add feature gate tests for DispatchFromDyn

tests/ui/feature-gates/feature-gate-dispatch-from-dyn-cell.rs

@@ -0,0 +1,9 @@
+// Check that even though Cell: DispatchFromDyn it remains an invalid self parameter type
+
+use std::cell::Cell;
+
+trait Trait{
+    fn cell(self: Cell<&Self>); //~ ERROR invalid `self` parameter type: Cell<&Self>
+}
+
+fn main() {}

tests/ui/feature-gates/feature-gate-dispatch-from-dyn-cell.stderr

@@ -0,0 +1,12 @@
+error[E0307]: invalid `self` parameter type: Cell<&Self>
+  --> $DIR/feature-gate-dispatch-from-dyn-cell.rs:6:19
+   |
+LL |     fn cell(self: Cell<&Self>);
+   |                   ^^^^^^^^^^^
+   |
+   = note: type of `self` must be `Self` or a type that dereferences to it
+   = help: consider changing to `self`, `&self`, `&mut self`, `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one of the previous types except `Self`)
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0307`.

tests/ui/feature-gates/feature-gate-dispatch-from-dyn-missing-impl.rs

@@ -0,0 +1,35 @@
+// Check that a self parameter type requires a DispatchFromDyn impl to be object safe
+
+#![feature(arbitrary_self_types, unsize, coerce_unsized)]
+
+use std::{
+    marker::Unsize,
+    ops::{CoerceUnsized, Deref},
+};
+
+struct Ptr<T: ?Sized>(Box<T>);
+
+impl<T: ?Sized> Deref for Ptr<T> {
+    type Target = T;
+
+    fn deref(&self) -> &T {
+        &*self.0
+    }
+}
+
+impl<T: Unsize<U> + ?Sized, U: ?Sized> CoerceUnsized<Ptr<U>> for Ptr<T> {}
+// Because this impl is missing the coercion below fails.
+// impl<T: Unsize<U> + ?Sized, U: ?Sized> DispatchFromDyn<Ptr<U>> for Ptr<T> {}
+
+trait Trait {
+    fn ptr(self: Ptr<Self>);
+}
+impl Trait for i32 {
+    fn ptr(self: Ptr<Self>) {}
+}
+
+fn main() {
+    Ptr(Box::new(4)) as Ptr<dyn Trait>;
+    //~^ ERROR the trait `Trait` cannot be made into an object
+    //~^^ ERROR the trait `Trait` cannot be made into an object
+}

tests/ui/feature-gates/feature-gate-dispatch-from-dyn-missing-impl.stderr

@@ -0,0 +1,45 @@
+error[E0038]: the trait `Trait` cannot be made into an object
+  --> $DIR/feature-gate-dispatch-from-dyn-missing-impl.rs:32:25
+   |
+LL |     fn ptr(self: Ptr<Self>);
+   |                  --------- help: consider changing method `ptr`'s `self` parameter to be `&self`: `&Self`
+...
+LL |     Ptr(Box::new(4)) as Ptr<dyn Trait>;
+   |                         ^^^^^^^^^^^^^^ `Trait` cannot be made into an object
+   |
+note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
+  --> $DIR/feature-gate-dispatch-from-dyn-missing-impl.rs:25:18
+   |
+LL | trait Trait {
+   |       ----- this trait cannot be made into an object...
+LL |     fn ptr(self: Ptr<Self>);
+   |                  ^^^^^^^^^ ...because method `ptr`'s `self` parameter cannot be dispatched on
+
+error[E0038]: the trait `Trait` cannot be made into an object
+  --> $DIR/feature-gate-dispatch-from-dyn-missing-impl.rs:32:5
+   |
+LL |     fn ptr(self: Ptr<Self>);
+   |                  --------- help: consider changing method `ptr`'s `self` parameter to be `&self`: `&Self`
+...
+LL |     Ptr(Box::new(4)) as Ptr<dyn Trait>;
+   |     ^^^^^^^^^^^^^^^^ `Trait` cannot be made into an object
+   |
+note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
+  --> $DIR/feature-gate-dispatch-from-dyn-missing-impl.rs:25:18
+   |
+LL | trait Trait {
+   |       ----- this trait cannot be made into an object...
+LL |     fn ptr(self: Ptr<Self>);
+   |                  ^^^^^^^^^ ...because method `ptr`'s `self` parameter cannot be dispatched on
+note: required for `Ptr<{integer}>` to implement `CoerceUnsized<Ptr<dyn Trait>>`
+  --> $DIR/feature-gate-dispatch-from-dyn-missing-impl.rs:20:40
+   |
+LL | impl<T: Unsize<U> + ?Sized, U: ?Sized> CoerceUnsized<Ptr<U>> for Ptr<T> {}
+   |         ---------                      ^^^^^^^^^^^^^^^^^^^^^     ^^^^^^
+   |         |
+   |         unsatisfied trait bound introduced here
+   = note: required by cast to type `Ptr<dyn Trait>`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0038`.