feat: simplify subtraction from self to return zero

compiler/noirc_evaluator/src/ssa/ir/instruction/binary.rs

@@ -39,9 +39,9 @@
     Or,
     /// Bitwise xor (^)
     Xor,
     /// Bitshift left (<<)
     Shl,
     /// Bitshift right (>>)
     Shr,
 }
 
@@ -107,7 +107,7 @@
         }
 
         let operator = if lhs_type == NumericType::NativeField {
             // Unchecked operations between fields or bools don't make sense, so we convert those to non-unchecked
             // to reduce noise and confusion in the generated SSA.
             match operator {
                 BinaryOp::Add { unchecked: true } => BinaryOp::Add { unchecked: false },
@@ -116,7 +116,7 @@
                 _ => operator,
             }
         } else if lhs_type == NumericType::bool() {
             // Unchecked mul between bools doesn't make sense, so we convert that to non-unchecked
             if let BinaryOp::Mul { unchecked: true } = operator {
                 BinaryOp::Mul { unchecked: false }
             } else {
@@ -155,6 +155,11 @@
                 }
             }
             BinaryOp::Sub { .. } => {
+                if lhs == rhs {
+                    let zero = dfg.make_constant(FieldElement::zero(), lhs_type);
+                    return SimplifyResult::SimplifiedTo(zero);
+                }
+
                 if rhs_is_zero {
                     return SimplifyResult::SimplifiedTo(lhs);
                 }
@@ -284,7 +289,7 @@
                 }
                 if lhs_type == NumericType::bool() {
                     // Boolean AND is equivalent to multiplication, which is a cheaper operation.
                     // (mul unchecked because these are bools so it doesn't matter really)
                     let instruction =
                         Instruction::binary(BinaryOp::Mul { unchecked: true }, lhs, rhs);
                     return SimplifyResult::SimplifiedToInstruction(instruction);

test_programs/noir_test_success/regression_4080/src/main.nr

@@ -3,6 +3,6 @@
 
 #[test(should_fail_with = "attempt to add with overflow")]
 fn main() {
-    let var1: u8 = ((255 + 1) ^ (255 + 1)) - ((255 + 1) - (255 + 1));
-    assert_eq(var1, 0);
+    let var1: u8 = ((255 + 1) ^ (255 + 1) ^ (255 + 1));
+    assert_eq(var1 as Field, 256);
 }