Testing for ErrorKind from a Box<dyn Error>

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I've got this function that I want to test:

pub fn run(config: Config) -> Result<(), Box<dyn Error>> {
    let contents = fs::read_to_string(config.filename)?;
    // -- snip
    Ok(())
}

And this test:

#[test]
fn error_for_missing_file() {
    let error = run(Config {
        query: String::from("query"),
        filename: String::from("not_a_real_file.txt"),
    })
    .unwrap_err();

    let kind = error.kind(); // no method named `kind` found for struct `Box<dyn std::error::Error>` in the current scope
   // assert to follow
}

How can I get the ErrorKind from a Box?

rust
1 Answers

std::error::Error doesn't have a kind method; it's a method of std::io::Error. If you only have access to the base trait you won't be able to call it.

If you make that Box<dyn std::error::Error> a std::io::Error instead you'll have access to it. (And there's no need for a boxed trait object since std::io::Error is a concrete struct.)

Better yet, you can return a std::io::Result<_>, which is a Result type specialized to hold std::io::Errors:

use std::io;

pub fn run(config: Config) -> io::Result<()> {
    let contents = fs::read_to_string(config.filename)?;
    // -- snip
    Ok(())
}
user_685870
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