Allow borrow attribute on newtype variants

test_suite/tests/compile-fail/borrow/duplicate_variant.rs

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+// Copyright 2017 Serde Developers
+//
+// 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.
+
+#[macro_use]
+extern crate serde_derive;
+
+#[derive(Deserialize)]
+struct Str<'a>(&'a str);
+
+#[derive(Deserialize)] //~ ERROR: proc-macro derive panicked
+enum Test<'a> {
+    #[serde(borrow)] //~^^ HELP: duplicate serde attribute `borrow`
+    S(#[serde(borrow)] Str<'a>)
+}
+
+fn main() {}

test_suite/tests/compile-fail/borrow/struct_variant.rs

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+// Copyright 2017 Serde Developers
+//
+// 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.
+
+#[macro_use]
+extern crate serde_derive;
+
+#[derive(Deserialize)]
+struct Str<'a>(&'a str);
+
+#[derive(Deserialize)] //~ ERROR: proc-macro derive panicked
+enum Test<'a> {
+    #[serde(borrow)] //~^^ HELP: #[serde(borrow)] may only be used on newtype variants
+    S { s: Str<'a> }
+}
+
+fn main() {}