show mark-queue on GC critical error (#49902)

Re-adds the capability of showing the mark-queue on a GC critical error.

src/gc.c

@@ -1871,6 +1871,28 @@ STATIC_INLINE jl_gc_chunk_t gc_chunkqueue_pop(jl_gc_markqueue_t *mq) JL_NOTSAFEP
     return c;
 }
 
+// Dump mark queue on critical error
+JL_NORETURN NOINLINE void gc_dump_queue_and_abort(jl_ptls_t ptls, jl_datatype_t *vt) JL_NOTSAFEPOINT
+{
+    jl_safe_printf("GC error (probable corruption)\n");
+    jl_gc_debug_print_status();
+    jl_(vt);
+    jl_gc_debug_critical_error();
+    if (jl_n_gcthreads == 0) {
+        jl_safe_printf("\n");
+        jl_value_t *new_obj;
+        jl_gc_markqueue_t *mq = &ptls->mark_queue;
+        jl_safe_printf("thread %d ptr queue:\n", ptls->tid);
+        jl_safe_printf("~~~~~~~~~~ ptr queue top ~~~~~~~~~~\n");
+        while ((new_obj = gc_ptr_queue_steal_from(mq)) != NULL) {
+            jl_(new_obj);
+            jl_safe_printf("==========\n");
+        }
+        jl_safe_printf("~~~~~~~~~~ ptr queue bottom ~~~~~~~~~~\n");
+    }
+    abort();
+}
+
 // Steal chunk from `mq2`
 STATIC_INLINE jl_gc_chunk_t gc_chunkqueue_steal_from(jl_gc_markqueue_t *mq2) JL_NOTSAFEPOINT
 {
@@ -2568,6 +2590,11 @@ FORCE_INLINE void gc_mark_outrefs(jl_ptls_t ptls, jl_gc_markqueue_t *mq, void *_
             }
             return;
         }
+        else {
+            jl_datatype_t *vt = (jl_datatype_t *)vtag;
+            if (__unlikely(!jl_is_datatype(vt) || vt->smalltag))
+                gc_dump_queue_and_abort(ptls, vt);
+        }
         jl_datatype_t *vt = (jl_datatype_t *)vtag;
         if (vt->name == jl_array_typename) {
             jl_array_t *a = (jl_array_t *)new_obj;