lint: dead_code ignores items with leading underscores.

This generalises the behaviour with struct fields (which recieve no
dead_code warning if they have a leading _), and other similar lints, to
all items, e.g. `fn _foo() {} fn main() {}` has no warnings.

src/librustc/middle/dead.rs

@@ -412,10 +412,7 @@ struct DeadVisitor<'a> {
 
 impl<'a> DeadVisitor<'a> {
     fn should_warn_about_field(&mut self, node: &ast::StructField_) -> bool {
-        let (is_named, has_leading_underscore) = match node.ident() {
-            Some(ref ident) => (true, token::get_ident(*ident).get().as_bytes()[0] == ('_' as u8)),
-            _ => (false, false)
-        };
+        let is_named = node.ident().is_some();
         let field_type = ty::node_id_to_type(self.tcx, node.id);
         let is_marker_field = match ty::ty_to_def_id(field_type) {
             Some(def_id) => self.tcx.lang_items.items().any(|(_, item)| *item == Some(def_id)),
@@ -423,7 +420,6 @@ impl<'a> DeadVisitor<'a> {
         };
         is_named
             && !self.symbol_is_live(node.id, None)
-            && !has_leading_underscore
             && !is_marker_field
             && !has_allow_dead_code_or_lang_attr(node.attrs.as_slice())
     }
@@ -468,13 +464,15 @@ impl<'a> DeadVisitor<'a> {
                       id: ast::NodeId,
                       span: codemap::Span,
                       ident: ast::Ident) {
-        self.tcx
-            .sess
-            .add_lint(lint::builtin::DEAD_CODE,
-                      id,
-                      span,
-                      format!("code is never used: `{}`",
-                              token::get_ident(ident)));
+        let name = ident.as_str();
+        if !name.starts_with("_") {
+            self.tcx
+                .sess
+                .add_lint(lint::builtin::DEAD_CODE,
+                          id,
+                          span,
+                          format!("code is never used: `{}`", name));
+        }
     }
 }
 

src/test/run-pass/dead-code-leading-underscore.rs

@@ -0,0 +1,38 @@
+// 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.
+
+#![deny(dead_code)]
+
+static _X: uint = 0;
+
+fn _foo() {}
+
+struct _Y {
+    _z: uint
+}
+
+enum _Z {}
+
+impl _Y {
+    fn _bar() {}
+}
+
+type _A = int;
+
+mod _bar {
+    fn _qux() {}
+}
+
+extern {
+    #[link_name = "abort"]
+    fn _abort() -> !;
+}
+
+fn main() {}