> Creating new objects in functions and returning them should be avoided if possible. It's a Rust idiom to create the object in the caller and pass it into a function to be modified.

> immutability is the default

Haven't yet written any Rust code, but aren't those two things contradicting each other? I'd have thought that with immutability the most natural way to operate is to do pure functions (for uninitiated: pure function doesn't modify anything, can only produce new things).