How to match on a struct with private fields in Rust?

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I have a function which returns an error type that contains private fields. I want the caller to match on this error type, and when matched, print a hardcoded error message. That's because this error type already has a specific meaning, so I'm not interested in the message that err.display() would return.

However, the function could return a different error type in the future. If that happens then I want existing callers to get a compilation error, so that they can update the error message that they print.

However, if callers match on Err(_), then a change in the error type doesn't result in a compilation error.

How do I solve this? One way would be to match on the specific error type, but that doesn't seem possible if the error type has private fields.

Here's example code:

use std::ffi::{CString, NulError};

fn create_cstring() -> Result<CString, NulError> {
    return CString::new("hello");
}

fn main() {
    match create_cstring() {
        Ok(val) => println!("Output: {:?}", val),
        Err(_) => println!("Error: input contains forbidden null bytes"),
    };
}

Suppose I change create_cstring()'s signature and make it return something other than NulError. How do I make the match block fail to compile?

rust
1 Answers

Use .. to match the private fields:

use std::ffi::{CString, NulError};

fn create_cstring() -> Result<CString, NulError> {
    return CString::new("hello");
}

fn main() {
    match create_cstring() {
        Ok(val) => println!("Output: {:?}", val),
        Err(NulError{..}) => println!("Error: input contains forbidden null bytes"),
    };
}

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