Enforce that dyn* casts are actually pointer-sized

src/test/ui/dyn-star/check-size-at-cast-polymorphic-bad.rs

@@ -0,0 +1,15 @@
+#![feature(dyn_star)]
+#![allow(incomplete_features)]
+
+use std::fmt::Debug;
+
+fn dyn_debug(_: (dyn* Debug + '_)) {
+
+}
+
+fn polymorphic<T: Debug + ?Sized>(t: &T) {
+    dyn_debug(t);
+    //~^ ERROR `&T` needs to be a pointer-sized type
+}
+
+fn main() {}

src/test/ui/dyn-star/check-size-at-cast-polymorphic-bad.stderr

@@ -0,0 +1,15 @@
+error[E0277]: `&T` needs to be a pointer-sized type
+  --> $DIR/check-size-at-cast-polymorphic-bad.rs:11:15
+   |
+LL |     dyn_debug(t);
+   |               ^ `&T` needs to be a pointer-sized type
+   |
+   = help: the trait `PointerSized` is not implemented for `&T`
+help: consider introducing a `where` clause, but there might be an alternative better way to express this requirement
+   |
+LL | fn polymorphic<T: Debug + ?Sized>(t: &T) where &T: PointerSized {
+   |                                          ++++++++++++++++++++++
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.

src/test/ui/dyn-star/check-size-at-cast-polymorphic.rs

@@ -0,0 +1,16 @@
+// check-pass
+
+#![feature(dyn_star)]
+#![allow(incomplete_features)]
+
+use std::fmt::Debug;
+
+fn dyn_debug(_: (dyn* Debug + '_)) {
+
+}
+
+fn polymorphic<T: Debug>(t: &T) {
+    dyn_debug(t);
+}
+
+fn main() {}

src/test/ui/dyn-star/check-size-at-cast.rs

@@ -0,0 +1,10 @@
+#![feature(dyn_star)]
+#![allow(incomplete_features)]
+
+use std::fmt::Debug;
+
+fn main() {
+    let i = [1, 2, 3, 4] as dyn* Debug;
+    //~^ ERROR `[i32; 4]` needs to be a pointer-sized type
+    dbg!(i);
+}

src/test/ui/dyn-star/check-size-at-cast.stderr

@@ -0,0 +1,11 @@
+error[E0277]: `[i32; 4]` needs to be a pointer-sized type
+  --> $DIR/check-size-at-cast.rs:7:13
+   |
+LL |     let i = [1, 2, 3, 4] as dyn* Debug;
+   |             ^^^^^^^^^^^^ `[i32; 4]` needs to be a pointer-sized type
+   |
+   = help: the trait `PointerSized` is not implemented for `[i32; 4]`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.