Use symbols in lint tool list

src/bootstrap/bin/rustc.rs

@@ -306,14 +306,15 @@ fn main() {
     }
 
     // This is required for internal lints.
-    cmd.arg("-Zunstable-options");
     if let Some(crate_name) = args.windows(2).find(|a| &*a[0] == "--crate-name") {
         let crate_name = crate_name[1].to_string_lossy();
-        if crate_name.starts_with("rustc")
-            || crate_name.starts_with("syntax")
-            || crate_name == "arena"
-            || crate_name == "fmt_macros"
+        if crate_name != "rustc_version"
+            && (crate_name.starts_with("rustc")
+                || crate_name.starts_with("syntax")
+                || crate_name == "arena"
+                || crate_name == "fmt_macros")
         {
+            cmd.arg("-Zunstable-options");
             if stage != "0" {
                 cmd.arg("-Wrustc::internal");
             }

src/libsyntax/attr/mod.rs

@@ -60,7 +60,7 @@ pub fn is_known(attr: &Attribute) -> bool {
 }
 
 pub fn is_known_lint_tool(m_item: Ident) -> bool {
-    ["clippy", "rustc"].contains(&m_item.as_str().as_ref())
+    [sym::clippy, sym::rustc].contains(&m_item.name)
 }
 
 impl NestedMetaItem {

src/libsyntax_pos/symbol.rs

@@ -173,6 +173,7 @@ symbols! {
         cfg_target_thread_local,
         cfg_target_vendor,
         char,
+        clippy,
         clone,
         Clone,
         clone_closures,