Don't allow implement CoerceUnsized into dyn-star

compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs

@@ -840,7 +840,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             }
 
             // `T` -> `Trait`
-            (_, &ty::Dynamic(..)) => {
+            (_, &ty::Dynamic(_, _, ty::Dyn)) => {
                 candidates.vec.push(BuiltinUnsizeCandidate);
             }
 

src/test/ui/dyn-star/unsize-into-ref-dyn-star.rs

@@ -0,0 +1,9 @@
+#![feature(dyn_star)]
+#![allow(incomplete_features)]
+
+use std::fmt::Debug;
+
+fn main() {
+    let i = 42 as &dyn* Debug;
+    //~^ ERROR non-primitive cast: `i32` as `&dyn* Debug`
+}

src/test/ui/dyn-star/unsize-into-ref-dyn-star.stderr

@@ -0,0 +1,9 @@
+error[E0605]: non-primitive cast: `i32` as `&dyn* Debug`
+  --> $DIR/unsize-into-ref-dyn-star.rs:7:13
+   |
+LL |     let i = 42 as &dyn* Debug;
+   |             ^^^^^^^^^^^^^^^^^ an `as` expression can only be used to convert between primitive types or to coerce to a specific trait object
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0605`.