diff --git a/src/librustc_lint/unused.rs b/src/librustc_lint/unused.rs
index a29ff18ab53..ca47db6ccee 100644
--- a/src/librustc_lint/unused.rs
+++ b/src/librustc_lint/unused.rs
@@ -19,7 +19,7 @@ use std::collections::hash_map::Entry::{Occupied, Vacant};
 
 use syntax::ast;
 use syntax::attr;
-use syntax::feature_gate::{KNOWN_ATTRIBUTES, AttributeType};
+use syntax::feature_gate::{BUILTIN_ATTRIBUTES, AttributeType};
 use syntax::parse::token::keywords;
 use syntax::ptr::P;
 use syntax_pos::Span;
@@ -245,7 +245,7 @@ impl LateLintPass for UnusedAttributes {
         debug!("checking attribute: {:?}", attr);
 
         // Note that check_name() marks the attribute as used if it matches.
-        for &(ref name, ty, _) in KNOWN_ATTRIBUTES {
+        for &(ref name, ty, _) in BUILTIN_ATTRIBUTES {
             match ty {
                 AttributeType::Whitelisted if attr.check_name(name) => {
                     debug!("{:?} is Whitelisted", name);
@@ -267,7 +267,7 @@ impl LateLintPass for UnusedAttributes {
             debug!("Emitting warning for: {:?}", attr);
             cx.span_lint(UNUSED_ATTRIBUTES, attr.span, "unused attribute");
             // Is it a builtin attribute that must be used at the crate level?
-            let known_crate = KNOWN_ATTRIBUTES.iter()
+            let known_crate = BUILTIN_ATTRIBUTES.iter()
                 .find(|&&(name, ty, _)| attr.name() == name && ty == AttributeType::CrateLevel)
                 .is_some();
 
diff --git a/src/libsyntax/attr.rs b/src/libsyntax/attr.rs
index 0335f210347..57a936bf9b0 100644
--- a/src/libsyntax/attr.rs
+++ b/src/libsyntax/attr.rs
@@ -32,7 +32,8 @@ use std::cell::{RefCell, Cell};
 use std::collections::HashSet;
 
 thread_local! {
-    static USED_ATTRS: RefCell<Vec<u64>> = RefCell::new(Vec::new())
+    static USED_ATTRS: RefCell<Vec<u64>> = RefCell::new(Vec::new());
+    static KNOWN_ATTRS: RefCell<Vec<u64>> = RefCell::new(Vec::new());
 }
 
 enum AttrError {
@@ -81,6 +82,29 @@ pub fn is_used(attr: &Attribute) -> bool {
     })
 }
 
+pub fn mark_known(attr: &Attribute) {
+    debug!("Marking {:?} as known.", attr);
+    let AttrId(id) = attr.node.id;
+    KNOWN_ATTRS.with(|slot| {
+        let idx = (id / 64) as usize;
+        let shift = id % 64;
+        if slot.borrow().len() <= idx {
+            slot.borrow_mut().resize(idx + 1, 0);
+        }
+        slot.borrow_mut()[idx] |= 1 << shift;
+    });
+}
+
+pub fn is_known(attr: &Attribute) -> bool {
+    let AttrId(id) = attr.node.id;
+    KNOWN_ATTRS.with(|slot| {
+        let idx = (id / 64) as usize;
+        let shift = id % 64;
+        slot.borrow().get(idx).map(|bits| bits & (1 << shift) != 0)
+            .unwrap_or(false)
+    })
+}
+
 impl NestedMetaItem {
     /// Returns the MetaItem if self is a NestedMetaItemKind::MetaItem.
     pub fn meta_item(&self) -> Option<&P<MetaItem>> {
diff --git a/src/libsyntax/feature_gate.rs b/src/libsyntax/feature_gate.rs
index 129e4a82338..f543eae8179 100644
--- a/src/libsyntax/feature_gate.rs
+++ b/src/libsyntax/feature_gate.rs
@@ -417,11 +417,11 @@ macro_rules! cfg_fn {
 }
 
 pub fn deprecated_attributes() -> Vec<&'static (&'static str, AttributeType, AttributeGate)> {
-    KNOWN_ATTRIBUTES.iter().filter(|a| a.2.is_deprecated()).collect()
+    BUILTIN_ATTRIBUTES.iter().filter(|a| a.2.is_deprecated()).collect()
 }
 
 // Attributes that have a special meaning to rustc or rustdoc
-pub const KNOWN_ATTRIBUTES: &'static [(&'static str, AttributeType, AttributeGate)] = &[
+pub const BUILTIN_ATTRIBUTES: &'static [(&'static str, AttributeType, AttributeGate)] = &[
     // Normal attributes
 
     ("warn", Normal, Ungated),
@@ -790,12 +790,12 @@ impl<'a> Context<'a> {
     fn check_attribute(&self, attr: &ast::Attribute, is_macro: bool) {
         debug!("check_attribute(attr = {:?})", attr);
         let name = &*attr.name();
-        for &(n, ty, ref gateage) in KNOWN_ATTRIBUTES {
+        for &(n, ty, ref gateage) in BUILTIN_ATTRIBUTES {
             if n == name {
                 if let &Gated(_, ref name, ref desc, ref has_feature) = gateage {
                     gate_feature_fn!(self, has_feature, attr.span, name, desc);
                 }
-                debug!("check_attribute: {:?} is known, {:?}, {:?}", name, ty, gateage);
+                debug!("check_attribute: {:?} is builtin, {:?}, {:?}", name, ty, gateage);
                 return;
             }
         }
@@ -815,6 +815,8 @@ impl<'a> Context<'a> {
                            are reserved for internal compiler diagnostics");
         } else if name.starts_with("derive_") {
             gate_feature!(self, custom_derive, attr.span, EXPLAIN_DERIVE_UNDERSCORE);
+        } else if attr::is_known(attr) {
+            debug!("check_attribute: {:?} is known", name);
         } else {
             // Only run the custom attribute lint during regular
             // feature gate checking. Macro gating runs