40 -> 39, as ceil(log10(2^128)) == 39

just as ceil(log10(2^64)) == 20

Author: est31

src/libcore/fmt/num.rs

@@ -211,7 +211,7 @@ macro_rules! impl_Display {
                 // convert the negative num to positive by summing 1 to it's 2 complement
                 (!self.$conv_fn()).wrapping_add(1)
             };
-            let mut buf: [u8; 40] = unsafe { mem::uninitialized() };
+            let mut buf: [u8; 39] = unsafe { mem::uninitialized() };
             let mut curr = buf.len() as isize;
             let buf_ptr = buf.as_mut_ptr();
             let lut_ptr = DEC_DIGITS_LUT.as_ptr();

src/test/run-pass/u128.rs

@@ -59,6 +59,8 @@ fn main() {
     assert_eq!("20000000000000000000000", format!("{:o}", j));
     assert_eq!("10000000000000000000000000000000000000000000000000000000000000000000",
                format!("{:b}", j));
+    assert_eq!("340282366920938463463374607431768211455",
+        format!("{}", u128::max_value()));
     assert_eq!("147573952589676412928", format!("{:?}", j));
     // common traits
     x.clone();