let a = "123".to_owned();
let b = &a;
let c = *b; // error

赋值语句有两种理解，首先是Copy，没有实现Copy则move。

然后在有借用的时候，不允许move它，这是编译器规定的，否则你move了那借用指向的不是空指针了？ 而String又没有实现Copy，所以无法做到解引用

编译器自动做了哈，借用主要用于方法传参数，

这里：https://doc.rust-lang.org/book/ch05-03-method-syntax.html

Where’s the -> Operator?
In C and C++, two different operators are used for calling methods: you use . if you’re calling a method on the object directly and -> if you’re calling the method on a pointer to the object and need to dereference the pointer first. In other words, if object is a pointer, object->something() is similar to (*object).something().

Rust doesn’t have an equivalent to the -> operator; instead, Rust has a feature called automatic referencing and dereferencing. Calling methods is one of the few places in Rust that has this behavior.

Here’s how it works: when you call a method with object.something(), Rust automatically adds in &, &mut, or * so object matches the signature of the method. In other words, the following are the same: