How to make a Rust mutable reference immutable?

Dereference then re-reference the value:

fn main() {
    let data = &mut vec![1, 2, 3];
    let x = &*data;
}

For what your code was doing, you should probably read What’s the difference in `mut` before a variable name and after the `:`?. Your variable data is already immutable, but it contains a mutable reference. You cannot re-assign data, but you can change the pointed-to value.

How can I turn a mutable reference into an immutable binding?

It already is an immutable binding, as you cannot change what data is.