impl: fix signed shrinker

Previously, it was possible for the signed shrinker to call abs on the
minimum value of an integer. This is incorrect and can lead to a panic,
so we avoid abs in that case.

We add a regression test that isn't guaranteed to trip the failure but
is highly likely to.

Fixes #268

src/arbitrary.rs

@@ -813,7 +813,7 @@ macro_rules! signed_shrinker {
  } else {
  let shrinker = SignedShrinker { x: x, i: x / 2 };
  let mut items = vec![0];
- if shrinker.i < 0 {
+ if shrinker.i < 0 && shrinker.x != <$ty>::MIN {
  items.push(shrinker.x.abs());
  }
  Box::new(items.into_iter().chain(shrinker))

src/tester.rs

@@ -431,11 +431,21 @@ mod test {
  let expected_argument = format!("{:?}", [true, true]);
  assert_eq!(failing_case.arguments, vec![expected_argument]);
  }
+
  #[test]
  fn size_for_small_types_issue_143() {
  fn t(_: i8) -> bool {
  true
  }
  QuickCheck::new().gen(Gen::new(129)).quickcheck(t as fn(i8) -> bool);
  }
+
+ #[test]
+ fn regression_signed_shrinker_panic() {
+ fn foo_can_shrink(v: i8) -> bool {
+ let _ = crate::Arbitrary::shrink(&v);
+ true
+ }
+ crate::quickcheck(foo_can_shrink as fn(i8) -> bool);
+ }
 }