Twelve years and running!

This seems like the string version of --ffast-math. Optimize performance at the cost of everything else[1]. Except with --fast-math you explicitly turn it on.

I assume the underlying storage isn't reference counted, because if it was it seems it would be trivial to detect there's only a single reference left, and trigger a reallocation.

[1]: https://gcc.gnu.org/wiki/FloatingPointMath