You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: id.rs
    path of file: ./repos/objc2/crates/objc2/src/rc
    
    the code of the file until where you have to start completion: use core::fmt;
use core::marker::PhantomData;
use core::mem::ManuallyDrop;
use core::ops::{Deref, DerefMut};
use core::panic::{RefUnwindSafe, UnwindSafe};
use core::ptr::{self, NonNull};

use super::AutoreleasePool;
use crate::mutability::{IsIdCloneable, IsMutable};
use crate::runtime::{objc_release_fast, objc_retain_fast};
use crate::{ffi, ClassType, Message};

/// A reference counted pointer type for Objective-C objects
    
    fn clone(&self) -> Self {
        // SAFETY:
        // - The object is known to not be mutable due to the `IsIdCloneable`
        //   bound. Additionally, since the object is already an `Id`, types
        //   like `NSObject` and `NSString` that have a mutable subclass is
        //   also allowed (since even if the object is originally an
        //   `Id<NSMutableString>`, by converting it into `Id<NSObject>` or
        //   `Id<NSString>` that fact is wholly forgotten, and the object
        //   cannot ever be mutated again).
        // - The pointer is valid.
        let obj = unsafe { Id::retain(self.ptr.as_ptr()) };
        // SAFETY: `objc_retain` always returns the same object pointer, and
        // the pointer is guaranteed non-null.
        unsafe { obj.unwrap_unchecked() }
    }