diff --git a/src/librustc/middle/dataflow.rs b/src/librustc/middle/dataflow.rs
index 41b27a48b29f..4d01b59001c5 100644
--- a/src/librustc/middle/dataflow.rs
+++ b/src/librustc/middle/dataflow.rs
@@ -660,8 +660,8 @@ fn set_bit(words: &mut [usize], bit: usize) -> bool {
 }
 
 fn bit_str(bit: usize) -> String {
-    let byte = bit >> 8;
-    let lobits = 1 << (bit & 0xFF);
+    let byte = bit >> 3;
+    let lobits = 1 << (bit & 0b111);
     format!("[{}:{}-{:02x}]", bit, byte, lobits)
 }
 
diff --git a/src/librustc_borrowck/bitslice.rs b/src/librustc_borrowck/bitslice.rs
index a4aa7ae15744..0454b0f3e7cc 100644
--- a/src/librustc_borrowck/bitslice.rs
+++ b/src/librustc_borrowck/bitslice.rs
@@ -73,8 +73,8 @@ fn bit_lookup(bit: usize) -> BitLookup {
 
 
 fn bit_str(bit: usize) -> String {
-    let byte = bit >> 8;
-    let lobits = 1 << (bit & 0xFF);
+    let byte = bit >> 3;
+    let lobits = 1 << (bit & 0b111);
     format!("[{}:{}-{:02x}]", bit, byte, lobits)
 }