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Yes, immutability is great for safety. But the copies you have to make to keep everything immutable extracts a price in copies and garbage collection.

Rust is advertised as having fearless concurrency. That's true, but not that important as concurrency is not that common. What's important to everyday programming is Rust provides fearless mutability. The fearless concurrency you get with that is just a bonus.

Fearless mutability provides Rust the same safety as a functional language in a without the speed or space cost. IMO, it's Rust's true secret sauce.