core_io 0.1.20210325

This is a copy of libstd::io with all the parts that don't work in core removed. Most importantly, it provides the Read and Write traits. This crate is (mostly) automatically generated from the rust git source. The version of the source that corresponds to your compiler version will be selected automatically by the build script. 
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use super::{Custom, Error, ErrorKind, Repr};
use crate::error;
use crate::fmt;
use crate::sys::decode_error_kind;
use crate::sys::os::error_string;

#[test]
fn test_debug_error() {
    let code = 6;
    let msg = error_string(code);
    let kind = decode_error_kind(code);
    let err = Error {
        repr: Repr::Custom(box Custom {
            kind: ErrorKind::InvalidInput,
            error: box Error { repr: super::Repr::Os(code) },
        }),
    };
    let expected = format!(
        "Custom {{ \
         kind: InvalidInput, \
         error: Os {{ \
         code: {:?}, \
         kind: {:?}, \
         message: {:?} \
         }} \
         }}",
        code, kind, msg
    );
    assert_eq!(format!("{:?}", err), expected);
}

#[test]
fn test_downcasting() {
    #[derive(Debug)]
    struct TestError;

    impl fmt::Display for TestError {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.write_str("asdf")
        }
    }

    impl error::Error for TestError {}

    // we have to call all of these UFCS style right now since method
    // resolution won't implicitly drop the Send+Sync bounds
    let mut err = Error::new(ErrorKind::Other, TestError);
    assert!(err.get_ref().unwrap().is::<TestError>());
    assert_eq!("asdf", err.get_ref().unwrap().to_string());
    assert!(err.get_mut().unwrap().is::<TestError>());
    let extracted = err.into_inner().unwrap();
    extracted.downcast::<TestError>().unwrap();
}