Persistent Data Structure Support

So we have variables immutable by default and data structures mutable by default.
Is this not in conflict even though I understand that it is safe?
Would that not promote mutability based implementations? Is Functional Style not dependent on Promoting Immutability?
We still consider writing Functional style in Rust across functions Idiomatic as stated in Idiomatic Rust favors Functional or Imperative Style?

Sorry for so many questions but they seem to be related hence wanted to put them together. Thanks in advance.

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