You can’t borrow a mutable reference to a read-only value

I bought the book Programming Rust: Fast, Safe Systems Development 2nd Edition a couple weeks ago to learn Rust. At the moment, I am struggling with the topic &T and mut &T.

In the book, the author has mentioned the following regarding to references:

You can’t borrow a mutable reference to a read-only value.

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What does it mean? An example would be nice.

>Solution :

You can’t do the following:

let a = 99;
let b = &mut a;