How do you test for a specific Rust error? - unit-testing

Question

I can find ways to detect if Rust gives me an error,
assert!(fs::metadata(path).is_err())
source
How do I test for a specific error?

Answer 1

You can directly compare the returned Err variant if it impl Debug + PartialEq:
#[derive(Debug, PartialEq)]
enum MyError {
TooBig,
TooSmall,
}
pub fn encode(&self, decoded: &'a Bytes) -> Result<&'a Bytes, MyError> {
if decoded.len() > self.length() as usize {
Err(MyError::TooBig)
} else {
Ok(&decoded)
}
}
assert_eq!(fixed.encode(&[1]), Err(MyError::TooBig));

Answer 2

Following solution doesn't require PartialEq trait to be implemented. For instance std::io::Error does not implement this, and more general solution is required.
In these cases, you can borrow a macro assert_matches from matches crate. It works by giving more succinct way to pattern match, the macro is so short you can just type it too:
macro_rules! assert_err {
($expression:expr, $($pattern:tt)+) => {
match $expression {
$($pattern)+ => (),
ref e => panic!("expected `{}` but got `{:?}`", stringify!($($pattern)+), e),
}
}
}
// Example usages:
assert_err!(your_func(), Err(Error::UrlParsingFailed(_)));
assert_err!(your_func(), Err(Error::CanonicalizationFailed(_)));
assert_err!(your_func(), Err(Error::FileOpenFailed(er)) if er.kind() == ErrorKind::NotFound);
Full playground buildable example, with example Error enum:
#[derive(Debug)]
pub enum Error {
UrlCreationFailed,
CanonicalizationFailed(std::io::Error),
FileOpenFailed(std::io::Error),
UrlParsingFailed(url::ParseError),
}
pub fn your_func() -> Result<(), Error> {
Ok(())
}
#[cfg(test)]
mod test {
use std::io::ErrorKind;
use super::{your_func, Error};
macro_rules! assert_err {
($expression:expr, $($pattern:tt)+) => {
match $expression {
$($pattern)+ => (),
ref e => panic!("expected `{}` but got `{:?}`", stringify!($($pattern)+), e),
}
}
}
#[test]
fn test_failures() {
// Few examples are here:
assert_err!(your_func(), Err(Error::UrlParsingFailed(_)));
assert_err!(your_func(), Err(Error::CanonicalizationFailed(_)));
assert_err!(your_func(), Err(Error::FileOpenFailed(er)) if er.kind() == ErrorKind::NotFound);
}
}