Forbid the Sized bound on unsized types

closes #16800

src/test/compile-fail/bad-sized.rs

@@ -0,0 +1,25 @@
+// Copyright 2012 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.
+
+use std::cell::RefCell;
+
+trait Trait {}
+
+pub fn main() {
+    let x: Vec<Trait + Sized> = Vec::new();
+    //~^ ERROR explicitly adding `Sized` bound to an unsized type `Trait+Sized`
+    //~^^ ERROR explicitly adding `Sized` bound to an unsized type `Trait+Sized`
+    let x: Vec<Box<Trait + Sized>> = Vec::new();
+    //~^ ERROR explicitly adding `Sized` bound to an unsized type `Trait+Sized`
+    //~^^ ERROR explicitly adding `Sized` bound to an unsized type `Trait+Sized`
+    let x: Vec<Box<RefCell<Trait + Sized>>> = Vec::new();
+    //~^ ERROR explicitly adding `Sized` bound to an unsized type `Trait+Sized`
+    //~^^ ERROR explicitly adding `Sized` bound to an unsized type `Trait+Sized`
+}