Add a syntax extension to log syntax, for debugging macros.

src/comp/syntax/ext/log_syntax.rs

@@ -0,0 +1,13 @@
+import std::option;
+import base::*;
+import syntax::ast;
+
+fn expand_syntax_ext(cx: &ext_ctxt, sp: codemap::span, arg: @ast::expr,
+                     body: option::t[str]) -> @ast::expr {
+
+    cx.print_backtrace();
+    std::io::stdout().write_line(print::pprust::expr_to_str(arg));
+
+    //trivial expression
+    ret @{id: cx.next_id(), node: ast::expr_rec(~[], option::none), span: sp};
+}