Comment by byroot

> There must surely be programs doing a lot of string building or in-place modification that would benefit from non-frozen.

The point is that the magic comment (or the --enable-frozen-string-literal) only applies to literals. If you have some code using mutable strings to iteratively append to it, flipping that switch doesn't change that. It just means you'll have to explicitly create a mutable string. So it doesn't change the performance profile.