fix: error on &mut x when x is not mutable (#6037)

# Description

## Problem

Resolves #5721

## Summary

Today I bumped into this bug so I thought about trying to fix it.

## Additional Context

## Documentation

Check one:
- [x] No documentation needed.
- [ ] Documentation included in this PR.
- [ ] **[For Experimental Features]** 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_frontend/src/elaborator/expressions.rs

@@ -5,7 +5,7 @@ use rustc_hash::FxHashSet as HashSet;
 
 use crate::{
     ast::{
-        ArrayLiteral, ConstructorExpression, IfExpression, InfixExpression, Lambda,
+        ArrayLiteral, ConstructorExpression, IfExpression, InfixExpression, Lambda, UnaryOp,
         UnresolvedTypeData, UnresolvedTypeExpression,
     },
     hir::{
@@ -248,10 +248,17 @@ impl<'context> Elaborator<'context> {
     }
 
     fn elaborate_prefix(&mut self, prefix: PrefixExpression, span: Span) -> (ExprId, Type) {
+        let rhs_span = prefix.rhs.span;
+
         let (rhs, rhs_type) = self.elaborate_expression(prefix.rhs);
         let trait_id = self.interner.get_prefix_operator_trait_method(&prefix.operator);
 
         let operator = prefix.operator;
+
+        if let UnaryOp::MutableReference = operator {
+            self.check_can_mutate(rhs, rhs_span);
+        }
+
         let expr =
             HirExpression::Prefix(HirPrefixExpression { operator, rhs, trait_method_id: trait_id });
         let expr_id = self.interner.push_expr(expr);
@@ -264,6 +271,25 @@ impl<'context> Elaborator<'context> {
         (expr_id, typ)
     }
 
+    fn check_can_mutate(&mut self, expr_id: ExprId, span: Span) {
+        let expr = self.interner.expression(&expr_id);
+        match expr {
+            HirExpression::Ident(hir_ident, _) => {
+                let definition = self.interner.definition(hir_ident.id);
+                if !definition.mutable {
+                    self.push_err(TypeCheckError::CannotMutateImmutableVariable {
+                        name: definition.name.clone(),
+                        span,
+                    });
+                }
+            }
+            HirExpression::MemberAccess(member_access) => {
+                self.check_can_mutate(member_access.lhs, span);
+            }
+            _ => (),
+        }
+    }
+
     fn elaborate_index(&mut self, index_expr: IndexExpression) -> (HirExpression, Type) {
         let span = index_expr.index.span;
         let (index, index_type) = self.elaborate_expression(index_expr.index);

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

@@ -65,8 +65,10 @@ pub enum TypeCheckError {
     UnsupportedCast { span: Span },
     #[error("Index {index} is out of bounds for this tuple {lhs_type} of length {length}")]
     TupleIndexOutOfBounds { index: usize, lhs_type: Type, length: usize, span: Span },
-    #[error("Variable {name} must be mutable to be assigned to")]
+    #[error("Variable `{name}` must be mutable to be assigned to")]
     VariableMustBeMutable { name: String, span: Span },
+    #[error("Cannot mutate immutable variable `{name}`")]
+    CannotMutateImmutableVariable { name: String, span: Span },
     #[error("No method named '{method_name}' found for type '{object_type}'")]
     UnresolvedMethodCall { method_name: String, object_type: Type, span: Span },
     #[error("Integers must have the same signedness LHS is {sign_x:?}, RHS is {sign_y:?}")]
@@ -268,6 +270,7 @@ impl<'a> From<&'a TypeCheckError> for Diagnostic {
             | TypeCheckError::UnsupportedCast { span }
             | TypeCheckError::TupleIndexOutOfBounds { span, .. }
             | TypeCheckError::VariableMustBeMutable { span, .. }
+            | TypeCheckError::CannotMutateImmutableVariable { span, .. }
             | TypeCheckError::UnresolvedMethodCall { span, .. }
             | TypeCheckError::IntegerSignedness { span, .. }
             | TypeCheckError::IntegerBitWidth { span, .. }

compiler/noirc_frontend/src/tests.rs

@@ -3462,3 +3462,55 @@ fn comptime_type_in_runtime_code() {
         CompilationError::ResolverError(ResolverError::ComptimeTypeInRuntimeCode { .. })
     ));
 }
+
+#[test]
+fn cannot_mutate_immutable_variable() {
+    let src = r#"
+    fn main() {
+        let array = [1];
+        mutate(&mut array);
+    }
+
+    fn mutate(_: &mut [Field; 1]) {}
+    "#;
+
+    let errors = get_program_errors(src);
+    assert_eq!(errors.len(), 1);
+
+    let CompilationError::TypeError(TypeCheckError::CannotMutateImmutableVariable { name, .. }) =
+        &errors[0].0
+    else {
+        panic!("Expected a CannotMutateImmutableVariable error");
+    };
+
+    assert_eq!(name, "array");
+}
+
+#[test]
+fn cannot_mutate_immutable_variable_on_member_access() {
+    let src = r#"
+    struct Foo {
+        x: Field
+    }
+
+    fn main() {
+        let foo = Foo { x: 0 };
+        mutate(&mut foo.x);
+    }
+
+    fn mutate(foo: &mut Field) {
+        *foo = 1;
+    }
+    "#;
+
+    let errors = get_program_errors(src);
+    assert_eq!(errors.len(), 1);
+
+    let CompilationError::TypeError(TypeCheckError::CannotMutateImmutableVariable { name, .. }) =
+        &errors[0].0
+    else {
+        panic!("Expected a CannotMutateImmutableVariable error");
+    };
+
+    assert_eq!(name, "foo");
+}