fix: ensure that unconstrained entrypoint functions don't generate co…

…nstraints (#4292)

# Description

## Problem\*

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

## Summary\*

This PR enforces that if we're generating an unconstrained function that
we should only emit a single brillig opcode.

## 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.

---------

Co-authored-by: kevaundray <[email protected]>

compiler/noirc_evaluator/src/ssa/acir_gen/acir_ir/acir_variable.rs

@@ -319,6 +319,7 @@ impl AcirContext {
             vec![AcirValue::Var(var, AcirType::field())],
             vec![AcirType::field()],
             true,
+            false,
         )?;
         let inverted_var = Self::expect_one_var(results);
 
@@ -706,6 +707,7 @@ impl AcirContext {
                 ],
                 vec![AcirType::unsigned(max_q_bits), AcirType::unsigned(max_rhs_bits)],
                 true,
+                false,
             )?
             .try_into()
             .expect("quotient only returns two values");
@@ -1438,6 +1440,7 @@ impl AcirContext {
         inputs: Vec<AcirValue>,
         outputs: Vec<AcirType>,
         attempt_execution: bool,
+        unsafe_return_values: bool,
     ) -> Result<Vec<AcirValue>, RuntimeError> {
         let b_inputs = try_vecmap(inputs, |i| -> Result<_, InternalError> {
             match i {
@@ -1512,10 +1515,13 @@ impl AcirContext {
             Ok(())
         }
 
-        for output_var in &outputs_var {
-            range_constraint_value(self, output_var)?;
+        // This is a hack to ensure that if we're compiling a brillig entrypoint function then
+        // we don't also add a number of range constraints.
+        if !unsafe_return_values {
+            for output_var in &outputs_var {
+                range_constraint_value(self, output_var)?;
+            }
         }
-
         Ok(outputs_var)
     }
 

compiler/noirc_evaluator/src/ssa/acir_gen/acir_ir/generated_acir.rs

@@ -74,6 +74,10 @@ impl GeneratedAcir {
         }
     }
 
+    pub(crate) fn opcodes(&self) -> &[AcirOpcode] {
+        &self.opcodes
+    }
+
     pub(crate) fn take_opcodes(&mut self) -> Vec<AcirOpcode> {
         std::mem::take(&mut self.opcodes)
     }

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

@@ -270,6 +270,7 @@ impl Context {
             inputs,
             outputs,
             false,
+            true,
         )?;
         let output_vars: Vec<_> = output_values
             .iter()
@@ -280,7 +281,16 @@ impl Context {
         for acir_var in output_vars {
             self.acir_context.return_var(acir_var)?;
         }
-        Ok(self.acir_context.finish(witness_inputs, Vec::new()))
+
+        let generated_acir = self.acir_context.finish(witness_inputs, Vec::new());
+
+        assert_eq!(
+            generated_acir.opcodes().len(),
+            1,
+            "Unconstrained programs should only generate a single opcode but multiple were emitted"
+        );
+
+        Ok(generated_acir)
     }
 
     /// Adds and binds `AcirVar`s for each numeric block parameter or block parameter array element.
@@ -523,7 +533,7 @@ impl Context {
 
                                 let outputs: Vec<AcirType> = vecmap(result_ids, |result_id| dfg.type_of_value(*result_id).into());
 
-                                let output_values = self.acir_context.brillig(self.current_side_effects_enabled_var, code, inputs, outputs, true)?;
+                                let output_values = self.acir_context.brillig(self.current_side_effects_enabled_var, code, inputs, outputs, true, false)?;
 
                                 // Compiler sanity check
                                 assert_eq!(result_ids.len(), output_values.len(), "ICE: The number of Brillig output values should match the result ids in SSA");