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An &mut reference permits mutation without transferring ownership. Exclusivity lasts through its final use, preventing another reader or writer from observing a half-finished update.
An &mut T gives one caller exclusive temporary access to a value. The owner still exists, but it cannot read or write through another path while the mutable borrow is active. This is more than a rule about syntax: it prevents a reader from seeing a partially updated structure and prevents two writers from racing to change the same state. After the last use of the borrow, the owner can be used again.
In add_one, the star in *value += 1 follows the mutable reference to the integer it points at.
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