How to make generated parameters test certain cases

[burntsushi]

How to make generated parameters test certain cases

[bluss]

You can use the -> TestResult form of a property and return TestResult::discard() when strings are not equal. This will let you spin until you've had the full count of non-discarded test results. Of course, if it's very improbable to generate strings that are equal, then it will waste a lot of cpu time before it's done. There is no adaptive logic on discard, it just doesn't count that testcase towards the number of tests (100) it does however count towards max_tests (10000), so it will return Ok if you did not find 100 non-discarded tests in 10000.

[BurntSushi]

Ah, for some reason, I missed this issue. Thanks @bluss for commenting and bringing it to my attention. :-)

The approach @bluss mentions is good, but only if you expect the condition you want to test for to happen reasonably often. In your case, that's probably not a good assumption.

Instead, I'd recommend defining your own string type and implementing arbitrary such that there is a good likelihood that two strings are equal. For example, by limiting their length and/or alphabet, or by choosing from a pre-selected list of strings. You can see an example here: https://github.com/rust-lang/regex/blob/master/regex-syntax/src/properties.rs#L91-L107

[febeling]

Thank you both for answering! It makes sense.