[vnorilo]

Not in general. Immutable strings can be deduplicated, leading to a different performance tradeoff that is often quite good. This is mentioned in TFA.

[johncolanduoni]

It’s worth noting that C++ standard libraries have mostly moved away from copy-on-write strings, due to their poor performance in multithreaded scenarios. And JavaScript engines have ended up adding a bunch of optimizations that simulate mutable strings in certain common scenarios. It depends on what the code in question is doing, and I think the ideal scenario is to allow both in different contexts as long as they can be kept distinct.

[IshKebab]

Mutable strings can be duplicated too. You can use reference counting, or borrow checking in Rust.

[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.