fix: prevent check_can_mutate crashing on undefined variable

compiler/noirc_frontend/src/elaborator/expressions.rs

@@ -301,12 +301,13 @@ impl<'context> Elaborator<'context> {
         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,
-                    });
+                if let Some(definition) = self.interner.try_definition(hir_ident.id) {
+                    if !definition.mutable {
+                        self.push_err(TypeCheckError::CannotMutateImmutableVariable {
+                            name: definition.name.clone(),
+                            span,
+                        });
+                    }
                 }
             }
             HirExpression::MemberAccess(member_access) => {

compiler/noirc_frontend/src/tests.rs

@@ -2333,7 +2333,7 @@
 }
 
 #[test]
 fn underflowing_u8() {
     let src = r#"
         fn main() {
             let _: u8 = -1;
@@ -2371,7 +2371,7 @@
 }
 
 #[test]
 fn underflowing_i8() {
     let src = r#"
         fn main() {
             let _: i8 = -129;
@@ -3464,7 +3464,7 @@
 }
 
 #[test]
 fn arithmetic_generics_canonicalization_deduplication_regression() {
     let source = r#"
         struct ArrData<let N: u32> {
             a: [Field; N],
@@ -3533,3 +3533,27 @@
 
     assert_eq!(name, "foo");
 }
+
+#[test]
+fn does_not_crash_when_passing_mutable_undefined_variable() {
+    let src = r#"
+    fn main() {
+        mutate(&mut undefined);
+    }
+
+    fn mutate(foo: &mut Field) {
+        *foo = 1;
+    }
+    "#;
+
+    let errors = get_program_errors(src);
+    assert_eq!(errors.len(), 1);
+
+    let CompilationError::ResolverError(ResolverError::VariableNotDeclared { name, .. }) =
+        &errors[0].0
+    else {
+        panic!("Expected a VariableNotDeclared error");
+    };
+
+    assert_eq!(name, "undefined");
+}