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今日大战borrow checker：

enum E {
    A(usize),
    B,
}

struct S {
    x: i32,
    e: E,
}

fn f(s: &mut Vec<S>) {
    match &s[0].e {
        E::A(index) => s[*index].x = 114514,
        _ => {}
    }
}

error[E0502]: cannot borrow `*s` as mutable because it is also borrowed as immutable
  --> src/main.rs:13:24
   |
12 |     match &s[0].e {
   |            - immutable borrow occurs here
13 |         E::A(index) => s[*index].x = 114514,
   |                        ^ ------ immutable borrow later used here
   |                        |
   |                        mutable borrow occurs here
   |
   = help: use `.split_at_mut(position)` to obtain two mutable non-overlapping sub-slices

此处同时持有不可变引用和可变引用理论上没有问题，因为读的是e而写的是x。

编译器提示的split_at_mut并不完全适用，因为index可能是0。

最后我写出了一段超级抽象的match match：

fn f(s: &mut Vec<S>) {
    match match &s[0].e {
        E::A(index) => Some(*index),
        _ => None,
    } {
        Some(index) => s[index].x = 114514,
        None => {}
    }
}

以上是在实现Crafting Interpreters § 25.3之OP_SET_UPVALUE指令时遇到的问题。函数和闭包两章给我写到怀疑人生了 😾 指针乱飞，生命周期一桶浆糊，怪不得用Rust写clox的最后都坑掉了 😾