diff --git a/src/libsyntax/ast.rs b/src/libsyntax/ast.rs
index d59c9dd6b53e8..28c1e4324de7a 100644
--- a/src/libsyntax/ast.rs
+++ b/src/libsyntax/ast.rs
@@ -23,6 +23,7 @@ use ext::hygiene::{Mark, SyntaxContext};
 use print::pprust;
 use ptr::P;
 use rustc_data_structures::indexed_vec;
+use rustc_data_structures::indexed_vec::Idx;
 use symbol::{Symbol, keywords};
 use tokenstream::{ThinTokenStream, TokenStream};
 
@@ -1910,9 +1911,18 @@ pub enum AttrStyle {
     Inner,
 }
 
-#[derive(Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash, Debug, Copy)]
+#[derive(Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash, Debug, PartialOrd, Ord, Copy)]
 pub struct AttrId(pub usize);
 
+impl Idx for AttrId {
+    fn new(idx: usize) -> Self {
+        AttrId(idx)
+    }
+    fn index(self) -> usize {
+        self.0
+    }
+}
+
 /// Meta-data associated with an item
 /// Doc-comments are promoted to attributes that have is_sugared_doc = true
 #[derive(Clone, RustcEncodable, RustcDecodable, Debug)]
diff --git a/src/libsyntax/attr/mod.rs b/src/libsyntax/attr/mod.rs
index 137b94230a3cd..7fbc5d4c41a50 100644
--- a/src/libsyntax/attr/mod.rs
+++ b/src/libsyntax/attr/mod.rs
@@ -41,51 +41,27 @@ use std::iter;
 
 pub fn mark_used(attr: &Attribute) {
     debug!("Marking {:?} as used.", attr);
-    let AttrId(id) = attr.id;
     GLOBALS.with(|globals| {
-        let mut slot = globals.used_attrs.lock();
-        let idx = (id / 64) as usize;
-        let shift = id % 64;
-        if slot.len() <= idx {
-            slot.resize(idx + 1, 0);
-        }
-        slot[idx] |= 1 << shift;
+        globals.used_attrs.lock().insert(attr.id);
     });
 }
 
 pub fn is_used(attr: &Attribute) -> bool {
-    let AttrId(id) = attr.id;
     GLOBALS.with(|globals| {
-        let slot = globals.used_attrs.lock();
-        let idx = (id / 64) as usize;
-        let shift = id % 64;
-        slot.get(idx).map(|bits| bits & (1 << shift) != 0)
-            .unwrap_or(false)
+        globals.used_attrs.lock().contains(attr.id)
     })
 }
 
 pub fn mark_known(attr: &Attribute) {
     debug!("Marking {:?} as known.", attr);
-    let AttrId(id) = attr.id;
     GLOBALS.with(|globals| {
-        let mut slot = globals.known_attrs.lock();
-        let idx = (id / 64) as usize;
-        let shift = id % 64;
-        if slot.len() <= idx {
-            slot.resize(idx + 1, 0);
-        }
-        slot[idx] |= 1 << shift;
+        globals.known_attrs.lock().insert(attr.id);
     });
 }
 
 pub fn is_known(attr: &Attribute) -> bool {
-    let AttrId(id) = attr.id;
     GLOBALS.with(|globals| {
-        let slot = globals.known_attrs.lock();
-        let idx = (id / 64) as usize;
-        let shift = id % 64;
-        slot.get(idx).map(|bits| bits & (1 << shift) != 0)
-            .unwrap_or(false)
+        globals.known_attrs.lock().contains(attr.id)
     })
 }
 
diff --git a/src/libsyntax/lib.rs b/src/libsyntax/lib.rs
index 60de94821bba0..16507ec5a5dd5 100644
--- a/src/libsyntax/lib.rs
+++ b/src/libsyntax/lib.rs
@@ -43,6 +43,8 @@ extern crate rustc_target;
 extern crate serialize as rustc_serialize; // used by deriving
 
 use rustc_data_structures::sync::Lock;
+use rustc_data_structures::bitvec::BitVector;
+use ast::AttrId;
 
 // A variant of 'try!' that panics on an Err. This is used as a crutch on the
 // way towards a non-panic!-prone parser. It should be used for fatal parsing
@@ -75,16 +77,18 @@ macro_rules! unwrap_or {
 }
 
 pub struct Globals {
-    used_attrs: Lock<Vec<u64>>,
-    known_attrs: Lock<Vec<u64>>,
+    used_attrs: Lock<BitVector<AttrId>>,
+    known_attrs: Lock<BitVector<AttrId>>,
     syntax_pos_globals: syntax_pos::Globals,
 }
 
 impl Globals {
     fn new() -> Globals {
         Globals {
-            used_attrs: Lock::new(Vec::new()),
-            known_attrs: Lock::new(Vec::new()),
+            // We have no idea how many attributes their will be, so just
+            // initiate the vectors with 0 bits. We'll grow them as necessary.
+            used_attrs: Lock::new(BitVector::new(0)),
+            known_attrs: Lock::new(BitVector::new(0)),
             syntax_pos_globals: syntax_pos::Globals::new(),
         }
     }