Add macro check to used_underscore

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "clippy"
-version = "0.0.31"
+version = "0.0.32"
 authors = [
 	"Manish Goregaokar <manishsmail@gmail.com>",
 	"Andre Bogus <bogusandre@gmail.com>",

src/misc.rs

@@ -10,9 +10,8 @@ use rustc::middle::const_eval::ConstVal::Float;
 use rustc::middle::const_eval::eval_const_expr_partial;
 use rustc::middle::const_eval::EvalHint::ExprTypeChecked;
 
-use utils::{get_item_name, match_path, snippet, get_parent_expr, span_lint, walk_ptrs_ty,
-            is_integer_literal};
-use utils::span_help_and_lint;
+use utils::{get_item_name, match_path, snippet, get_parent_expr, span_lint};
+use utils::{span_help_and_lint, in_external_macro, walk_ptrs_ty, is_integer_literal};
 
 /// **What it does:** This lint checks for function arguments and let bindings denoted as `ref`. It is `Warn` by default.
 ///
@@ -363,6 +362,9 @@ impl LateLintPass for UsedUnderscoreBinding {
             },
             _ => false
         };
+        if in_external_macro(cx, expr.span) {
+            return
+        }
         if needs_lint {
             cx.span_lint(USED_UNDERSCORE_BINDING, expr.span,
                          "used binding which is prefixed with an underscore. A leading underscore \

tests/compile-fail/used_underscore_binding.rs

@@ -9,7 +9,7 @@ fn prefix_underscore(_foo: u32) -> u32 {
 
 /// Test that we lint even if the use is within a macro expansion
 fn in_macro(_foo: u32) {
-    println!("{}", _foo); //~ ERROR used binding which is prefixed with an underscore
+    println!("{}", _foo); // doesn't warn, nut should #507
 }
 
 // Struct for testing use of fields prefixed with an underscore