Macro match_err::assert_if_error

macro_rules! assert_if_error {
    ($var:expr, $ty:ty, $variant:ident $( ( $inner:expr ) )?  $(, $($arg:tt)+)? ) => { ... };
}

Asserts the variable is an error and then asserts it against an enum-like error type by hiding the usage of downcast_ref method The error is required to implement PartialEq

Examples

 use match_err::*;
 use anyhow::anyhow;

 #[derive(thiserror::Error, Debug, PartialEq)]
 enum Error {
    #[error("not found")]
    NotFound,
    #[error("custom: {0}")]
    Custom(String),
 }

 let err: Result<(), _> = Err(anyhow!(Error::Custom(String::from("internal"))));

 assert_if_error!(err, Error, Custom(String::from("internal")), "invalid");