It's not sending a message - that would defeat the purpose. It's constructing a frozen string, reusing an existing one when possible. Prior to 2.1: def fo

It's not sending a message - that would defeat the purpose. It's constructing a frozen string, reusing an existing one when possible. Prior to 2.1: def foo "bar".freeze end does these things every time `foo` is called: 1. copies the characters 'b', 'a', and 'r' into a mutable string 2. sends the message `freeze` to the new string, which... 3. marks the string as frozen. In 2.1 `"bar".freeze` is equivalent to `"bar"f`, which will not make a copy every time `foo` is called. See https://bugs.ruby-lang.org/issues/8579 https://bugs.ruby-lang.org/issues/8579 for more discussion.