Function check_mutable_fields 

pub fn check_mutable_fields(
    ast: &CppAst,
    safety_context: &SafetyContext,
    external_annotations: Option<&ExternalAnnotations>,
) -> Result<Vec<String>, String>

Check for mutable fields in safe functions and classes

In Rust, the mutable keyword is considered unsafe because it breaks the const contract and allows interior mutability without proper guarantees. Instead, users should use UnsafeCell<T> and explicitly unsafe code.