Suppress default message of assert_eq! when custom message provided?

To my surprise, when I supply a custom message, it seems that assert_eq!'s default message is also printed out regardless.


Here I'm comparing some very long strs. So it is preferable not to clutter up the console but instead take a limited slice of each.
let left = tds.get_bulk_post_str();
let error_msg = format!("Bulk text is not what is expected: get_bulk_post_str() starts\n{}...\n\nExpected text starts\n{}...\n",
&left[0..20], &expected_bulk_text[0..20]);
// assert_eq! not used because assert_eq! apparently does not suppress its default message even when you supply a custom message!
// ... and in this case the default message is far too long.
// assert_eq!(left, expected_bulk_text, "{}", error_msg);

// instead, I think I'm forced to do something like this:
if left != expected_bulk_text {
panic!("{}", error_msg)
}



... is there any way of suppressing the default message?


If not, this seems a strange design choice, unlike any other language I know. Is it deliberate?


PS I'm aware the first 20 chars of each string might be identical. Obviously if I really wanted to go to town on this I'd have to examine both strings to find out the first case of difference, and just print corresponding slices from the middle.