Pop the expansion context after expanding a method macro

We were leaving these on the stack, causing spurious backtraces.

I've confirmed that this test fails without the fix.

src/libsyntax/ext/expand.rs

@@ -897,7 +897,10 @@ fn expand_method(m: P<ast::Method>, fld: &mut MacroExpander) -> SmallVector<P<as
             };
 
             // expand again if necessary
-            new_methods.into_iter().flat_map(|m| fld.fold_method(m).into_iter()).collect()
+            let new_methods = new_methods.move_iter()
+                                  .flat_map(|m| fld.fold_method(m).into_iter()).collect();
+            fld.cx.bt_pop();
+            new_methods
         }
     })
 }

src/test/compile-fail/method-macro-backtrace.rs

@@ -0,0 +1,37 @@
+// Copyright 2014 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.
+
+// forbid-output: in expansion of
+
+#![feature(macro_rules)]
+
+macro_rules! make_method ( ($name:ident) => (
+    fn $name(&self) { }
+))
+
+struct S;
+
+impl S {
+    // We had a bug where these wouldn't clean up macro backtrace frames.
+    make_method!(foo1)
+    make_method!(foo2)
+    make_method!(foo3)
+    make_method!(foo4)
+    make_method!(foo5)
+    make_method!(foo6)
+    make_method!(foo7)
+    make_method!(foo8)
+
+    // Cause an error. It shouldn't have any macro backtrace frames.
+    fn bar(&self) { }
+    fn bar(&self) { } //~ ERROR duplicate definition
+}
+
+fn main() { }