Auto merge of #54383 - mikeyhew:custom-receivers-object-safety, r=nikomatsakis

Take 2: Implement object-safety and dynamic dispatch for arbitrary_self_types

This replaces #50173. Over the months that that PR was open, we made a lot of changes to the way this was going to be implemented, and the long, meandering comment thread and commit history would have been confusing to people reading it in the future. So I decided to package everything up with new, straighforward commits and open a new PR.

Here are the main points. Please read the commit messages for details.

- To simplify codegen, we only support receivers that have the ABI of a pointer. That means they are builtin pointer types, or newtypes thereof.
- We introduce a new trait: `DispatchFromDyn<T>`, similar to `CoerceUnsized<T>`. `DispatchFromDyn` has extra requirements that `CoerceUnsized` does not: when you implement `DispatchFromDyn` for a struct, there cannot be any extra fields besides the field being coerced and `PhantomData` fields. This ensures that the struct's ABI is the same as a pointer.
- For a method's receiver (e.g. `self: Rc<Self>`) to be object-safe, it needs to have the following property:
    - let `DynReceiver` be the receiver when `Self = dyn Trait`
    - let `ConcreteReceiver` be the receiver when `Self = T`, where `T` is some unknown `Sized` type that implements `Trait`, and is the erased type of the trait object.
    - `ConcreteReceiver` must implement `DispatchFromDyn<DynReceiver>`

In the case of `Rc<Self>`, this requires `Rc<T>: DispatchFromDyn<Rc<dyn Trait>>`

These rules are explained more thoroughly in the doc comment on `receiver_is_dispatchable` in object_safety.rs.

r? @nikomatsakis and @eddyb

cc @arielb1 @cramertj @withoutboats

Special thanks to @nikomatsakis for getting me un-stuck when implementing the object-safety checks, and @eddyb for helping with the codegen parts.

EDIT 2018-11-01: updated because CoerceSized has been replaced with DispatchFromDyn

4 files changed, 18 insertions, 3 deletions

diff --git a/src/liballoc/boxed.rs b/src/liballoc/boxed.rs
index f989e701913..74354f605e5 100644
--- a/src/liballoc/boxed.rs
+++ b/src/liballoc/boxed.rs
@@ -77,7 +77,7 @@ use core::iter::FusedIterator;
 use core::marker::{Unpin, Unsize};
 use core::mem;
 use core::pin::Pin;
-use core::ops::{CoerceUnsized, Deref, DerefMut, Generator, GeneratorState};
+use core::ops::{CoerceUnsized, DispatchFromDyn, Deref, DerefMut, Generator, GeneratorState};
 use core::ptr::{self, NonNull, Unique};
 use core::task::{LocalWaker, Poll};
 
@@ -696,6 +696,9 @@ impl<'a, A, R> FnOnce<A> for Box<dyn FnBox<A, Output = R> + Send + 'a> {
 #[unstable(feature = "coerce_unsized", issue = "27732")]
 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Box<U>> for Box<T> {}
 
+#[unstable(feature = "dispatch_from_dyn", issue = "0")]
+impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Box<U>> for Box<T> {}
+
 #[stable(feature = "box_slice_clone", since = "1.3.0")]
 impl<T: Clone> Clone for Box<[T]> {
     fn clone(&self) -> Self {
diff --git a/src/liballoc/lib.rs b/src/liballoc/lib.rs
index 7db6261a01c..ad6e594c884 100644
--- a/src/liballoc/lib.rs
+++ b/src/liballoc/lib.rs
@@ -86,6 +86,7 @@
 #![feature(box_syntax)]
 #![feature(cfg_target_has_atomic)]
 #![feature(coerce_unsized)]
+#![feature(dispatch_from_dyn)]
 #![feature(core_intrinsics)]
 #![feature(custom_attribute)]
 #![feature(dropck_eyepatch)]
diff --git a/src/liballoc/rc.rs b/src/liballoc/rc.rs
index 3e8dfd105de..be452ebb45a 100644
--- a/src/liballoc/rc.rs
+++ b/src/liballoc/rc.rs
@@ -255,7 +255,7 @@ use core::marker;
 use core::marker::{Unpin, Unsize, PhantomData};
 use core::mem::{self, align_of_val, forget, size_of_val};
 use core::ops::Deref;
-use core::ops::CoerceUnsized;
+use core::ops::{CoerceUnsized, DispatchFromDyn};
 use core::pin::Pin;
 use core::ptr::{self, NonNull};
 use core::convert::From;
@@ -297,6 +297,9 @@ impl<T: ?Sized> !marker::Sync for Rc<T> {}
 #[unstable(feature = "coerce_unsized", issue = "27732")]
 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Rc<U>> for Rc<T> {}
 
+#[unstable(feature = "dispatch_from_dyn", issue = "0")]
+impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Rc<U>> for Rc<T> {}
+
 impl<T> Rc<T> {
     /// Constructs a new `Rc<T>`.
     ///
@@ -1176,6 +1179,9 @@ impl<T: ?Sized> !marker::Sync for Weak<T> {}
 #[unstable(feature = "coerce_unsized", issue = "27732")]
 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
 
+#[unstable(feature = "dispatch_from_dyn", issue = "0")]
+impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Weak<U>> for Weak<T> {}
+
 impl<T> Weak<T> {
     /// Constructs a new `Weak<T>`, without allocating any memory.
     /// Calling [`upgrade`][Weak::upgrade] on the return value always gives [`None`].
diff --git a/src/liballoc/sync.rs b/src/liballoc/sync.rs
index bcf5212f1ff..d388f76d8e8 100644
--- a/src/liballoc/sync.rs
+++ b/src/liballoc/sync.rs
@@ -25,7 +25,7 @@ use core::cmp::Ordering;
 use core::intrinsics::abort;
 use core::mem::{self, align_of_val, size_of_val};
 use core::ops::Deref;
-use core::ops::CoerceUnsized;
+use core::ops::{CoerceUnsized, DispatchFromDyn};
 use core::pin::Pin;
 use core::ptr::{self, NonNull};
 use core::marker::{Unpin, Unsize, PhantomData};
@@ -214,6 +214,9 @@ unsafe impl<T: ?Sized + Sync + Send> Sync for Arc<T> {}
 #[unstable(feature = "coerce_unsized", issue = "27732")]
 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Arc<U>> for Arc<T> {}
 
+#[unstable(feature = "dispatch_from_dyn", issue = "0")]
+impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Arc<U>> for Arc<T> {}
+
 /// `Weak` is a version of [`Arc`] that holds a non-owning reference to the
 /// managed value. The value is accessed by calling [`upgrade`] on the `Weak`
 /// pointer, which returns an [`Option`]`<`[`Arc`]`<T>>`.
@@ -254,6 +257,8 @@ unsafe impl<T: ?Sized + Sync + Send> Sync for Weak<T> {}
 
 #[unstable(feature = "coerce_unsized", issue = "27732")]
 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
+#[unstable(feature = "dispatch_from_dyn", issue = "0")]
+impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Weak<U>> for Weak<T> {}
 
 #[stable(feature = "arc_weak", since = "1.4.0")]
 impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {