1 files changed, 24 insertions, 0 deletions
diff --git a/spec/ruby/optional/capi/string_spec.rb b/spec/ruby/optional/capi/string_spec.rb
index 06b0c9cbf7..5ad8169e8e 100644
--- a/spec/ruby/optional/capi/string_spec.rb
+++ b/spec/ruby/optional/capi/string_spec.rb
@@ -558,10 +558,33 @@ describe "C-API String function" do
     it_behaves_like :string_value_macro, :SafeStringValue
   end
 
+  describe "rb_str_modify_expand" do
+    it "grows the capacity to bytesize + expand, not changing the bytesize" do
+      str = @s.rb_str_buf_new(256, "abcd")
+      @s.rb_str_capacity(str).should == 256
+
+      @s.rb_str_set_len(str, 3)
+      str.bytesize.should == 3
+      @s.RSTRING_LEN(str).should == 3
+      @s.rb_str_capacity(str).should == 256
+
+      @s.rb_str_modify_expand(str, 4)
+      str.bytesize.should == 3
+      @s.RSTRING_LEN(str).should == 3
+      @s.rb_str_capacity(str).should == 7
+
+      @s.rb_str_modify_expand(str, 1024)
+      str.bytesize.should == 3
+      @s.RSTRING_LEN(str).should == 3
+      @s.rb_str_capacity(str).should == 1027
+    end
+  end
+
   describe "rb_str_resize" do
     it "reduces the size of the string" do
       str = @s.rb_str_resize("test", 2)
       str.size.should == 2
+      str.bytesize.should == 2
       @s.RSTRING_LEN(str).should == 2
       str.should == "te"
     end
@@ -574,6 +597,7 @@ describe "C-API String function" do
       expected = "test".force_encoding("US-ASCII")
       str = @s.rb_str_resize(expected.dup, 12)
       str.size.should == 12
+      str.bytesize.should == 12
       @s.RSTRING_LEN(str).should == 12
       str[0, 4].should == expected
     end