Add #![feature(unsized_locals)].

src/libsyntax/feature_gate.rs

@@ -503,6 +503,9 @@ declare_features! (
 
     // Allows `Self` in type definitions
     (active, self_in_typedefs, "1.30.0", Some(49303), None),
+
+    // unsized rvalues at arguments and parameters
+    (active, unsized_locals, "1.30.0", Some(48055), None),
 );
 
 declare_features! (

src/test/ui/feature-gate-unsized_locals.rs

@@ -0,0 +1,15 @@
+// Copyright 2018 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.
+
+fn f(f: FnOnce()) {}
+//~^ ERROR E0277
+
+fn main() {
+}

src/test/ui/feature-gate-unsized_locals.stderr

@@ -0,0 +1,13 @@
+error[E0277]: the size for values of type `(dyn std::ops::FnOnce() + 'static)` cannot be known at compilation time
+  --> $DIR/feature-gate-unsized_locals.rs:11:6
+   |
+LL | fn f(f: FnOnce()) {}
+   |      ^ doesn't have a size known at compile-time
+   |
+   = help: the trait `std::marker::Sized` is not implemented for `(dyn std::ops::FnOnce() + 'static)`
+   = note: to learn more, visit <https://doc.rust-lang.org/book/second-edition/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
+   = note: all local variables must have a statically known size
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.