First sketch of lint pass

Enough attributes are marked to cleanly compile an empty library.

src/librustc/front/feature_gate.rs

@@ -327,7 +327,7 @@ pub fn check_crate(sess: &Session, krate: &ast::Crate) {
     };
 
     for attr in krate.attrs.iter() {
-        if !attr.name().equiv(&("feature")) {
+        if !attr.check_name("feature") {
             continue
         }
 

src/librustc/front/std_inject.rs

@@ -115,6 +115,8 @@ impl<'a> fold::Folder for StandardLibraryInjector<'a> {
                                   InternedString::new("feature"),
                                   vec![attr::mk_word_item(InternedString::new("phase"))],
                               ));
+        // std_inject runs after feature checking so manually mark this attr
+        attr::mark_used(&feat_phase_attr);
         krate.attrs.push(feat_phase_attr);
 
         krate
@@ -141,6 +143,8 @@ impl<'a> fold::Folder for PreludeInjector<'a> {
 
         let no_std_attr = attr::mk_attr_inner(attr::mk_attr_id(),
                                               attr::mk_word_item(InternedString::new("no_std")));
+        // std_inject runs after feature checking so manually mark this attr
+        attr::mark_used(&no_std_attr);
         krate.attrs.push(no_std_attr);
 
         if !no_prelude(krate.attrs.as_slice()) {
@@ -154,6 +158,8 @@ impl<'a> fold::Folder for PreludeInjector<'a> {
                 vec!(
                     attr::mk_word_item(InternedString::new("globs")),
                 )));
+            // std_inject runs after feature checking so manually mark this attr
+            attr::mark_used(&globs_attr);
             krate.attrs.push(globs_attr);
 
             krate.module = self.fold_mod(&krate.module);

src/librustc/middle/lint.rs

@@ -90,6 +90,7 @@ pub enum Lint {
     UnusedUnsafe,
     UnsafeBlock,
     AttributeUsage,
+    UnusedAttribute,
     UnknownFeatures,
     UnknownCrateType,
     UnsignedNegate,
@@ -288,6 +289,13 @@ static lint_table: &'static [(&'static str, LintSpec)] = &[
         default: Warn
     }),
 
+    ("unused_attribute",
+     LintSpec {
+         lint: UnusedAttribute,
+         desc: "detects attributes that were not used by the compiler",
+         default: Allow
+    }),
+
     ("unused_variable",
      LintSpec {
         lint: UnusedVariable,
@@ -619,7 +627,7 @@ pub fn each_lint(sess: &session::Session,
     let xs = [Allow, Warn, Deny, Forbid];
     for &level in xs.iter() {
         let level_name = level_to_str(level);
-        for attr in attrs.iter().filter(|m| m.name().equiv(&level_name)) {
+        for attr in attrs.iter().filter(|m| m.check_name(level_name)) {
             let meta = attr.node.value;
             let metas = match meta.node {
                 ast::MetaList(_, ref metas) => metas,
@@ -1137,6 +1145,15 @@ fn check_attrs_usage(cx: &Context, attrs: &[ast::Attribute]) {
     }
 }
 
+fn check_unused_attribute(cx: &Context, attrs: &[ast::Attribute]) {
+    for attr in attrs.iter() {
+        if !attr::is_used(attr) {
+            cx.span_lint(UnusedAttribute, attr.span,
+                         format!("unused attribute {}", attr.name()).as_slice());
+        }
+    }
+}
+
 fn check_heap_expr(cx: &Context, e: &ast::Expr) {
     let ty = ty::expr_ty(cx.tcx, e);
     check_heap_type(cx, e.span, ty);
@@ -1694,6 +1711,7 @@ impl<'a> Visitor<()> for Context<'a> {
             check_heap_item(cx, it);
             check_missing_doc_item(cx, it);
             check_attrs_usage(cx, it.attrs.as_slice());
+            check_unused_attribute(cx, it.attrs.as_slice());
             check_raw_ptr_deriving(cx, it);
 
             cx.visit_ids(|v| v.visit_item(it, ()));
@@ -1900,6 +1918,7 @@ pub fn check_crate(tcx: &ty::ctxt,
         check_crate_attrs_usage(cx, krate.attrs.as_slice());
         // since the root module isn't visited as an item (because it isn't an item), warn for it
         // here.
+        check_unused_attribute(cx, krate.attrs.as_slice());
         check_missing_doc_attrs(cx,
                                 None,
                                 krate.attrs.as_slice(),

src/libsyntax/attr.rs

@@ -35,9 +35,9 @@ pub fn is_used(attr: &Attribute) -> bool {
 }
 
 pub trait AttrMetaMethods {
-    // This could be changed to `fn check_name(&self, name: InternedString) ->
-    // bool` which would facilitate a side table recording which
-    // attributes/meta items are used/unused.
+    fn check_name(&self, name: &str) -> bool {
+        name == self.name().get()
+    }
 
     /// Retrieve the name of the meta item, e.g. foo in #[foo],
     /// #[foo="bar"] and #[foo(bar)]
@@ -59,6 +59,14 @@ pub trait AttrMetaMethods {
 }
 
 impl AttrMetaMethods for Attribute {
+    fn check_name(&self, name: &str) -> bool {
+        if name == self.name().get() {
+            mark_used(self);
+            true
+        } else {
+            false
+        }
+    }
     fn name(&self) -> InternedString { self.meta().name() }
     fn value_str(&self) -> Option<InternedString> {
         self.meta().value_str()

src/libsyntax/ext/expand.rs

@@ -474,7 +474,7 @@ pub fn expand_view_item(vi: &ast::ViewItem,
     match vi.node {
         ast::ViewItemExternCrate(..) => {
             let should_load = vi.attrs.iter().any(|attr| {
-                attr.name().get() == "phase" &&
+                attr.check_name("phase") &&
                     attr.meta_item_list().map_or(false, |phases| {
                         attr::contains_name(phases, "syntax")
                     })