Frozen Struct can be shareable.

A frozen Struct object which refers to shareable objects should be
shareable.

bootstraptest/test_ractor.rb

@@ -427,6 +427,7 @@ assert_equal "ok", %q{
 
   class C; end
   module M; end
+  S = Struct.new(:a, :b, :c, :d)
 
   shareable_objects = [
     true,
@@ -445,6 +446,8 @@ assert_equal "ok", %q{
     [1, 2].freeze,   # frozen Array which only refers to shareable
     {a: 1}.freeze,   # frozen Hash which only refers to shareable
     [{a: 1}.freeze, 'str'.freeze].freeze, # nested frozen container
+    S.new(1, 2).freeze, # frozen Struct
+    S.new(1, 2, 3, 4).freeze, # frozen Struct
     C, # class
     M, # module
     Ractor.current, # Ractor
@@ -454,6 +457,9 @@ assert_equal "ok", %q{
     'mutable str'.dup,
     [:array],
     {hash: true},
+    S.new(1, 2),
+    S.new(1, 2, 3, 4),
+    S.new("a", 2).freeze, # frozen, but refers to an unshareable object
   ]
 
   results = []

ractor.c

@@ -7,6 +7,7 @@
 #include "vm_sync.h"
 #include "ractor.h"
 #include "internal/error.h"
+#include "internal/struct.h"
 
 static VALUE rb_cRactor;
 static VALUE rb_eRactorError;
@@ -1784,6 +1785,22 @@ rb_ractor_shareable_p_hash_i(VALUE key, VALUE value, VALUE arg)
     return ST_CONTINUE;
 }
 
+static bool
+ractor_struct_shareable_members_p(VALUE obj)
+{
+    VM_ASSERT(RB_TYPE_P(obj, T_STRUCT));
+
+    long len = RSTRUCT_LEN(obj);
+    const VALUE *ptr = RSTRUCT_CONST_PTR(obj);
+
+    for (long i=0; i<len; i++) {
+        if (!rb_ractor_shareable_p(ptr[i])) {
+            return false;
+        }
+    }
+    return true;
+}
+
 static bool
 ractor_obj_ivars_shareable_p(VALUE obj)
 {
@@ -1850,6 +1867,19 @@ rb_ractor_shareable_p_continue(VALUE obj)
                 return false;
             }
         }
+      case T_STRUCT:
+        if (!RB_OBJ_FROZEN_RAW(obj) ||
+            FL_TEST_RAW(obj, RUBY_FL_EXIVAR)) {
+            return false;
+        }
+        else {
+            if (ractor_struct_shareable_members_p(obj)) {
+                goto shareable;
+            }
+            else {
+                return false;
+            }
+        }
       case T_OBJECT:
         if (RB_OBJ_FROZEN_RAW(obj) && ractor_obj_ivars_shareable_p(obj)) {
             goto shareable;