bitv: make benchmarks always return a value

This makes sure that the benchmarked code does not get optimized away.
Also fixed a typo.

Fixes #12118.

src/libcollections/bitv.rs

@@ -2616,29 +2616,31 @@ mod tests {
         let mut b1 = Bitv::with_capacity(BENCH_BITS, false);
         let b2 = Bitv::with_capacity(BENCH_BITS, false);
         b.iter(|| {
-            b1.union(&b2);
+            b1.union(&b2)
         })
     }
 
     #[bench]
-    fn bench_btv_small_iter(b: &mut Bencher) {
+    fn bench_bitv_small_iter(b: &mut Bencher) {
         let bitv = Bitv::with_capacity(uint::BITS, false);
         b.iter(|| {
-            let mut _sum = 0;
+            let mut sum = 0;
             for pres in bitv.iter() {
-                _sum += pres as uint;
+                sum += pres as uint;
             }
+            sum
         })
     }
 
     #[bench]
     fn bench_bitv_big_iter(b: &mut Bencher) {
         let bitv = Bitv::with_capacity(BENCH_BITS, false);
         b.iter(|| {
-            let mut _sum = 0;
+            let mut sum = 0;
             for pres in bitv.iter() {
-                _sum += pres as uint;
+                sum += pres as uint;
             }
+            sum
         })
     }
 
@@ -2647,10 +2649,11 @@ mod tests {
         let bitv = BitvSet::from_bitv(from_fn(BENCH_BITS,
                                               |idx| {idx % 3 == 0}));
         b.iter(|| {
-            let mut _sum = 0;
+            let mut sum = 0;
             for idx in bitv.iter() {
-                _sum += idx;
+                sum += idx;
             }
+            sum
         })
     }
 }