fix #[loop_match] on diverging loop

compiler/rustc_mir_build/src/builder/expr/into.rs

@@ -295,9 +295,11 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                     this.diverge_from(loop_block);
 
                     // Logic for `match`.
-                    let scrutinee_place_builder =
-                        unpack!(body_block = this.as_place_builder(body_block, scrutinee));
                     let scrutinee_span = this.thir.exprs[scrutinee].span;
+                    let scrutinee_place_builder = unpack!(
+                        body_block = this.lower_scrutinee(body_block, scrutinee, scrutinee_span)
+                    );
+
                     let match_start_span = match_span.shrink_to_lo().to(scrutinee_span);
 
                     let mut patterns = Vec::with_capacity(arms.len());
@@ -345,7 +347,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                                         expr_span,
                                         |this| {
                                             this.lower_match_arms(
-                                                destination,
+                                                state_place,
                                                 scrutinee_place_builder,
                                                 scrutinee_span,
                                                 arms,

compiler/rustc_mir_build/src/builder/matches/mod.rs

@@ -407,7 +407,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
     }
 
     /// Evaluate the scrutinee and add the PlaceMention for it.
-    fn lower_scrutinee(
+    pub(crate) fn lower_scrutinee(
         &mut self,
         mut block: BasicBlock,
         scrutinee_id: ExprId,

tests/mir-opt/building/loop_match_diverges.break_to_block_unit.built.after.mir

@@ -0,0 +1,63 @@
+// MIR for `break_to_block_unit` after built
+
+fn break_to_block_unit() -> u8 {
+    let mut _0: u8;
+    let mut _1: i32;
+    let mut _2: !;
+    scope 1 {
+        debug state => _1;
+    }
+
+    bb0: {
+        StorageLive(_1);
+        _1 = const 0_i32;
+        FakeRead(ForLet(None), _1);
+        StorageLive(_2);
+        goto -> bb1;
+    }
+
+    bb1: {
+        falseUnwind -> [real: bb2, unwind: bb10];
+    }
+
+    bb2: {
+        PlaceMention(_1);
+        _1 = const 2_i32;
+        goto -> bb5;
+    }
+
+    bb3: {
+        FakeRead(ForMatchedPlace(None), _1);
+        unreachable;
+    }
+
+    bb4: {
+        goto -> bb6;
+    }
+
+    bb5: {
+        goto -> bb6;
+    }
+
+    bb6: {
+        goto -> bb7;
+    }
+
+    bb7: {
+        goto -> bb1;
+    }
+
+    bb8: {
+        unreachable;
+    }
+
+    bb9: {
+        StorageDead(_2);
+        StorageDead(_1);
+        return;
+    }
+
+    bb10 (cleanup): {
+        resume;
+    }
+}

tests/mir-opt/building/loop_match_diverges.infinite_a.built.after.mir

@@ -0,0 +1,51 @@
+// MIR for `infinite_a` after built
+
+fn infinite_a(_1: u8) -> () {
+    debug state => _1;
+    let mut _0: ();
+    let mut _2: !;
+    let _3: u8;
+    scope 1 {
+        debug a => _3;
+    }
+
+    bb0: {
+        StorageLive(_2);
+        goto -> bb1;
+    }
+
+    bb1: {
+        falseUnwind -> [real: bb2, unwind: bb7];
+    }
+
+    bb2: {
+        PlaceMention(_1);
+        StorageLive(_3);
+        _3 = copy _1;
+        _1 = copy _3;
+        StorageDead(_3);
+        goto -> bb4;
+    }
+
+    bb3: {
+        FakeRead(ForMatchedPlace(None), _1);
+        unreachable;
+    }
+
+    bb4: {
+        goto -> bb1;
+    }
+
+    bb5: {
+        unreachable;
+    }
+
+    bb6: {
+        StorageDead(_2);
+        return;
+    }
+
+    bb7 (cleanup): {
+        resume;
+    }
+}

tests/mir-opt/building/loop_match_diverges.rs

@@ -0,0 +1,68 @@
+// skip-filecheck
+#![allow(incomplete_features)]
+#![feature(loop_match)]
+#![crate_type = "lib"]
+
+// Test that a #[loop_match] without an explicit break from the loop generates valid MIR.
+
+enum State {
+    A,
+    B,
+    C,
+}
+
+// EMIT_MIR loop_match_diverges.simple.built.after.mir
+fn simple(mut state: State) -> State {
+    #[loop_match]
+    'a: loop {
+        state = 'blk: {
+            match state {
+                State::A => {
+                    #[const_continue]
+                    break 'blk State::B;
+                }
+                State::B => {
+                    if true {
+                        #[const_continue]
+                        break 'blk State::C;
+                    } else {
+                        #[const_continue]
+                        break 'blk State::A;
+                    }
+                }
+                State::C => break 'a,
+            }
+        };
+    }
+
+    state
+}
+
+// EMIT_MIR loop_match_diverges.break_to_block_unit.built.after.mir
+#[unsafe(no_mangle)]
+fn break_to_block_unit() -> u8 {
+    let mut state = 0;
+    #[loop_match]
+    loop {
+        state = 'blk: {
+            match state {
+                _ => 'b: {
+                    break 'b 2;
+                }
+            }
+        }
+    }
+}
+
+// EMIT_MIR loop_match_diverges.infinite_a.built.after.mir
+#[unsafe(no_mangle)]
+fn infinite_a(mut state: u8) {
+    #[loop_match]
+    loop {
+        state = 'blk: {
+            match state {
+                a => a,
+            }
+        }
+    }
+}