How to make a Rust mutable reference immutable?

Question

I'm trying to convert a mutable vector to an immutable vector in Rust. I thought this would work but it doesn't:

let data = &mut vec![];
let x = data;          // I thought x would now be an immutable reference

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

Answer 1

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.

Answer 2

The other solution that works now is to specify the type of x:

fn main() {
    let data = &mut vec![1, 2, 3];
    let x: &_ = data;
    println!("{x:?}");
    // This will fail as x immutable
    // *x = vec![]
}

When you don't specify the type, the compiler assumes you want the same type, which happens to be a &mut Vec<_>.