'main': Optimize a hot path.

% git co HEAD^ && repeat 3 { zsh -f tests/test-zprof.zsh main | tee … | grep -w _zsh_highlight | head -n1 }
HEAD is now at 64e3651 'main': Optimize a hot path.
19)    1       28765.13 28765.13  100.00%      5.57     5.57    0.02%  _zsh_highlight
19)    1       28566.46 28566.46  100.00%      5.91     5.91    0.02%  _zsh_highlight
19)    1       28248.12 28248.12  100.00%      5.57     5.57    0.02%  _zsh_highlight

----

This commit has been rebased.  The above statistics were measured after
the rebase.  The below statistics had been measured before the rebase.

Before this patch:

    num  calls                time                       self            name
    -----------------------------------------------------------------------------------
     1)    3       33410.81 11136.94   98.51%  19277.07  6425.69   56.84%  _zsh_highlight_main_highlighter_highlight_list
    19)    1       33916.21 33916.21  100.00%      5.27     5.27    0.02%  _zsh_highlight

With this patch:

    num  calls                time                       self            name
    -----------------------------------------------------------------------------------
     1)    3       27167.49  9055.83   98.17%  18754.77  6251.59   67.77%  _zsh_highlight_main_highlighter_highlight_list
    19)    1       27674.40 27674.40  100.00%      5.39     5.39    0.02%  _zsh_highlight

And if test-zprof.zsh is changed to not set interactivecomments:

    num  calls                time                       self            name
    -----------------------------------------------------------------------------------
     1) 13360       36029.12     2.70   83.56%  30304.23     2.27   70.28%  _zsh_highlight_main_highlighter_highlight_argument
    21)    1       43117.76 43117.76  100.00%      4.52     4.52    0.01%  _zsh_highlight

    num  calls                time                       self            name
    -----------------------------------------------------------------------------------
     1) 13360       14782.89     1.11   68.12%   9163.42     0.69   42.23%  _zsh_highlight_main_highlighter_highlight_argument
    21)    1       21699.93 21699.93  100.00%      4.17     4.17    0.02%  _zsh_highlight

Author: danielshahaf

highlighters/main/main-highlighter.zsh

@@ -1311,7 +1311,12 @@ _zsh_highlight_main_highlighter_highlight_argument()
       fi
   esac
 
+  # This loop is a hot path.  Keep it fast!
   for (( ; i <= $#arg ; i += 1 )); do
+    if [[ $arg[$i] != [\\\'\"\`\$\<\>\*\?] ]]; then
+      continue
+    fi
+
     case "$arg[$i]" in
       "\\") (( i += 1 )); continue;;
       "'")