Fix constant name of Ractor::IsolationError message

`dest` of `const_decl_path` is `NODE_COLON2` or `NODE_COLON3` in some cases.
For example, `B::C ||= [“Not ” + “shareable”]` passes `NODE_COLON2`
and `::C ||= [“Not ” + “shareable”]` passes `NODE_COLON3`.
This commit fixes `Ractor::IsolationError` message for such case.

```
# shareable_constant_value: literal
::C ||= ["Not " + "shareable"]

# Before
# => cannot assign unshareable object to C (Ractor::IsolationError)

# After
# => cannot assign unshareable object to ::C (Ractor::IsolationError)
```

ruby_parser.c

@@ -1045,30 +1045,49 @@ VALUE
 rb_node_const_decl_val(const NODE *node)
 {
     VALUE path;
-    if (RNODE_CDECL(node)->nd_vid) {
-        path = rb_id2str(RNODE_CDECL(node)->nd_vid);
+    switch (nd_type(node)) {
+      case NODE_CDECL:
+        if (RNODE_CDECL(node)->nd_vid) {
+            path = rb_id2str(RNODE_CDECL(node)->nd_vid);
+            goto end;
+        }
+        else {
+            node = RNODE_CDECL(node)->nd_else;
+        }
+        break;
+      case NODE_COLON2:
+        break;
+      case NODE_COLON3:
+        // ::Const
+        path = rb_str_new_cstr("::");
+        rb_str_append(path, rb_id2str(RNODE_COLON3(node)->nd_mid));
+        goto end;
+      default:
+        rb_bug("unexpected node: %s", ruby_node_name(nd_type(node)));
+        UNREACHABLE_RETURN(0);
     }
-    else {
-        NODE *n = RNODE_CDECL(node)->nd_else;
-        path = rb_ary_new();
-        for (; n && nd_type_p(n, NODE_COLON2); n = RNODE_COLON2(n)->nd_head) {
-            rb_ary_push(path, rb_id2str(RNODE_COLON2(n)->nd_mid));
+
+    path = rb_ary_new();
+    if (node) {
+        for (; node && nd_type_p(node, NODE_COLON2); node = RNODE_COLON2(node)->nd_head) {
+            rb_ary_push(path, rb_id2str(RNODE_COLON2(node)->nd_mid));
         }
-        if (n && nd_type_p(n, NODE_CONST)) {
+        if (node && nd_type_p(node, NODE_CONST)) {
             // Const::Name
-            rb_ary_push(path, rb_id2str(RNODE_CONST(n)->nd_vid));
+            rb_ary_push(path, rb_id2str(RNODE_CONST(node)->nd_vid));
         }
-        else if (n && nd_type_p(n, NODE_COLON3)) {
+        else if (node && nd_type_p(node, NODE_COLON3)) {
             // ::Const::Name
-            rb_ary_push(path, rb_id2str(RNODE_COLON3(n)->nd_mid));
+            rb_ary_push(path, rb_id2str(RNODE_COLON3(node)->nd_mid));
             rb_ary_push(path, rb_str_new(0, 0));
         }
         else {
             // expression::Name
             rb_ary_push(path, rb_str_new_cstr("..."));
         }
         path = rb_ary_join(rb_ary_reverse(path), rb_str_new_cstr("::"));
-        path = rb_fstring(path);
     }
+  end:
+    path = rb_fstring(path);
     return path;
 }

test/ruby/test_parse.rb

@@ -1568,6 +1568,40 @@ def test_shareable_constant_value_unshareable_literal
         ::B::C = ["Not " + "shareable"]
       end
     end;
+
+    assert_separately([], "#{<<~"begin;"}\n#{<<~'end;'}")
+    begin;
+      assert_raise_with_message(Ractor::IsolationError, /unshareable object to ::C/) do
+        # shareable_constant_value: literal
+        ::C ||= ["Not " + "shareable"]
+      end
+    end;
+
+    assert_raise_separately(Ractor::IsolationError, /unshareable object to B::C/,
+                            "#{<<~"begin;"}\n#{<<~'end;'}")
+    begin;
+      # shareable_constant_value: literal
+      B = Class.new
+      B::C ||= ["Not " + "shareable"]
+    end;
+
+    assert_separately([], "#{<<~"begin;"}\n#{<<~'end;'}")
+    begin;
+      assert_raise_with_message(Ractor::IsolationError, /unshareable object to ::B::C/) do
+        # shareable_constant_value: literal
+        ::B = Class.new
+        ::B::C ||= ["Not " + "shareable"]
+      end
+    end;
+
+    assert_raise_separately(Ractor::IsolationError, /unshareable object to ...::C/,
+                            "#{<<~"begin;"}\n#{<<~'end;'}")
+    begin;
+      # shareable_constant_value: literal
+      B = Class.new
+      def self.expr; B; end
+      expr::C ||= ["Not " + "shareable"]
+    end;
   end
 
   def test_shareable_constant_value_nonliteral