Require that objects can only be made from Sized types. Fixes #18333.

src/test/compile-fail/dst-object-from-unsized-type.rs

@@ -0,0 +1,30 @@
+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Test that we cannot create objects from unsized types.
+
+trait Foo for Sized? {}
+impl Foo for str {}
+
+fn test<Sized? T: Foo>(t: &T) {
+    let u: &Foo = t;
+    //~^ ERROR `core::kinds::Sized` is not implemented for the type `T`
+
+    let v: &Foo = t as &Foo;
+    //~^ ERROR `core::kinds::Sized` is not implemented for the type `T`
+}
+
+fn main() {
+    let _: &[&Foo] = &["hi"];
+    //~^ ERROR `core::kinds::Sized` is not implemented for the type `str`
+
+    let _: &Foo = "hi" as &Foo;
+    //~^ ERROR `core::kinds::Sized` is not implemented for the type `str`
+}

src/test/compile-fail/issue-14366.rs

@@ -11,5 +11,5 @@
 fn main() {
     let _x = "test" as &::std::any::Any;
 //~^ ERROR the trait `core::kinds::Sized` is not implemented for the type `str`
-//~^^ NOTE the trait `core::kinds::Sized` must be implemented for the cast to the object type
+//~^^ ERROR the trait `core::kinds::Sized` is not implemented for the type `str`
 }