Fix a compilation error in debug mode

src/optimize/mod.rs

@@ -19,7 +19,7 @@ pub(crate) fn optimize_function<'tcx>(
     if tcx.sess.opts.optimize == rustc_session::config::OptLevel::No {
         return; // FIXME classify optimizations over opt levels
     }
-    self::stack2reg::optimize_function(ctx);
+    self::stack2reg::optimize_function(ctx, clif_comments);
     crate::pretty_clif::write_clif_file(tcx, "stack2reg", None, instance, &ctx, &*clif_comments);
     crate::base::verify_func(tcx, &*clif_comments, &ctx.func);
 }

src/optimize/stack2reg.rs

@@ -182,7 +182,10 @@ fn for_context(ctx: &'a mut Context) -> Self {
     }
 }
 
-pub(super) fn optimize_function(ctx: &mut Context) {
+pub(super) fn optimize_function(
+    ctx: &mut Context,
+    #[cfg_attr(not(debug_assertions), allow(unused_variables))] clif_comments: &mut crate::pretty_clif::CommentWriter,
+) {
     combine_stack_addr_with_load_store(&mut ctx.func);
 
     let mut opt_ctx = OptimizeContext::for_context(ctx);