Include the minus sign in the same buffer, to only make a single write_all call.

src/lib.rs

@@ -37,7 +37,6 @@ macro_rules! impl_Integer {
             let mut n = if is_nonnegative {
                 self as $conv_fn
             } else {
-                try!(wr.write_all(b"-"));
                 // convert the negative num to positive by summing 1 to it's 2 complement
                 (!(self as $conv_fn)).wrapping_add(1)
             };
@@ -81,13 +80,15 @@ macro_rules! impl_Integer {
                     curr -= 2;
                     ptr::copy_nonoverlapping(lut_ptr.offset(d1), buf_ptr.offset(curr), 2);
                 }
+
+                if !is_nonnegative {
+                    curr -= 1;
+                    *buf_ptr.offset(curr) = b'-';
+                }
             }
 
-            let mut len = buf.len() - curr as usize;
+            let len = buf.len() - curr as usize;
             try!(wr.write_all(unsafe { slice::from_raw_parts(buf_ptr.offset(curr), len) }));
-            if !is_nonnegative {
-                len += 1;
-            }
             Ok(len)
         }
     })*);

tests/test.rs

@@ -19,6 +19,7 @@ test!(
     test_0u64(0u64, "0"),
     test_HALFu64(<u32>::max_value() as u64, "4294967295"),
     test_MAXu64(<u64>::max_value(), "18446744073709551615"),
+    test_MINi64(<i64>::min_value(), "-9223372036854775808"),
 
     test_0i16(0i16, "0"),
     test_MINi16(<i16>::min_value(), "-32768"),