feat: remove truncation from brillig casts (#3997)

# Description

## Problem\*

Resolves <!-- Link to GitHub Issue -->

## Summary\*

As part of #3984, I noticed that I forgot to remove the truncation
behaviour from casts in brillig. This PR updates them to be a simple
`mov` instruction.

In order to do this I've added a proper implementation of
`Instruction::Truncate`.

## Additional Context



## Documentation\*

Check one:
- [x] No documentation needed.
- [ ] Documentation included in this PR.
- [ ] **[Exceptional Case]** Documentation to be submitted in a separate
PR.

# PR Checklist\*

- [x] I have tested the changes locally.
- [x] I have formatted the changes with [Prettier](https://prettier.io/)
and/or `cargo fmt` on default settings.

compiler/noirc_evaluator/src/brillig/brillig_gen/brillig_block.rs

@@ -521,7 +521,7 @@ impl<'block> BrilligBlock<'block> {
                     unreachable!("unsupported function call type {:?}", dfg[*func])
                 }
             },
-            Instruction::Truncate { value, .. } => {
+            Instruction::Truncate { value, bit_size, .. } => {
                 let result_ids = dfg.instruction_results(instruction_id);
                 let destination_register = self.variables.define_register_variable(
                     self.function_context,
@@ -530,9 +530,13 @@ impl<'block> BrilligBlock<'block> {
                     dfg,
                 );
                 let source_register = self.convert_ssa_register_value(*value, dfg);
-                self.brillig_context.truncate_instruction(destination_register, source_register);
+                self.brillig_context.truncate_instruction(
+                    destination_register,
+                    source_register,
+                    *bit_size,
+                );
             }
-            Instruction::Cast(value, target_type) => {
+            Instruction::Cast(value, _) => {
                 let result_ids = dfg.instruction_results(instruction_id);
                 let destination_register = self.variables.define_register_variable(
                     self.function_context,
@@ -541,12 +545,7 @@ impl<'block> BrilligBlock<'block> {
                     dfg,
                 );
                 let source_register = self.convert_ssa_register_value(*value, dfg);
-                self.convert_cast(
-                    destination_register,
-                    source_register,
-                    target_type,
-                    &dfg.type_of_value(*value),
-                );
+                self.convert_cast(destination_register, source_register);
             }
             Instruction::ArrayGet { array, index } => {
                 let result_ids = dfg.instruction_results(instruction_id);
@@ -1092,43 +1091,11 @@ impl<'block> BrilligBlock<'block> {
 
     /// Converts an SSA cast to a sequence of Brillig opcodes.
     /// Casting is only necessary when shrinking the bit size of a numeric value.
-    fn convert_cast(
-        &mut self,
-        destination: RegisterIndex,
-        source: RegisterIndex,
-        target_type: &Type,
-        source_type: &Type,
-    ) {
-        fn numeric_to_bit_size(typ: &NumericType) -> u32 {
-            match typ {
-                NumericType::Signed { bit_size } | NumericType::Unsigned { bit_size } => *bit_size,
-                NumericType::NativeField => FieldElement::max_num_bits(),
-            }
-        }
-        // Casting is only valid for numeric types
-        // This should be checked by the frontend, so we panic if this is the case
-        let (source_numeric_type, target_numeric_type) = match (source_type, target_type) {
-            (Type::Numeric(source_numeric_type), Type::Numeric(target_numeric_type)) => {
-                (source_numeric_type, target_numeric_type)
-            }
-            _ => unimplemented!("The cast operation is only valid for integers."),
-        };
-        let source_bit_size = numeric_to_bit_size(source_numeric_type);
-        let target_bit_size = numeric_to_bit_size(target_numeric_type);
-        // Casting from a larger bit size to a smaller bit size (narrowing cast)
-        // requires a cast instruction.
-        // If its a widening cast, ie casting from a smaller bit size to a larger bit size
-        // we simply put a mov instruction as a no-op
-        //
-        // Field elements by construction always have the largest bit size
-        // This means that casting to a Field element, will always be a widening cast
-        // and therefore a no-op. Conversely, casting from a Field element
-        // will always be a narrowing cast and therefore a cast instruction
-        if source_bit_size > target_bit_size {
-            self.brillig_context.cast_instruction(destination, source, target_bit_size);
-        } else {
-            self.brillig_context.mov_instruction(destination, source);
-        }
+    fn convert_cast(&mut self, destination: RegisterIndex, source: RegisterIndex) {
+        // We assume that `source` is a valid `target_type` as it's expected that a truncate instruction was emitted
+        // to ensure this is the case.
+
+        self.brillig_context.mov_instruction(destination, source);
     }
 
     /// Converts the Binary instruction into a sequence of Brillig opcodes.

compiler/noirc_evaluator/src/brillig/brillig_ir.rs

@@ -687,10 +687,29 @@ impl BrilligContext {
         &mut self,
         destination_of_truncated_value: RegisterIndex,
         value_to_truncate: RegisterIndex,
+        bit_size: u32,
     ) {
-        // Effectively a no-op because brillig already has implicit truncation on integer
-        // operations. We need only copy the value to it's destination.
-        self.mov_instruction(destination_of_truncated_value, value_to_truncate);
+        self.debug_show.truncate_instruction(
+            destination_of_truncated_value,
+            value_to_truncate,
+            bit_size,
+        );
+        assert!(
+            bit_size <= BRILLIG_INTEGER_ARITHMETIC_BIT_SIZE,
+            "tried to truncate to a bit size greater than allowed {bit_size}"
+        );
+
+        // The brillig VM performs all arithmetic operations modulo 2**bit_size
+        // So to truncate any value to a target bit size we can just issue a no-op arithmetic operation
+        // With bit size equal to target_bit_size
+        let zero_register = self.make_constant(Value::from(FieldElement::zero()));
+        self.binary_instruction(
+            value_to_truncate,
+            zero_register,
+            destination_of_truncated_value,
+            BrilligBinaryOp::Integer { op: BinaryIntOp::Add, bit_size },
+        );
+        self.deallocate_register(zero_register);
     }
 
     /// Emits a stop instruction
@@ -761,36 +780,6 @@ impl BrilligContext {
         self.deallocate_register(scratch_register_j);
     }
 
-    /// Emits a modulo instruction against 2**target_bit_size
-    ///
-    /// Integer arithmetic in Brillig is currently constrained to 127 bit integers.
-    /// We restrict the cast operation, so that integer types over 127 bits
-    /// cannot be created.
-    pub(crate) fn cast_instruction(
-        &mut self,
-        destination: RegisterIndex,
-        source: RegisterIndex,
-        target_bit_size: u32,
-    ) {
-        self.debug_show.cast_instruction(destination, source, target_bit_size);
-        assert!(
-            target_bit_size <= BRILLIG_INTEGER_ARITHMETIC_BIT_SIZE,
-            "tried to cast to a bit size greater than allowed {target_bit_size}"
-        );
-
-        // The brillig VM performs all arithmetic operations modulo 2**bit_size
-        // So to cast any value to a target bit size we can just issue a no-op arithmetic operation
-        // With bit size equal to target_bit_size
-        let zero_register = self.make_constant(Value::from(FieldElement::zero()));
-        self.binary_instruction(
-            source,
-            zero_register,
-            destination,
-            BrilligBinaryOp::Integer { op: BinaryIntOp::Add, bit_size: target_bit_size },
-        );
-        self.deallocate_register(zero_register);
-    }
-
     /// Adds a unresolved external `Call` instruction to the bytecode.
     /// This calls into another function compiled into this brillig artifact.
     pub(crate) fn add_external_call_instruction<T: ToString>(&mut self, func_label: T) {

compiler/noirc_evaluator/src/brillig/brillig_ir/debug_show.rs

@@ -326,15 +326,15 @@ impl DebugShow {
     }
 
     /// Debug function for cast_instruction
-    pub(crate) fn cast_instruction(
+    pub(crate) fn truncate_instruction(
         &self,
         destination: RegisterIndex,
         source: RegisterIndex,
         target_bit_size: u32,
     ) {
         debug_println!(
             self.enable_debug_trace,
-            "  CAST {} FROM {} TO {} BITS",
+            "  TRUNCATE {} FROM {} TO {} BITS",
             destination,
             source,
             target_bit_size