• IterMute实现要点难点

...
pub struct List<T> {head: Link<T>,
}
​
type Link<T> = Option<Box<Node<T>>>;
​
struct Node<T> {elem: T,next: Link<T>,
}
...
...
...
pub struct IterMut<'a, T> {next: Option<&'a mut Node<T>>,
}
​
//https://rust-unofficial.github.io/too-many-lists/second-iter-mut.html
//IterMut之所以可以工作， 原文给出了2个理由：
//1. 通过take确实保证了&mut型引用的唯一排他性。
//2. https://doc.rust-lang.org/nomicon/borrow-splitting.html
impl<T> List<T> {pub fn iter_mut(&mut self) -> IterMut<'_, T> {//this works coz: Rust understands that it's ok to shard a mutable reference into the subfields of the pointed-to struct, //because there's no way to "go back up", and they're definitely disjoint.IterMut { next: self.head.as_mut().map(|node| &mut **node) }}
}
​
impl<'a, T> Iterator for IterMut<'a, T> {type Item = &'a mut T;
​fn next(&mut self) -> Option<Self::Item> {self.next.take().map(|node| {//此处next被take收割了， 保证了map外部无法再访问到！从而确保&mut引用排他性。//this works coz: Rust understands that it's ok to shard a mutable reference into the subfields of the pointed-to struct,// because there's no way to "go back up", and they're definitely disjoint.//We take the Option<&mut> so we have exclusive access to the mutable reference. //No need to worry about someone looking at it again.self.next = node.next.as_mut().map(|node| &mut **node);&mut node.elem})}......

上面的IterMute代码之所以可以工作，too-many-lists作者给出两点理由：

1. 通过take确实保证了&mut型引用的唯一排他性。

2. Rust知道可以同时借用结构的不相交字段，如下面代码例子。

//https://doc.rust-lang.org/nomicon/borrow-splitting.html
//https://rust-unofficial.github.io/too-many-lists/second-iter-mut.html
​
#[derive(Debug)]
struct Foo {a: i32,b: i32,c: i32,
}
​
fn main() {let mut x = Foo {a: 0, b: 0, c: 0};let xx =&mut x;let a = &mut x.a;let b = &mut x.b;let c = &x.c;*b += 1;let c2 = &x.c;*a += 10;println!("{} {} {} {}", a, b, c, c2);//println!("whole struct: {:?}", xx);//Rust 编译器很聪明，允许你分别持有指向结构体子成员的可变引用，因为Rust编译器知道他们彼此没有交集，是安全的。//但是`引用xx`与后续声明的其他引用有交集，很明显struct整体当然包含其子成员！Rust铁律：允许多只读，排他写！所以不安全！//但是Rust编译器很聪明，只要`引用xx`无处使用就是安全的！所以注释掉println!就OK, 否则报错：` cannot borrow `x.a` as mutable more than once at a time`
​
}

too-many-lists中亮点很多，非常值得学习的Rust著作， 而IterMute的实现方法更是创意非凡！不用unsafe也可有这么好的trick! 绝对让人眼前一亮！灵光一闪！值得反复学习。例子代码位置：https://github.com/yujinliang/rust_learn/tree/master/too_many_lists

• Author

作者：心尘了

email: 285779289@qq.com

git: https://github.com/yujinliang

水平有限，万望海涵，指导

• Reference

https://doc.rust-lang.org/nomicon/borrow-splitting.html

https://rust-unofficial.github.io/too-many-lists/second-iter-mut.html