Add support for casting.

src/syntax/arbitrary.rs

@@ -124,7 +124,30 @@ impl Arbitrary for Expression {
             Union::new([value_strategy, reference_strategy]).boxed()
         };
 
-        leaf_strategy
+        let cast_strategy = if let Some(bigger_type) = target_type {
+            let mut smaller_types = bigger_type.safe_casts_to();
+
+            if smaller_types.is_empty() {
+                leaf_strategy
+            } else {
+                let duplicated_env = defined_variables.clone();
+                let cast_exp = |t, e| Expression::Cast(Location::manufactured(), t, Box::new(e));
+
+                let smaller_strats: Vec<BoxedStrategy<Expression>> = smaller_types
+                    .drain(..)
+                    .map(|t| {
+                        Expression::arbitrary_with((Some(duplicated_env.clone()), Some(t)))
+                            .prop_map(move |e| cast_exp(t.name(), e))
+                            .boxed()
+                    })
+                    .collect();
+                Union::new(smaller_strats).boxed()
+            }
+        } else {
+            leaf_strategy
+        };
+
+        cast_strategy
             .prop_recursive(3, 64, 2, move |inner| {
                 (select(OPERATORS), proptest::collection::vec(inner, 2)).prop_map(
                     move |((operator, arg_count), mut exprs)| {