Support string key in `Struct#[]=`.

mrbgems/mruby-struct/src/struct.c

@@ -660,6 +660,14 @@ mrb_struct_aset(mrb_state *mrb, mrb_value s)
 
   mrb_get_args(mrb, "oo", &idx, &val);
 
+  if (mrb_string_p(idx)) {
+    mrb_value sym = mrb_check_intern_str(mrb, idx);
+
+    if (mrb_nil_p(sym)) {
+      mrb_raisef(mrb, E_INDEX_ERROR, "no member '%S' in struct", idx);
+    }
+    idx = sym;
+  }
   if (mrb_symbol_p(idx)) {
     return mrb_struct_aset_sym(mrb, s, mrb_symbol(idx), val);
   }

mrbgems/mruby-struct/test/struct.rb

@@ -39,6 +39,8 @@ assert('Struct#[]=', '15.2.18.4.3') do
   cc = c.new(1,2)
   cc[:m1] = 3
   cc[:m1] == 3
+  cc["m2"] = 3
+  assert_equal 3, cc["m2"]
 end
 
 assert('Struct#each', '15.2.18.4.4') do