Good idea. I would be more inclined toward making the whole format string customisable though:

assert_eq!(..., ..., "blah blah '{left}' '{right}' blah {}", some_value);

    ($left:expr , $right:expr, $fmt:expr, $($arg:tt)*) => ({
        match (&($left), &($right)) {
            (left_val, right_val) => {
                if !(*left_val == *right_val) {
                    panic!($fmt, left=left_val, right=right_val, $($arg)*)
                }
            }
        }
    });

This has always been strange, especially considering that without this functionality, it could be a normal function instead.

@bjz I proposed concatenating instead of replacing the format string based on the assumption that, if you use assert_eq! instead of assert! with == you always want to print/format the two values.

@ticki With a function instead of a macro, the potentially-expensive string formatting would have to be done even when it’s not necessary (when the two values are equal and the assertion succeeds).

With a function instead of a macro, the potentially-expensive string formatting would have to be done even when it’s not necessary (when the two values are equal and the assertion succeeds).

Oh, don't get me wrong. I simple state that, as it is currently, having it as a macro is unnecessary since there is no formatting involved.

rust-lang/rust#33976 was merged, can this be closed?

Indeed!