Add a feature gate

@alexcrichton figured out a way how to do it :)

src/libsyntax/ext/source_util.rs

@@ -52,6 +52,16 @@ pub fn expand_column(cx: &mut ExtCtxt, sp: Span, tts: &[tokenstream::TokenTree])
     base::MacEager::expr(cx.expr_u32(topmost, loc.col.to_usize() as u32))
 }
 
+/* __rust_unstable_column!(): expands to the current column number */
+pub fn expand_column_gated(cx: &mut ExtCtxt, sp: Span, tts: &[tokenstream::TokenTree])
+                  -> Box<base::MacResult+'static> {
+    if sp.allows_unstable() {
+        expand_column(cx, sp, tts)
+    } else {
+        cx.span_fatal(sp, "the __rust_unstable_column macro is unstable");
+    }
+}
+
 /// file!(): expands to the current filename */
 /// The filemap (`loc.file`) contains a bunch more information we could spit
 /// out if we wanted.

src/libsyntax_ext/lib.rs

@@ -89,7 +89,7 @@ pub fn register_builtins(resolver: &mut syntax::ext::base::Resolver,
     use syntax::ext::source_util::*;
     register! {
         line: expand_line,
-        __rust_unstable_column: expand_column,
+        __rust_unstable_column: expand_column_gated,
         column: expand_column,
         file: expand_file,
         stringify: expand_stringify,

src/test/compile-fail/rust-unstable-column-gated.rs

@@ -0,0 +1,14 @@
+// Copyright 2017 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 main() {
+    println!("{}", __rust_unstable_column!());
+    //~^ERROR the __rust_unstable_column macro is unstable
+}