diff --git a/src/libcollections/bit.rs b/src/libcollections/bit.rs
index 0bdc383492f3a..25d9fbee0a766 100644
--- a/src/libcollections/bit.rs
+++ b/src/libcollections/bit.rs
@@ -87,8 +87,8 @@ use std::hash;
 
 use vec::Vec;
 
-type Blocks<'a> = Cloned<Items<'a, u32>>
-type MutBlocks<'a> MutItems<'a, u32>;
+type Blocks<'a> = Cloned<Items<'a, u32>>;
+type MutBlocks<'a> = MutItems<'a, u32>;
 type MatchWords<'a> = Chain<Enumerate<Blocks<'a>>, Skip<Take<Enumerate<Repeat<u32>>>>>;
 
 // Take two BitV's, and return iterators of their words, where the shorter one
@@ -174,6 +174,12 @@ fn blocks_for_bits(bits: uint) -> uint {
 
 }
 
+/// Computes the bitmask for the final word of the vector
+fn mask_for_bits(bits: uint) -> u32 {
+    // Note especially that a perfect multiple of u32::BITS should mask all 1s.
+    !0u32 >> (u32::BITS - bits % u32::BITS)
+}
+
 impl Bitv {
     /// Applies the given operation to the blocks of self and other, and sets self to
     /// be the result.
@@ -199,7 +205,7 @@ impl Bitv {
     /// Iterator over mutable refs to  the underlying blocks of data.
     fn blocks_mut(&mut self) -> MutBlocks {
         let blocks = blocks_for_bits(self.len());
-        self.storage[..blocks].iter_mut()
+        self.storage.slice_to_mut(blocks).iter_mut()
     }
 
     /// Iterator over the underlying blocks of data
@@ -336,7 +342,7 @@ impl Bitv {
         assert!(i < self.nbits);
         let w = i / u32::BITS;
         let b = i % u32::BITS;
-        self.storage.get(w).map(|block|
+        self.storage.get(w).map(|&block|
             (block & (1 << b)) != 0
         )
     }
@@ -526,7 +532,7 @@ impl Bitv {
             last_word = elem;
             tmp == !0u32
         // and then check the last one has enough ones
-        }) && (last_word == ((1 << self.nbits % u32::BITS) - 1) || last_word == !0u32)
+        }) && (last_word == mask_for_bits(self.nbits))
     }
 
     /// Returns an iterator over the elements of the vector in order.
@@ -788,15 +794,15 @@ impl Bitv {
         let new_nblocks = blocks_for_bits(new_nbits);
         let full_value = if value { !0 } else { 0 };
 
-        // Correct the old tail word
+        // Correct the old tail word, setting or clearing formerly unused bits
         let old_last_word = blocks_for_bits(self.nbits) - 1;
         if self.nbits % u32::BITS > 0 {
-            let overhang = self.nbits % u32::BITS; // # of already-used bits
-            let mask = !((1 << overhang) - 1);  // e.g. 5 unused bits => 111110..0
+            let mask = mask_for_bits(self.nbits);
             if value {
-                self.storage[old_last_word] |= mask;
+                self.storage[old_last_word] |= !mask;
             } else {
-                self.storage[old_last_word] &= !mask;
+                // Extra bits are already supposed to be zero by invariant, but play it safe...
+                self.storage[old_last_word] &= mask;
             }
         }
 
@@ -835,10 +841,11 @@ impl Bitv {
         if self.is_empty() {
             None
         } else {
-            let ret = self[self.nbits - 1];
+            let i = self.nbits - 1;
+            let ret = self[i];
             // Second rule of Bitv Club
-            self.set(self.nbits - 1, false);
-            self.nbits -= 1;
+            self.set(i, false);
+            self.nbits = i;
             Some(ret)
         }
     }
@@ -1794,14 +1801,17 @@ mod bitv_test {
         let act = Bitv::new();
         let exp = Vec::from_elem(0u, false);
         assert!(act.eq_vec(exp.as_slice()));
+        assert!(act.none() && act.all());
     }
 
     #[test]
     fn test_1_element() {
         let mut act = Bitv::from_elem(1u, false);
         assert!(act.eq_vec(&[false]));
+        assert!(act.none() && !act.all());
         act = Bitv::from_elem(1u, true);
         assert!(act.eq_vec(&[true]));
+        assert!(!act.none() && act.all());
     }
 
     #[test]
@@ -1810,6 +1820,7 @@ mod bitv_test {
         b.set(0, true);
         b.set(1, false);
         assert_eq!(b.to_string().as_slice(), "10");
+        assert!(!b.none() && !b.all());
     }
 
     #[test]
@@ -1820,10 +1831,12 @@ mod bitv_test {
         act = Bitv::from_elem(10u, false);
         assert!((act.eq_vec(
                     &[false, false, false, false, false, false, false, false, false, false])));
+        assert!(act.none() && !act.all());
         // all 1
 
         act = Bitv::from_elem(10u, true);
         assert!((act.eq_vec(&[true, true, true, true, true, true, true, true, true, true])));
+        assert!(!act.none() && act.all());
         // mixed
 
         act = Bitv::from_elem(10u, false);
@@ -1833,6 +1846,7 @@ mod bitv_test {
         act.set(3u, true);
         act.set(4u, true);
         assert!((act.eq_vec(&[true, true, true, true, true, false, false, false, false, false])));
+        assert!(!act.none() && !act.all());
         // mixed
 
         act = Bitv::from_elem(10u, false);
@@ -1842,6 +1856,7 @@ mod bitv_test {
         act.set(8u, true);
         act.set(9u, true);
         assert!((act.eq_vec(&[false, false, false, false, false, true, true, true, true, true])));
+        assert!(!act.none() && !act.all());
         // mixed
 
         act = Bitv::from_elem(10u, false);
@@ -1850,6 +1865,7 @@ mod bitv_test {
         act.set(6u, true);
         act.set(9u, true);
         assert!((act.eq_vec(&[true, false, false, true, false, false, true, false, false, true])));
+        assert!(!act.none() && !act.all());
     }
 
     #[test]
@@ -1862,6 +1878,7 @@ mod bitv_test {
                 &[false, false, false, false, false, false, false, false, false, false, false,
                   false, false, false, false, false, false, false, false, false, false, false,
                   false, false, false, false, false, false, false, false, false]));
+        assert!(act.none() && !act.all());
         // all 1
 
         act = Bitv::from_elem(31u, true);
@@ -1869,6 +1886,7 @@ mod bitv_test {
                 &[true, true, true, true, true, true, true, true, true, true, true, true, true,
                   true, true, true, true, true, true, true, true, true, true, true, true, true,
                   true, true, true, true, true]));
+        assert!(!act.none() && act.all());
         // mixed
 
         act = Bitv::from_elem(31u, false);
@@ -1884,6 +1902,7 @@ mod bitv_test {
                 &[true, true, true, true, true, true, true, true, false, false, false, false, false,
                   false, false, false, false, false, false, false, false, false, false, false,
                   false, false, false, false, false, false, false]));
+        assert!(!act.none() && !act.all());
         // mixed
 
         act = Bitv::from_elem(31u, false);
@@ -1899,6 +1918,7 @@ mod bitv_test {
                 &[false, false, false, false, false, false, false, false, false, false, false,
                   false, false, false, false, false, true, true, true, true, true, true, true, true,
                   false, false, false, false, false, false, false]));
+        assert!(!act.none() && !act.all());
         // mixed
 
         act = Bitv::from_elem(31u, false);
@@ -1913,6 +1933,7 @@ mod bitv_test {
                 &[false, false, false, false, false, false, false, false, false, false, false,
                   false, false, false, false, false, false, false, false, false, false, false,
                   false, false, true, true, true, true, true, true, true]));
+        assert!(!act.none() && !act.all());
         // mixed
 
         act = Bitv::from_elem(31u, false);
@@ -1923,6 +1944,7 @@ mod bitv_test {
                 &[false, false, false, true, false, false, false, false, false, false, false, false,
                   false, false, false, false, false, true, false, false, false, false, false, false,
                   false, false, false, false, false, false, true]));
+        assert!(!act.none() && !act.all());
     }
 
     #[test]
@@ -1935,6 +1957,7 @@ mod bitv_test {
                 &[false, false, false, false, false, false, false, false, false, false, false,
                   false, false, false, false, false, false, false, false, false, false, false,
                   false, false, false, false, false, false, false, false, false, false]));
+        assert!(act.none() && !act.all());
         // all 1
 
         act = Bitv::from_elem(32u, true);
@@ -1942,6 +1965,7 @@ mod bitv_test {
                 &[true, true, true, true, true, true, true, true, true, true, true, true, true,
                   true, true, true, true, true, true, true, true, true, true, true, true, true,
                   true, true, true, true, true, true]));
+        assert!(!act.none() && act.all());
         // mixed
 
         act = Bitv::from_elem(32u, false);
@@ -1957,6 +1981,7 @@ mod bitv_test {
                 &[true, true, true, true, true, true, true, true, false, false, false, false, false,
                   false, false, false, false, false, false, false, false, false, false, false,
                   false, false, false, false, false, false, false, false]));
+        assert!(!act.none() && !act.all());
         // mixed
 
         act = Bitv::from_elem(32u, false);
@@ -1972,6 +1997,7 @@ mod bitv_test {
                 &[false, false, false, false, false, false, false, false, false, false, false,
                   false, false, false, false, false, true, true, true, true, true, true, true, true,
                   false, false, false, false, false, false, false, false]));
+        assert!(!act.none() && !act.all());
         // mixed
 
         act = Bitv::from_elem(32u, false);
@@ -1987,6 +2013,7 @@ mod bitv_test {
                 &[false, false, false, false, false, false, false, false, false, false, false,
                   false, false, false, false, false, false, false, false, false, false, false,
                   false, false, true, true, true, true, true, true, true, true]));
+        assert!(!act.none() && !act.all());
         // mixed
 
         act = Bitv::from_elem(32u, false);
@@ -1998,6 +2025,7 @@ mod bitv_test {
                 &[false, false, false, true, false, false, false, false, false, false, false, false,
                   false, false, false, false, false, true, false, false, false, false, false, false,
                   false, false, false, false, false, false, true, true]));
+        assert!(!act.none() && !act.all());
     }
 
     #[test]
@@ -2010,6 +2038,7 @@ mod bitv_test {
                 &[false, false, false, false, false, false, false, false, false, false, false,
                   false, false, false, false, false, false, false, false, false, false, false,
                   false, false, false, false, false, false, false, false, false, false, false]));
+        assert!(act.none() && !act.all());
         // all 1
 
         act = Bitv::from_elem(33u, true);
@@ -2017,6 +2046,7 @@ mod bitv_test {
                 &[true, true, true, true, true, true, true, true, true, true, true, true, true,
                   true, true, true, true, true, true, true, true, true, true, true, true, true,
                   true, true, true, true, true, true, true]));
+        assert!(!act.none() && act.all());
         // mixed
 
         act = Bitv::from_elem(33u, false);
@@ -2032,6 +2062,7 @@ mod bitv_test {
                 &[true, true, true, true, true, true, true, true, false, false, false, false, false,
                   false, false, false, false, false, false, false, false, false, false, false,
                   false, false, false, false, false, false, false, false, false]));
+        assert!(!act.none() && !act.all());
         // mixed
 
         act = Bitv::from_elem(33u, false);
@@ -2047,6 +2078,7 @@ mod bitv_test {
                 &[false, false, false, false, false, false, false, false, false, false, false,
                   false, false, false, false, false, true, true, true, true, true, true, true, true,
                   false, false, false, false, false, false, false, false, false]));
+        assert!(!act.none() && !act.all());
         // mixed
 
         act = Bitv::from_elem(33u, false);
@@ -2062,6 +2094,7 @@ mod bitv_test {
                 &[false, false, false, false, false, false, false, false, false, false, false,
                   false, false, false, false, false, false, false, false, false, false, false,
                   false, false, true, true, true, true, true, true, true, true, false]));
+        assert!(!act.none() && !act.all());
         // mixed
 
         act = Bitv::from_elem(33u, false);
@@ -2074,6 +2107,7 @@ mod bitv_test {
                 &[false, false, false, true, false, false, false, false, false, false, false, false,
                   false, false, false, false, false, true, false, false, false, false, false, false,
                   false, false, false, false, false, false, true, true, true]));
+        assert!(!act.none() && !act.all());
     }
 
     #[test]
@@ -2191,15 +2225,17 @@ mod bitv_test {
     #[test]
     fn test_small_clear() {
         let mut b = Bitv::from_elem(14, true);
+        assert!(!b.none() && b.all());
         b.clear();
-        assert!(b.none());
+        assert!(b.none() && !b.all());
     }
 
     #[test]
     fn test_big_clear() {
         let mut b = Bitv::from_elem(140, true);
+        assert!(!b.none() && b.all());
         b.clear();
-        assert!(b.none());
+        assert!(b.none() && !b.all());
     }
 
     #[test]