Operator

In Rust, error handling uses Result. A typical example would be:

fn halves_if_even(i: i32) -> Result<i32, Error> {
    if i % 2 == 0 {
        Ok(i / 2)
    } else {
        Err(/* something */)
    }
}
 
fn do_the_thing(i: i32) -> Result<i32, Error> {
    let i = match halves_if_even(i) {
        Ok(i) => i,
        Err(e) => return Err(e),
    };
 
    // use `i`
}

This is great because:

• when writing the code you cannot accidentally forget to deal with the error,
• when reading the code you can immediately see that there is a potential for error right here.

This is where the question mark operator ? comes in. The above can be rewritten as:

fn do_the_thing(i: i32) -> Result<i32, Error> {
    let i = halves_if_even(i)?;
 
    // use `i`
}

What ? does here is equivalent to the match statement above with an addition. In short:

1. It unpacks the Result if OK
2. It returns the error if not, calling From::from on the error value to potentially convert it to another type.

https://stackoverflow.com/questions/42917566/what-is-this-question-mark-operator-about

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