True Immutability vs Record Illusions
真正的不可变性与 record 幻觉
What you’ll learn: Why Java records are useful but not deeply immutable by default, and how Rust’s default immutability changes the design conversation.
本章将学习： 为什么 Java record 很有用，但默认并不等于深度不可变，以及 Rust 的默认不可变性会怎样改变设计讨论。Difficulty: 🟡 Intermediate
难度： 🟡 中级
Java records reduce boilerplate, but they do not automatically guarantee deep immutability.
Java record 确实能大幅减少样板代码，但它并不会自动保证深度不可变。
The Java Record Caveat
Java record 的限制
record UserProfile(String name, List<String> tags) {}
The tags reference is final, but the list behind it can still mutate unless the code deliberately wraps or copies it.
这里 tags 引用本身是 final，但后面的列表内容依然可能变化，除非显式包裹或拷贝。
Rust’s Default Position
Rust 的默认立场
#![allow(unused)]
fn main() {
struct UserProfile {
name: String,
tags: Vec<String>,
}
}If the binding is immutable, mutation is blocked unless a mutable binding or a special interior mutability type is involved.
如果绑定本身不可变，那么修改动作就会被拦住，除非显式使用可变绑定，或者引入内部可变性类型。
What This Means in Practice
落到实践里的差别
| Concern 关注点 | Java record | Rust struct |
|---|---|---|
| shallow immutability 浅层不可变 | common 常见 | common 常见 |
| deep immutability 深度不可变 | manual design choice 靠设计保证 | manual design choice 也靠设计保证 |
| mutation signal 修改信号 | often hidden behind references 常藏在引用背后 | explicit through mut or interior mutability通过 mut 或内部可变性显式出现 |
Rust does not magically make every data structure deeply immutable, but it makes mutation far easier to spot.
Rust 也不会魔法般地让所有数据结构都变成深度不可变，但它确实让“哪里会变”更容易被看出来。
Design Guidance
设计建议
- treat Java records as concise carriers, not as proof of immutability
把 Java record 当成简洁的数据载体，不要把它误当成不可变证明。 - in Rust, start immutable and add
mutonly where required
在 Rust 里先从不可变开始，再在必要处加mut。 - if mutation must cross shared boundaries, make that choice obvious in the type design
如果可变状态必须跨共享边界存在，就让这种选择在类型设计里足够显眼。
The useful lesson is not “records are bad.” The useful lesson is that Rust defaults push teams toward more explicit state transitions.
真正有价值的结论不是“record 不好”，而是 Rust 的默认值会把团队推向更明确的状态转换设计。