T> {
/// Copies a `Ref`.
///
/// The `RefCell` is already immutably borrowed, so this cannot fail.
///
/// This is an associated function that needs to be used as
/// `Ref::clone(...)`. A `Clone` implementation or a method would interfere
/// with the widespread use of `r.borrow().clone()` to clone the contents of
/// a `RefCell`.
#[stable(feature = "cell_extras", since = "1.15.0")]
#[must_use]
#[inline]
pub fn clone(orig: &Ref<'b, T>) -> Ref<'b, T> {
Ref { value: orig.value, borrow: orig.borrow.clone() }
}
/// Makes a new `Ref` for a component of the borrowed data.
///
/// The `RefCell` is already immutably borrowed, so this cannot fail.
///
/// This is an associated function that needs to be used as `Ref::map(...)`.
/// A method would interfere with methods of the same name on the contents
/// of a `RefCell` used through `Deref`.
///
/// # Examples
///
/// ```
/// use std::cell::{RefCell, Ref};
///
/// let c = RefCell::new((5, 'b'));
/// let b1: Ref<'_, (u32, char)> = c.borrow();
/// let b2: Ref<'_, u32> = Ref::map(b1, |t| &t.0);
/// assert_eq!(*b2, 5)
/// ```
#[stable(feature = "cell_map", since = "1.8.0")]
#[inline]
pub fn map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Ref<'b, U>
where
F: FnOnce(&T) -> &U,
{
Ref { value: NonNull::from(f(&*orig)), borrow: orig.borrow }
}
/// Makes a new `Ref` for an optional component of the borrowed data. The
/// original guard is returned as an `Err(..)` if the closure returns
/// `None`.
///
/// The `RefCell` is already immutably borrowed, so this cannot fail.
///
/// This is an associated function that needs to be used as
/// `Ref::filter_map(...)`. A method would interfere with methods of the same
/// name on the contents of a `RefCell` used through `Deref`.
///
/// # Examples
///
/// ```
/// use std::cell::{RefCell, Ref};
///
/// let c = RefCell::new(vec![1, 2, 3]);
/// let b1: Ref<'_, Vec<u32>> = c.borrow();
/// let b2: Result<Ref<'_, u32>, _> = Ref::filter_map(b1, |v| v.get(1));
/// assert_eq!(*b2.unwrap(), 2);
/// ```
#[stable(feature = "cell_filter_map", since = "1.63.0")]
#[inline]
pub fn filter_map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Result<Ref<'b, U>, Self>
where
F: FnOnce(&T) -> Option<&U>,
{
match f(&*orig) {
Some(value) => Ok(Ref { value: NonNull::from(value), borrow: orig.borrow }),
None => Err(orig),
}
}
/// Splits a `Ref` into multiple `Ref`s for different components of the
/// borrowed data.
///
/// The `RefCell` is already immutably borrowed, so this cannot fail.
///
/// This is an associated function that needs to be used as
/// `Ref::map_split(...)`. A method would interfere with methods of the same
/// name on the contents of a `RefCell` used through `Deref`.
///
/// # Examples
///
/// ```
/// use std::cell::{Ref, RefCell};
///
/// let cell = RefCell::new([1, 2, 3, 4]);
/// let borrow = cell.borrow();
/// let (begin, end) = Ref::map_split(borrow, |slice| slice.split_at(2));
/// assert_eq!(*begin, [1, 2]);
/// assert_eq!(*end, [3, 4]);
/// ```
#[stable(feature = "refcell_map_split", since = "1.35.0")]
#[inline]
pub fn map_split<U: ?Sized, V: ?Sized, F>(orig: Ref<'b, T>, f: F) -> (Ref<'b, U>, Ref<'b, V>)
where
F: FnOnce(&T) -> (&U, &V),
{
let (a, b) = f(&*orig);
let borrow = orig.borrow.clone();
(Ref { value: NonNull::from(a), borrow }, Ref {
value: NonNull::from(b),
borrow: orig.borrow,
})
}
/// Converts into a reference to the underlying data.
///
/// The underlying `RefCell` can never be mutably borrowed from again and will always appear
/// already immutably borrowed. It is not a good idea to leak more than a constant number of
/// references. The `RefCell` can be immutably borrowed again if only a smaller number of leaks
/// have occurred in total.
///
/// This is an associated function that needs to be used as
/// `Ref::leak(...)`. A method would interfere with methods of the
/// same name on the contents of a `RefCell` used through `Deref`.
///
/// # Examples
///
/// ```
/// #![feature(cell_leak)]
/// use std::cell::{RefCell, Ref};
/// let cell = RefCell::new(0);
///
/// let value = Ref::leak(cell.borrow());
/// assert_eq!(*value, 0);
///
/// assert!(cell.try_borrow().is_ok());
/// assert!(cell.try_borrow_mut().is_err());
/// ```
#[unstable(feature = "cell_leak", issue = "69099")]
pub fn leak(orig: Ref<'b, T>) -> &'b T {
// By forgetting this Ref we ensure that the borrow counter in the RefCell can't go back to
// UNUSED within the lifetime `'b`. Resetting the reference tracking state would require a
// unique reference to the borrowed RefCell. No further mutable references can be created
// from the original cell.
mem::forget(orig.borrow);
// SAFETY: after forgetting, we can form a reference for the rest of lifetime `'b`.
unsafe { orig.value.as_ref() }
}
}
#[unstable(feature = "coerce_unsized", issue = "18598")]
impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Ref<'b, U>> for Ref<'b, T> {}
#[stable(feature = "std_guard_impls", since = "1.20.0")]
impl<T: ?Sized + fmt::Display> fmt::Display for Ref<'_, T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
(**self).fmt(f)
}
}
impl<'b, T: ?Sized> RefMut<'b, T> {
/// Makes a new `RefMut` for a component of the borrowed data, e.g., an enum
/// variant.
///
/// The `RefCell` is already mutably borrowed, so this cannot fail.
///
/// This is an associated function that needs to be used as
/// `RefMut::map(...)`. A method would interfere with methods of the same
/// name on the contents of a `RefCell` used through `Deref`.
///
/// # Examples
///
/// ```
/// use std::cell::{RefCell, RefMut};
///
/// let c = RefCell::new((5, 'b'));
/// {
/// let b1: RefMut<'_, (u32, char)> = c.borrow_mut();
/// let mut b2: RefMut<'_, u32> = RefMut::map(b1, |t| &mut t.0);
/// assert_eq!(*b2, 5);
/// *b2 = 42;
/// }
/// assert_eq!(*c.borrow(), (42, 'b'));
/// ```
#[stable(feature = "cell_map", since = "1.8.0")]
#[inline]
pub fn map<U: ?Sized, F>(mut orig: RefMut<'b, T>, f: F) -> RefMut<'b, U>
where
F: FnOnce(&mut T) -> &mut U,
{
let value = NonNull::from(f(&mut *orig));
RefMut { value, borrow: orig.borrow, marker: PhantomData }
}
/// Makes a new `RefMut` for an optional component of the borrowed data. The
/// original guard is returned as an `Err(..)` if the closure returns
/// `None`.
///
/// The `RefCell` is already mutably borrowed, so this cannot fail.
///
/// This is an associated function that needs to be used as
/// `RefMut::filter_map(...)`. A method would interfere with methods of the
/// same name on the contents of a `RefCell` used through `Deref`.
///
/// # Examples
///
/// ```
/// use std::cell::{RefCell, RefMut};
///
/// let c = RefCell::new(vec![1, 2, 3]);
///
/// {
/// let b1: RefMut<'_, Vec<u32>> = c.borrow_mut();
/// let mut b2: Result<RefMut<'_, u32>, _> = RefMut::filter_map(b1, |v| v.get_mut(1));
///
/// if let Ok(mut b2) = b2 {
/// *b2 += 2;
/// }
/// }
///
/// assert_eq!(*c.borrow(), vec![1, 4, 3]);
/// ```
#[stable(feature = "cell_filter_map", since = "1.63.0")]
#[inline]
pub fn filter_map<U: ?Sized, F>(mut orig: RefMut<'b, T>, f: F) -> Result<RefMut<'b, U>, Self>
where
F: FnOnce(&mut T) -> Option<&mut U>,
{
// SAFETY: function holds onto an exclusive reference for the duration
// of its call through `orig`, and the pointer is only de-referenced
// inside of the function call never allowing the exclusive reference to
// escape.
match f(&mut *orig) {
Some(value) => {
Ok(RefMut { value: NonNull::from(value), borrow: orig.borrow, marker: PhantomData })
}
None => Err(orig),
}
}
/// Splits a `RefMut` into multiple `RefMut`s for different components of the
/// borrowed data.
///
/// The underlying `RefCell` will remain mutably borrowed until both
/// returned `RefMut`s go out of scope.
///
/// The `RefCell` is already mutably borrowed, so this cannot fail.
///
/// This is an associated function that needs to be used as
/// `RefMut::map_split(...)`. A method would interfere with methods of the
/// same name on the contents of a `RefCell` used through `Deref`.
///
/// # Examples
///
/// ```
/// use std::cell::{RefCell, RefMut};
///
/// let cell = RefCell::new([1, 2, 3, 4]);
/// let borrow = cell.borrow_mut();
/// let (mut begin, mut end) = RefMut::map_split(borrow, |slice| slice.split_at_mut(2));
/// assert_eq!(*begin, [1, 2]);
/// assert_eq!(*end, [3, 4]);
/// begin.copy_from_slice(&[4, 3]);
/// end.copy_from_slice(&[2, 1]);
/// ```
#[stable(feature = "refcell_map_split", since = "1.35.0")]
#[inline]
pub fn map_split<U: ?Sized, V: ?Sized, F>(
mut orig: RefMut<'b, T>,
f: F,
) -> (RefMut<'b, U>, RefMut<'b, V>)
where
F: FnOnce(&mut T) -> (&mut U, &mut V),
{
let borrow = orig.borrow.clone();
let (a, b) = f(&mut *orig);
(RefMut { value: NonNull::from(a), borrow, marker: PhantomData }, RefMut {
value: NonNull::from(b),
borrow: orig.borrow,
marker: PhantomData,
})
}
/// Converts into a mutable reference to the underlying data.
///
/// The underlying `RefCell` can not be borrowed from again and will always appear already
/// mutably borrowed, making the returned reference the only to the interior.
///
/// This is an associated function that needs to be used as
/// `RefMut::leak(...)`. A method would interfere with methods of the
/// same name on the contents of a `RefCell` used through `Deref`.
///
/// # Examples
///
/// ```
/// #![feature(cell_leak)]
/// use std::cell::{RefCell, RefMut};
/// let cell = RefCell::new(0);
///
/// let value = RefMut::leak(cell.borrow_mut());
/// assert_eq!(*value, 0);
/// *value = 1;
///
/// assert!(cell.try_borrow_mut().is_err());
/// ```
#[unstable(feature = "cell_leak", issue = "69099")]
pub fn leak(mut orig: RefMut<'b, T>) -> &'b mut T {
// By forgetting this BorrowRefMut we ensure that the borrow counter in the RefCell can't
// go back to UNUSED within the lifetime `'b`. Resetting the reference tracking state would
// require a unique reference to the borrowed RefCell. No further references can be created
// from the original cell within that lifetime, making the current borrow the only
// reference for the remaining lifetime.
mem::forget(orig.borrow);
// SAFETY: after forgetting, we can form a reference for the rest of lifetime `'b`.
unsafe { orig.value.as_mut() }
}
}
struct BorrowRefMut<'b> {
borrow: &'b Cell<BorrowFlag>,
}
impl Drop for BorrowRefMut<'_> {
#[inline]
fn drop(&mut self) {
let borrow = self.borrow.get();
debug_assert!(is_writing(borrow));
self.borrow.set(borrow + 1);
}
}
impl<'b> BorrowRefMut<'b> {
#[inline]
fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRefMut<'b>> {
// NOTE: Unlike BorrowRefMut::clone, new is called to create the initial
// mutable reference, and so there must currently be no existing
// references. Thus, while clone increments the mutable refcount, here
// we explicitly only allow going from UNUSED to UNUSED - 1.
match borrow.get() {
UNUSED => {
borrow.set(UNUSED - 1);
Some(BorrowRefMut { borrow })
}
_ => None,
}
}
// Clones a `BorrowRefMut`.
//
// This is only valid if each `BorrowRefMut` is used to track a mutable
// reference to a distinct, nonoverlapping range of the original object.
// This isn't in a Clone impl so that code doesn't call this implicitly.
#[inline]
fn clone(&self) -> BorrowRefMut<'b> {
let borrow = self.borrow.get();
debug_assert!(is_writing(borrow));
// Prevent the borrow counter from underflowing.
assert!(borrow != BorrowFlag::MIN);
self.borrow.set(borrow - 1);
BorrowRefMut { borrow: self.borrow }
}
}
/// A wrapper type for a mutably borrowed value from a `RefCell<T>`.
///
/// See the [module-level documentation](self) for more.
#[stable(feature = "rust1", since = "1.0.0")]
#[must_not_suspend = "holding a RefMut across suspend points can cause BorrowErrors"]
#[rustc_diagnostic_item = "RefCellRefMut"]
pub struct RefMut<'b, T: ?Sized + 'b> {
// NB: we use a pointer instead of `&'b mut T` to avoid `noalias` violations, because a
// `RefMut` argument doesn't hold exclusivity for its whole scope, only until it drops.
value: NonNull<T>,
borrow: BorrowRefMut<'b>,
// `NonNull` is covariant over `T`, so we need to reintroduce invariance.
marker: PhantomData<&'b mut T>,
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> Deref for RefMut<'_, T> {
type Target = T;
#[inline]
fn deref(&self) -> &T {
// SAFETY: the value is accessible as long as we hold our borrow.
unsafe { self.value.as_ref() }
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> DerefMut for RefMut<'_, T> {
#[inline]
fn deref_mut(&mut self) -> &mut T {
// SAFETY: the value is accessible as long as we hold our borrow.
unsafe { self.value.as_mut() }
}
}
#[unstable(feature = "deref_pure_trait", issue = "87121")]
unsafe impl<T: ?Sized> DerefPure for RefMut<'_, T> {}
#[unstable(feature = "coerce_unsized", issue = "18598")]
impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<RefMut<'b, U>> for RefMut<'b, T> {}
#[stable(feature = "std_guard_impls", since = "1.20.0")]
impl<T: ?Sized + fmt::Display> fmt::Display for RefMut<'_, T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
(**self).fmt(f)
}
}
/// The core primitive for interior mutability in Rust.
///
/// If you have a reference `&T`, then normally in Rust the compiler performs optimizations based on
/// the knowledge that `&T` points to immutable data. Mutating that data, for example through an
/// alias or by transmuting a `&T` into a `&mut T`, is considered undefined behavior.
/// `UnsafeCell<T>` opts-out of the immutability guarantee for `&T`: a shared reference
/// `&UnsafeCell<T>` may point to data that is being mutated. This is called "interior mutability".
///
/// All other types that allow internal mutability, such as [`Cell<T>`] and [`RefCell<T>`], internally
/// use `UnsafeCell` to wrap their data.
///
/// Note that only the immutability guarantee for shared references is affected by `UnsafeCell`. The
/// uniqueness guarantee for mutable references is unaffected. There is *no* legal way to obtain
/// aliasing `&mut`, not even with `UnsafeCell<T>`.
///
/// `UnsafeCell` does nothing to avoid data races; they are still undefined behavior. If multiple
/// threads have access to the same `UnsafeCell`, they must follow the usual rules of the
/// [concurrent memory model]: conflicting non-synchronized accesses must be done via the APIs in
/// [`core::sync::atomic`].
///
/// The `UnsafeCell` API itself is technically very simple: [`.get()`] gives you a raw pointer
/// `*mut T` to its contents. It is up to _you_ as the abstraction designer to use that raw pointer
/// correctly.
///
/// [`.get()`]: `UnsafeCell::get`
/// [concurrent memory model]: ../sync/atomic/index.html#memory-model-for-atomic-accesses
///
/// The precise Rust aliasing rules are somewhat in flux, but the main points are not contentious:
///
/// - If you create a safe reference with lifetime `'a` (either a `&T` or `&mut T` reference), then
/// you must not access the data in any way that contradicts that reference for the remainder of
/// `'a`. For example, this means that if you take the `*mut T` from an `UnsafeCell<T>` and cast it
/// to an `&T`, then the data in `T` must remain immutable (modulo any `UnsafeCell` data found
/// within `T`, of course) until that reference's lifetime expires. Similarly, if you create a `&mut
/// T` reference that is released to safe code, then you must not access the data within the
/// `UnsafeCell` until that reference expires.
///
/// - For both `&T` without `UnsafeCell<_>` and `&mut T`, you must also not deallocate the data
/// until the reference expires. As a special exception, given an `&T`, any part of it that is
/// inside an `UnsafeCell<_>` may be deallocated during the lifetime of the reference, after the
/// last time the reference is used (dereferenced or reborrowed). Since you cannot deallocate a part
/// of what a reference points to, this means the memory an `&T` points to can be deallocated only if
/// *every part of it* (including padding) is inside an `UnsafeCell`.
///
/// However, whenever a `&UnsafeCell<T>` is constructed or dereferenced, it must still point to
/// live memory and the compiler is allowed to insert spurious reads if it can prove that this
/// memory has not yet been deallocated.
///
/// To assist with proper design, the following scenarios are explicitly declared legal
/// for single-threaded code:
///
/// 1. A `&T` reference can be released to safe code and there it can co-exist with other `&T`
/// references, but not with a `&mut T`
///
/// 2. A `&mut T` reference may be released to safe code provided neither other `&mut T` nor `&T`
/// co-exist with it. A `&mut T` must always be unique.
///
/// Note that whilst mutating the contents of an `&UnsafeCell<T>` (even while other
/// `&UnsafeCell<T>` references alias the cell) is
/// ok (provided you enforce the above invariants some other way), it is still undefined behavior
/// to have multiple `&mut UnsafeCell<T>` aliases. That is, `UnsafeCell` is a wrapper
/// designed to have a special interaction with _shared_ accesses (_i.e._, through an
/// `&UnsafeCell<_>` reference); there is no magic whatsoever when dealing with _exclusive_
/// accesses (_e.g._, through a `&mut UnsafeCell<_>`): neither the cell nor the wrapped value
/// may be aliased for the duration of that `&mut` borrow.
/// This is showcased by the [`.get_mut()`] accessor, which is a _safe_ getter that yields
/// a `&mut T`.
///
/// [`.get_mut()`]: `UnsafeCell::get_mut`
///
/// # Memory layout
///
/// `UnsafeCell<T>` has the same in-memory representation as its inner type `T`. A consequence
/// of this guarantee is that it is possible to convert between `T` and `UnsafeCell<T>`.
/// Special care has to be taken when converting a nested `T` inside of an `Outer<T>` type
/// to an `Outer<UnsafeCell<T>>` type: this is not sound when the `Outer<T>` type enables [niche]
/// optimizations. For example, the type `Option<NonNull<u8>>` is typically 8 bytes large on
/// 64-bit platforms, but the type `Option<UnsafeCell<NonNull<u8>>>` takes up 16 bytes of space.
/// Therefore this is not a valid conversion, despite `NonNull<u8>` and `UnsafeCell<NonNull<u8>>>`
/// having the same memory layout. This is because `UnsafeCell` disables niche optimizations in
/// order to avoid its interior mutability property from spreading from `T` into the `Outer` type,
/// thus this can cause distortions in the type size in these cases.
///
/// Note that the only valid way to obtain a `*mut T` pointer to the contents of a
/// _shared_ `UnsafeCell<T>` is through [`.get()`] or [`.raw_get()`]. A `&mut T` reference
/// can be obtained by either dereferencing this pointer or by calling [`.get_mut()`]
/// on an _exclusive_ `UnsafeCell<T>`. Even though `T` and `UnsafeCell<T>` have the
/// same memory layout, the following is not allowed and undefined behavior:
///
/// ```rust,compile_fail
/// # use std::cell::UnsafeCell;
/// unsafe fn not_allowed<T>(ptr: &UnsafeCell<T>) -> &mut T {
/// let t = ptr as *const UnsafeCell<T> as *mut T;
/// // This is undefined behavior, because the `*mut T` pointer
/// // was not obtained through `.get()` nor `.raw_get()`:
/// unsafe { &mut *t }
/// }
/// ```
///
/// Instead, do this:
///
/// ```rust
/// # use std::cell::UnsafeCell;
/// // Safety: the caller must ensure that there are no references that
/// // point to the *contents* of the `UnsafeCell`.
/// unsafe fn get_mut<T>(ptr: &UnsafeCell<T>) -> &mut T {
/// unsafe { &mut *ptr.get() }
/// }
/// ```
///
/// Converting in the other direction from a `&mut T`
/// to an `&UnsafeCell<T>` is allowed:
///
/// ```rust
/// # use std::cell::UnsafeCell;
/// fn get_shared<T>(ptr: &mut T) -> &UnsafeCell<T> {
/// let t = ptr as *mut T as *const UnsafeCell<T>;
/// // SAFETY: `T` and `UnsafeCell<T>` have the same memory layout
/// unsafe { &*t }
/// }
/// ```
///
/// [niche]: https://rust-lang.github.io/unsafe-code-guidelines/glossary.html#niche
/// [`.raw_get()`]: `UnsafeCell::raw_get`
///
/// # Examples
///
/// Here is an example showcasing how to soundly mutate the contents of an `UnsafeCell<_>` despite
/// there being multiple references aliasing the cell:
///
/// ```
/// use std::cell::UnsafeCell;
///
/// let x: UnsafeCell<i32> = 42.into();
/// // Get multiple / concurrent / shared references to the same `x`.
/// let (p1, p2): (&UnsafeCell<i32>, &UnsafeCell<i32>) = (&x, &x);
///
/// unsafe {
/// // SAFETY: within this scope there are no other references to `x`'s contents,
/// // so ours is effectively unique.
/// let p1_exclusive: &mut i32 = &mut *p1.get(); // -- borrow --+
/// *p1_exclusive += 27; // |
/// } // <---------- cannot go beyond this point -------------------+
///
/// unsafe {
/// // SAFETY: within this scope nobody expects to have exclusive access to `x`'s contents,
/// // so we can have multiple shared accesses concurrently.
/// let p2_shared: &i32 = &*p2.get();
/// assert_eq!(*p2_shared, 42 + 27);
/// let p1_shared: &i32 = &*p1.get();
/// assert_eq!(*p1_shared, *p2_shared);
/// }
/// ```
///
/// The following example showcases the fact that exclusive access to an `UnsafeCell<T>`
/// implies exclusive access to its `T`:
///
/// ```rust
/// #![forbid(unsafe_code)] // with exclusive accesses,
/// // `UnsafeCell` is a transparent no-op wrapper,
/// // so no need for `unsafe` here.
/// use std::cell::UnsafeCell;
///
/// let mut x: UnsafeCell<i32> = 42.into();
///
/// // Get a compile-time-checked unique reference to `x`.
/// let p_unique: &mut UnsafeCell<i32> = &mut x;
/// // With an exclusive reference, we can mutate the contents for free.
/// *p_unique.get_mut() = 0;
/// // Or, equivalently:
/// x = UnsafeCell::new(0);
///
/// // When we own the value, we can extract the contents for free.
/// let contents: i32 = x.into_inner();
/// assert_eq!(contents, 0);
/// ```
#[lang = "unsafe_cell"]
#[stable(feature = "rust1", since = "1.0.0")]
#[repr(transparent)]
#[rustc_pub_transparent]
pub struct UnsafeCell<T: ?Sized> {
value: T,
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized> !Sync for UnsafeCell<T> {}
impl<T> UnsafeCell<T> {
/// Constructs a new instance of `UnsafeCell` which will wrap the specified
/// value.
///
/// All access to the inner value through `&UnsafeCell<T>` requires `unsafe` code.
///
/// # Examples
///
/// ```
/// use std::cell::UnsafeCell;
///
/// let uc = UnsafeCell::new(5);
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_const_stable(feature = "const_unsafe_cell_new", since = "1.32.0")]
#[inline(always)]
pub const fn new(value: T) -> UnsafeCell<T> {
UnsafeCell { value }
}
/// Unwraps the value, consuming the cell.
///
/// # Examples
///
/// ```
/// use std::cell::UnsafeCell;
///
/// let uc = UnsafeCell::new(5);
///
/// let five = uc.into_inner();
/// ```
#[inline(always)]
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_const_stable(feature = "const_cell_into_inner", since = "CURRENT_RUSTC_VERSION")]
#[rustc_allow_const_fn_unstable(const_precise_live_drops)]
pub const fn into_inner(self) -> T {
self.value
}
}
impl<T: ?Sized> UnsafeCell<T> {
/// Converts from `&mut T` to `&mut UnsafeCell<T>`.
///
/// # Examples
///
/// ```
/// # #![feature(unsafe_cell_from_mut)]
/// use std::cell::UnsafeCell;
///
/// let mut val = 42;
/// let uc = UnsafeCell::from_mut(&mut val);
///
/// *uc.get_mut() -= 1;
/// assert_eq!(*uc.get_mut(), 41);
/// ```
#[inline(always)]
#[unstable(feature = "unsafe_cell_from_mut", issue = "111645")]
pub const fn from_mut(value: &mut T) -> &mut UnsafeCell<T> {
// SAFETY: `UnsafeCell<T>` has the same memory layout as `T` due to #[repr(transparent)].
unsafe { &mut *(value as *mut T as *mut UnsafeCell<T>) }
}
/// Gets a mutable pointer to the wrapped value.
///
/// This can be cast to a pointer of any kind.
/// Ensure that the access is unique (no active references, mutable or not)
/// when casting to `&mut T`, and ensure that there are no mutations
/// or mutable aliases going on when casting to `&T`
///
/// # Examples
///
/// ```
/// use std::cell::UnsafeCell;
///
/// let uc = UnsafeCell::new(5);
///
/// let five = uc.get();
/// ```
#[inline(always)]
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_const_stable(feature = "const_unsafecell_get", since = "1.32.0")]
#[rustc_never_returns_null_ptr]
pub const fn get(&self) -> *mut T {
// We can just cast the pointer from `UnsafeCell<T>` to `T` because of
// #[repr(transparent)]. This exploits std's special status, there is
// no guarantee for user code that this will work in future versions of the compiler!
self as *const UnsafeCell<T> as *const T as *mut T
}
/// Returns a mutable reference to the underlying data.
///
/// This call borrows the `UnsafeCell` mutably (at compile-time) which
/// guarantees that we possess the only reference.
///
/// # Examples
///
/// ```
/// use std::cell::UnsafeCell;
///
/// let mut c = UnsafeCell::new(5);
/// *c.get_mut() += 1;
///
/// assert_eq!(*c.get_mut(), 6);
/// ```
#[inline(always)]
#[stable(feature = "unsafe_cell_get_mut", since = "1.50.0")]
#[cfg_attr(bootstrap, rustc_allow_const_fn_unstable(const_mut_refs))]
#[rustc_const_stable(feature = "const_unsafecell_get_mut", since = "CURRENT_RUSTC_VERSION")]
pub const fn get_mut(&mut self) -> &mut T {
&mut self.value
}
/// Gets a mutable pointer to the wrapped value.
/// The difference from [`get`] is that this function accepts a raw pointer,
/// which is useful to avoid the creation of temporary references.
///
/// The result can be cast to a pointer of any kind.
/// Ensure that the access is unique (no active references, mutable or not)
/// when casting to `&mut T`, and ensure that there are no mutations
/// or mutable aliases going on when casting to `&T`.
///
/// [`get`]: UnsafeCell::get()
///
/// # Examples
///
/// Gradual initialization of an `UnsafeCell` requires `raw_get`, as
/// calling `get` would require creating a reference to uninitialized data:
///
/// ```
/// use std::cell::UnsafeCell;
/// use std::mem::MaybeUninit;
///
/// let m = MaybeUninit::<UnsafeCell<i32>>::uninit();
/// unsafe { UnsafeCell::raw_get(m.as_ptr()).write(5); }
/// // avoid below which references to uninitialized data
/// // unsafe { UnsafeCell::get(&*m.as_ptr()).write(5); }
/// let uc = unsafe { m.assume_init() };
///
/// assert_eq!(uc.into_inner(), 5);
/// ```
#[inline(always)]
#[stable(feature = "unsafe_cell_raw_get", since = "1.56.0")]
#[rustc_const_stable(feature = "unsafe_cell_raw_get", since = "1.56.0")]
#[rustc_diagnostic_item = "unsafe_cell_raw_get"]
pub const fn raw_get(this: *const Self) -> *mut T {
// We can just cast the pointer from `UnsafeCell<T>` to `T` because of
// #[repr(transparent)]. This exploits std's special status, there is
// no guarantee for user code that this will work in future versions of the compiler!
this as *const T as *mut T
}
}
#[stable(feature = "unsafe_cell_default", since = "1.10.0")]
impl<T: Default> Default for UnsafeCell<T> {
/// Creates an `UnsafeCell`, with the `Default` value for T.
fn default() -> UnsafeCell<T> {
UnsafeCell::new(Default::default())
}
}
#[stable(feature = "cell_from", since = "1.12.0")]
impl<T> From<T> for UnsafeCell<T> {
/// Creates a new `UnsafeCell<T>` containing the given value.
fn from(t: T) -> UnsafeCell<T> {
UnsafeCell::new(t)
}
}
#[unstable(feature = "coerce_unsized", issue = "18598")]
impl<T: CoerceUnsized<U>, U> CoerceUnsized<UnsafeCell<U>> for UnsafeCell<T> {}
// Allow types that wrap `UnsafeCell` to also implement `DispatchFromDyn`
// and become dyn-compatible method receivers.
// Note that currently `UnsafeCell` itself cannot be a method receiver
// because it does not implement Deref.
// In other words:
// `self: UnsafeCell<&Self>` won't work
// `self: UnsafeCellWrapper<Self>` becomes possible
#[unstable(feature = "dispatch_from_dyn", issue = "none")]
impl<T: DispatchFromDyn<U>, U> DispatchFromDyn<UnsafeCell<U>> for UnsafeCell<T> {}
/// [`UnsafeCell`], but [`Sync`].
///
/// This is just an `UnsafeCell`, except it implements `Sync`
/// if `T` implements `Sync`.
///
/// `UnsafeCell` doesn't implement `Sync`, to prevent accidental mis-use.
/// You can use `SyncUnsafeCell` instead of `UnsafeCell` to allow it to be
/// shared between threads, if that's intentional.
/// Providing proper synchronization is still the task of the user,
/// making this type just as unsafe to use.
///
/// See [`UnsafeCell`] for details.
#[unstable(feature = "sync_unsafe_cell", issue = "95439")]
#[repr(transparent)]
#[rustc_pub_transparent]
pub struct SyncUnsafeCell<T: ?Sized> {
value: UnsafeCell<T>,
}
#[unstable(feature = "sync_unsafe_cell", issue = "95439")]
unsafe impl<T: ?Sized + Sync> Sync for SyncUnsafeCell<T> {}
#[unstable(feature = "sync_unsafe_cell", issue = "95439")]
impl<T> SyncUnsafeCell<T> {
/// Constructs a new instance of `SyncUnsafeCell` which will wrap the specified value.
#[inline]
pub const fn new(value: T) -> Self {
Self { value: UnsafeCell { value } }
}
/// Unwraps the value, consuming the cell.
#[inline]
pub const fn into_inner(self) -> T {
self.value.into_inner()
}
}
#[unstable(feature = "sync_unsafe_cell", issue = "95439")]
impl<T: ?Sized> SyncUnsafeCell<T> {
/// Gets a mutable pointer to the wrapped value.
///
/// This can be cast to a pointer of any kind.
/// Ensure that the access is unique (no active references, mutable or not)
/// when casting to `&mut T`, and ensure that there are no mutations
/// or mutable aliases going on when casting to `&T`
#[inline]
#[rustc_never_returns_null_ptr]
pub const fn get(&self) -> *mut T {
self.value.get()
}
/// Returns a mutable reference to the underlying data.
///
/// This call borrows the `SyncUnsafeCell` mutably (at compile-time) which
/// guarantees that we possess the only reference.
#[inline]
pub const fn get_mut(&mut self) -> &mut T {
self.value.get_mut()
}
/// Gets a mutable pointer to the wrapped value.
///
/// See [`UnsafeCell::get`] for details.
#[inline]
pub const fn raw_get(this: *const Self) -> *mut T {
// We can just cast the pointer from `SyncUnsafeCell<T>` to `T` because
// of #[repr(transparent)] on both SyncUnsafeCell and UnsafeCell.
// See UnsafeCell::raw_get.
this as *const T as *mut T
}
}
#[unstable(feature = "sync_unsafe_cell", issue = "95439")]
impl<T: Default> Default for SyncUnsafeCell<T> {
/// Creates an `SyncUnsafeCell`, with the `Default` value for T.
fn default() -> SyncUnsafeCell<T> {
SyncUnsafeCell::new(Default::default())
}
}
#[unstable(feature = "sync_unsafe_cell", issue = "95439")]
impl<T> From<T> for SyncUnsafeCell<T> {
/// Creates a new `SyncUnsafeCell<T>` containing the given value.
fn from(t: T) -> SyncUnsafeCell<T> {
SyncUnsafeCell::new(t)
}
}
#[unstable(feature = "coerce_unsized", issue = "18598")]
//#[unstable(feature = "sync_unsafe_cell", issue = "95439")]
impl<T: CoerceUnsized<U>, U> CoerceUnsized<SyncUnsafeCell<U>> for SyncUnsafeCell<T> {}
// Allow types that wrap `SyncUnsafeCell` to also implement `DispatchFromDyn`
// and become dyn-compatible method receivers.
// Note that currently `SyncUnsafeCell` itself cannot be a method receiver
// because it does not implement Deref.
// In other words:
// `self: SyncUnsafeCell<&Self>` won't work
// `self: SyncUnsafeCellWrapper<Self>` becomes possible
#[unstable(feature = "dispatch_from_dyn", issue = "none")]
//#[unstable(feature = "sync_unsafe_cell", issue = "95439")]
impl<T: DispatchFromDyn<U>, U> DispatchFromDyn<SyncUnsafeCell<U>> for SyncUnsafeCell<T> {}
#[allow(unused)]
fn assert_coerce_unsized(
a: UnsafeCell<&i32>,
b: SyncUnsafeCell<&i32>,
c: Cell<&i32>,
d: RefCell<&i32>,
) {
let _: UnsafeCell<&dyn Send> = a;
let _: SyncUnsafeCell<&dyn Send> = b;
let _: Cell<&dyn Send> = c;
let _: RefCell<&dyn Send> = d;
}
#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
unsafe impl<T: ?Sized> PinCoerceUnsized for UnsafeCell<T> {}
#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
unsafe impl<T: ?Sized> PinCoerceUnsized for SyncUnsafeCell<T> {}
#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
unsafe impl<T: ?Sized> PinCoerceUnsized for Cell<T> {}
#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
unsafe impl<T: ?Sized> PinCoerceUnsized for RefCell<T> {}
#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
unsafe impl<'b, T: ?Sized> PinCoerceUnsized for Ref<'b, T> {}
#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
unsafe impl<'b, T: ?Sized> PinCoerceUnsized for RefMut<'b, T> {}