auto merge of #14275 : kballard/rust/bytes-return-static, r=huonw

Change `bytes!()` to return

    {
        static BYTES: &'static [u8] = &[...];
        BYTES
    }

This gives it the `'static` lifetime, whereas before it had an rvalue
lifetime. Until recently this would have prevented assigning `bytes!()`
to a static, as in

    static FOO: &'static [u8] = bytes!(1,2,3);

but #14183 fixed it so blocks are now allowed in constant expressions
(with restrictions).

Fixes #11641.

src/test/run-pass/bytes-macro-static.rs

@@ -0,0 +1,21 @@
+// 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.
+
+static FOO: &'static [u8] = bytes!("hello, world");
+
+pub fn main() {
+    let b = match true {
+        true => bytes!("test"),
+        false => unreachable!()
+    };
+
+    assert_eq!(b, "test".as_bytes());
+    assert_eq!(FOO, "hello, world".as_bytes());
+}