Pass a more proper span to the syntax expanders

Closes #5794

Author: alexcrichton

src/libsyntax/ext/expand.rs

@@ -76,8 +76,26 @@ pub fn expand_expr(extsbox: @mut SyntaxEnv,
                             // mark before:
                             let marked_before = mark_tts(*tts,fm);
                             let marked_ctxt = new_mark(fm, ctxt);
+
+                            // The span that we pass to the expanders we want to
+                            // be the root of the call stack. That's the most
+                            // relevant span and it's the actual invocation of
+                            // the macro.
+                            let mut relevant_info = cx.backtrace();
+                            let mut einfo = relevant_info.unwrap();
+                            loop {
+                                match relevant_info {
+                                    None => { break }
+                                    Some(e) => {
+                                        einfo = e;
+                                        relevant_info = einfo.call_site.expn_info;
+                                    }
+                                }
+                            }
+
                             let expanded =
-                                match expandfun(cx, mac.span, marked_before, marked_ctxt) {
+                                match expandfun(cx, einfo.call_site,
+                                                marked_before, marked_ctxt) {
                                     MRExpr(e) => e,
                                     MRAny(expr_maker,_,_) => expr_maker(),
                                     _ => {

src/test/compile-fail/debug-correct-span.rs

@@ -0,0 +1,13 @@
+// Copyright 2013 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() {
+    debug!("%s %s", 3); //~ ERROR: not enough arguments
+}