rustc: Always mark #[crate_name] as used

It's just an annoying error if you use --crate-name on the command line and you
also have a #[crate_name] specified

src/librustc/back/link.rs

@@ -555,6 +555,12 @@ pub fn find_crate_name(sess: Option<&Session>,
         s
     };
 
+    // Look in attributes 100% of the time to make sure the attribute is marked
+    // as used. After doing this, however, favor crate names from the command
+    // line.
+    let attr_crate_name = attrs.iter().find(|at| at.check_name("crate_name"))
+                               .and_then(|at| at.value_str().map(|s| (at, s)));
+
     match sess {
         Some(sess) => {
             match sess.opts.crate_name {
@@ -565,9 +571,7 @@ pub fn find_crate_name(sess: Option<&Session>,
         None => {}
     }
 
-    let crate_name = attrs.iter().find(|at| at.check_name("crate_name"))
-                          .and_then(|at| at.value_str().map(|s| (at, s)));
-    match crate_name {
+    match attr_crate_name {
         Some((attr, s)) => return validate(s.get().to_string(), Some(attr.span)),
         None => {}
     }

src/test/run-pass/crate-name-attr-used.rs

@@ -0,0 +1,15 @@
+// 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.
+
+// compile-flags:--crate-name crate-name-attr-used -F unused-attribute
+
+#![crate_name = "test"]
+
+fn main() {}