Add a check for reprs that could change the ABI

disallow `#[repr(C)] and `#[repr(packed)]` on structs implementing DispatchFromDyn because they will change the ABI from Scalar/ScalarPair to Aggregrate, resulting in an ICE during object-safety checks or codegen

src/librustc_typeck/coherence/builtin.rs

@@ -223,6 +223,13 @@ fn visit_implementation_of_dispatch_from_dyn<'a, 'tcx>(
                         return
                     }
 
+                    if def_a.repr.c() || def_a.repr.packed() {
+                        create_err(
+                            "structs implementing `DispatchFromDyn` may not have \
+                             `#[repr(packed)]` or `#[repr(C)]`"
+                        ).emit();
+                    }
+
                     let fields = &def_a.non_enum_variant().fields;
 
                     let coerced_fields = fields.iter().filter_map(|field| {

src/test/ui/invalid_dispatch_from_dyn_impls.rs

@@ -41,4 +41,12 @@ struct NothingToCoerce<T: ?Sized> {
 impl<T: ?Sized, U: ?Sized> DispatchFromDyn<NothingToCoerce<T>> for NothingToCoerce<U> {}
 //~^ ERROR [E0378]
 
+#[repr(C)]
+struct HasReprC<T: ?Sized>(Box<T>);
+
+impl<T: ?Sized, U: ?Sized> DispatchFromDyn<HasReprC<U>> for HasReprC<T>
+where
+    T: Unsize<U>,
+{} //~^^^ ERROR [E0378]
+
 fn main() {}

src/test/ui/invalid_dispatch_from_dyn_impls.stderr

@@ -27,6 +27,15 @@ error[E0378]: the trait `DispatchFromDyn` may only be implemented for a coercion
 LL | impl<T: ?Sized, U: ?Sized> DispatchFromDyn<NothingToCoerce<T>> for NothingToCoerce<U> {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to 3 previous errors
+error[E0378]: structs implementing `DispatchFromDyn` may not have `#[repr(packed)]` or `#[repr(C)]`
+  --> $DIR/invalid_dispatch_from_dyn_impls.rs:47:1
+   |
+LL | / impl<T: ?Sized, U: ?Sized> DispatchFromDyn<HasReprC<U>> for HasReprC<T>
+LL | | where
+LL | |     T: Unsize<U>,
+LL | | {} //~^^^ ERROR [E0378]
+   | |__^
+
+error: aborting due to 4 previous errors
 
 For more information about this error, try `rustc --explain E0378`.