Rollup merge of #127164 - Nadrieril:clean-lowering-loop, r=matthewjasper

match lowering: Clarify the main loop of the algorithm

Now that we expand or-patterns in a single place in the algorithm, we can move it (back) to the main part of the loop. This makes the call-graph of the main loop rather simple: `match_candidates` has three branches that each call back to `match_candidates`. The remaining tricky part is `finalize_or_candidate`.

I also factored out the whole "process a prefix of the candidates then process the rest" thing which I think helps legibility.

The first two commits are a fix for an indexing mistake I introduced in #126553, already sumitted in #127028 but feel free to merge this first.

r? `@matthewjasper`

tests/mir-opt/building/match/match_false_edges.full_tested_match.built.after.mir

@@ -62,7 +62,7 @@ fn full_tested_match() -> () {
         _6 = &((_2 as Some).0: i32);
         _3 = &fake shallow _2;
         StorageLive(_7);
-        _7 = guard() -> [return: bb8, unwind: bb16];
+        _7 = guard() -> [return: bb8, unwind: bb15];
     }
 
     bb8: {
@@ -118,11 +118,7 @@ fn full_tested_match() -> () {
         unreachable;
     }
 
-    bb15: {
-        goto -> bb14;
-    }
-
-    bb16 (cleanup): {
+    bb15 (cleanup): {
         resume;
     }
 }

tests/mir-opt/building/match/match_false_edges.full_tested_match2.built.after.mir

@@ -68,7 +68,7 @@ fn full_tested_match2() -> () {
         _6 = &((_2 as Some).0: i32);
         _3 = &fake shallow _2;
         StorageLive(_7);
-        _7 = guard() -> [return: bb8, unwind: bb16];
+        _7 = guard() -> [return: bb8, unwind: bb15];
     }
 
     bb8: {
@@ -118,11 +118,7 @@ fn full_tested_match2() -> () {
         unreachable;
     }
 
-    bb15: {
-        goto -> bb14;
-    }
-
-    bb16 (cleanup): {
+    bb15 (cleanup): {
         resume;
     }
 }

tests/mir-opt/building/match/match_false_edges.main.built.after.mir

@@ -38,65 +38,61 @@ fn main() -> () {
         StorageLive(_2);
         _2 = Option::<i32>::Some(const 1_i32);
         PlaceMention(_2);
-        _5 = discriminant(_2);
-        switchInt(move _5) -> [1: bb8, otherwise: bb2];
+        _4 = discriminant(_2);
+        switchInt(move _4) -> [1: bb2, otherwise: bb1];
     }
 
     bb1: {
-        FakeRead(ForMatchedPlace(None), _2);
-        unreachable;
+        falseEdge -> [real: bb14, imaginary: bb4];
     }
 
     bb2: {
-        falseEdge -> [real: bb15, imaginary: bb3];
+        falseEdge -> [real: bb9, imaginary: bb1];
     }
 
     bb3: {
-        _4 = discriminant(_2);
-        switchInt(move _4) -> [1: bb6, otherwise: bb4];
+        goto -> bb1;
     }
 
     bb4: {
+        _5 = discriminant(_2);
+        switchInt(move _5) -> [1: bb6, otherwise: bb5];
+    }
+
+    bb5: {
         StorageLive(_14);
         _14 = _2;
         _1 = const 4_i32;
         StorageDead(_14);
-        goto -> bb21;
-    }
-
-    bb5: {
-        goto -> bb1;
+        goto -> bb20;
     }
 
     bb6: {
-        falseEdge -> [real: bb16, imaginary: bb4];
+        falseEdge -> [real: bb15, imaginary: bb5];
     }
 
     bb7: {
-        goto -> bb4;
+        goto -> bb5;
     }
 
     bb8: {
-        falseEdge -> [real: bb10, imaginary: bb2];
+        FakeRead(ForMatchedPlace(None), _2);
+        unreachable;
     }
 
     bb9: {
-        goto -> bb2;
-    }
-
-    bb10: {
         StorageLive(_7);
         _7 = &((_2 as Some).0: i32);
         _3 = &fake shallow _2;
         StorageLive(_8);
-        _8 = guard() -> [return: bb11, unwind: bb24];
+        _8 = guard() -> [return: bb10, unwind: bb22];
     }
 
-    bb11: {
-        switchInt(move _8) -> [0: bb13, otherwise: bb12];
+    bb10: {
+        switchInt(move _8) -> [0: bb12, otherwise: bb11];
     }
 
-    bb12: {
+    bb11: {
         StorageDead(_8);
         FakeRead(ForMatchGuard, _3);
         FakeRead(ForGuardBinding, _7);
@@ -105,42 +101,42 @@ fn main() -> () {
         _1 = const 1_i32;
         StorageDead(_6);
         StorageDead(_7);
-        goto -> bb21;
+        goto -> bb20;
     }
 
-    bb13: {
-        goto -> bb14;
+    bb12: {
+        goto -> bb13;
     }
 
-    bb14: {
+    bb13: {
         StorageDead(_8);
         StorageDead(_7);
-        falseEdge -> [real: bb9, imaginary: bb2];
+        falseEdge -> [real: bb3, imaginary: bb1];
     }
 
-    bb15: {
+    bb14: {
         StorageLive(_9);
         _9 = _2;
         _1 = const 2_i32;
         StorageDead(_9);
-        goto -> bb21;
+        goto -> bb20;
     }
 
-    bb16: {
+    bb15: {
         StorageLive(_11);
         _11 = &((_2 as Some).0: i32);
         _3 = &fake shallow _2;
         StorageLive(_12);
         StorageLive(_13);
         _13 = (*_11);
-        _12 = guard2(move _13) -> [return: bb17, unwind: bb24];
+        _12 = guard2(move _13) -> [return: bb16, unwind: bb22];
     }
 
-    bb17: {
-        switchInt(move _12) -> [0: bb19, otherwise: bb18];
+    bb16: {
+        switchInt(move _12) -> [0: bb18, otherwise: bb17];
     }
 
-    bb18: {
+    bb17: {
         StorageDead(_13);
         StorageDead(_12);
         FakeRead(ForMatchGuard, _3);
@@ -150,38 +146,34 @@ fn main() -> () {
         _1 = const 3_i32;
         StorageDead(_10);
         StorageDead(_11);
-        goto -> bb21;
+        goto -> bb20;
     }
 
-    bb19: {
-        goto -> bb20;
+    bb18: {
+        goto -> bb19;
     }
 
-    bb20: {
+    bb19: {
         StorageDead(_13);
         StorageDead(_12);
         StorageDead(_11);
-        falseEdge -> [real: bb7, imaginary: bb4];
+        falseEdge -> [real: bb7, imaginary: bb5];
     }
 
-    bb21: {
+    bb20: {
         PlaceMention(_1);
         StorageDead(_2);
         StorageDead(_1);
         _0 = const ();
         return;
     }
 
-    bb22: {
+    bb21: {
         FakeRead(ForMatchedPlace(None), _1);
         unreachable;
     }
 
-    bb23: {
-        goto -> bb22;
-    }
-
-    bb24 (cleanup): {
+    bb22 (cleanup): {
         resume;
     }
 }

tests/mir-opt/building/match/simple_match.match_bool.built.after.mir

@@ -6,37 +6,33 @@ fn match_bool(_1: bool) -> usize {
 
     bb0: {
         PlaceMention(_1);
-        switchInt(_1) -> [0: bb2, otherwise: bb4];
+        switchInt(_1) -> [0: bb1, otherwise: bb2];
     }
 
     bb1: {
-        FakeRead(ForMatchedPlace(None), _1);
-        unreachable;
+        _0 = const 20_usize;
+        goto -> bb6;
     }
 
     bb2: {
-        _0 = const 20_usize;
-        goto -> bb7;
+        falseEdge -> [real: bb5, imaginary: bb1];
     }
 
     bb3: {
         goto -> bb1;
     }
 
     bb4: {
-        falseEdge -> [real: bb6, imaginary: bb2];
+        FakeRead(ForMatchedPlace(None), _1);
+        unreachable;
     }
 
     bb5: {
-        goto -> bb2;
-    }
-
-    bb6: {
         _0 = const 10_usize;
-        goto -> bb7;
+        goto -> bb6;
     }
 
-    bb7: {
+    bb6: {
         return;
     }
 }

tests/mir-opt/building/match/simple_match.match_enum.built.after.mir

@@ -8,7 +8,7 @@ fn match_enum(_1: E1) -> bool {
     bb0: {
         PlaceMention(_1);
         _2 = discriminant(_1);
-        switchInt(move _2) -> [0: bb3, 1: bb5, 2: bb7, otherwise: bb2];
+        switchInt(move _2) -> [0: bb2, 1: bb4, 2: bb6, otherwise: bb1];
     }
 
     bb1: {
@@ -17,44 +17,40 @@ fn match_enum(_1: E1) -> bool {
     }
 
     bb2: {
-        goto -> bb1;
+        goto -> bb8;
     }
 
     bb3: {
-        goto -> bb9;
+        goto -> bb1;
     }
 
     bb4: {
-        goto -> bb2;
+        goto -> bb8;
     }
 
     bb5: {
-        goto -> bb9;
+        goto -> bb1;
     }
 
     bb6: {
-        goto -> bb2;
+        _0 = const false;
+        goto -> bb10;
     }
 
     bb7: {
-        _0 = const false;
-        goto -> bb11;
+        goto -> bb1;
     }
 
     bb8: {
-        goto -> bb2;
+        falseEdge -> [real: bb9, imaginary: bb6];
     }
 
     bb9: {
-        falseEdge -> [real: bb10, imaginary: bb7];
-    }
-
-    bb10: {
         _0 = const true;
-        goto -> bb11;
+        goto -> bb10;
     }
 
-    bb11: {
+    bb10: {
         return;
     }
 }