Limitations of Property Testing

Given infinite time, property testing will eventually explore the whole input space to a test. However, time is not infinite, so only a randomly sampled portion of the input space can be explored. This means that property testing is extremely unlikely to find single-value edge cases in a large space. For example, the following test will virtually always pass:

use proptest::prelude::*;

proptest! {
    fn i64_abs_is_never_negative(a: i64) {
        // This actually fails if a == i64::MIN, but randomly picking one
        // specific value out of 2⁶⁴ is overwhelmingly unlikely.
        assert!(a.abs() >= 0);
fn main() { } // NOREADME

Because of this, traditional unit testing with intelligently selected cases is still necessary for many kinds of problems.

Similarly, in some cases it can be hard or impossible to define a strategy which actually produces useful inputs. A strategy of .{1,4096} may be great to fuzz a C parser, but is highly unlikely to produce anything that makes it to a code generator.