Mark early otherwise optimization unsound

compiler/rustc_mir/src/transform/early_otherwise_branch.rs

@@ -26,6 +26,11 @@ pub struct EarlyOtherwiseBranch;
 
 impl<'tcx> MirPass<'tcx> for EarlyOtherwiseBranch {
     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
+        //  FIXME(#78496)
+        if !tcx.sess.opts.debugging_opts.unsound_mir_opts {
+            return;
+        }
+
         if tcx.sess.mir_opt_level() < 3 {
             return;
         }

src/test/mir-opt/early_otherwise_branch.rs

@@ -1,4 +1,4 @@
-// compile-flags: -Z mir-opt-level=4
+// compile-flags: -Z mir-opt-level=4 -Z unsound-mir-opts
 // EMIT_MIR early_otherwise_branch.opt1.EarlyOtherwiseBranch.diff
 fn opt1(x: Option<u32>, y: Option<u32>) -> u32 {
     match (x, y) {

src/test/mir-opt/early_otherwise_branch_3_element_tuple.rs

@@ -1,4 +1,4 @@
-// compile-flags: -Z mir-opt-level=4
+// compile-flags: -Z mir-opt-level=4 -Z unsound-mir-opts
 
 // EMIT_MIR early_otherwise_branch_3_element_tuple.opt1.EarlyOtherwiseBranch.diff
 fn opt1(x: Option<u32>, y: Option<u32>, z: Option<u32>) -> u32 {