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 2498 2499 2500 2501 2502 2503 2504 2505 2506 2507 2508 2509 2510 2511 2512 2513 2514 2515 2516 2517 2518 2519 2520 2521 2522 2523 2524 2525 2526 2527 2528 2529 2530 2531 2532 2533 2534 2535 2536 2537 2538 2539 2540 2541 2542 2543 2544 2545 2546 2547 2548 2549 2550 2551 2552 2553 2554 2555 2556 2557 2558 2559 2560 2561 2562 2563 2564 2565 2566 2567 2568 2569 2570 2571 2572 2573 2574 2575 2576 2577 2578 2579 2580 2581 2582 2583 2584 2585 2586 2587 2588 2589 2590 2591 2592 2593 2594 2595 2596 2597 2598 2599 2600 2601 2602 2603 2604 2605 2606 2607 2608 2609 2610 2611 2612 2613 2614 2615 2616 2617 2618 2619 2620 2621 2622 2623 2624 2625 2626 2627 2628 2629 2630 2631 2632 2633 2634 2635 2636 2637 2638 2639 2640 2641 2642 2643 2644 2645 2646 2647 2648 2649 2650 2651 2652 2653 2654 2655 2656 2657 2658 2659 2660 2661 2662 2663 2664 2665 2666 2667 2668 2669 2670 2671 2672 2673 2674 2675 2676 2677 2678 2679 2680 2681 2682 2683 2684 2685 2686 2687 2688 2689 2690 2691 2692 2693 2694 2695 2696 2697 2698 2699 2700 2701 2702 2703 2704 2705 2706 2707 2708 2709 2710 2711 2712 2713 2714 2715 2716 2717 2718 2719 2720 2721 2722 2723 2724 2725 2726 2727 2728 2729 2730 2731 2732 2733 2734 2735 2736 2737 2738 2739 2740 2741 2742 2743 2744 2745 2746 2747 2748 2749 2750 2751 2752 2753 2754 2755 2756 2757 2758 2759 2760 2761 2762 2763 2764 2765 2766 2767 2768 2769 2770 2771 2772 2773 2774 2775 2776 2777 2778 2779 2780 2781 2782 2783 2784 2785 2786 2787 2788 2789 2790 2791 2792 2793 2794 2795 2796 2797 2798 2799 2800 2801 2802 2803 2804 2805 2806 2807 2808 2809 2810 2811 2812 2813 2814 2815 2816 2817 2818 2819 2820 2821 2822 2823 2824 2825
//! Code related to match expressions. These are sufficiently complex to
//! warrant their own module and submodules. :) This main module includes the
//! high-level algorithm, the submodules contain the details.
//!
//! This also includes code for pattern bindings in `let` statements and
//! function parameters.
use rustc_data_structures::fx::FxIndexMap;
use rustc_data_structures::stack::ensure_sufficient_stack;
use rustc_hir::{BindingMode, ByRef};
use rustc_middle::bug;
use rustc_middle::middle::region;
use rustc_middle::mir::{self, *};
use rustc_middle::thir::{self, *};
use rustc_middle::ty::{self, CanonicalUserTypeAnnotation, Ty};
use rustc_span::symbol::Symbol;
use rustc_span::{BytePos, Pos, Span};
use rustc_target::abi::VariantIdx;
use tracing::{debug, instrument};
use crate::build::expr::as_place::PlaceBuilder;
use crate::build::scope::DropKind;
use crate::build::ForGuard::{self, OutsideGuard, RefWithinGuard};
use crate::build::{
BlockAnd, BlockAndExtension, Builder, GuardFrame, GuardFrameLocal, LocalsForNode,
};
// helper functions, broken out by category:
mod match_pair;
mod simplify;
mod test;
mod util;
use std::assert_matches::assert_matches;
use std::borrow::Borrow;
use std::mem;
/// Arguments to [`Builder::then_else_break_inner`] that are usually forwarded
/// to recursive invocations.
#[derive(Clone, Copy)]
struct ThenElseArgs {
/// Used as the temp scope for lowering `expr`. If absent (for match guards),
/// `self.local_scope()` is used.
temp_scope_override: Option<region::Scope>,
variable_source_info: SourceInfo,
/// Determines how bindings should be handled when lowering `let` expressions.
///
/// Forwarded to [`Builder::lower_let_expr`] when lowering [`ExprKind::Let`].
declare_let_bindings: DeclareLetBindings,
}
/// Should lowering a `let` expression also declare its bindings?
///
/// Used by [`Builder::lower_let_expr`] when lowering [`ExprKind::Let`].
#[derive(Clone, Copy)]
pub(crate) enum DeclareLetBindings {
/// Yes, declare `let` bindings as normal for `if` conditions.
Yes,
/// No, don't declare `let` bindings, because the caller declares them
/// separately due to special requirements.
///
/// Used for match guards and let-else.
No,
/// Let expressions are not permitted in this context, so it is a bug to
/// try to lower one (e.g inside lazy-boolean-or or boolean-not).
LetNotPermitted,
}
/// Used by [`Builder::bind_matched_candidate_for_arm_body`] to determine
/// whether or not to call [`Builder::storage_live_binding`] to emit
/// [`StatementKind::StorageLive`].
#[derive(Clone, Copy)]
pub(crate) enum EmitStorageLive {
/// Yes, emit `StorageLive` as normal.
Yes,
/// No, don't emit `StorageLive`. The caller has taken responsibility for
/// emitting `StorageLive` as appropriate.
No,
}
/// Used by [`Builder::storage_live_binding`] and [`Builder::bind_matched_candidate_for_arm_body`]
/// to decide whether to schedule drops.
#[derive(Clone, Copy, Debug)]
pub(crate) enum ScheduleDrops {
/// Yes, the relevant functions should also schedule drops as appropriate.
Yes,
/// No, don't schedule drops. The caller has taken responsibility for any
/// appropriate drops.
No,
}
impl<'a, 'tcx> Builder<'a, 'tcx> {
/// Lowers a condition in a way that ensures that variables bound in any let
/// expressions are definitely initialized in the if body.
///
/// If `declare_let_bindings` is false then variables created in `let`
/// expressions will not be declared. This is for if let guards on arms with
/// an or pattern, where the guard is lowered multiple times.
pub(crate) fn then_else_break(
&mut self,
block: BasicBlock,
expr_id: ExprId,
temp_scope_override: Option<region::Scope>,
variable_source_info: SourceInfo,
declare_let_bindings: DeclareLetBindings,
) -> BlockAnd<()> {
self.then_else_break_inner(
block,
expr_id,
ThenElseArgs { temp_scope_override, variable_source_info, declare_let_bindings },
)
}
fn then_else_break_inner(
&mut self,
block: BasicBlock, // Block that the condition and branch will be lowered into
expr_id: ExprId, // Condition expression to lower
args: ThenElseArgs,
) -> BlockAnd<()> {
let this = self;
let expr = &this.thir[expr_id];
let expr_span = expr.span;
match expr.kind {
ExprKind::LogicalOp { op: op @ LogicalOp::And, lhs, rhs } => {
this.visit_coverage_branch_operation(op, expr_span);
let lhs_then_block = this.then_else_break_inner(block, lhs, args).into_block();
let rhs_then_block =
this.then_else_break_inner(lhs_then_block, rhs, args).into_block();
rhs_then_block.unit()
}
ExprKind::LogicalOp { op: op @ LogicalOp::Or, lhs, rhs } => {
this.visit_coverage_branch_operation(op, expr_span);
let local_scope = this.local_scope();
let (lhs_success_block, failure_block) =
this.in_if_then_scope(local_scope, expr_span, |this| {
this.then_else_break_inner(
block,
lhs,
ThenElseArgs {
declare_let_bindings: DeclareLetBindings::LetNotPermitted,
..args
},
)
});
let rhs_success_block = this
.then_else_break_inner(
failure_block,
rhs,
ThenElseArgs {
declare_let_bindings: DeclareLetBindings::LetNotPermitted,
..args
},
)
.into_block();
// Make the LHS and RHS success arms converge to a common block.
// (We can't just make LHS goto RHS, because `rhs_success_block`
// might contain statements that we don't want on the LHS path.)
let success_block = this.cfg.start_new_block();
this.cfg.goto(lhs_success_block, args.variable_source_info, success_block);
this.cfg.goto(rhs_success_block, args.variable_source_info, success_block);
success_block.unit()
}
ExprKind::Unary { op: UnOp::Not, arg } => {
// Improve branch coverage instrumentation by noting conditions
// nested within one or more `!` expressions.
// (Skipped if branch coverage is not enabled.)
if let Some(coverage_info) = this.coverage_info.as_mut() {
coverage_info.visit_unary_not(this.thir, expr_id);
}
let local_scope = this.local_scope();
let (success_block, failure_block) =
this.in_if_then_scope(local_scope, expr_span, |this| {
// Help out coverage instrumentation by injecting a dummy statement with
// the original condition's span (including `!`). This fixes #115468.
if this.tcx.sess.instrument_coverage() {
this.cfg.push_coverage_span_marker(block, this.source_info(expr_span));
}
this.then_else_break_inner(
block,
arg,
ThenElseArgs {
declare_let_bindings: DeclareLetBindings::LetNotPermitted,
..args
},
)
});
this.break_for_else(success_block, args.variable_source_info);
failure_block.unit()
}
ExprKind::Scope { region_scope, lint_level, value } => {
let region_scope = (region_scope, this.source_info(expr_span));
this.in_scope(region_scope, lint_level, |this| {
this.then_else_break_inner(block, value, args)
})
}
ExprKind::Use { source } => this.then_else_break_inner(block, source, args),
ExprKind::Let { expr, ref pat } => this.lower_let_expr(
block,
expr,
pat,
Some(args.variable_source_info.scope),
args.variable_source_info.span,
args.declare_let_bindings,
EmitStorageLive::Yes,
),
_ => {
let mut block = block;
let temp_scope = args.temp_scope_override.unwrap_or_else(|| this.local_scope());
let mutability = Mutability::Mut;
// Increment the decision depth, in case we encounter boolean expressions
// further down.
this.mcdc_increment_depth_if_enabled();
let place =
unpack!(block = this.as_temp(block, Some(temp_scope), expr_id, mutability));
this.mcdc_decrement_depth_if_enabled();
let operand = Operand::Move(Place::from(place));
let then_block = this.cfg.start_new_block();
let else_block = this.cfg.start_new_block();
let term = TerminatorKind::if_(operand, then_block, else_block);
// Record branch coverage info for this condition.
// (Does nothing if branch coverage is not enabled.)
this.visit_coverage_branch_condition(expr_id, then_block, else_block);
let source_info = this.source_info(expr_span);
this.cfg.terminate(block, source_info, term);
this.break_for_else(else_block, source_info);
then_block.unit()
}
}
}
/// Generates MIR for a `match` expression.
///
/// The MIR that we generate for a match looks like this.
///
/// ```text
/// [ 0. Pre-match ]
/// |
/// [ 1. Evaluate Scrutinee (expression being matched on) ]
/// [ (PlaceMention of scrutinee) ]
/// |
/// [ 2. Decision tree -- check discriminants ] <--------+
/// | |
/// | (once a specific arm is chosen) |
/// | |
/// [pre_binding_block] [otherwise_block]
/// | |
/// [ 3. Create "guard bindings" for arm ] |
/// [ (create fake borrows) ] |
/// | |
/// [ 4. Execute guard code ] |
/// [ (read fake borrows) ] --(guard is false)-----------+
/// |
/// | (guard results in true)
/// |
/// [ 5. Create real bindings and execute arm ]
/// |
/// [ Exit match ]
/// ```
///
/// All of the different arms have been stacked on top of each other to
/// simplify the diagram. For an arm with no guard the blocks marked 3 and
/// 4 and the fake borrows are omitted.
///
/// We generate MIR in the following steps:
///
/// 1. Evaluate the scrutinee and add the PlaceMention of it ([Builder::lower_scrutinee]).
/// 2. Create the decision tree ([Builder::lower_match_tree]).
/// 3. Determine the fake borrows that are needed from the places that were
/// matched against and create the required temporaries for them
/// ([util::collect_fake_borrows]).
/// 4. Create everything else: the guards and the arms ([Builder::lower_match_arms]).
///
/// ## False edges
///
/// We don't want to have the exact structure of the decision tree be visible through borrow
/// checking. Specifically we want borrowck to think that:
/// - at any point, any or none of the patterns and guards seen so far may have been tested;
/// - after the match, any of the patterns may have matched.
///
/// For example, all of these would fail to error if borrowck could see the real CFG (examples
/// taken from `tests/ui/nll/match-cfg-fake-edges.rs`):
/// ```ignore (too many errors, this is already in the test suite)
/// let x = String::new();
/// let _ = match true {
/// _ => {},
/// _ => drop(x),
/// };
/// // Borrowck must not know the second arm is never run.
/// drop(x); //~ ERROR use of moved value
///
/// let x;
/// # let y = true;
/// match y {
/// _ if { x = 2; true } => {},
/// // Borrowck must not know the guard is always run.
/// _ => drop(x), //~ ERROR used binding `x` is possibly-uninitialized
/// };
///
/// let x = String::new();
/// # let y = true;
/// match y {
/// false if { drop(x); true } => {},
/// // Borrowck must not know the guard is not run in the `true` case.
/// true => drop(x), //~ ERROR use of moved value: `x`
/// false => {},
/// };
///
/// # let mut y = (true, true);
/// let r = &mut y.1;
/// match y {
/// //~^ ERROR cannot use `y.1` because it was mutably borrowed
/// (false, true) => {}
/// // Borrowck must not know we don't test `y.1` when `y.0` is `true`.
/// (true, _) => drop(r),
/// (false, _) => {}
/// };
/// ```
///
/// We add false edges to act as if we were naively matching each arm in order. What we need is
/// a (fake) path from each candidate to the next, specifically from candidate C's pre-binding
/// block to next candidate D's pre-binding block. For maximum precision (needed for deref
/// patterns), we choose the earliest node on D's success path that doesn't also lead to C (to
/// avoid loops).
///
/// This turns out to be easy to compute: that block is the `start_block` of the first call to
/// `match_candidates` where D is the first candidate in the list.
///
/// For example:
/// ```rust
/// # let (x, y) = (true, true);
/// match (x, y) {
/// (true, true) => 1,
/// (false, true) => 2,
/// (true, false) => 3,
/// _ => 4,
/// }
/// # ;
/// ```
/// In this example, the pre-binding block of arm 1 has a false edge to the block for result
/// `false` of the first test on `x`. The other arms have false edges to the pre-binding blocks
/// of the next arm.
///
/// On top of this, we also add a false edge from the otherwise_block of each guard to the
/// aforementioned start block of the next candidate, to ensure borrock doesn't rely on which
/// guards may have run.
#[instrument(level = "debug", skip(self, arms))]
pub(crate) fn match_expr(
&mut self,
destination: Place<'tcx>,
mut block: BasicBlock,
scrutinee_id: ExprId,
arms: &[ArmId],
span: Span,
scrutinee_span: Span,
) -> BlockAnd<()> {
let scrutinee_place =
unpack!(block = self.lower_scrutinee(block, scrutinee_id, scrutinee_span));
let arms = arms.iter().map(|arm| &self.thir[*arm]);
let match_start_span = span.shrink_to_lo().to(scrutinee_span);
let patterns = arms
.clone()
.map(|arm| {
let has_match_guard =
if arm.guard.is_some() { HasMatchGuard::Yes } else { HasMatchGuard::No };
(&*arm.pattern, has_match_guard)
})
.collect();
let built_tree = self.lower_match_tree(
block,
scrutinee_span,
&scrutinee_place,
match_start_span,
patterns,
false,
);
self.lower_match_arms(
destination,
scrutinee_place,
scrutinee_span,
arms,
built_tree,
self.source_info(span),
)
}
/// Evaluate the scrutinee and add the PlaceMention for it.
fn lower_scrutinee(
&mut self,
mut block: BasicBlock,
scrutinee_id: ExprId,
scrutinee_span: Span,
) -> BlockAnd<PlaceBuilder<'tcx>> {
let scrutinee_place_builder = unpack!(block = self.as_place_builder(block, scrutinee_id));
if let Some(scrutinee_place) = scrutinee_place_builder.try_to_place(self) {
let source_info = self.source_info(scrutinee_span);
self.cfg.push_place_mention(block, source_info, scrutinee_place);
}
block.and(scrutinee_place_builder)
}
/// Lower the bindings, guards and arm bodies of a `match` expression.
///
/// The decision tree should have already been created
/// (by [Builder::lower_match_tree]).
///
/// `outer_source_info` is the SourceInfo for the whole match.
fn lower_match_arms<'pat>(
&mut self,
destination: Place<'tcx>,
scrutinee_place_builder: PlaceBuilder<'tcx>,
scrutinee_span: Span,
arms: impl IntoIterator<Item = &'pat Arm<'tcx>>,
built_match_tree: BuiltMatchTree<'tcx>,
outer_source_info: SourceInfo,
) -> BlockAnd<()>
where
'tcx: 'pat,
{
let arm_end_blocks: Vec<BasicBlock> = arms
.into_iter()
.zip(built_match_tree.branches)
.map(|(arm, branch)| {
debug!("lowering arm {:?}\ncorresponding branch = {:?}", arm, branch);
let arm_source_info = self.source_info(arm.span);
let arm_scope = (arm.scope, arm_source_info);
let match_scope = self.local_scope();
self.in_scope(arm_scope, arm.lint_level, |this| {
let old_dedup_scope =
mem::replace(&mut this.fixed_temps_scope, Some(arm.scope));
// `try_to_place` may fail if it is unable to resolve the given
// `PlaceBuilder` inside a closure. In this case, we don't want to include
// a scrutinee place. `scrutinee_place_builder` will fail to be resolved
// if the only match arm is a wildcard (`_`).
// Example:
// ```
// let foo = (0, 1);
// let c = || {
// match foo { _ => () };
// };
// ```
let scrutinee_place = scrutinee_place_builder.try_to_place(this);
let opt_scrutinee_place =
scrutinee_place.as_ref().map(|place| (Some(place), scrutinee_span));
let scope = this.declare_bindings(
None,
arm.span,
&arm.pattern,
arm.guard,
opt_scrutinee_place,
);
let arm_block = this.bind_pattern(
outer_source_info,
branch,
&built_match_tree.fake_borrow_temps,
scrutinee_span,
Some((arm, match_scope)),
EmitStorageLive::Yes,
);
this.fixed_temps_scope = old_dedup_scope;
if let Some(source_scope) = scope {
this.source_scope = source_scope;
}
this.expr_into_dest(destination, arm_block, arm.body)
})
.into_block()
})
.collect();
// all the arm blocks will rejoin here
let end_block = self.cfg.start_new_block();
let end_brace = self.source_info(
outer_source_info.span.with_lo(outer_source_info.span.hi() - BytePos::from_usize(1)),
);
for arm_block in arm_end_blocks {
let block = &self.cfg.basic_blocks[arm_block];
let last_location = block.statements.last().map(|s| s.source_info);
self.cfg.goto(arm_block, last_location.unwrap_or(end_brace), end_block);
}
self.source_scope = outer_source_info.scope;
end_block.unit()
}
/// For a top-level `match` arm or a `let` binding, binds the variables and
/// ascribes types, and also checks the match arm guard (if present).
///
/// `arm_scope` should be `Some` if and only if this is called for a
/// `match` arm.
///
/// In the presence of or-patterns, a match arm might have multiple
/// sub-branches representing different ways to match, with each sub-branch
/// requiring its own bindings and its own copy of the guard. This method
/// handles those sub-branches individually, and then has them jump together
/// to a common block.
///
/// Returns a single block that the match arm can be lowered into.
/// (For `let` bindings, this is the code that can use the bindings.)
fn bind_pattern(
&mut self,
outer_source_info: SourceInfo,
branch: MatchTreeBranch<'tcx>,
fake_borrow_temps: &[(Place<'tcx>, Local, FakeBorrowKind)],
scrutinee_span: Span,
arm_match_scope: Option<(&Arm<'tcx>, region::Scope)>,
emit_storage_live: EmitStorageLive,
) -> BasicBlock {
if branch.sub_branches.len() == 1 {
let [sub_branch] = branch.sub_branches.try_into().unwrap();
// Avoid generating another `BasicBlock` when we only have one sub branch.
self.bind_and_guard_matched_candidate(
sub_branch,
fake_borrow_temps,
scrutinee_span,
arm_match_scope,
ScheduleDrops::Yes,
emit_storage_live,
)
} else {
// It's helpful to avoid scheduling drops multiple times to save
// drop elaboration from having to clean up the extra drops.
//
// If we are in a `let` then we only schedule drops for the first
// candidate.
//
// If we're in a `match` arm then we could have a case like so:
//
// Ok(x) | Err(x) if return => { /* ... */ }
//
// In this case we don't want a drop of `x` scheduled when we
// return: it isn't bound by move until right before enter the arm.
// To handle this we instead unschedule it's drop after each time
// we lower the guard.
let target_block = self.cfg.start_new_block();
let mut schedule_drops = ScheduleDrops::Yes;
let arm = arm_match_scope.unzip().0;
// We keep a stack of all of the bindings and type ascriptions
// from the parent candidates that we visit, that also need to
// be bound for each candidate.
for sub_branch in branch.sub_branches {
if let Some(arm) = arm {
self.clear_top_scope(arm.scope);
}
let binding_end = self.bind_and_guard_matched_candidate(
sub_branch,
fake_borrow_temps,
scrutinee_span,
arm_match_scope,
schedule_drops,
emit_storage_live,
);
if arm.is_none() {
schedule_drops = ScheduleDrops::No;
}
self.cfg.goto(binding_end, outer_source_info, target_block);
}
target_block
}
}
pub(super) fn expr_into_pattern(
&mut self,
mut block: BasicBlock,
irrefutable_pat: &Pat<'tcx>,
initializer_id: ExprId,
) -> BlockAnd<()> {
match irrefutable_pat.kind {
// Optimize the case of `let x = ...` to write directly into `x`
PatKind::Binding { mode: BindingMode(ByRef::No, _), var, subpattern: None, .. } => {
let place = self.storage_live_binding(
block,
var,
irrefutable_pat.span,
OutsideGuard,
ScheduleDrops::Yes,
);
block = self.expr_into_dest(place, block, initializer_id).into_block();
// Inject a fake read, see comments on `FakeReadCause::ForLet`.
let source_info = self.source_info(irrefutable_pat.span);
self.cfg.push_fake_read(block, source_info, FakeReadCause::ForLet(None), place);
self.schedule_drop_for_binding(var, irrefutable_pat.span, OutsideGuard);
block.unit()
}
// Optimize the case of `let x: T = ...` to write directly
// into `x` and then require that `T == typeof(x)`.
PatKind::AscribeUserType {
subpattern:
box Pat {
kind:
PatKind::Binding {
mode: BindingMode(ByRef::No, _),
var,
subpattern: None,
..
},
..
},
ascription: thir::Ascription { ref annotation, variance: _ },
} => {
let place = self.storage_live_binding(
block,
var,
irrefutable_pat.span,
OutsideGuard,
ScheduleDrops::Yes,
);
block = self.expr_into_dest(place, block, initializer_id).into_block();
// Inject a fake read, see comments on `FakeReadCause::ForLet`.
let pattern_source_info = self.source_info(irrefutable_pat.span);
let cause_let = FakeReadCause::ForLet(None);
self.cfg.push_fake_read(block, pattern_source_info, cause_let, place);
let ty_source_info = self.source_info(annotation.span);
let base = self.canonical_user_type_annotations.push(annotation.clone());
self.cfg.push(
block,
Statement {
source_info: ty_source_info,
kind: StatementKind::AscribeUserType(
Box::new((place, UserTypeProjection { base, projs: Vec::new() })),
// We always use invariant as the variance here. This is because the
// variance field from the ascription refers to the variance to use
// when applying the type to the value being matched, but this
// ascription applies rather to the type of the binding. e.g., in this
// example:
//
// ```
// let x: T = <expr>
// ```
//
// We are creating an ascription that defines the type of `x` to be
// exactly `T` (i.e., with invariance). The variance field, in
// contrast, is intended to be used to relate `T` to the type of
// `<expr>`.
ty::Invariant,
),
},
);
self.schedule_drop_for_binding(var, irrefutable_pat.span, OutsideGuard);
block.unit()
}
_ => {
let initializer = &self.thir[initializer_id];
let place_builder =
unpack!(block = self.lower_scrutinee(block, initializer_id, initializer.span));
self.place_into_pattern(block, irrefutable_pat, place_builder, true)
}
}
}
pub(crate) fn place_into_pattern(
&mut self,
block: BasicBlock,
irrefutable_pat: &Pat<'tcx>,
initializer: PlaceBuilder<'tcx>,
set_match_place: bool,
) -> BlockAnd<()> {
let built_tree = self.lower_match_tree(
block,
irrefutable_pat.span,
&initializer,
irrefutable_pat.span,
vec![(irrefutable_pat, HasMatchGuard::No)],
false,
);
let [branch] = built_tree.branches.try_into().unwrap();
// For matches and function arguments, the place that is being matched
// can be set when creating the variables. But the place for
// let PATTERN = ... might not even exist until we do the assignment.
// so we set it here instead.
if set_match_place {
// `try_to_place` may fail if it is unable to resolve the given `PlaceBuilder` inside a
// closure. In this case, we don't want to include a scrutinee place.
// `scrutinee_place_builder` will fail for destructured assignments. This is because a
// closure only captures the precise places that it will read and as a result a closure
// may not capture the entire tuple/struct and rather have individual places that will
// be read in the final MIR.
// Example:
// ```
// let foo = (0, 1);
// let c = || {
// let (v1, v2) = foo;
// };
// ```
if let Some(place) = initializer.try_to_place(self) {
// Because or-alternatives bind the same variables, we only explore the first one.
let first_sub_branch = branch.sub_branches.first().unwrap();
for binding in &first_sub_branch.bindings {
let local = self.var_local_id(binding.var_id, OutsideGuard);
if let LocalInfo::User(BindingForm::Var(VarBindingForm {
opt_match_place: Some((ref mut match_place, _)),
..
})) = **self.local_decls[local].local_info.as_mut().assert_crate_local()
{
*match_place = Some(place);
} else {
bug!("Let binding to non-user variable.")
};
}
}
}
self.bind_pattern(
self.source_info(irrefutable_pat.span),
branch,
&[],
irrefutable_pat.span,
None,
EmitStorageLive::Yes,
)
.unit()
}
/// Declares the bindings of the given patterns and returns the visibility
/// scope for the bindings in these patterns, if such a scope had to be
/// created. NOTE: Declaring the bindings should always be done in their
/// drop scope.
#[instrument(skip(self), level = "debug")]
pub(crate) fn declare_bindings(
&mut self,
mut visibility_scope: Option<SourceScope>,
scope_span: Span,
pattern: &Pat<'tcx>,
guard: Option<ExprId>,
opt_match_place: Option<(Option<&Place<'tcx>>, Span)>,
) -> Option<SourceScope> {
self.visit_primary_bindings(
pattern,
UserTypeProjections::none(),
&mut |this, name, mode, var, span, ty, user_ty| {
if visibility_scope.is_none() {
visibility_scope =
Some(this.new_source_scope(scope_span, LintLevel::Inherited));
}
let source_info = SourceInfo { span, scope: this.source_scope };
let visibility_scope = visibility_scope.unwrap();
this.declare_binding(
source_info,
visibility_scope,
name,
mode,
var,
ty,
user_ty,
ArmHasGuard(guard.is_some()),
opt_match_place.map(|(x, y)| (x.cloned(), y)),
pattern.span,
);
},
);
if let Some(guard_expr) = guard {
self.declare_guard_bindings(guard_expr, scope_span, visibility_scope);
}
visibility_scope
}
/// Declare bindings in a guard. This has to be done when declaring bindings
/// for an arm to ensure that or patterns only have one version of each
/// variable.
pub(crate) fn declare_guard_bindings(
&mut self,
guard_expr: ExprId,
scope_span: Span,
visibility_scope: Option<SourceScope>,
) {
match self.thir.exprs[guard_expr].kind {
ExprKind::Let { expr: _, pat: ref guard_pat } => {
// FIXME: pass a proper `opt_match_place`
self.declare_bindings(visibility_scope, scope_span, guard_pat, None, None);
}
ExprKind::Scope { value, .. } => {
self.declare_guard_bindings(value, scope_span, visibility_scope);
}
ExprKind::Use { source } => {
self.declare_guard_bindings(source, scope_span, visibility_scope);
}
ExprKind::LogicalOp { op: LogicalOp::And, lhs, rhs } => {
self.declare_guard_bindings(lhs, scope_span, visibility_scope);
self.declare_guard_bindings(rhs, scope_span, visibility_scope);
}
_ => {}
}
}
/// Emits a [`StatementKind::StorageLive`] for the given var, and also
/// schedules a drop if requested (and possible).
pub(crate) fn storage_live_binding(
&mut self,
block: BasicBlock,
var: LocalVarId,
span: Span,
for_guard: ForGuard,
schedule_drop: ScheduleDrops,
) -> Place<'tcx> {
let local_id = self.var_local_id(var, for_guard);
let source_info = self.source_info(span);
self.cfg.push(block, Statement { source_info, kind: StatementKind::StorageLive(local_id) });
// Although there is almost always scope for given variable in corner cases
// like #92893 we might get variable with no scope.
if let Some(region_scope) = self.region_scope_tree.var_scope(var.0.local_id)
&& matches!(schedule_drop, ScheduleDrops::Yes)
{
self.schedule_drop(span, region_scope, local_id, DropKind::Storage);
}
Place::from(local_id)
}
pub(crate) fn schedule_drop_for_binding(
&mut self,
var: LocalVarId,
span: Span,
for_guard: ForGuard,
) {
let local_id = self.var_local_id(var, for_guard);
if let Some(region_scope) = self.region_scope_tree.var_scope(var.0.local_id) {
self.schedule_drop(span, region_scope, local_id, DropKind::Value);
}
}
/// Visit all of the primary bindings in a patterns, that is, visit the
/// leftmost occurrence of each variable bound in a pattern. A variable
/// will occur more than once in an or-pattern.
pub(super) fn visit_primary_bindings(
&mut self,
pattern: &Pat<'tcx>,
pattern_user_ty: UserTypeProjections,
f: &mut impl FnMut(
&mut Self,
Symbol,
BindingMode,
LocalVarId,
Span,
Ty<'tcx>,
UserTypeProjections,
),
) {
debug!(
"visit_primary_bindings: pattern={:?} pattern_user_ty={:?}",
pattern, pattern_user_ty
);
match pattern.kind {
PatKind::Binding { name, mode, var, ty, ref subpattern, is_primary, .. } => {
if is_primary {
f(self, name, mode, var, pattern.span, ty, pattern_user_ty.clone());
}
if let Some(subpattern) = subpattern.as_ref() {
self.visit_primary_bindings(subpattern, pattern_user_ty, f);
}
}
PatKind::Array { ref prefix, ref slice, ref suffix }
| PatKind::Slice { ref prefix, ref slice, ref suffix } => {
let from = u64::try_from(prefix.len()).unwrap();
let to = u64::try_from(suffix.len()).unwrap();
for subpattern in prefix.iter() {
self.visit_primary_bindings(subpattern, pattern_user_ty.clone().index(), f);
}
if let Some(subpattern) = slice {
self.visit_primary_bindings(
subpattern,
pattern_user_ty.clone().subslice(from, to),
f,
);
}
for subpattern in suffix.iter() {
self.visit_primary_bindings(subpattern, pattern_user_ty.clone().index(), f);
}
}
PatKind::Constant { .. }
| PatKind::Range { .. }
| PatKind::Wild
| PatKind::Never
| PatKind::Error(_) => {}
PatKind::Deref { ref subpattern } => {
self.visit_primary_bindings(subpattern, pattern_user_ty.deref(), f);
}
PatKind::DerefPattern { ref subpattern, .. } => {
self.visit_primary_bindings(subpattern, UserTypeProjections::none(), f);
}
PatKind::AscribeUserType {
ref subpattern,
ascription: thir::Ascription { ref annotation, variance: _ },
} => {
// This corresponds to something like
//
// ```
// let A::<'a>(_): A<'static> = ...;
// ```
//
// Note that the variance doesn't apply here, as we are tracking the effect
// of `user_ty` on any bindings contained with subpattern.
let projection = UserTypeProjection {
base: self.canonical_user_type_annotations.push(annotation.clone()),
projs: Vec::new(),
};
let subpattern_user_ty =
pattern_user_ty.push_projection(&projection, annotation.span);
self.visit_primary_bindings(subpattern, subpattern_user_ty, f)
}
PatKind::InlineConstant { ref subpattern, .. } => {
self.visit_primary_bindings(subpattern, pattern_user_ty, f)
}
PatKind::Leaf { ref subpatterns } => {
for subpattern in subpatterns {
let subpattern_user_ty = pattern_user_ty.clone().leaf(subpattern.field);
debug!("visit_primary_bindings: subpattern_user_ty={:?}", subpattern_user_ty);
self.visit_primary_bindings(&subpattern.pattern, subpattern_user_ty, f);
}
}
PatKind::Variant { adt_def, args: _, variant_index, ref subpatterns } => {
for subpattern in subpatterns {
let subpattern_user_ty =
pattern_user_ty.clone().variant(adt_def, variant_index, subpattern.field);
self.visit_primary_bindings(&subpattern.pattern, subpattern_user_ty, f);
}
}
PatKind::Or { ref pats } => {
// In cases where we recover from errors the primary bindings
// may not all be in the leftmost subpattern. For example in
// `let (x | y) = ...`, the primary binding of `y` occurs in
// the right subpattern
for subpattern in pats.iter() {
self.visit_primary_bindings(subpattern, pattern_user_ty.clone(), f);
}
}
}
}
}
/// Data extracted from a pattern that doesn't affect which branch is taken. Collected during
/// pattern simplification and not mutated later.
#[derive(Debug, Clone)]
struct PatternExtraData<'tcx> {
/// [`Span`] of the original pattern.
span: Span,
/// Bindings that must be established.
bindings: Vec<Binding<'tcx>>,
/// Types that must be asserted.
ascriptions: Vec<Ascription<'tcx>>,
/// Whether this corresponds to a never pattern.
is_never: bool,
}
impl<'tcx> PatternExtraData<'tcx> {
fn is_empty(&self) -> bool {
self.bindings.is_empty() && self.ascriptions.is_empty()
}
}
/// A pattern in a form suitable for lowering the match tree, with all irrefutable
/// patterns simplified away, and or-patterns sorted to the end.
///
/// Here, "flat" indicates that the pattern's match pairs have been recursively
/// simplified by [`Builder::simplify_match_pairs`]. They are not necessarily
/// flat in an absolute sense.
///
/// Will typically be incorporated into a [`Candidate`].
#[derive(Debug, Clone)]
struct FlatPat<'pat, 'tcx> {
/// To match the pattern, all of these must be satisfied...
// Invariant: all the match pairs are recursively simplified.
// Invariant: or-patterns must be sorted to the end.
match_pairs: Vec<MatchPairTree<'pat, 'tcx>>,
extra_data: PatternExtraData<'tcx>,
}
impl<'tcx, 'pat> FlatPat<'pat, 'tcx> {
/// Creates a `FlatPat` containing a simplified [`MatchPairTree`] list/forest
/// for the given pattern.
fn new(
place: PlaceBuilder<'tcx>,
pattern: &'pat Pat<'tcx>,
cx: &mut Builder<'_, 'tcx>,
) -> Self {
// First, recursively build a tree of match pairs for the given pattern.
let mut match_pairs = vec![MatchPairTree::for_pattern(place, pattern, cx)];
let mut extra_data = PatternExtraData {
span: pattern.span,
bindings: Vec::new(),
ascriptions: Vec::new(),
is_never: pattern.is_never_pattern(),
};
// Recursively remove irrefutable match pairs, while recording their
// bindings/ascriptions, and sort or-patterns after other match pairs.
cx.simplify_match_pairs(&mut match_pairs, &mut extra_data);
Self { match_pairs, extra_data }
}
}
/// Candidates are a generalization of (a) top-level match arms, and
/// (b) sub-branches of or-patterns, allowing the match-lowering process to handle
/// them both in a mostly-uniform way. For example, the list of candidates passed
/// to [`Builder::match_candidates`] will often contain a mixture of top-level
/// candidates and or-pattern subcandidates.
///
/// At the start of match lowering, there is one candidate for each match arm.
/// During match lowering, arms with or-patterns will be expanded into a tree
/// of candidates, where each "leaf" candidate represents one of the ways for
/// the arm pattern to successfully match.
#[derive(Debug)]
struct Candidate<'pat, 'tcx> {
/// For the candidate to match, all of these must be satisfied...
///
/// ---
/// Initially contains a list of match pairs created by [`FlatPat`], but is
/// subsequently mutated (in a queue-like way) while lowering the match tree.
/// When this list becomes empty, the candidate is fully matched and becomes
/// a leaf (see [`Builder::select_matched_candidate`]).
///
/// Key mutations include:
///
/// - When a match pair is fully satisfied by a test, it is removed from the
/// list, and its subpairs are added instead (see [`Builder::sort_candidate`]).
/// - During or-pattern expansion, any leading or-pattern is removed, and is
/// converted into subcandidates (see [`Builder::expand_and_match_or_candidates`]).
/// - After a candidate's subcandidates have been lowered, a copy of any remaining
/// or-patterns is added to each leaf subcandidate
/// (see [`Builder::test_remaining_match_pairs_after_or`]).
///
/// Invariants:
/// - All [`TestCase::Irrefutable`] patterns have been removed by simplification.
/// - All or-patterns ([`TestCase::Or`]) have been sorted to the end.
match_pairs: Vec<MatchPairTree<'pat, 'tcx>>,
/// ...and if this is non-empty, one of these subcandidates also has to match...
///
/// ---
/// Initially a candidate has no subcandidates; they are added (and then immediately
/// lowered) during or-pattern expansion. Their main function is to serve as _output_
/// of match tree lowering, allowing later steps to see the leaf candidates that
/// represent a match of the entire match arm.
///
/// A candidate no subcandidates is either incomplete (if it has match pairs left),
/// or is a leaf in the match tree. A candidate with one or more subcandidates is
/// an internal node in the match tree.
///
/// Invariant: at the end of match tree lowering, this must not contain an
/// `is_never` candidate, because that would break binding consistency.
/// - See [`Builder::remove_never_subcandidates`].
subcandidates: Vec<Candidate<'pat, 'tcx>>,
/// ...and if there is a guard it must be evaluated; if it's `false` then branch to `otherwise_block`.
///
/// ---
/// For subcandidates, this is copied from the parent candidate, so it indicates
/// whether the enclosing match arm has a guard.
has_guard: bool,
/// Holds extra pattern data that was prepared by [`FlatPat`], including bindings and
/// ascriptions that must be established if this candidate succeeds.
extra_data: PatternExtraData<'tcx>,
/// When setting `self.subcandidates`, we store here the span of the or-pattern they came from.
///
/// ---
/// Invariant: it is `None` iff `subcandidates.is_empty()`.
/// - FIXME: We sometimes don't unset this when clearing `subcandidates`.
or_span: Option<Span>,
/// The block before the `bindings` have been established.
///
/// After the match tree has been lowered, [`Builder::lower_match_arms`]
/// will use this as the start point for lowering bindings and guards, and
/// then jump to a shared block containing the arm body.
pre_binding_block: Option<BasicBlock>,
/// The block to branch to if the guard or a nested candidate fails to match.
otherwise_block: Option<BasicBlock>,
/// The earliest block that has only candidates >= this one as descendents. Used for false
/// edges, see the doc for [`Builder::match_expr`].
false_edge_start_block: Option<BasicBlock>,
}
impl<'tcx, 'pat> Candidate<'pat, 'tcx> {
fn new(
place: PlaceBuilder<'tcx>,
pattern: &'pat Pat<'tcx>,
has_guard: HasMatchGuard,
cx: &mut Builder<'_, 'tcx>,
) -> Self {
// Use `FlatPat` to build simplified match pairs, then immediately
// incorporate them into a new candidate.
Self::from_flat_pat(
FlatPat::new(place, pattern, cx),
matches!(has_guard, HasMatchGuard::Yes),
)
}
/// Incorporates an already-simplified [`FlatPat`] into a new candidate.
fn from_flat_pat(flat_pat: FlatPat<'pat, 'tcx>, has_guard: bool) -> Self {
Candidate {
match_pairs: flat_pat.match_pairs,
extra_data: flat_pat.extra_data,
has_guard,
subcandidates: Vec::new(),
or_span: None,
otherwise_block: None,
pre_binding_block: None,
false_edge_start_block: None,
}
}
/// Returns whether the first match pair of this candidate is an or-pattern.
fn starts_with_or_pattern(&self) -> bool {
matches!(&*self.match_pairs, [MatchPairTree { test_case: TestCase::Or { .. }, .. }, ..])
}
/// Visit the leaf candidates (those with no subcandidates) contained in
/// this candidate.
fn visit_leaves<'a>(&'a mut self, mut visit_leaf: impl FnMut(&'a mut Self)) {
traverse_candidate(
self,
&mut (),
&mut move |c, _| visit_leaf(c),
move |c, _| c.subcandidates.iter_mut(),
|_| {},
);
}
/// Visit the leaf candidates in reverse order.
fn visit_leaves_rev<'a>(&'a mut self, mut visit_leaf: impl FnMut(&'a mut Self)) {
traverse_candidate(
self,
&mut (),
&mut move |c, _| visit_leaf(c),
move |c, _| c.subcandidates.iter_mut().rev(),
|_| {},
);
}
}
/// A depth-first traversal of the `Candidate` and all of its recursive
/// subcandidates.
///
/// This signature is very generic, to support traversing candidate trees by
/// reference or by value, and to allow a mutable "context" to be shared by the
/// traversal callbacks. Most traversals can use the simpler
/// [`Candidate::visit_leaves`] wrapper instead.
fn traverse_candidate<'pat, 'tcx: 'pat, C, T, I>(
candidate: C,
context: &mut T,
// Called when visiting a "leaf" candidate (with no subcandidates).
visit_leaf: &mut impl FnMut(C, &mut T),
// Called when visiting a "node" candidate (with one or more subcandidates).
// Returns an iterator over the candidate's children (by value or reference).
// Can perform setup before visiting the node's children.
get_children: impl Copy + Fn(C, &mut T) -> I,
// Called after visiting a "node" candidate's children.
complete_children: impl Copy + Fn(&mut T),
) where
C: Borrow<Candidate<'pat, 'tcx>>, // Typically `Candidate` or `&mut Candidate`
I: Iterator<Item = C>,
{
if candidate.borrow().subcandidates.is_empty() {
visit_leaf(candidate, context)
} else {
for child in get_children(candidate, context) {
traverse_candidate(child, context, visit_leaf, get_children, complete_children);
}
complete_children(context)
}
}
#[derive(Clone, Debug)]
struct Binding<'tcx> {
span: Span,
source: Place<'tcx>,
var_id: LocalVarId,
binding_mode: BindingMode,
}
/// Indicates that the type of `source` must be a subtype of the
/// user-given type `user_ty`; this is basically a no-op but can
/// influence region inference.
#[derive(Clone, Debug)]
struct Ascription<'tcx> {
source: Place<'tcx>,
annotation: CanonicalUserTypeAnnotation<'tcx>,
variance: ty::Variance,
}
/// Partial summary of a [`thir::Pat`], indicating what sort of test should be
/// performed to match/reject the pattern, and what the desired test outcome is.
/// This avoids having to perform a full match on [`thir::PatKind`] in some places,
/// and helps [`TestKind::Switch`] and [`TestKind::SwitchInt`] know what target
/// values to use.
///
/// Created by [`MatchPairTree::for_pattern`], and then inspected primarily by:
/// - [`Builder::pick_test_for_match_pair`] (to choose a test)
/// - [`Builder::sort_candidate`] (to see how the test interacts with a match pair)
///
/// Two variants are unlike the others and deserve special mention:
///
/// - [`Self::Irrefutable`] is only used temporarily when building a [`MatchPairTree`].
/// They are then flattened away by [`Builder::simplify_match_pairs`], with any
/// bindings/ascriptions incorporated into the enclosing [`FlatPat`].
/// - [`Self::Or`] are not tested directly like the other variants. Instead they
/// participate in or-pattern expansion, where they are transformed into subcandidates.
/// - See [`Builder::expand_and_match_or_candidates`].
#[derive(Debug, Clone)]
enum TestCase<'pat, 'tcx> {
Irrefutable { binding: Option<Binding<'tcx>>, ascription: Option<Ascription<'tcx>> },
Variant { adt_def: ty::AdtDef<'tcx>, variant_index: VariantIdx },
Constant { value: mir::Const<'tcx> },
Range(&'pat PatRange<'tcx>),
Slice { len: usize, variable_length: bool },
Deref { temp: Place<'tcx>, mutability: Mutability },
Never,
Or { pats: Box<[FlatPat<'pat, 'tcx>]> },
}
impl<'pat, 'tcx> TestCase<'pat, 'tcx> {
fn as_range(&self) -> Option<&'pat PatRange<'tcx>> {
if let Self::Range(v) = self { Some(*v) } else { None }
}
}
/// Node in a tree of "match pairs", where each pair consists of a place to be
/// tested, and a test to perform on that place.
///
/// Each node also has a list of subpairs (possibly empty) that must also match,
/// and a reference to the THIR pattern it represents.
#[derive(Debug, Clone)]
pub(crate) struct MatchPairTree<'pat, 'tcx> {
/// This place...
///
/// ---
/// This can be `None` if it referred to a non-captured place in a closure.
///
/// Invariant: Can only be `None` when `test_case` is `Irrefutable`.
/// Therefore this must be `Some(_)` after simplification.
place: Option<Place<'tcx>>,
/// ... must pass this test...
///
/// ---
/// Invariant: after creation and simplification in [`FlatPat::new`],
/// this must not be [`TestCase::Irrefutable`].
test_case: TestCase<'pat, 'tcx>,
/// ... and these subpairs must match.
///
/// ---
/// Subpairs typically represent tests that can only be performed after their
/// parent has succeeded. For example, the pattern `Some(3)` might have an
/// outer match pair that tests for the variant `Some`, and then a subpair
/// that tests its field for the value `3`.
subpairs: Vec<Self>,
/// The pattern this was created from.
pattern: &'pat Pat<'tcx>,
}
/// See [`Test`] for more.
#[derive(Clone, Debug, PartialEq)]
enum TestKind<'tcx> {
/// Test what enum variant a value is.
///
/// The subset of expected variants is not stored here; instead they are
/// extracted from the [`TestCase`]s of the candidates participating in the
/// test.
Switch {
/// The enum type being tested.
adt_def: ty::AdtDef<'tcx>,
},
/// Test what value an integer or `char` has.
///
/// The test's target values are not stored here; instead they are extracted
/// from the [`TestCase`]s of the candidates participating in the test.
SwitchInt,
/// Test whether a `bool` is `true` or `false`.
If,
/// Test for equality with value, possibly after an unsizing coercion to
/// `ty`,
Eq {
value: Const<'tcx>,
// Integer types are handled by `SwitchInt`, and constants with ADT
// types are converted back into patterns, so this can only be `&str`,
// `&[T]`, `f32` or `f64`.
ty: Ty<'tcx>,
},
/// Test whether the value falls within an inclusive or exclusive range.
Range(Box<PatRange<'tcx>>),
/// Test that the length of the slice is `== len` or `>= len`.
Len { len: u64, op: BinOp },
/// Call `Deref::deref[_mut]` on the value.
Deref {
/// Temporary to store the result of `deref()`/`deref_mut()`.
temp: Place<'tcx>,
mutability: Mutability,
},
/// Assert unreachability of never patterns.
Never,
}
/// A test to perform to determine which [`Candidate`] matches a value.
///
/// [`Test`] is just the test to perform; it does not include the value
/// to be tested.
#[derive(Debug)]
pub(crate) struct Test<'tcx> {
span: Span,
kind: TestKind<'tcx>,
}
/// The branch to be taken after a test.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
enum TestBranch<'tcx> {
/// Success branch, used for tests with two possible outcomes.
Success,
/// Branch corresponding to this constant.
Constant(Const<'tcx>, u128),
/// Branch corresponding to this variant.
Variant(VariantIdx),
/// Failure branch for tests with two possible outcomes, and "otherwise" branch for other tests.
Failure,
}
impl<'tcx> TestBranch<'tcx> {
fn as_constant(&self) -> Option<&Const<'tcx>> {
if let Self::Constant(v, _) = self { Some(v) } else { None }
}
}
/// `ArmHasGuard` is a wrapper around a boolean flag. It indicates whether
/// a match arm has a guard expression attached to it.
#[derive(Copy, Clone, Debug)]
pub(crate) struct ArmHasGuard(pub(crate) bool);
///////////////////////////////////////////////////////////////////////////
// Main matching algorithm
/// A sub-branch in the output of match lowering. Match lowering has generated MIR code that will
/// branch to `success_block` when the matched value matches the corresponding pattern. If there is
/// a guard, its failure must continue to `otherwise_block`, which will resume testing patterns.
#[derive(Debug)]
struct MatchTreeSubBranch<'tcx> {
span: Span,
/// The block that is branched to if the corresponding subpattern matches.
success_block: BasicBlock,
/// The block to branch to if this arm had a guard and the guard fails.
otherwise_block: BasicBlock,
/// The bindings to set up in this sub-branch.
bindings: Vec<Binding<'tcx>>,
/// The ascriptions to set up in this sub-branch.
ascriptions: Vec<Ascription<'tcx>>,
/// Whether the sub-branch corresponds to a never pattern.
is_never: bool,
}
/// A branch in the output of match lowering.
#[derive(Debug)]
struct MatchTreeBranch<'tcx> {
sub_branches: Vec<MatchTreeSubBranch<'tcx>>,
}
/// The result of generating MIR for a pattern-matching expression. Each input branch/arm/pattern
/// gives rise to an output `MatchTreeBranch`. If one of the patterns matches, we branch to the
/// corresponding `success_block`. If none of the patterns matches, we branch to `otherwise_block`.
///
/// Each branch is made of one of more sub-branches, corresponding to or-patterns. E.g.
/// ```ignore(illustrative)
/// match foo {
/// (x, false) | (false, x) => {}
/// (true, true) => {}
/// }
/// ```
/// Here the first arm gives the first `MatchTreeBranch`, which has two sub-branches, one for each
/// alternative of the or-pattern. They are kept separate because each needs to bind `x` to a
/// different place.
#[derive(Debug)]
struct BuiltMatchTree<'tcx> {
branches: Vec<MatchTreeBranch<'tcx>>,
otherwise_block: BasicBlock,
/// If any of the branches had a guard, we collect here the places and locals to fakely borrow
/// to ensure match guards can't modify the values as we match them. For more details, see
/// [`util::collect_fake_borrows`].
fake_borrow_temps: Vec<(Place<'tcx>, Local, FakeBorrowKind)>,
}
impl<'tcx> MatchTreeSubBranch<'tcx> {
fn from_sub_candidate(
candidate: Candidate<'_, 'tcx>,
parent_data: &Vec<PatternExtraData<'tcx>>,
) -> Self {
debug_assert!(candidate.match_pairs.is_empty());
MatchTreeSubBranch {
span: candidate.extra_data.span,
success_block: candidate.pre_binding_block.unwrap(),
otherwise_block: candidate.otherwise_block.unwrap(),
bindings: parent_data
.iter()
.flat_map(|d| &d.bindings)
.chain(&candidate.extra_data.bindings)
.cloned()
.collect(),
ascriptions: parent_data
.iter()
.flat_map(|d| &d.ascriptions)
.cloned()
.chain(candidate.extra_data.ascriptions)
.collect(),
is_never: candidate.extra_data.is_never,
}
}
}
impl<'tcx> MatchTreeBranch<'tcx> {
fn from_candidate(candidate: Candidate<'_, 'tcx>) -> Self {
let mut sub_branches = Vec::new();
traverse_candidate(
candidate,
&mut Vec::new(),
&mut |candidate: Candidate<'_, '_>, parent_data: &mut Vec<PatternExtraData<'_>>| {
sub_branches.push(MatchTreeSubBranch::from_sub_candidate(candidate, parent_data));
},
|inner_candidate, parent_data| {
parent_data.push(inner_candidate.extra_data);
inner_candidate.subcandidates.into_iter()
},
|parent_data| {
parent_data.pop();
},
);
MatchTreeBranch { sub_branches }
}
}
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
enum HasMatchGuard {
Yes,
No,
}
impl<'a, 'tcx> Builder<'a, 'tcx> {
/// The entrypoint of the matching algorithm. Create the decision tree for the match expression,
/// starting from `block`.
///
/// `patterns` is a list of patterns, one for each arm. The associated boolean indicates whether
/// the arm has a guard.
///
/// `refutable` indicates whether the candidate list is refutable (for `if let` and `let else`)
/// or not (for `let` and `match`). In the refutable case we return the block to which we branch
/// on failure.
fn lower_match_tree<'pat>(
&mut self,
block: BasicBlock,
scrutinee_span: Span,
scrutinee_place_builder: &PlaceBuilder<'tcx>,
match_start_span: Span,
patterns: Vec<(&'pat Pat<'tcx>, HasMatchGuard)>,
refutable: bool,
) -> BuiltMatchTree<'tcx>
where
'tcx: 'pat,
{
// Assemble the initial list of candidates. These top-level candidates are 1:1 with the
// input patterns, but other parts of match lowering also introduce subcandidates (for
// sub-or-patterns). So inside the algorithm, the candidates list may not correspond to
// match arms directly.
let mut candidates: Vec<Candidate<'_, '_>> = patterns
.into_iter()
.map(|(pat, has_guard)| {
Candidate::new(scrutinee_place_builder.clone(), pat, has_guard, self)
})
.collect();
let fake_borrow_temps = util::collect_fake_borrows(
self,
&candidates,
scrutinee_span,
scrutinee_place_builder.base(),
);
// This will generate code to test scrutinee_place and branch to the appropriate arm block.
// If none of the arms match, we branch to `otherwise_block`. When lowering a `match`
// expression, exhaustiveness checking ensures that this block is unreachable.
let mut candidate_refs = candidates.iter_mut().collect::<Vec<_>>();
let otherwise_block =
self.match_candidates(match_start_span, scrutinee_span, block, &mut candidate_refs);
// Set up false edges so that the borrow-checker cannot make use of the specific CFG we
// generated. We falsely branch from each candidate to the one below it to make it as if we
// were testing match branches one by one in order. In the refutable case we also want a
// false edge to the final failure block.
let mut next_candidate_start_block = if refutable { Some(otherwise_block) } else { None };
for candidate in candidates.iter_mut().rev() {
let has_guard = candidate.has_guard;
candidate.visit_leaves_rev(|leaf_candidate| {
if let Some(next_candidate_start_block) = next_candidate_start_block {
let source_info = self.source_info(leaf_candidate.extra_data.span);
// Falsely branch to `next_candidate_start_block` before reaching pre_binding.
let old_pre_binding = leaf_candidate.pre_binding_block.unwrap();
let new_pre_binding = self.cfg.start_new_block();
self.false_edges(
old_pre_binding,
new_pre_binding,
next_candidate_start_block,
source_info,
);
leaf_candidate.pre_binding_block = Some(new_pre_binding);
if has_guard {
// Falsely branch to `next_candidate_start_block` also if the guard fails.
let new_otherwise = self.cfg.start_new_block();
let old_otherwise = leaf_candidate.otherwise_block.unwrap();
self.false_edges(
new_otherwise,
old_otherwise,
next_candidate_start_block,
source_info,
);
leaf_candidate.otherwise_block = Some(new_otherwise);
}
}
assert!(leaf_candidate.false_edge_start_block.is_some());
next_candidate_start_block = leaf_candidate.false_edge_start_block;
});
}
if !refutable {
// Match checking ensures `otherwise_block` is actually unreachable in irrefutable
// cases.
let source_info = self.source_info(scrutinee_span);
// Matching on a scrutinee place of an uninhabited type doesn't generate any memory
// reads by itself, and so if the place is uninitialized we wouldn't know. In order to
// disallow the following:
// ```rust
// let x: !;
// match x {}
// ```
// we add a dummy read on the place.
//
// NOTE: If we require never patterns for empty matches, those will check that the place
// is initialized, and so this read would no longer be needed.
let cause_matched_place = FakeReadCause::ForMatchedPlace(None);
if let Some(scrutinee_place) = scrutinee_place_builder.try_to_place(self) {
self.cfg.push_fake_read(
otherwise_block,
source_info,
cause_matched_place,
scrutinee_place,
);
}
self.cfg.terminate(otherwise_block, source_info, TerminatorKind::Unreachable);
}
BuiltMatchTree {
branches: candidates.into_iter().map(MatchTreeBranch::from_candidate).collect(),
otherwise_block,
fake_borrow_temps,
}
}
/// The main match algorithm. It begins with a set of candidates `candidates` and has the job of
/// generating code that branches to an appropriate block if the scrutinee matches one of these
/// candidates. The
/// candidates are ordered such that the first item in the list
/// has the highest priority. When a candidate is found to match
/// the value, we will set and generate a branch to the appropriate
/// pre-binding block.
///
/// If none of the candidates apply, we continue to the returned `otherwise_block`.
///
/// Note that while `match` expressions in the Rust language are exhaustive,
/// candidate lists passed to this method are often _non-exhaustive_.
/// For example, the match lowering process will frequently divide up the
/// list of candidates, and recursively call this method with a non-exhaustive
/// subset of candidates.
/// See [`Builder::test_candidates`] for more details on this
/// "backtracking automata" approach.
///
/// For an example of how we use `otherwise_block`, consider:
/// ```
/// # fn foo((x, y): (bool, bool)) -> u32 {
/// match (x, y) {
/// (true, true) => 1,
/// (_, false) => 2,
/// (false, true) => 3,
/// }
/// # }
/// ```
/// For this match, we generate something like:
/// ```
/// # fn foo((x, y): (bool, bool)) -> u32 {
/// if x {
/// if y {
/// return 1
/// } else {
/// // continue
/// }
/// } else {
/// // continue
/// }
/// if y {
/// if x {
/// // This is actually unreachable because the `(true, true)` case was handled above,
/// // but we don't know that from within the lowering algorithm.
/// // continue
/// } else {
/// return 3
/// }
/// } else {
/// return 2
/// }
/// // this is the final `otherwise_block`, which is unreachable because the match was exhaustive.
/// unreachable!()
/// # }
/// ```
///
/// Every `continue` is an instance of branching to some `otherwise_block` somewhere deep within
/// the algorithm. For more details on why we lower like this, see [`Builder::test_candidates`].
///
/// Note how we test `x` twice. This is the tradeoff of backtracking automata: we prefer smaller
/// code size so we accept non-optimal code paths.
#[instrument(skip(self), level = "debug")]
fn match_candidates(
&mut self,
span: Span,
scrutinee_span: Span,
start_block: BasicBlock,
candidates: &mut [&mut Candidate<'_, 'tcx>],
) -> BasicBlock {
ensure_sufficient_stack(|| {
self.match_candidates_inner(span, scrutinee_span, start_block, candidates)
})
}
/// Construct the decision tree for `candidates`. Don't call this, call `match_candidates`
/// instead to reserve sufficient stack space.
fn match_candidates_inner(
&mut self,
span: Span,
scrutinee_span: Span,
mut start_block: BasicBlock,
candidates: &mut [&mut Candidate<'_, 'tcx>],
) -> BasicBlock {
if let [first, ..] = candidates {
if first.false_edge_start_block.is_none() {
first.false_edge_start_block = Some(start_block);
}
}
// Process a prefix of the candidates.
let rest = match candidates {
[] => {
// If there are no candidates that still need testing, we're done.
return start_block;
}
[first, remaining @ ..] if first.match_pairs.is_empty() => {
// The first candidate has satisfied all its match pairs.
// We record the blocks that will be needed by match arm lowering,
// and then continue with the remaining candidates.
let remainder_start = self.select_matched_candidate(first, start_block);
remainder_start.and(remaining)
}
candidates if candidates.iter().any(|candidate| candidate.starts_with_or_pattern()) => {
// If any candidate starts with an or-pattern, we want to expand or-patterns
// before we do any more tests.
//
// The only candidate we strictly _need_ to expand here is the first one.
// But by expanding other candidates as early as possible, we unlock more
// opportunities to include them in test outcomes, making the match tree
// smaller and simpler.
self.expand_and_match_or_candidates(span, scrutinee_span, start_block, candidates)
}
candidates => {
// The first candidate has some unsatisfied match pairs; we proceed to do more tests.
self.test_candidates(span, scrutinee_span, candidates, start_block)
}
};
// Process any candidates that remain.
let remaining_candidates = unpack!(start_block = rest);
self.match_candidates(span, scrutinee_span, start_block, remaining_candidates)
}
/// Link up matched candidates.
///
/// For example, if we have something like this:
///
/// ```ignore (illustrative)
/// ...
/// Some(x) if cond1 => ...
/// Some(x) => ...
/// Some(x) if cond2 => ...
/// ...
/// ```
///
/// We generate real edges from:
///
/// * `start_block` to the [pre-binding block] of the first pattern,
/// * the [otherwise block] of the first pattern to the second pattern,
/// * the [otherwise block] of the third pattern to a block with an
/// [`Unreachable` terminator](TerminatorKind::Unreachable).
///
/// In addition, we later add fake edges from the otherwise blocks to the
/// pre-binding block of the next candidate in the original set of
/// candidates.
///
/// [pre-binding block]: Candidate::pre_binding_block
/// [otherwise block]: Candidate::otherwise_block
fn select_matched_candidate(
&mut self,
candidate: &mut Candidate<'_, 'tcx>,
start_block: BasicBlock,
) -> BasicBlock {
assert!(candidate.otherwise_block.is_none());
assert!(candidate.pre_binding_block.is_none());
assert!(candidate.subcandidates.is_empty());
candidate.pre_binding_block = Some(start_block);
let otherwise_block = self.cfg.start_new_block();
// Create the otherwise block for this candidate, which is the
// pre-binding block for the next candidate.
candidate.otherwise_block = Some(otherwise_block);
otherwise_block
}
/// Takes a list of candidates such that some of the candidates' first match pairs are
/// or-patterns. This expands as many or-patterns as possible and processes the resulting
/// candidates. Returns the unprocessed candidates if any.
fn expand_and_match_or_candidates<'pat, 'b, 'c>(
&mut self,
span: Span,
scrutinee_span: Span,
start_block: BasicBlock,
candidates: &'b mut [&'c mut Candidate<'pat, 'tcx>],
) -> BlockAnd<&'b mut [&'c mut Candidate<'pat, 'tcx>]> {
// We can't expand or-patterns freely. The rule is:
// - If a candidate doesn't start with an or-pattern, we include it in
// the expansion list as-is (i.e. it "expands" to itself).
// - If a candidate has an or-pattern as its only remaining match pair,
// we can expand it.
// - If it starts with an or-pattern but also has other match pairs,
// we can expand it, but we can't process more candidates after it.
//
// If we didn't stop, the `otherwise` cases could get mixed up. E.g. in the
// following, or-pattern simplification (in `merge_trivial_subcandidates`) makes it
// so the `1` and `2` cases branch to a same block (which then tests `false`). If we
// took `(2, _)` in the same set of candidates, when we reach the block that tests
// `false` we don't know whether we came from `1` or `2`, hence we can't know where
// to branch on failure.
//
// ```ignore(illustrative)
// match (1, true) {
// (1 | 2, false) => {},
// (2, _) => {},
// _ => {}
// }
// ```
//
// We therefore split the `candidates` slice in two, expand or-patterns in the first part,
// and process the rest separately.
let expand_until = candidates
.iter()
.position(|candidate| {
// If a candidate starts with an or-pattern and has more match pairs,
// we can expand it, but we must stop expanding _after_ it.
candidate.match_pairs.len() > 1 && candidate.starts_with_or_pattern()
})
.map(|pos| pos + 1) // Stop _after_ the found candidate
.unwrap_or(candidates.len()); // Otherwise, include all candidates
let (candidates_to_expand, remaining_candidates) = candidates.split_at_mut(expand_until);
// Expand one level of or-patterns for each candidate in `candidates_to_expand`.
// We take care to preserve the relative ordering of candidates, so that
// or-patterns are expanded in their parent's relative position.
let mut expanded_candidates = Vec::new();
for candidate in candidates_to_expand.iter_mut() {
if candidate.starts_with_or_pattern() {
let or_match_pair = candidate.match_pairs.remove(0);
// Expand the or-pattern into subcandidates.
self.create_or_subcandidates(candidate, or_match_pair);
// Collect the newly created subcandidates.
for subcandidate in candidate.subcandidates.iter_mut() {
expanded_candidates.push(subcandidate);
}
// Note that the subcandidates have been added to `expanded_candidates`,
// but `candidate` itself has not. If the last candidate has more match pairs,
// they are handled separately by `test_remaining_match_pairs_after_or`.
} else {
// A candidate that doesn't start with an or-pattern has nothing to
// expand, so it is included in the post-expansion list as-is.
expanded_candidates.push(candidate);
}
}
// Recursively lower the part of the match tree represented by the
// expanded candidates. This is where subcandidates actually get lowered!
let remainder_start = self.match_candidates(
span,
scrutinee_span,
start_block,
expanded_candidates.as_mut_slice(),
);
// Postprocess subcandidates, and process any leftover match pairs.
// (Only the last candidate can possibly have more match pairs.)
debug_assert!({
let mut all_except_last = candidates_to_expand.iter().rev().skip(1);
all_except_last.all(|candidate| candidate.match_pairs.is_empty())
});
for candidate in candidates_to_expand.iter_mut() {
if !candidate.subcandidates.is_empty() {
self.merge_trivial_subcandidates(candidate);
self.remove_never_subcandidates(candidate);
}
}
// It's important to perform the above simplifications _before_ dealing
// with remaining match pairs, to avoid exponential blowup if possible
// (for trivial or-patterns), and avoid useless work (for never patterns).
if let Some(last_candidate) = candidates_to_expand.last_mut() {
self.test_remaining_match_pairs_after_or(span, scrutinee_span, last_candidate);
}
remainder_start.and(remaining_candidates)
}
/// Given a match-pair that corresponds to an or-pattern, expand each subpattern into a new
/// subcandidate. Any candidate that has been expanded this way should also be postprocessed
/// at the end of [`Self::expand_and_match_or_candidates`].
fn create_or_subcandidates<'pat>(
&mut self,
candidate: &mut Candidate<'pat, 'tcx>,
match_pair: MatchPairTree<'pat, 'tcx>,
) {
let TestCase::Or { pats } = match_pair.test_case else { bug!() };
debug!("expanding or-pattern: candidate={:#?}\npats={:#?}", candidate, pats);
candidate.or_span = Some(match_pair.pattern.span);
candidate.subcandidates = pats
.into_vec()
.into_iter()
.map(|flat_pat| Candidate::from_flat_pat(flat_pat, candidate.has_guard))
.collect();
candidate.subcandidates[0].false_edge_start_block = candidate.false_edge_start_block;
}
/// Try to merge all of the subcandidates of the given candidate into one. This avoids
/// exponentially large CFGs in cases like `(1 | 2, 3 | 4, ...)`. The candidate should have been
/// expanded with `create_or_subcandidates`.
///
/// Given a pattern `(P | Q, R | S)` we (in principle) generate a CFG like
/// so:
///
/// ```text
/// [ start ]
/// |
/// [ match P, Q ]
/// |
/// +----------------------------------------+------------------------------------+
/// | | |
/// V V V
/// [ P matches ] [ Q matches ] [ otherwise ]
/// | | |
/// V V |
/// [ match R, S ] [ match R, S ] |
/// | | |
/// +--------------+------------+ +--------------+------------+ |
/// | | | | | | |
/// V V V V V V |
/// [ R matches ] [ S matches ] [otherwise ] [ R matches ] [ S matches ] [otherwise ] |
/// | | | | | | |
/// +--------------+------------|------------+--------------+ | |
/// | | | |
/// | +----------------------------------------+--------+
/// | |
/// V V
/// [ Success ] [ Failure ]
/// ```
///
/// In practice there are some complications:
///
/// * If there's a guard, then the otherwise branch of the first match on
/// `R | S` goes to a test for whether `Q` matches, and the control flow
/// doesn't merge into a single success block until after the guard is
/// tested.
/// * If neither `P` or `Q` has any bindings or type ascriptions and there
/// isn't a match guard, then we create a smaller CFG like:
///
/// ```text
/// ...
/// +---------------+------------+
/// | | |
/// [ P matches ] [ Q matches ] [ otherwise ]
/// | | |
/// +---------------+ |
/// | ...
/// [ match R, S ]
/// |
/// ...
/// ```
///
/// Note that this takes place _after_ the subcandidates have participated
/// in match tree lowering.
fn merge_trivial_subcandidates(&mut self, candidate: &mut Candidate<'_, 'tcx>) {
assert!(!candidate.subcandidates.is_empty());
if candidate.has_guard {
// FIXME(or_patterns; matthewjasper) Don't give up if we have a guard.
return;
}
// FIXME(or_patterns; matthewjasper) Try to be more aggressive here.
let can_merge = candidate.subcandidates.iter().all(|subcandidate| {
subcandidate.subcandidates.is_empty() && subcandidate.extra_data.is_empty()
});
if !can_merge {
return;
}
let mut last_otherwise = None;
let shared_pre_binding_block = self.cfg.start_new_block();
// This candidate is about to become a leaf, so unset `or_span`.
let or_span = candidate.or_span.take().unwrap();
let source_info = self.source_info(or_span);
if candidate.false_edge_start_block.is_none() {
candidate.false_edge_start_block = candidate.subcandidates[0].false_edge_start_block;
}
// Remove the (known-trivial) subcandidates from the candidate tree,
// so that they aren't visible after match tree lowering, and wire them
// all to join up at a single shared pre-binding block.
// (Note that the subcandidates have already had their part of the match
// tree lowered by this point, which is why we can add a goto to them.)
for subcandidate in mem::take(&mut candidate.subcandidates) {
let subcandidate_block = subcandidate.pre_binding_block.unwrap();
self.cfg.goto(subcandidate_block, source_info, shared_pre_binding_block);
last_otherwise = subcandidate.otherwise_block;
}
candidate.pre_binding_block = Some(shared_pre_binding_block);
assert!(last_otherwise.is_some());
candidate.otherwise_block = last_otherwise;
}
/// Never subcandidates may have a set of bindings inconsistent with their siblings,
/// which would break later code. So we filter them out. Note that we can't filter out
/// top-level candidates this way.
fn remove_never_subcandidates(&mut self, candidate: &mut Candidate<'_, 'tcx>) {
if candidate.subcandidates.is_empty() {
return;
}
candidate.subcandidates.retain_mut(|candidate| {
if candidate.extra_data.is_never {
candidate.visit_leaves(|subcandidate| {
let block = subcandidate.pre_binding_block.unwrap();
// That block is already unreachable but needs a terminator to make the MIR well-formed.
let source_info = self.source_info(subcandidate.extra_data.span);
self.cfg.terminate(block, source_info, TerminatorKind::Unreachable);
});
false
} else {
true
}
});
if candidate.subcandidates.is_empty() {
// If `candidate` has become a leaf candidate, ensure it has a `pre_binding_block`.
candidate.pre_binding_block = Some(self.cfg.start_new_block());
}
}
/// If more match pairs remain, test them after each subcandidate.
/// We could have added them to the or-candidates during or-pattern expansion, but that
/// would make it impossible to detect simplifiable or-patterns. That would guarantee
/// exponentially large CFGs for cases like `(1 | 2, 3 | 4, ...)`.
fn test_remaining_match_pairs_after_or(
&mut self,
span: Span,
scrutinee_span: Span,
candidate: &mut Candidate<'_, 'tcx>,
) {
if candidate.match_pairs.is_empty() {
return;
}
let or_span = candidate.or_span.unwrap_or(candidate.extra_data.span);
let source_info = self.source_info(or_span);
let mut last_otherwise = None;
candidate.visit_leaves(|leaf_candidate| {
last_otherwise = leaf_candidate.otherwise_block;
});
let remaining_match_pairs = mem::take(&mut candidate.match_pairs);
// We're testing match pairs that remained after an `Or`, so the remaining
// pairs should all be `Or` too, due to the sorting invariant.
debug_assert!(
remaining_match_pairs
.iter()
.all(|match_pair| matches!(match_pair.test_case, TestCase::Or { .. }))
);
// Visit each leaf candidate within this subtree, add a copy of the remaining
// match pairs to it, and then recursively lower the rest of the match tree
// from that point.
candidate.visit_leaves(|leaf_candidate| {
// At this point the leaf's own match pairs have all been lowered
// and removed, so `extend` and assignment are equivalent,
// but extending can also recycle any existing vector capacity.
assert!(leaf_candidate.match_pairs.is_empty());
leaf_candidate.match_pairs.extend(remaining_match_pairs.iter().cloned());
let or_start = leaf_candidate.pre_binding_block.unwrap();
let otherwise =
self.match_candidates(span, scrutinee_span, or_start, &mut [leaf_candidate]);
// In a case like `(P | Q, R | S)`, if `P` succeeds and `R | S` fails, we know `(Q,
// R | S)` will fail too. If there is no guard, we skip testing of `Q` by branching
// directly to `last_otherwise`. If there is a guard,
// `leaf_candidate.otherwise_block` can be reached by guard failure as well, so we
// can't skip `Q`.
let or_otherwise = if leaf_candidate.has_guard {
leaf_candidate.otherwise_block.unwrap()
} else {
last_otherwise.unwrap()
};
self.cfg.goto(otherwise, source_info, or_otherwise);
});
}
/// Pick a test to run. Which test doesn't matter as long as it is guaranteed to fully match at
/// least one match pair. We currently simply pick the test corresponding to the first match
/// pair of the first candidate in the list.
///
/// *Note:* taking the first match pair is somewhat arbitrary, and we might do better here by
/// choosing more carefully what to test.
///
/// For example, consider the following possible match-pairs:
///
/// 1. `x @ Some(P)` -- we will do a [`Switch`] to decide what variant `x` has
/// 2. `x @ 22` -- we will do a [`SwitchInt`] to decide what value `x` has
/// 3. `x @ 3..5` -- we will do a [`Range`] test to decide what range `x` falls in
/// 4. etc.
///
/// [`Switch`]: TestKind::Switch
/// [`SwitchInt`]: TestKind::SwitchInt
/// [`Range`]: TestKind::Range
fn pick_test(&mut self, candidates: &[&mut Candidate<'_, 'tcx>]) -> (Place<'tcx>, Test<'tcx>) {
// Extract the match-pair from the highest priority candidate
let match_pair = &candidates[0].match_pairs[0];
let test = self.pick_test_for_match_pair(match_pair);
// Unwrap is ok after simplification.
let match_place = match_pair.place.unwrap();
debug!(?test, ?match_pair);
(match_place, test)
}
/// Given a test, we partition the input candidates into several buckets.
/// If a candidate matches in exactly one of the branches of `test`
/// (and no other branches), we put it into the corresponding bucket.
/// If it could match in more than one of the branches of `test`, the test
/// doesn't usefully apply to it, and we stop partitioning candidates.
///
/// Importantly, we also **mutate** the branched candidates to remove match pairs
/// that are entailed by the outcome of the test, and add any sub-pairs of the
/// removed pairs.
///
/// This returns a pair of
/// - the candidates that weren't sorted;
/// - for each possible outcome of the test, the candidates that match in that outcome.
///
/// For example:
/// ```
/// # let (x, y, z) = (true, true, true);
/// match (x, y, z) {
/// (true , _ , true ) => true, // (0)
/// (false, false, _ ) => false, // (1)
/// (_ , true , _ ) => true, // (2)
/// (true , _ , false) => false, // (3)
/// }
/// # ;
/// ```
///
/// Assume we are testing on `x`. Conceptually, there are 2 overlapping candidate sets:
/// - If the outcome is that `x` is true, candidates {0, 2, 3} are possible
/// - If the outcome is that `x` is false, candidates {1, 2} are possible
///
/// Following our algorithm:
/// - Candidate 0 is sorted into outcome `x == true`
/// - Candidate 1 is sorted into outcome `x == false`
/// - Candidate 2 remains unsorted, because testing `x` has no effect on it
/// - Candidate 3 remains unsorted, because a previous candidate (2) was unsorted
/// - This helps preserve the illusion that candidates are tested "in order"
///
/// The sorted candidates are mutated to remove entailed match pairs:
/// - candidate 0 becomes `[z @ true]` since we know that `x` was `true`;
/// - candidate 1 becomes `[y @ false]` since we know that `x` was `false`.
fn sort_candidates<'b, 'c, 'pat>(
&mut self,
match_place: Place<'tcx>,
test: &Test<'tcx>,
mut candidates: &'b mut [&'c mut Candidate<'pat, 'tcx>],
) -> (
&'b mut [&'c mut Candidate<'pat, 'tcx>],
FxIndexMap<TestBranch<'tcx>, Vec<&'b mut Candidate<'pat, 'tcx>>>,
) {
// For each of the possible outcomes, collect vector of candidates that apply if the test
// has that particular outcome.
let mut target_candidates: FxIndexMap<_, Vec<&mut Candidate<'_, '_>>> = Default::default();
let total_candidate_count = candidates.len();
// Sort the candidates into the appropriate vector in `target_candidates`. Note that at some
// point we may encounter a candidate where the test is not relevant; at that point, we stop
// sorting.
while let Some(candidate) = candidates.first_mut() {
let Some(branch) =
self.sort_candidate(match_place, test, candidate, &target_candidates)
else {
break;
};
let (candidate, rest) = candidates.split_first_mut().unwrap();
target_candidates.entry(branch).or_insert_with(Vec::new).push(candidate);
candidates = rest;
}
// At least the first candidate ought to be tested
assert!(
total_candidate_count > candidates.len(),
"{total_candidate_count}, {candidates:#?}"
);
debug!("tested_candidates: {}", total_candidate_count - candidates.len());
debug!("untested_candidates: {}", candidates.len());
(candidates, target_candidates)
}
/// This is the most subtle part of the match lowering algorithm. At this point, there are
/// no fully-satisfied candidates, and no or-patterns to expand, so we actually need to
/// perform some sort of test to make progress.
///
/// Once we pick what sort of test we are going to perform, this test will help us winnow down
/// our candidates. So we walk over the candidates (from high to low priority) and check. We
/// compute, for each outcome of the test, a list of (modified) candidates. If a candidate
/// matches in exactly one branch of our test, we add it to the corresponding outcome. We also
/// **mutate its list of match pairs** if appropriate, to reflect the fact that we know which
/// outcome occurred.
///
/// For example, if we are testing `x.0`'s variant, and we have a candidate `(x.0 @ Some(v), x.1
/// @ 22)`, then we would have a resulting candidate of `((x.0 as Some).0 @ v, x.1 @ 22)` in the
/// branch corresponding to `Some`. To ensure we make progress, we always pick a test that
/// results in simplifying the first candidate.
///
/// But there may also be candidates that the test doesn't
/// apply to. The classical example is wildcards:
///
/// ```
/// # let (x, y, z) = (true, true, true);
/// match (x, y, z) {
/// (true , _ , true ) => true, // (0)
/// (false, false, _ ) => false, // (1)
/// (_ , true , _ ) => true, // (2)
/// (true , _ , false) => false, // (3)
/// }
/// # ;
/// ```
///
/// Here, the traditional "decision tree" method would generate 2 separate code-paths for the 2
/// possible values of `x`. This would however duplicate some candidates, which would need to be
/// lowered several times.
///
/// In some cases, this duplication can create an exponential amount of
/// code. This is most easily seen by noticing that this method terminates
/// with precisely the reachable arms being reachable - but that problem
/// is trivially NP-complete:
///
/// ```ignore (illustrative)
/// match (var0, var1, var2, var3, ...) {
/// (true , _ , _ , false, true, ...) => false,
/// (_ , true, true , false, _ , ...) => false,
/// (false, _ , false, false, _ , ...) => false,
/// ...
/// _ => true
/// }
/// ```
///
/// Here the last arm is reachable only if there is an assignment to
/// the variables that does not match any of the literals. Therefore,
/// compilation would take an exponential amount of time in some cases.
///
/// In rustc, we opt instead for the "backtracking automaton" approach. This guarantees we never
/// duplicate a candidate (except in the presence of or-patterns). In fact this guarantee is
/// ensured by the fact that we carry around `&mut Candidate`s which can't be duplicated.
///
/// To make this work, whenever we decide to perform a test, if we encounter a candidate that
/// could match in more than one branch of the test, we stop. We generate code for the test and
/// for the candidates in its branches; the remaining candidates will be tested if the
/// candidates in the branches fail to match.
///
/// For example, if we test on `x` in the following:
/// ```
/// # fn foo((x, y, z): (bool, bool, bool)) -> u32 {
/// match (x, y, z) {
/// (true , _ , true ) => 0,
/// (false, false, _ ) => 1,
/// (_ , true , _ ) => 2,
/// (true , _ , false) => 3,
/// }
/// # }
/// ```
/// this function generates code that looks more of less like:
/// ```
/// # fn foo((x, y, z): (bool, bool, bool)) -> u32 {
/// if x {
/// match (y, z) {
/// (_, true) => return 0,
/// _ => {} // continue matching
/// }
/// } else {
/// match (y, z) {
/// (false, _) => return 1,
/// _ => {} // continue matching
/// }
/// }
/// // the block here is `remainder_start`
/// match (x, y, z) {
/// (_ , true , _ ) => 2,
/// (true , _ , false) => 3,
/// _ => unreachable!(),
/// }
/// # }
/// ```
///
/// We return the unprocessed candidates.
fn test_candidates<'pat, 'b, 'c>(
&mut self,
span: Span,
scrutinee_span: Span,
candidates: &'b mut [&'c mut Candidate<'pat, 'tcx>],
start_block: BasicBlock,
) -> BlockAnd<&'b mut [&'c mut Candidate<'pat, 'tcx>]> {
// Choose a match pair from the first candidate, and use it to determine a
// test to perform that will confirm or refute that match pair.
let (match_place, test) = self.pick_test(candidates);
// For each of the N possible test outcomes, build the vector of candidates that applies if
// the test has that particular outcome. This also mutates the candidates to remove match
// pairs that are fully satisfied by the relevant outcome.
let (remaining_candidates, target_candidates) =
self.sort_candidates(match_place, &test, candidates);
// The block that we should branch to if none of the `target_candidates` match.
let remainder_start = self.cfg.start_new_block();
// For each outcome of the test, recursively lower the rest of the match tree
// from that point. (Note that we haven't lowered the actual test yet!)
let target_blocks: FxIndexMap<_, _> = target_candidates
.into_iter()
.map(|(branch, mut candidates)| {
let branch_start = self.cfg.start_new_block();
// Recursively lower the rest of the match tree after the relevant outcome.
let branch_otherwise =
self.match_candidates(span, scrutinee_span, branch_start, &mut *candidates);
// Link up the `otherwise` block of the subtree to `remainder_start`.
let source_info = self.source_info(span);
self.cfg.goto(branch_otherwise, source_info, remainder_start);
(branch, branch_start)
})
.collect();
// Perform the chosen test, branching to one of the N subtrees prepared above
// (or to `remainder_start` if no outcome was satisfied).
self.perform_test(
span,
scrutinee_span,
start_block,
remainder_start,
match_place,
&test,
target_blocks,
);
remainder_start.and(remaining_candidates)
}
}
///////////////////////////////////////////////////////////////////////////
// Pat binding - used for `let` and function parameters as well.
impl<'a, 'tcx> Builder<'a, 'tcx> {
/// Lowers a `let` expression that appears in a suitable context
/// (e.g. an `if` condition or match guard).
///
/// Also used for lowering let-else statements, since they have similar
/// needs despite not actually using `let` expressions.
///
/// Use [`DeclareLetBindings`] to control whether the `let` bindings are
/// declared or not.
pub(crate) fn lower_let_expr(
&mut self,
mut block: BasicBlock,
expr_id: ExprId,
pat: &Pat<'tcx>,
source_scope: Option<SourceScope>,
scope_span: Span,
declare_let_bindings: DeclareLetBindings,
emit_storage_live: EmitStorageLive,
) -> BlockAnd<()> {
let expr_span = self.thir[expr_id].span;
let scrutinee = unpack!(block = self.lower_scrutinee(block, expr_id, expr_span));
let built_tree = self.lower_match_tree(
block,
expr_span,
&scrutinee,
pat.span,
vec![(pat, HasMatchGuard::No)],
true,
);
let [branch] = built_tree.branches.try_into().unwrap();
self.break_for_else(built_tree.otherwise_block, self.source_info(expr_span));
match declare_let_bindings {
DeclareLetBindings::Yes => {
let expr_place = scrutinee.try_to_place(self);
let opt_expr_place = expr_place.as_ref().map(|place| (Some(place), expr_span));
self.declare_bindings(
source_scope,
pat.span.to(scope_span),
pat,
None,
opt_expr_place,
);
}
DeclareLetBindings::No => {} // Caller is responsible for bindings.
DeclareLetBindings::LetNotPermitted => {
self.tcx.dcx().span_bug(expr_span, "let expression not expected in this context")
}
}
let success = self.bind_pattern(
self.source_info(pat.span),
branch,
&[],
expr_span,
None,
emit_storage_live,
);
// If branch coverage is enabled, record this branch.
self.visit_coverage_conditional_let(pat, success, built_tree.otherwise_block);
success.unit()
}
/// Initializes each of the bindings from the candidate by
/// moving/copying/ref'ing the source as appropriate. Tests the guard, if
/// any, and then branches to the arm. Returns the block for the case where
/// the guard succeeds.
///
/// Note: we do not check earlier that if there is a guard,
/// there cannot be move bindings. We avoid a use-after-move by only
/// moving the binding once the guard has evaluated to true (see below).
fn bind_and_guard_matched_candidate(
&mut self,
sub_branch: MatchTreeSubBranch<'tcx>,
fake_borrows: &[(Place<'tcx>, Local, FakeBorrowKind)],
scrutinee_span: Span,
arm_match_scope: Option<(&Arm<'tcx>, region::Scope)>,
schedule_drops: ScheduleDrops,
emit_storage_live: EmitStorageLive,
) -> BasicBlock {
debug!("bind_and_guard_matched_candidate(subbranch={:?})", sub_branch);
let block = sub_branch.success_block;
if sub_branch.is_never {
// This arm has a dummy body, we don't need to generate code for it. `block` is already
// unreachable (except via false edge).
let source_info = self.source_info(sub_branch.span);
self.cfg.terminate(block, source_info, TerminatorKind::Unreachable);
return self.cfg.start_new_block();
}
self.ascribe_types(block, sub_branch.ascriptions);
// Lower an instance of the arm guard (if present) for this candidate,
// and then perform bindings for the arm body.
if let Some((arm, match_scope)) = arm_match_scope
&& let Some(guard) = arm.guard
{
let tcx = self.tcx;
// Bindings for guards require some extra handling to automatically
// insert implicit references/dereferences.
self.bind_matched_candidate_for_guard(
block,
schedule_drops,
sub_branch.bindings.iter(),
);
let guard_frame = GuardFrame {
locals: sub_branch
.bindings
.iter()
.map(|b| GuardFrameLocal::new(b.var_id))
.collect(),
};
debug!("entering guard building context: {:?}", guard_frame);
self.guard_context.push(guard_frame);
let re_erased = tcx.lifetimes.re_erased;
let scrutinee_source_info = self.source_info(scrutinee_span);
for &(place, temp, kind) in fake_borrows {
let borrow = Rvalue::Ref(re_erased, BorrowKind::Fake(kind), place);
self.cfg.push_assign(block, scrutinee_source_info, Place::from(temp), borrow);
}
let mut guard_span = rustc_span::DUMMY_SP;
let (post_guard_block, otherwise_post_guard_block) =
self.in_if_then_scope(match_scope, guard_span, |this| {
guard_span = this.thir[guard].span;
this.then_else_break(
block,
guard,
None, // Use `self.local_scope()` as the temp scope
this.source_info(arm.span),
DeclareLetBindings::No, // For guards, `let` bindings are declared separately
)
});
let source_info = self.source_info(guard_span);
let guard_end = self.source_info(tcx.sess.source_map().end_point(guard_span));
let guard_frame = self.guard_context.pop().unwrap();
debug!("Exiting guard building context with locals: {:?}", guard_frame);
for &(_, temp, _) in fake_borrows {
let cause = FakeReadCause::ForMatchGuard;
self.cfg.push_fake_read(post_guard_block, guard_end, cause, Place::from(temp));
}
self.cfg.goto(otherwise_post_guard_block, source_info, sub_branch.otherwise_block);
// We want to ensure that the matched candidates are bound
// after we have confirmed this candidate *and* any
// associated guard; Binding them on `block` is too soon,
// because that would be before we've checked the result
// from the guard.
//
// But binding them on the arm is *too late*, because
// then all of the candidates for a single arm would be
// bound in the same place, that would cause a case like:
//
// ```rust
// match (30, 2) {
// (mut x, 1) | (2, mut x) if { true } => { ... }
// ... // ^^^^^^^ (this is `arm_block`)
// }
// ```
//
// would yield an `arm_block` something like:
//
// ```
// StorageLive(_4); // _4 is `x`
// _4 = &mut (_1.0: i32); // this is handling `(mut x, 1)` case
// _4 = &mut (_1.1: i32); // this is handling `(2, mut x)` case
// ```
//
// and that is clearly not correct.
let by_value_bindings = sub_branch
.bindings
.iter()
.filter(|binding| matches!(binding.binding_mode.0, ByRef::No));
// Read all of the by reference bindings to ensure that the
// place they refer to can't be modified by the guard.
for binding in by_value_bindings.clone() {
let local_id = self.var_local_id(binding.var_id, RefWithinGuard);
let cause = FakeReadCause::ForGuardBinding;
self.cfg.push_fake_read(post_guard_block, guard_end, cause, Place::from(local_id));
}
assert_matches!(
schedule_drops,
ScheduleDrops::Yes,
"patterns with guards must schedule drops"
);
self.bind_matched_candidate_for_arm_body(
post_guard_block,
ScheduleDrops::Yes,
by_value_bindings,
emit_storage_live,
);
post_guard_block
} else {
// (Here, it is not too early to bind the matched
// candidate on `block`, because there is no guard result
// that we have to inspect before we bind them.)
self.bind_matched_candidate_for_arm_body(
block,
schedule_drops,
sub_branch.bindings.iter(),
emit_storage_live,
);
block
}
}
/// Append `AscribeUserType` statements onto the end of `block`
/// for each ascription
fn ascribe_types(
&mut self,
block: BasicBlock,
ascriptions: impl IntoIterator<Item = Ascription<'tcx>>,
) {
for ascription in ascriptions {
let source_info = self.source_info(ascription.annotation.span);
let base = self.canonical_user_type_annotations.push(ascription.annotation);
self.cfg.push(
block,
Statement {
source_info,
kind: StatementKind::AscribeUserType(
Box::new((
ascription.source,
UserTypeProjection { base, projs: Vec::new() },
)),
ascription.variance,
),
},
);
}
}
/// Binding for guards is a bit different from binding for the arm body,
/// because an extra layer of implicit reference/dereference is added.
///
/// The idea is that any pattern bindings of type T will map to a `&T` within
/// the context of the guard expression, but will continue to map to a `T`
/// in the context of the arm body. To avoid surfacing this distinction in
/// the user source code (which would be a severe change to the language and
/// require far more revision to the compiler), any occurrence of the
/// identifier in the guard expression will automatically get a deref op
/// applied to it. (See the caller of [`Self::is_bound_var_in_guard`].)
///
/// So an input like:
///
/// ```ignore (illustrative)
/// let place = Foo::new();
/// match place { foo if inspect(foo)
/// => feed(foo), ... }
/// ```
///
/// will be treated as if it were really something like:
///
/// ```ignore (illustrative)
/// let place = Foo::new();
/// match place { Foo { .. } if { let tmp1 = &place; inspect(*tmp1) }
/// => { let tmp2 = place; feed(tmp2) }, ... }
/// ```
///
/// And an input like:
///
/// ```ignore (illustrative)
/// let place = Foo::new();
/// match place { ref mut foo if inspect(foo)
/// => feed(foo), ... }
/// ```
///
/// will be treated as if it were really something like:
///
/// ```ignore (illustrative)
/// let place = Foo::new();
/// match place { Foo { .. } if { let tmp1 = & &mut place; inspect(*tmp1) }
/// => { let tmp2 = &mut place; feed(tmp2) }, ... }
/// ```
/// ---
///
/// ## Implementation notes
///
/// To encode the distinction above, we must inject the
/// temporaries `tmp1` and `tmp2`.
///
/// There are two cases of interest: binding by-value, and binding by-ref.
///
/// 1. Binding by-value: Things are simple.
///
/// * Establishing `tmp1` creates a reference into the
/// matched place. This code is emitted by
/// [`Self::bind_matched_candidate_for_guard`].
///
/// * `tmp2` is only initialized "lazily", after we have
/// checked the guard. Thus, the code that can trigger
/// moves out of the candidate can only fire after the
/// guard evaluated to true. This initialization code is
/// emitted by [`Self::bind_matched_candidate_for_arm_body`].
///
/// 2. Binding by-reference: Things are tricky.
///
/// * Here, the guard expression wants a `&&` or `&&mut`
/// into the original input. This means we need to borrow
/// the reference that we create for the arm.
/// * So we eagerly create the reference for the arm and then take a
/// reference to that.
///
/// ---
///
/// See these PRs for some historical context:
/// - <https://github.com/rust-lang/rust/pull/49870> (introduction of autoref)
/// - <https://github.com/rust-lang/rust/pull/59114> (always use autoref)
fn bind_matched_candidate_for_guard<'b>(
&mut self,
block: BasicBlock,
schedule_drops: ScheduleDrops,
bindings: impl IntoIterator<Item = &'b Binding<'tcx>>,
) where
'tcx: 'b,
{
debug!("bind_matched_candidate_for_guard(block={:?})", block);
// Assign each of the bindings. Since we are binding for a
// guard expression, this will never trigger moves out of the
// candidate.
let re_erased = self.tcx.lifetimes.re_erased;
for binding in bindings {
debug!("bind_matched_candidate_for_guard(binding={:?})", binding);
let source_info = self.source_info(binding.span);
// For each pattern ident P of type T, `ref_for_guard` is
// a reference R: &T pointing to the location matched by
// the pattern, and every occurrence of P within a guard
// denotes *R.
let ref_for_guard = self.storage_live_binding(
block,
binding.var_id,
binding.span,
RefWithinGuard,
schedule_drops,
);
match binding.binding_mode.0 {
ByRef::No => {
// The arm binding will be by value, so for the guard binding
// just take a shared reference to the matched place.
let rvalue = Rvalue::Ref(re_erased, BorrowKind::Shared, binding.source);
self.cfg.push_assign(block, source_info, ref_for_guard, rvalue);
}
ByRef::Yes(mutbl) => {
// The arm binding will be by reference, so eagerly create it now.
let value_for_arm = self.storage_live_binding(
block,
binding.var_id,
binding.span,
OutsideGuard,
schedule_drops,
);
let rvalue =
Rvalue::Ref(re_erased, util::ref_pat_borrow_kind(mutbl), binding.source);
self.cfg.push_assign(block, source_info, value_for_arm, rvalue);
// For the guard binding, take a shared reference to that reference.
let rvalue = Rvalue::Ref(re_erased, BorrowKind::Shared, value_for_arm);
self.cfg.push_assign(block, source_info, ref_for_guard, rvalue);
}
}
}
}
fn bind_matched_candidate_for_arm_body<'b>(
&mut self,
block: BasicBlock,
schedule_drops: ScheduleDrops,
bindings: impl IntoIterator<Item = &'b Binding<'tcx>>,
emit_storage_live: EmitStorageLive,
) where
'tcx: 'b,
{
debug!("bind_matched_candidate_for_arm_body(block={:?})", block);
let re_erased = self.tcx.lifetimes.re_erased;
// Assign each of the bindings. This may trigger moves out of the candidate.
for binding in bindings {
let source_info = self.source_info(binding.span);
let local = match emit_storage_live {
// Here storages are already alive, probably because this is a binding
// from let-else.
// We just need to schedule drop for the value.
EmitStorageLive::No => self.var_local_id(binding.var_id, OutsideGuard).into(),
EmitStorageLive::Yes => self.storage_live_binding(
block,
binding.var_id,
binding.span,
OutsideGuard,
schedule_drops,
),
};
if matches!(schedule_drops, ScheduleDrops::Yes) {
self.schedule_drop_for_binding(binding.var_id, binding.span, OutsideGuard);
}
let rvalue = match binding.binding_mode.0 {
ByRef::No => Rvalue::Use(self.consume_by_copy_or_move(binding.source)),
ByRef::Yes(mutbl) => {
Rvalue::Ref(re_erased, util::ref_pat_borrow_kind(mutbl), binding.source)
}
};
self.cfg.push_assign(block, source_info, local, rvalue);
}
}
/// Each binding (`ref mut var`/`ref var`/`mut var`/`var`, where the bound
/// `var` has type `T` in the arm body) in a pattern maps to 2 locals. The
/// first local is a binding for occurrences of `var` in the guard, which
/// will have type `&T`. The second local is a binding for occurrences of
/// `var` in the arm body, which will have type `T`.
#[instrument(skip(self), level = "debug")]
fn declare_binding(
&mut self,
source_info: SourceInfo,
visibility_scope: SourceScope,
name: Symbol,
mode: BindingMode,
var_id: LocalVarId,
var_ty: Ty<'tcx>,
user_ty: UserTypeProjections,
has_guard: ArmHasGuard,
opt_match_place: Option<(Option<Place<'tcx>>, Span)>,
pat_span: Span,
) {
let tcx = self.tcx;
let debug_source_info = SourceInfo { span: source_info.span, scope: visibility_scope };
let local = LocalDecl {
mutability: mode.1,
ty: var_ty,
user_ty: if user_ty.is_empty() { None } else { Some(Box::new(user_ty)) },
source_info,
local_info: ClearCrossCrate::Set(Box::new(LocalInfo::User(BindingForm::Var(
VarBindingForm {
binding_mode: mode,
// hypothetically, `visit_primary_bindings` could try to unzip
// an outermost hir::Ty as we descend, matching up
// idents in pat; but complex w/ unclear UI payoff.
// Instead, just abandon providing diagnostic info.
opt_ty_info: None,
opt_match_place,
pat_span,
},
)))),
};
let for_arm_body = self.local_decls.push(local);
self.var_debug_info.push(VarDebugInfo {
name,
source_info: debug_source_info,
value: VarDebugInfoContents::Place(for_arm_body.into()),
composite: None,
argument_index: None,
});
let locals = if has_guard.0 {
let ref_for_guard = self.local_decls.push(LocalDecl::<'tcx> {
// This variable isn't mutated but has a name, so has to be
// immutable to avoid the unused mut lint.
mutability: Mutability::Not,
ty: Ty::new_imm_ref(tcx, tcx.lifetimes.re_erased, var_ty),
user_ty: None,
source_info,
local_info: ClearCrossCrate::Set(Box::new(LocalInfo::User(
BindingForm::RefForGuard,
))),
});
self.var_debug_info.push(VarDebugInfo {
name,
source_info: debug_source_info,
value: VarDebugInfoContents::Place(ref_for_guard.into()),
composite: None,
argument_index: None,
});
LocalsForNode::ForGuard { ref_for_guard, for_arm_body }
} else {
LocalsForNode::One(for_arm_body)
};
debug!(?locals);
self.var_indices.insert(var_id, locals);
}
}