fix: Type check ACIR mutable reference passed to brillig (#4281)

# Description

## Problem\*

Resolves #4245 

## Summary\*

This adds a type check for call expressions to check the arguments of a
call to an unconstrained function from a constrained function. As we
have two different runtime environments this makes type checking a bit
more complicated. As a followup to this PR we could try and catch the
error added here (#4280) sooner.
However, if the unconstrained function uses generics we must still fall
back to the "type check" in SSA/ACIR gen from PR #4280.

This PR now errors for the snippet in issue with the below using `nargo
check`:

<img width="900" alt="Screenshot 2024-02-06 at 3 36 27 PM"
src="https://github.com/noir-lang/noir/assets/43554004/6c037458-3182-4ea4-8470-050a1734b875">


## Additional Context



## Documentation\*

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

# PR Checklist\*

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

compiler/noirc_frontend/src/hir/type_check/errors.rs

@@ -115,6 +115,10 @@ pub enum TypeCheckError {
     NoMatchingImplFound { constraints: Vec<(Type, String)>, span: Span },
     #[error("Constraint for `{typ}: {trait_name}` is not needed, another matching impl is already in scope")]
     UnneededTraitConstraint { trait_name: String, typ: Type, span: Span },
+    #[error(
+        "Cannot pass a mutable reference from a constrained runtime to an unconstrained runtime"
+    )]
+    ConstrainedReferenceToUnconstrained { span: Span },
 }
 
 impl TypeCheckError {
@@ -202,7 +206,8 @@ impl From<TypeCheckError> for Diagnostic {
             | TypeCheckError::AmbiguousBitWidth { span, .. }
             | TypeCheckError::IntegerAndFieldBinaryOperation { span }
             | TypeCheckError::OverflowingAssignment { span, .. }
-            | TypeCheckError::FieldModulo { span } => {
+            | TypeCheckError::FieldModulo { span }
+            | TypeCheckError::ConstrainedReferenceToUnconstrained { span } => {
                 Diagnostic::simple_error(error.to_string(), String::new(), span)
             }
             TypeCheckError::PublicReturnType { typ, span } => Diagnostic::simple_error(

compiler/noirc_frontend/src/hir/type_check/expr.rs

@@ -36,6 +36,18 @@ impl<'interner> TypeChecker<'interner> {
         }
     }
 
+    fn is_unconstrained_call(&self, expr: &ExprId) -> bool {
+        if let HirExpression::Ident(expr::HirIdent { id, .. }) = self.interner.expression(expr) {
+            if let Some(DefinitionKind::Function(func_id)) =
+                self.interner.try_definition(id).map(|def| &def.kind)
+            {
+                let modifiers = self.interner.function_modifiers(func_id);
+                return modifiers.is_unconstrained;
+            }
+        }
+        false
+    }
+
     /// Infers a type for a given expression, and return this type.
     /// As a side-effect, this function will also remember this type in the NodeInterner
     /// for the given expr_id key.
@@ -139,6 +151,15 @@ impl<'interner> TypeChecker<'interner> {
             }
             HirExpression::Index(index_expr) => self.check_index_expression(expr_id, index_expr),
             HirExpression::Call(call_expr) => {
+                // Need to setup these flags here as `self` is borrowed mutably to type check the rest of the call expression
+                // These flags are later used to type check calls to unconstrained functions from constrained functions
+                let current_func = self
+                    .current_function
+                    .expect("Can only have call expression inside of a function body");
+                let func_mod = self.interner.function_modifiers(&current_func);
+                let is_current_func_constrained = !func_mod.is_unconstrained;
+                let is_unconstrained_call = self.is_unconstrained_call(&call_expr.func);
+
                 self.check_if_deprecated(&call_expr.func);
 
                 let function = self.check_expression(&call_expr.func);
@@ -147,6 +168,19 @@ impl<'interner> TypeChecker<'interner> {
                     let typ = self.check_expression(arg);
                     (typ, *arg, self.interner.expr_span(arg))
                 });
+
+                for (typ, _, _) in args.iter() {
+                    if is_current_func_constrained
+                        && is_unconstrained_call
+                        && matches!(&typ, Type::MutableReference(_))
+                    {
+                        self.errors.push(TypeCheckError::ConstrainedReferenceToUnconstrained {
+                            span: self.interner.expr_span(expr_id),
+                        });
+                        return Type::Error;
+                    }
+                }
+
                 let span = self.interner.expr_span(expr_id);
                 self.bind_function_type(function, args, span)
             }

test_programs/compile_failure/brillig_mut_ref_from_acir/Nargo.toml

@@ -0,0 +1,7 @@
+[package]
+name = "brillig_mut_ref_from_acir"
+type = "bin"
+authors = [""]
+compiler_version = ">=0.23.0"
+
+[dependencies]

test_programs/compile_failure/brillig_mut_ref_from_acir/src/main.nr

@@ -0,0 +1,8 @@
+unconstrained fn mut_ref_identity(value: &mut Field) -> Field {
+    *value
+}
+
+fn main(mut x: Field, y: pub Field) {
+    let returned_x = mut_ref_identity(&mut x);
+    assert(returned_x == x);
+}