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The nice thing about this pattern wrt mutability is that the mutability is scoped very tightly to the statement, since it's not often that you'd need a builder to live longer than a single statement or at the very most the lifetime of the builder. Although I've had to allow a builder to live longer than a single statement for things in the past, nothing is coming to mind at the moment. Because Rust enforces single ownership of mutable resources, it's not all that bad.

> With that disclaimer, I find it unfortunate that this pattern requires something to be mutable that wouldn't necessarily need to be otherwise.

It does not, you can also have the builder consume itself. But it’s often less convenient.

The consumption also allows for required parameters through the builder pattern: have the “configuration” method return a different type. It's essentially a statically typed state machine.

It doesn't need to be mutable, it's just more efficient. Also the mutability only exists within the builder methods - you can build the struct and assign it to an immutable variable.