[spankalee]

Mutable string literals can't be easily deduplicated, unless your language semantics are that a literal is a singleton and all mutations are visible by all other evaluations of that literal. But no sane language would do that.

[johncolanduoni]

If the strings are backed by reference counted buffers, you can use copy-on-write semantics to provide the API of a mutable string but share buffers when a string is copied. Most C++ standard libraries actually did this prior to the multicore era.