@@ -212,6 +212,7 @@ static DEC_DIGITS_LUT: &[u8; 200] = b"0001020304050607080910111213141516171819\
212212
213213macro_rules! impl_Display{
214214( $( $t: ident) ,* as $u: ident via $conv_fn: ident named $name: ident) =>{
215+ #[ cfg( not( feature ="optimize_for_size" ) ) ]
215216fn $name( mut n: $u, is_nonnegative: bool , f: & mut fmt:: Formatter <' _>) -> fmt:: Result {
216217// 2^128 is about 3*10^38, so 39 gives an extra byte of space
217218let mut buf =[ MaybeUninit :: <u8 >:: uninit( ) ; 39 ] ;
@@ -277,6 +278,38 @@ macro_rules! impl_Display {
277278 f. pad_integral( is_nonnegative, "" , buf_slice)
278279}
279280
281+ #[ cfg( feature ="optimize_for_size" ) ]
282+ fn $name( mut n: $u, is_nonnegative: bool , f: & mut fmt:: Formatter <' _>) -> fmt:: Result {
283+ // 2^128 is about 3*10^38, so 39 gives an extra byte of space
284+ let mut buf =[ MaybeUninit :: <u8 >:: uninit( ) ; 39 ] ;
285+ let mut curr = buf. len( ) ;
286+ let buf_ptr =MaybeUninit :: slice_as_mut_ptr( & mut buf) ;
287+
288+ // SAFETY: To show that it's OK to copy into `buf_ptr`, notice that at the beginning
289+ // `curr == buf.len() == 39 > log(n)` since `n < 2^128 < 10^39`, and at
290+ // each step this is kept the same as `n` is divided. Since `n` is always
291+ // non-negative, this means that `curr > 0` so `buf_ptr[curr..curr + 1]`
292+ // is safe to access.
293+ unsafe {
294+ loop {
295+ curr -=1 ;
296+ buf_ptr. add( curr) . write( ( n %10 ) as u8 +b'0' ) ;
297+ n /=10 ;
298+
299+ if n ==0 {
300+ break ;
301+ }
302+ }
303+ }
304+
305+ // SAFETY: `curr` > 0 (since we made `buf` large enough), and all the chars are valid UTF-8
306+ let buf_slice =unsafe {
307+ str :: from_utf8_unchecked(
308+ slice:: from_raw_parts( buf_ptr. add( curr) , buf. len( ) - curr) )
309+ } ;
310+ f. pad_integral( is_nonnegative, "" , buf_slice)
311+ }
312+
280313 $( #[ stable( feature ="rust1" , since ="1.0.0" ) ]
281314impl fmt:: Display for $t{
282315 #[ allow( unused_comparisons) ]