Rollup merge of rust-lang#50406 - ExpHP:concat-nonzero-idents, r=dtolnay

Forbid constructing empty identifiers from concat_idents

The empty identifier is a [reserved identifier](https://github.com/rust-lang/rust/blob/8a37c75a3a661385cc607d934c70e86a9eaf5fd7/src/libsyntax_pos/symbol.rs#L300-L305) in rust, apparently used for black magicks like representing the crate root or somesuch... and therefore, being able to construct it is Ungood.  Presumably.

...even if the macro that lets you construct it is so useless that you can't actually do any damage with it. (and believe me, I tried)

Fixes rust-lang#50403.

**Note:** I noticed that when you try to do something similar with `proc_macro::Term`, the compiler actually catches it and flags the identifier as reserved.  Perhaps a better solution would be to somehow have that same check applied here.

src/libcore/macros.rs

@@ -606,8 +606,8 @@ mod builtin {
     #[macro_export]
     #[cfg(dox)]
     macro_rules! concat_idents {
-        ($($e:ident),*) => ({ /* compiler built-in */ });
-        ($($e:ident,)*) => ({ /* compiler built-in */ });
+        ($($e:ident),+) => ({ /* compiler built-in */ });
+        ($($e:ident,)+) => ({ /* compiler built-in */ });
     }
 
     /// Concatenates literals into a static string slice.

src/libstd/macros.rs

@@ -450,8 +450,8 @@ pub mod builtin {
     #[unstable(feature = "concat_idents_macro", issue = "29599")]
     #[macro_export]
     macro_rules! concat_idents {
-        ($($e:ident),*) => ({ /* compiler built-in */ });
-        ($($e:ident,)*) => ({ /* compiler built-in */ });
+        ($($e:ident),+) => ({ /* compiler built-in */ });
+        ($($e:ident,)+) => ({ /* compiler built-in */ });
     }
 
     /// Concatenates literals into a static string slice.

src/libsyntax_ext/concat_idents.rs

@@ -31,6 +31,11 @@ pub fn expand_syntax_ext<'cx>(cx: &'cx mut ExtCtxt,
         return base::DummyResult::expr(sp);
     }
 
+    if tts.is_empty() {
+        cx.span_err(sp, "concat_idents! takes 1 or more arguments.");
+        return DummyResult::expr(sp);
+    }
+
     let mut res_str = String::new();
     for (i, e) in tts.iter().enumerate() {
         if i & 1 == 1 {

src/test/ui/issue-50403.rs

@@ -0,0 +1,15 @@
+// Copyright 2016 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(concat_idents)]
+
+fn main() {
+    let x = concat_idents!(); //~ ERROR concat_idents! takes 1 or more arguments
+}

src/test/ui/issue-50403.stderr

@@ -0,0 +1,8 @@
+error: concat_idents! takes 1 or more arguments.
+  --> $DIR/issue-50403.rs:14:13
+   |
+LL |     let x = concat_idents!(); //~ ERROR concat_idents! takes 1 or more arguments
+   |             ^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+