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/ui/loop-match/diverges.rs

@@ -0,0 +1,44 @@
+//@ build-pass
+//@ compile-flags: -Zvalidate-mir
+#![allow(incomplete_features)]
+#![feature(loop_match)]
+#![crate_type = "lib"]
+
+// Test that a #[loop_match] without an explicit break from the loop generates valid MIR.
+
+fn break_to_block_unit() -> u8 {
+    let mut state = 0;
+    #[loop_match]
+    loop {
+        state = 'blk: {
+            match state {
+                _ => 'b: {
+                    break 'b 2;
+                }
+            }
+        }
+    }
+}
+
+fn break_to_block_value() -> u8 {
+    let mut state = 0u8;
+    #[loop_match]
+    'a: loop {
+        state = 'blk: {
+            match state {
+                _ => break 'blk state,
+            }
+        }
+    }
+}
+
+fn infinite_a(mut state: u8) {
+    #[loop_match]
+    loop {
+        state = 'blk: {
+            match state {
+                a => a,
+            }
+        }
+    }
+}