Is there a way to check a field's mutability?

Question

Something like:

let x = 5;
if x.isMut {
    // do stuff
} else {
    // do other stuff
}

You could try overloading the error handling, but that is an anti-pattern.

Answer 1

You mention "fields" in your title. A field does not have the property "mutability". Your code suggest that you actually want to ask about the mutability of variable bindings.

There is no way to find out at runtime if a variable binding is mutable or not. This property of variable bindings is always known at compile time and is only relevant at compile time. Therefore there is not really a point in checking this at runtime, as the check would always be either true or always false.

Apart from "binding mutability" there is "reference mutability", basically the difference between &T and &mut T. That you could check at runtime by using a bit of trait trickery. However, it also does not make sense to check at runtime, as it won't allow you to do anything interesting with that information.

Lastly, there is runtime borrowing via interior mutability containers like RefCell and Mutex. These do have dedicated methods for checking of something can be borrowed mutably.

Coming from a dynamic language, these things might seem really strange to you, that's normal. Just keep on learning Rust (for example by reading the book) and you will learn soon why your initial question indeed seems strange in the context of Rust.