Read from an enum without pattern matching

Question

The Rust documentation gives this example where we have an instance of Result named some_value:

match some_value {
             


        

Answer 1

At the lowest level, no, you can't read enum fields without a match¹.

Methods on an enum can provide more convenient access to data within the enum (e.g. Result::unwrap), but under the hood, they're always implemented with a match.

If you know that a particular case in a match is unreachable, a common practice is to write unreachable!() on that branch (unreachable!() simply expands to a panic!() with a specific message).

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¹ If you have an enum with only one variant, you could also write a simple let statement to deconstruct the enum. Patterns in let and match statements must be exhaustive, and pattern matching the single variant from an enum is exhaustive. But enums with only one variant are pretty much never used; a struct would do the job just fine. And if you intend to add variants later, you're better off writing a match right away.

enum Single {
    S(i32),
}

fn main() {
    let a = Single::S(1);
    let Single::S(b) = a;
    println!("{}", b);
}

On the other hand, if you have an enum with more than one variant, you can also use if let and while let if you're interested in the data from a single variant only. While let and match require exhaustive patterns, if let and while let accept non-exhaustive patterns. You'll often see them used with Option:

fn main() {
    if let Some(x) = std::env::args().len().checked_add(1) {
        println!("{}", x);
    } else {
        println!("too many args :(");
    }
}