// C# records look immutable but have escape hatches
public record Person(string Name, int Age, List<string> Hobbies);

var person = new Person("John", 30, new List<string> { "reading" });

// These all "look" like they create new instances:
var older = person with { Age = 31 };  // New record
var renamed = person with { Name = "Jonathan" };  // New record

// But the reference types are still mutable!
person.Hobbies.Add("gaming");  // Mutates the original!
Console.WriteLine(older.Hobbies.Count);  // 2 - older person affected!
Console.WriteLine(renamed.Hobbies.Count); // 2 - renamed person also affected!

// Init-only properties can still be set via reflection
typeof(Person).GetProperty("Age")?.SetValue(person, 25);

// Collection expressions help but don't solve the fundamental issue
public record BetterPerson(string Name, int Age, IReadOnlyList<string> Hobbies);

var betterPerson = new BetterPerson("Jane", 25, new List<string> { "painting" });
// Still mutable via casting: 
((List<string>)betterPerson.Hobbies).Add("hacking the system");

// Even "immutable" collections aren't truly immutable
using System.Collections.Immutable;
public record SafePerson(string Name, int Age, ImmutableList<string> Hobbies);
// This is better, but requires discipline and has performance overhead

#[derive(Debug, Clone)]
struct Person {
    name: String,
    age: u32,
    hobbies: Vec<String>,
}

let person = Person {
    name: "John".to_string(),
    age: 30,
    hobbies: vec!["reading".to_string()],
};

// This simply won't compile:
// person.age = 31;  // ERROR: cannot assign to immutable field
// person.hobbies.push("gaming".to_string());  // ERROR: cannot borrow as mutable

// To modify, you must explicitly opt-in with 'mut':
let mut older_person = person.clone();
older_person.age = 31;  // Now it's clear this is mutation

// Or use functional update patterns:
let renamed = Person {
    name: "Jonathan".to_string(),
    ..person  // Copies other fields (move semantics apply)
};

// The original is guaranteed unchanged (until moved):
println!("{:?}", person.hobbies);  // Always ["reading"] - immutable

// Structural sharing with efficient immutable data structures
use std::rc::Rc;

#[derive(Debug, Clone)]
struct EfficientPerson {
    name: String,
    age: u32,
    hobbies: Rc<Vec<String>>,  // Shared, immutable reference
}

// Creating new versions shares data efficiently
let person1 = EfficientPerson {
    name: "Alice".to_string(),
    age: 30,
    hobbies: Rc::new(vec!["reading".to_string(), "cycling".to_string()]),
};

let person2 = EfficientPerson {
    name: "Bob".to_string(),
    age: 25,
    hobbies: Rc::clone(&person1.hobbies),  // Shared reference, no deep copy
};