While demoing TestBox to a developer just now I showed how to specify a custom message in an expectation and they asked if there was a way to still show the default message since they found it useful in addition to their message. How do you feel about making the custom message get appended to the default one.
Another idea that gives the developer control would be to introduce the concept of placeholders in the custom message that would be replaced with the actual values:
%default This would be the default message
%testValue This would be the value being tested (converted to a string if necessary)
%expectedValue This would be the expected value (converted to a string if necessary. May not exist based on the toBexxx in use.)
An example would be:
That would render the following message: