/// 实现者必须保证特定的内存安全约束
unsafe trait Traversable {
    fn traverse(&self);
}

struct SafeType {
    data: Vec<i32>,
}

unsafe impl Traversable for SafeType {
    fn traverse(&self) {
        for item in &self.data {
            println!("{}", item);
        }
    }
}

// Send 和 Sync 就是 unsafe trait
// 大部分类型自动实现，手动实现需要 unsafe

struct MyType {
    // ...
}

// 如果你确定类型是线程安全的：
unsafe impl Send for MyType {}
unsafe impl Sync for MyType {}

use std::alloc::{GlobalAlloc, Layout};

struct MyAllocator;

unsafe impl GlobalAlloc for MyAllocator {
    unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
        std::alloc::System.alloc(layout)
    }

    unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout) {
        std::alloc::System.dealloc(ptr, layout);
    }
}