Ruby 3.4 allows Strings as Fiber storage keys

But not anything else that responds to #to_sym. It does accept objects that
respond to #to_str.

Reference: https://bugs.ruby-lang.org/issues/20978

core/fiber/storage_spec.rb

@@ -92,11 +92,51 @@
       end
 
       it "can't use invalid keys" do
-        invalid_keys = [Object.new, "Foo", 12]
+        invalid_keys = [Object.new, 12]
         invalid_keys.each do |key|
           -> { Fiber[key] }.should raise_error(TypeError)
         end
       end
+
+      ruby_version_is ""..."3.4" do
+        it "can't use String as a key" do
+          -> { Fiber["Foo"] }.should raise_error(TypeError)
+        end
+
+        it "can't use an object that responds to #to_str as a key" do
+          key = mock("key")
+          key.should_not_receive(:to_str)
+          -> { Fiber[key] }.should raise_error(TypeError)
+        end
+      end
+    end
+
+    ruby_version_is "3.4" do
+      it "can use String as a key" do
+        Fiber["Foo"] = 123
+        Fiber["Foo"].should == 123
+      end
+
+      it "converts a String key into a Symbol" do
+        Fiber["key"] = 123
+        Fiber[:key].should == 123
+
+        Fiber[:other_key] = 123
+        Fiber["other_key"].should == 123
+      end
+
+      it "can use #to_str as a key" do
+        key = mock("key")
+        key.should_receive(:to_str).twice.and_return("key")
+        Fiber[key] = 123
+        Fiber[key].should == 123
+      end
+    end
+
+    it "does not call #to_sym on the key" do
+      key = mock("key")
+      key.should_not_receive(:to_sym)
+      -> { Fiber[key] }.should raise_error(TypeError)
     end
 
     it "can access the storage of the parent fiber" do