Mul assign felt proptest (#717)

* DO NOT PUSH this commit

* test for multiplication with assignment

* minor fix

Co-authored-by: Martina <martina@martina>
Co-authored-by: Juan-M-V <102986292+Juan-M-V@users.noreply.github.com>

felt/src/lib.rs

@@ -213,10 +213,21 @@ mod test {
             prop_assert!(as_uint < p, "{}", as_uint);
         }
 
+        #[test]
+        // Property-based test that ensures, for 100 {x} and {y} values that are randomly generated each time tests are run, that a multiplication with assignment between two felts {x} and {y} and doesn't fall outside the range [0, p]. The values of {x} and {y} can be either [0] or a very large number.
+        fn mul_assign_in_range(ref x in "(0|[1-9][0-9]*)", ref y in "(0|[1-9][0-9]*)") {
+            let mut x = Felt::parse_bytes(x.as_bytes(), 10).unwrap();
+            let y = &Felt::parse_bytes(y.as_bytes(), 10).unwrap();
+            let p = &BigUint::parse_bytes(PRIME_STR[2..].as_bytes(), 16).unwrap();
+
+            x *= y;
+            let as_uint = &x.to_biguint();
+            prop_assert!(as_uint < p, "{}", as_uint);
+        }
+
         #[test]
         // Property-based test that ensures, for 100 {x} and {y} values that are randomly generated each time tests are run, that the result of the division of {x} by {y} is the inverse multiplicative of {x} --that is, multiplying the result by {y} returns the original number {x}. The values of {x} and {y} can be either [0] or a very large number.
         fn div_is_mul_inv(ref x in "(0|[1-9][0-9]*)", ref y in "[1-9][0-9]*") {
-
             let x = &Felt::parse_bytes(x.as_bytes(), 10).unwrap();
             let y = &Felt::parse_bytes(y.as_bytes(), 10).unwrap();
             prop_assume!(!y.is_zero());