fix: don't overflow when casting signed value to u128

compiler/noirc_evaluator/src/ssa/function_builder/mod.rs

@@ -549,10 +549,10 @@ impl std::ops::Index<BasicBlockId> for FunctionBuilder {
 fn validate_numeric_type(typ: &NumericType) {
     match &typ {
         NumericType::Signed { bit_size } => match bit_size {
-            8 | 16 | 32 | 64 => (),
+            8 | 16 | 32 | 64 | 128 => (),
             _ => {
                 panic!(
-                    "Invalid bit size for signed numeric type: {bit_size}. Expected one of 8, 16, 32 or 64."
+                    "Invalid bit size for signed numeric type: {bit_size}. Expected one of 8, 16, 32, 64 or 128."
                 );
             }
         },

compiler/noirc_evaluator/src/ssa/ssa_gen/context.rs

@@ -665,10 +665,19 @@ impl<'a> FunctionContext<'a> {
                         // value_sign is 1 if the value is positive, 0 otherwise
                         let value_sign =
                             self.builder.insert_binary(value_as_unsigned, BinaryOp::Lt, half_width);
-
+                        let max_for_incoming_type_size = if *incoming_type_size == 128 {
+                            u128::MAX
+                        } else {
+                            2_u128.pow(*incoming_type_size) - 1
+                        };
+                        let max_for_target_type_size = if target_type_size == 128 {
+                            u128::MAX
+                        } else {
+                            2_u128.pow(target_type_size) - 1
+                        };
                         let patch = self.builder.numeric_constant(
                             FieldElement::from(
-                                2_u128.pow(target_type_size) - 2_u128.pow(*incoming_type_size),
+                                max_for_target_type_size - max_for_incoming_type_size,
                             ),
                             NumericType::unsigned(target_type_size),
                         );

test_programs/execution_success/cast_to_u128/Nargo.toml

@@ -0,0 +1,7 @@
+[package]
+name = "cast_to_u128"
+version = "0.1.0"
+type = "bin"
+authors = [""]
+
+[dependencies]

test_programs/execution_success/cast_to_u128/Prover.toml

@@ -0,0 +1 @@
+x = -1

test_programs/execution_success/cast_to_u128/src/main.nr

@@ -0,0 +1,6 @@
+fn main(x: i8) {
+    let y: i8 = -1;
+
+    assert_eq(x as u128, 340282366920938463463374607431768211455);
+    assert_eq(y as u128, 340282366920938463463374607431768211455);
+}