Manually clone self in subtract for U256 and pow for U128 (F…

…uelLabs#4294)

sway-lib-std/src/u128.sw

@@ -378,7 +378,9 @@ impl Power for U128 {
         }
 
         if exp == one {
-            return self;
+            // Manually clone `self`. Otherwise, we may have a `MemoryOverflow`
+            // issue with code that looks like: `x = x.pow(other)`
+            return U128::from((self.upper, self.lower));
         }
 
         while exp & one == zero {

sway-lib-std/src/u256.sw

@@ -370,7 +370,9 @@ impl core::ops::Subtract for U256 {
         if self == other {
             return Self::min();
         } else if other == Self::min() {
-            return self;
+            // Manually clone `self`. Otherwise, we may have a `MemoryOverflow`
+            // issue with code that looks like: `x = x - other`
+            return U256::from((self.a, self.b, self.c, self.d));
         }
         // If trying to subtract a larger number, panic.
         assert(self > other);

test/src/e2e_vm_tests/test_programs/should_pass/stdlib/u128_pow_test/src/main.sw

@@ -43,5 +43,10 @@ fn main() -> bool {
     pow_of_u_128 = u_128.pow(U128::from((0, 3)));
     assert(pow_of_u_128 == U128::from((0, 1728)));
 
+    // Test reassignment
+    u_128 = U128::from((0, 13));
+    u_128 = u_128.pow(U128::from((0, 1)));
+    assert(u_128 == U128::from((0, 13)));
+
     true
-}
+}

test/src/e2e_vm_tests/test_programs/should_pass/stdlib/u256_ops_test/src/main.sw

@@ -4,7 +4,7 @@ use std::u256::U256;
 
 fn main() -> bool {
     let first = U256::from((0, 0, 0, 0));
-    let second = U256::from((0, 0, 0, 1));
+    let mut second = U256::from((0, 0, 0, 1));
     let max_u64 = U256::from((0, 0, 0, u64::max()));
 
     let one = first + second;
@@ -40,6 +40,13 @@ fn main() -> bool {
     assert(sub_max_again.c == 0);
     assert(sub_max_again.d == u64::max());
 
+    // Test reassignment
+    second = second - U256::min();
+    assert(second.a == 0);
+    assert(second.b == 0);
+    assert(second.c == 0);
+    assert(second.d == 1);
+
     let zero_in_between = U256::from((0, 1, 0, 10));
     let d_nonzero = U256::from((0, 0, 0, 12));