[red-knot] improve type shrinking coverage in red-knot property tests (#15297)

## Summary

While looking at #14899, I looked at seeing if I could get shrinking on
the examples. It turned out to be straightforward, with a couple of
caveats.

I'm calling `clone` a lot during shrinking. Since by the shrink step
we're already looking at a test failure this feels fine? Unless I
misunderstood `quickcheck`'s core loop

When shrinking `Intersection`s, in order to just rely on `quickcheck`'s
`Vec` shrinking without thinking about it too much, the shrinking
strategy is:
- try to shrink the negative side (keeping the positive side the same)
- try to shrink the positive side (keeping the negative side the same)

This means that you can't shrink from `(A & B & ~C & ~D)` directly to
`(A & ~C)`! You would first need an intermediate failure at `(A & B &
~C)` or `(A & ~C & ~D)`. This feels good enough. Shrinking the negative
side first also has the benefit of trying to strip down negative
elements in these intersections.

## Test Plan
`cargo test -p red_knot_python_semantic -- --ignored
types::property_tests::stable` still fails as it current does on `main`,
but now the errors seem more minimal.

crates/red_knot_python_semantic/src/types/property_tests.rs

@@ -123,14 +123,61 @@ impl Arbitrary for Ty {
     }
 
     fn shrink(&self) -> Box<dyn Iterator<Item = Self>> {
-        // This is incredibly naive. We can do much better here by
-        // trying various subsets of the elements in unions, tuples,
-        // and intersections. For now, we only try to shrink by
-        // reducing unions/tuples/intersections to a single element.
         match self.clone() {
-            Ty::Union(types) => Box::new(types.into_iter()),
-            Ty::Tuple(types) => Box::new(types.into_iter()),
-            Ty::Intersection { pos, neg } => Box::new(pos.into_iter().chain(neg)),
+            Ty::Union(types) => Box::new(types.shrink().filter_map(|elts| match elts.len() {
+                0 => None,
+                1 => Some(elts.into_iter().next().unwrap()),
+                _ => Some(Ty::Union(elts)),
+            })),
+            Ty::Tuple(types) => Box::new(types.shrink().filter_map(|elts| match elts.len() {
+                0 => None,
+                1 => Some(elts.into_iter().next().unwrap()),
+                _ => Some(Ty::Tuple(elts)),
+            })),
+            Ty::Intersection { pos, neg } => {
+                // Shrinking on intersections is not exhaustive!
+                //
+                // We try to shrink the positive side or the negative side,
+                // but we aren't shrinking both at the same time.
+                //
+                // This should remove positive or negative constraints but
+                // won't shrink (A & B & ~C & ~D) to (A & ~C) in one shrink
+                // iteration.
+                //
+                // Instead, it hopes that (A & B & ~C) or (A & ~C & ~D) fails
+                // so that shrinking can happen there.
+                let pos_orig = pos.clone();
+                let neg_orig = neg.clone();
+                Box::new(
+                    // we shrink negative constraints first, as
+                    // intersections with only negative constraints are
+                    // more confusing
+                    neg.shrink()
+                        .map(move |shrunk_neg| Ty::Intersection {
+                            pos: pos_orig.clone(),
+                            neg: shrunk_neg,
+                        })
+                        .chain(pos.shrink().map(move |shrunk_pos| Ty::Intersection {
+                            pos: shrunk_pos,
+                            neg: neg_orig.clone(),
+                        }))
+                        .filter_map(|ty| {
+                            if let Ty::Intersection { pos, neg } = &ty {
+                                match (pos.len(), neg.len()) {
+                                    // an empty intersection does not mean
+                                    // anything
+                                    (0, 0) => None,
+                                    // a single positive element should be
+                                    // unwrapped
+                                    (1, 0) => Some(pos[0].clone()),
+                                    _ => Some(ty),
+                                }
+                            } else {
+                                unreachable!()
+                            }
+                        }),
+                )
+            }
             _ => Box::new(std::iter::empty()),
         }
     }