Auto merge of #77549 - tmiasko:simplify-branch-same-fix, r=oli-obk

 Fix miscompile in SimplifyBranchSame

Cherry-picked from #77486, but with a different test case that used to be compiled incorrectly on both master & beta branches.

compiler/rustc_mir/src/transform/simplify_try.rs

@@ -630,7 +630,8 @@ impl<'a, 'tcx> SimplifyBranchSameOptimizationFinder<'a, 'tcx> {
                 // All successor basic blocks must be equal or contain statements that are pairwise considered equal.
                 for ((target_and_value_l,bb_l), (target_and_value_r,bb_r)) in iter_bbs_reachable.tuple_windows() {
                     let trivial_checks = bb_l.is_cleanup == bb_r.is_cleanup
-                    && bb_l.terminator().kind == bb_r.terminator().kind;
+                                            && bb_l.terminator().kind == bb_r.terminator().kind
+                                            && bb_l.statements.len() == bb_r.statements.len();
                     let statement_check = || {
                         bb_l.statements.iter().zip(&bb_r.statements).try_fold(StatementEquality::TrivialEqual, |acc,(l,r)| {
                             let stmt_equality = self.statement_equality(*adt_matched_on, &l, target_and_value_l, &r, target_and_value_r);

src/test/ui/mir/simplify-branch-same.rs

@@ -0,0 +1,21 @@
+// Regression test for SimplifyBranchSame miscompilation.
+// run-pass
+
+macro_rules! m {
+    ($a:expr, $b:expr, $c:block) => {
+        match $a {
+            Lto::Fat | Lto::Thin => { $b; (); $c }
+            Lto::No => { $b; () }
+        }
+    }
+}
+
+pub enum Lto { No, Thin, Fat }
+
+fn f(mut cookie: u32, lto: Lto) -> u32 {
+    let mut _a = false;
+    m!(lto, _a = true, {cookie = 0});
+    cookie
+}
+
+fn main() { assert_eq!(f(42, Lto::Thin), 0) }