Reference Type

• Shared ref: &
• Multable ref: &mut

Two rules:

1. reference can not outlive its owner
2. multable reference can not be aliased[^1]

[^1]: aliased means create an shared reference to the same owner as the multable reference.

You can't not use the multable reference when there is a shared reference created.

let mut i = 1;

let a = &mut i;
let b = &i; // <= b's value depends on action on A, this is not safe

*b = 2; // <= this is not allowed

Conclusion: User should not provides READ and WRITE at the same time.