Auto merge of #50345 - kennytm:rollup, r=kennytm

Rollup of 7 pull requests

Successful merges:

 - #50233 (Make `Vec::new` a `const fn`)
 - #50312 (Add more links in panic docs)
 - #50316 (Fix some broken links in docs.)
 - #50325 (Add a few more tests for proc macro feature gating)
 - #50327 (Display correct unused field suggestion for nested struct patterns)
 - #50330 (check that #[used] is used only on statics)
 - #50344 (Update Cargo to 2018-04-28 122fd5be5201913d42e219e132d6569493583bca)

Failed merges:

1 files changed, 28 insertions, 0 deletions

diff --git a/src/test/compile-fail/used.rs b/src/test/compile-fail/used.rs
new file mode 100644
index 00000000000..f170d9c25f5
--- /dev/null
+++ b/src/test/compile-fail/used.rs
@@ -0,0 +1,28 @@
+// 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.
+
+#![feature(used)]
+
+#[used]
+static FOO: u32 = 0; // OK
+
+#[used] //~ ERROR attribute must be applied to a `static` variable
+fn foo() {}
+
+#[used] //~ ERROR attribute must be applied to a `static` variable
+struct Foo {}
+
+#[used] //~ ERROR attribute must be applied to a `static` variable
+trait Bar {}
+
+#[used] //~ ERROR attribute must be applied to a `static` variable
+impl Bar for Foo {}
+
+fn main() {}