let  mut hh=Person {
        name:String::from("jack")
};

 let  a = vec![ hh];
 a[0].name=String::from("jack");

the element inner vec's control is managed by vec instance；
So even though hh variable is mutable,but it is not useful.
a variable is mutable or immutable that really work;

 let mut a = vec![ hh];
 let refrenceA=&a

refrenceA variable's type is a immutable reference；
No matter a variable is mutable or immutable, Only it' refrence like&a is default setted up to mutable refrence. if want to become mutable refrence；

1. & mut a； and mut a