diff --git a/Cargo.toml b/Cargo.toml
index 731cc02..937b1a2 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -13,7 +13,7 @@ optional = true
 
 [dependencies]
 bit-set = "0.3.0"
-vec_map = "0.4.0"
+vec_map = "0.6.0"
 libc = "0.2.0"
 
 [lib]
diff --git a/src/spaces/collector/mod.rs b/src/spaces/collector/mod.rs
index 6f44ef3..98d70f4 100644
--- a/src/spaces/collector/mod.rs
+++ b/src/spaces/collector/mod.rs
@@ -240,11 +240,11 @@ impl Collector {
             } else {
                 unsafe{ (*block).count_holes(); }
                 let (holes, marked_lines) = unsafe{ (*block).count_holes_and_marked_lines() };
-                if self.mark_histogram.contains_key(&(holes as usize)) {
-                    if let Some(val) = self.mark_histogram.get_mut(&(holes as usize)) {
+                if self.mark_histogram.contains_key(holes) {
+                    if let Some(val) = self.mark_histogram.get_mut(holes) {
                         *val += marked_lines;
                     }
-                } else { self.mark_histogram.insert(holes as usize, marked_lines); }
+                } else { self.mark_histogram.insert(holes, marked_lines); }
                 debug!("Found {} holes and {} marked lines in block {:p}",
                        holes, marked_lines, block);
                 match holes {
@@ -269,18 +269,18 @@ impl Collector {
         let mut available_histogram : VecMap<usize> = VecMap::with_capacity(NUM_LINES_PER_BLOCK);
         for &block in &self.all_blocks {
             let (holes, free_lines) = unsafe{ (*block).count_holes_and_available_lines() };
-            if available_histogram.contains_key(&(holes as usize)) {
-                if let Some(val) = available_histogram.get_mut(&(holes as usize)) {
+            if available_histogram.contains_key(holes) {
+                if let Some(val) = available_histogram.get_mut(holes) {
                     *val += free_lines;
                 }
-            } else { available_histogram.insert(holes as usize, free_lines); }
+            } else { available_histogram.insert(holes, free_lines); }
         }
         let mut required_lines = 0;
         let mut available_lines = evac_headroom * (NUM_LINES_PER_BLOCK - 1);
 
         for threshold in 0..NUM_LINES_PER_BLOCK {
-            required_lines += *self.mark_histogram.get(&threshold).unwrap_or(&0);
-            available_lines = available_lines.saturating_sub(*available_histogram.get(&threshold).unwrap_or(&0));
+            required_lines += *self.mark_histogram.get(threshold).unwrap_or(&0);
+            available_lines = available_lines.saturating_sub(*available_histogram.get(threshold).unwrap_or(&0));
             if available_lines <= required_lines {
                 return threshold;
             }
diff --git a/src/spaces/immix_space/block_info.rs b/src/spaces/immix_space/block_info.rs
index 33c656a..e789004 100644
--- a/src/spaces/immix_space/block_info.rs
+++ b/src/spaces/immix_space/block_info.rs
@@ -256,7 +256,7 @@ impl BlockInfo {
                self, last_high_index);
         let mut low_index = NUM_LINES_PER_BLOCK - 1;
         for index in (last_high_index + 1)..NUM_LINES_PER_BLOCK {
-            if self.line_counter.get(&index).map_or(true, |c| *c == 0) {
+            if self.line_counter.get(index).map_or(true, |c| *c == 0) {
                 // +1 to skip the next line in case an object straddles lines
                 low_index = index + 1;
                 break;
@@ -264,7 +264,7 @@ impl BlockInfo {
         }
         let mut high_index = NUM_LINES_PER_BLOCK;
         for index in low_index..NUM_LINES_PER_BLOCK {
-            if self.line_counter.get(&index).map_or(false, |c| *c != 0) {
+            if self.line_counter.get(index).map_or(false, |c| *c != 0) {
                 high_index = index;
                 break;
             }
@@ -341,10 +341,10 @@ impl BlockInfo{
             };
             if increment {
                 debug!("Incremented line count for line {} to {}", line,
-                       self.line_counter.get(&line).expect("a line count"));
+                       self.line_counter.get(line).expect("a line count"));
             } else {
                 debug!("Decremented line count for line {} to {}", line,
-                       self.line_counter.get(&line).expect("a line count"));
+                       self.line_counter.get(line).expect("a line count"));
             }
         }
     }