Fix mod by zero constant folding (#6112)

## Description

This PR fixes a problem with constant folding optimization pass with the
mod operator. Unfortunately, we were not checking that the RHS was not
zero. So expressions like `mod 1 0` were packing the compiler.

## Checklist

- [ ] I have linked to any relevant issues.
- [x] I have commented my code, particularly in hard-to-understand
areas.
- [x] I have updated the documentation where relevant (API docs, the
reference, and the Sway book).
- [ ] If my change requires substantial documentation changes, I have
[requested support from the DevRel
team](https://github.com/FuelLabs/devrel-requests/issues/new/choose)
- [x] I have added tests that prove my fix is effective or that my
feature works.
- [x] I have added (or requested a maintainer to add) the necessary
`Breaking*` or `New Feature` labels where relevant.
- [x] I have done my best to ensure that my PR adheres to [the Fuel Labs
Code Review
Standards](https://github.com/FuelLabs/rfcs/blob/master/text/code-standards/external-contributors.md).
- [ ] I have requested a review from the relevant team or maintainers.

sway-ir/src/optimize/constants.rs

@@ -213,8 +213,8 @@ fn combine_binary_op(context: &mut Context, function: &Function) -> bool {
                         (Xor, Uint(l), Uint(r)) => Some(Uint(l ^ r)),
                         (Xor, U256(l), U256(r)) => Some(U256(l ^ r)),
 
-                        (Mod, Uint(l), Uint(r)) => Some(Uint(l % r)),
-                        (Mod, U256(l), U256(r)) => Some(U256(l % r)),
+                        (Mod, Uint(l), Uint(r)) => l.checked_rem(*r).map(Uint),
+                        (Mod, U256(l), U256(r)) => l.checked_rem(r).map(U256),
 
                         (Rsh, Uint(l), Uint(r)) => u32::try_from(*r)
                             .ok()
@@ -350,6 +350,7 @@ mod tests {
         assert_operator("u64", "sub", "0", Some("1"), None);
         assert_operator("u64", "mul", &u64::MAX.to_string(), Some("2"), None);
         assert_operator("u64", "div", "1", Some("0"), None);
+        assert_operator("u64", "mod", "1", Some("0"), None);
 
         assert_operator("u64", "rsh", "1", Some("64"), None);
         assert_operator("u64", "lsh", "1", Some("64"), None);

sway-types/src/u256.rs

@@ -47,6 +47,14 @@ impl U256 {
         let r = (&self.0).shl(other);
         (r.bits() <= 256).then_some(Self(r))
     }
+
+    pub fn checked_rem(&self, other: &U256) -> Option<U256> {
+        if other.0 == BigUint::ZERO {
+            None
+        } else {
+            Some(U256(&self.0 % &other.0))
+        }
+    }
 }
 
 impl std::fmt::Display for U256 {