Use mrb_str_cat() instead of mrb_str_cat2() as possible.

src/class.c

@@ -1354,7 +1354,7 @@ mrb_mod_to_s(mrb_state *mrb, mrb_value klass)
     mrb_value s = mrb_str_new(mrb, "#<", 2);
     mrb_value v = mrb_iv_get(mrb, klass, mrb_intern2(mrb, "__attached__", 12));
 
-    mrb_str_cat2(mrb, s, "Class:");
+    mrb_str_cat(mrb, s, "Class:", 6);
     switch (mrb_type(v)) {
       case MRB_TT_CLASS:
       case MRB_TT_MODULE:
@@ -1365,7 +1365,7 @@ mrb_mod_to_s(mrb_state *mrb, mrb_value klass)
         mrb_str_append(mrb, s, mrb_any_to_s(mrb, v));
         break;
     }
-    mrb_str_cat2(mrb, s, ">");
+    mrb_str_cat(mrb, s, ">", 1);
 
     return s;
   }

src/error.c

@@ -129,25 +129,25 @@ exc_inspect(mrb_state *mrb, mrb_value exc)
 
   if (!mrb_nil_p(file) && !mrb_nil_p(line)) {
     str = file;
-    mrb_str_cat2(mrb, str, ":");
+    mrb_str_cat(mrb, str, ":", 1);
     mrb_str_append(mrb, str, line);
-    mrb_str_cat2(mrb, str, ": ");
+    mrb_str_cat(mrb, str, ": ", 2);
     if (!mrb_nil_p(mesg) && RSTRING_LEN(mesg) > 0) {
       mrb_str_append(mrb, str, mesg);
-      mrb_str_cat2(mrb, str, " (");
+      mrb_str_cat(mrb, str, " (", 2);
     }
     mrb_str_cat2(mrb, str, mrb_obj_classname(mrb, exc));
     if (!mrb_nil_p(mesg) && RSTRING_LEN(mesg) > 0) {
-      mrb_str_cat2(mrb, str, ")");
+      mrb_str_cat(mrb, str, ")", 1);
     }
   }
   else {
     str = mrb_str_new_cstr(mrb, mrb_obj_classname(mrb, exc));
     if (!mrb_nil_p(mesg) && RSTRING_LEN(mesg) > 0) {
-      mrb_str_cat2(mrb, str, ": ");
+      mrb_str_cat(mrb, str, ": ", 2);
       mrb_str_append(mrb, str, mesg);
     } else {
-      mrb_str_cat2(mrb, str, ": ");
+      mrb_str_cat(mrb, str, ": ", 2);
       mrb_str_cat2(mrb, str, mrb_obj_classname(mrb, exc));
     }
   }

src/hash.c

@@ -894,7 +894,7 @@ inspect_hash(mrb_state *mrb, mrb_value hash, int recur)
 
       ai = mrb_gc_arena_save(mrb);
 
-      if (RSTRING_LEN(str) > 1) mrb_str_cat2(mrb, str, ", ");
+      if (RSTRING_LEN(str) > 1) mrb_str_cat(mrb, str, ", ", 2);
 
       str2 = mrb_inspect(mrb, kh_key(h,k));
       mrb_str_append(mrb, str, str2);

src/variable.c

@@ -525,10 +525,10 @@ inspect_i(mrb_state *mrb, mrb_sym sym, mrb_value v, void *p)
   /* need not to show internal data */
   if (RSTRING_PTR(str)[0] == '-') { /* first element */
     RSTRING_PTR(str)[0] = '#';
-    mrb_str_cat2(mrb, str, " ");
+    mrb_str_cat(mrb, str, " ", 1);
   }
   else {
-    mrb_str_cat2(mrb, str, ", ");
+    mrb_str_cat(mrb, str, ", ", 2);
   }
   s = mrb_sym2name_len(mrb, sym, &len);
   mrb_str_cat(mrb, str, s, len);