I'd expect 'on_error' to take a function as a callback, not a string. But yes, a better named function could be added

In my Rust crates, I use a custom trait[1], with implementations for Option and Result, that provides an expected() method which, if it panics, prefixes the message with "expected: ". So you can write

  foo().pop().expected("foo length >= 1");

and if it fails, the error is something like "panicked at 'expected foo length >= 1'".

[1]https://gist.github.com/edmccard/8898dd397eec0ff3595c28ada52...

A failed expectation for one user isn't always an error for other users; calling out on_error may confuse people because it indicates an error may occur and must be handled.

Or `unwrap_msg()`?