What is the minimum to implement Arc<dyn Trait>?

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1

Starting from the example code for implementing an Arc, what else do we need to implement a Arc<dyn Trait>?

use std::marker::PhantomData;
use std::ops::Deref;
use std::panic::{RefUnwindSafe, UnwindSafe};
use std::ptr::NonNull;

use std::sync::atomic::{self, AtomicUsize, Ordering};
pub struct Arc<T: ?Sized> {
    ptr: NonNull<ArcInner<T>>,
    phantom: PhantomData<ArcInner<T>>,
}

pub struct ArcInner<T: ?Sized> {
    rc: AtomicUsize,
    data: T,
}

impl<T> Arc<T> {
    pub fn new(data: T) -> Arc<T> {
        // We start the reference count at 1, as that first reference is the
        // current pointer.
        let boxed = Box::new(ArcInner {
            rc: AtomicUsize::new(1),
            data,
        });
        Arc {
            // It is okay to call `.unwrap()` here as we get a pointer from
            // `Box::into_raw` which is guaranteed to not be null.
            ptr: NonNull::new(Box::into_raw(boxed)).unwrap(),
            phantom: PhantomData,
        }
    }
}

unsafe impl<T: Sync + Send> Send for Arc<T> {}
unsafe impl<T: Sync + Send> Sync for Arc<T> {}

impl<T: RefUnwindSafe + ?Sized> UnwindSafe for Arc<T> {}


impl<T: ?Sized> Deref for Arc<T> {
    type Target = T;

    fn deref(&self) -> &T {
        let inner = unsafe { self.ptr.as_ref() };
        &inner.data
    }
}

impl<T: ?Sized> Clone for Arc<T> {
    fn clone(&self) -> Arc<T> {
        let inner = unsafe { self.ptr.as_ref() };
        // Using a relaxed ordering is alright here as we don't need any atomic
        // synchronization here as we're not modifying or accessing the inner
        // data.
        let old_rc = inner.rc.fetch_add(1, Ordering::Relaxed);

        if old_rc >= isize::MAX as usize {
            std::process::abort();
        }

        Self {
            ptr: self.ptr,
            phantom: PhantomData,
        }
    }
}

impl<T: ?Sized> Drop for Arc<T> {
    fn drop(&mut self) {
        let inner = unsafe { self.ptr.as_ref() };
        if inner.rc.fetch_sub(1, Ordering::Release) != 1 {
            return;
        }
        // This fence is needed to prevent reordering of the use and deletion
        // of the data.
        atomic::fence(Ordering::Acquire);
        // This is safe as we know we have the last pointer to the `ArcInner`
        // and that its pointer is valid.
        unsafe {
            Box::from_raw(self.ptr.as_ptr());
        }
    }
}

fn main() {
    trait Trait {}

    struct Type();

    impl Trait for Type {}

    let d: Arc<dyn Trait> = Arc::new(Type());
}

(Playground)

Errors:

   Compiling playground v0.0.1 (/playground)
error[E0308]: mismatched types
  --> src/main.rs:92:29
   |
92 |     let d: Arc<dyn Trait> = Arc::new(Type());
   |            --------------   ^^^^^^^^^^^^^^^^ expected `Arc<dyn Trait>`, found `Arc<Type>`
   |            |
   |            expected due to this
   |
   = note: expected struct `Arc<dyn Trait>`
              found struct `Arc<Type>`

For more information about this error, try `rustc --explain E0308`.
error: could not compile `playground` (bin "playground") due to previous error
2

You also need the unstable CoerceUnsized, like:

#![feature(coerce_unsized, unsize)]

use std::marker::Unsize;
use std::ops::CoerceUnsized;

impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Arc<U>> for Arc<T> {}
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