Add a lint for duplicate feature attributes

src/librustc/lint/builtin.rs

@@ -111,6 +111,12 @@ declare_lint! {
     "unknown features found in crate-level #[feature] directives"
 }
 
+declare_lint! {
+    pub DUPLICATE_FEATURES,
+    Deny,
+    "duplicate features found in crate-level #[feature] directives"
+}
+
 declare_lint! {
     pub STABLE_FEATURES,
     Warn,
@@ -369,6 +375,7 @@ impl LintPass for HardwiredLints {
             WARNINGS,
             UNUSED_FEATURES,
             UNKNOWN_FEATURES,
+            DUPLICATE_FEATURES,
             STABLE_FEATURES,
             UNKNOWN_CRATE_TYPES,
             TRIVIAL_CASTS,

src/librustc/middle/stability.rs

@@ -826,6 +826,13 @@ pub fn check_unused_or_stable_features<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>) {
 
     let mut remaining_lib_features = FxHashMap();
     for (feature, span) in declared_lib_features.clone().into_iter() {
+        // Warn if the user enables a feature multiple times.
+        if remaining_lib_features.contains_key(&feature) {
+            tcx.lint_node(lint::builtin::DUPLICATE_FEATURES,
+                          ast::CRATE_NODE_ID,
+                          span,
+                          &format!("duplicate `{}` feature attribute", feature));
+        }
         remaining_lib_features.insert(feature, span);
     }
     // FIXME(varkor): we don't properly handle lib features behind `cfg` attributes yet,

src/librustc_lint/lib.rs

@@ -189,6 +189,7 @@ pub fn register_builtins(store: &mut lint::LintStore, sess: Option<&Session>) {
                     UNUSED_EXTERN_CRATES,
                     UNUSED_FEATURES,
                     UNKNOWN_FEATURES,
+                    DUPLICATE_FEATURES,
                     UNUSED_LABELS,
                     UNUSED_PARENS);