Utilize specialized zip iterator impl

 name                        old ns/iter  new ns/iter  diff ns/iter   diff %  speedup
 fmt::write_str_macro1       13,927       12,489             -1,438  -10.33%   x 1.12
 fmt::write_str_macro2       24,633       23,418             -1,215   -4.93%   x 1.05
 fmt::write_str_macro_debug  234,633      233,092            -1,541   -0.66%   x 1.01
 fmt::write_str_ref          5,819        5,823                   4    0.07%   x 1.00
 fmt::write_str_value        6,012        5,828                -184   -3.06%   x 1.03
 fmt::write_vec_macro1       18,550       17,143             -1,407   -7.58%   x 1.08
 fmt::write_vec_macro2       30,369       28,920             -1,449   -4.77%   x 1.05
 fmt::write_vec_macro_debug  244,338      244,901               563    0.23%   x 1.00
 fmt::write_vec_ref          5,952        5,885                 -67   -1.13%   x 1.01
 fmt::write_vec_value        5,944        5,894                 -50   -0.84%   x 1.01

Author: sinkuu

src/libcore/fmt/mod.rs

@@ -1026,28 +1026,30 @@ pub fn write(output: &mut dyn Write, args: Arguments) -> Result {
         curarg: args.args.iter(),
     };
 
-    let mut pieces = args.pieces.iter();
+    let mut idx = 0;
 
     match args.fmt {
         None => {
             // We can use default formatting parameters for all arguments.
-            for (arg, piece) in args.args.iter().zip(pieces.by_ref()) {
+            for (arg, piece) in args.args.iter().zip(args.pieces.iter()) {
                 formatter.buf.write_str(*piece)?;
                 (arg.formatter)(arg.value, &mut formatter)?;
+                idx += 1;
             }
         }
         Some(fmt) => {
             // Every spec has a corresponding argument that is preceded by
             // a string piece.
-            for (arg, piece) in fmt.iter().zip(pieces.by_ref()) {
+            for (arg, piece) in fmt.iter().zip(args.pieces.iter()) {
                 formatter.buf.write_str(*piece)?;
                 formatter.run(arg)?;
+                idx += 1;
             }
         }
     }
 
     // There can be only one trailing string piece left.
-    if let Some(piece) = pieces.next() {
+    if let Some(piece) = args.pieces.get(idx) {
         formatter.buf.write_str(*piece)?;
     }