What is the reason for the occurrence of the error message "cannot move out of `self.x` which is behind a mutable reference"?

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This error message occurs when trying to move a value out of a field or method of a mutable reference in Rust. The error occurs because the mutable reference owns the value, and moving it would violate the Rust ownership rules designed to prevent data races and memory unsafety. The solution to this error is to use a reference or borrow instead of moving the value out of the mutable reference.