'main': Optimize the unquoted arguments characterwise walker

Fixes #730.

% repeat 3 { zsh -f tests/test-zprof.zsh main | tee … | grep -w _zsh_highlight | head -n1 }
19)    1       26787.11 26787.11  100.00%      5.33     5.33    0.02%  _zsh_highlight
19)    1       26900.64 26900.64  100.00%      5.45     5.45    0.02%  _zsh_highlight
19)    1       26915.76 26915.76  100.00%      5.47     5.47    0.02%  _zsh_highlight

Merge remote-tracking branch 'danielsh/perf-argument-1-v2'

* danielsh/perf-argument-1-v2:
  'main': Further optimize argument parsing.
  'main': Optimize a hot path.
  tests: Add a performance testing script, for measuring the performance of the 'main' highlighter on a large file.

highlighters/main/main-highlighter.zsh

@@ -1311,8 +1311,12 @@ _zsh_highlight_main_highlighter_highlight_argument()
       fi
   esac
 
-  for (( ; i <= $#arg ; i += 1 )); do
+  # This loop is a hot path.  Keep it fast!
+  (( --i ))
+  while (( ++i <= $#arg )); do
+    i=${arg[(ib.i.)[\\\'\"\`\$\<\>\*\?]]}
     case "$arg[$i]" in
+      "") break;;
       "\\") (( i += 1 )); continue;;
       "'")
         _zsh_highlight_main_highlighter_highlight_single_quote $i

tests/test-zprof.zsh

@@ -0,0 +1,77 @@
+#!/usr/bin/env zsh
+# -------------------------------------------------------------------------------------------------
+# Copyright (c) 2010-2015 zsh-syntax-highlighting contributors
+# All rights reserved.
+#
+# Redistribution and use in source and binary forms, with or without modification, are permitted
+# provided that the following conditions are met:
+#
+#  * Redistributions of source code must retain the above copyright notice, this list of conditions
+#    and the following disclaimer.
+#  * Redistributions in binary form must reproduce the above copyright notice, this list of
+#    conditions and the following disclaimer in the documentation and/or other materials provided
+#    with the distribution.
+#  * Neither the name of the zsh-syntax-highlighting contributors nor the names of its contributors
+#    may be used to endorse or promote products derived from this software without specific prior
+#    written permission.
+#
+# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR
+# IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND
+# FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR
+# CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+# DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+# DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER
+# IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
+# OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+# -------------------------------------------------------------------------------------------------
+# -*- mode: zsh; sh-indentation: 2; indent-tabs-mode: nil; sh-basic-offset: 2; -*-
+# vim: ft=zsh sw=2 ts=2 et
+# -------------------------------------------------------------------------------------------------
+
+# Load the main script.
+. ${0:h:h}/zsh-syntax-highlighting.zsh
+
+# Activate the highlighter.
+ZSH_HIGHLIGHT_HIGHLIGHTERS=(main)
+
+source_file=0.7.1:highlighters/$1/$1-highlighter.zsh
+
+# Runs a highlighting test
+# $1: data file
+run_test_internal() {
+  setopt interactivecomments
+
+  local -a highlight_zone
+
+  local tests_tempdir="$1"; shift
+  local srcdir="$PWD"
+  builtin cd -q -- "$tests_tempdir" || { echo >&2 "Bail out! cd failed: $?"; return 1 }
+
+  # Load the data and prepare checking it.
+  PREBUFFER=
+  BUFFER=$(cd -- "$srcdir" && git cat-file blob $source_file)
+  expected_region_highlight=()
+
+  zmodload zsh/zprof
+  zprof -c
+  # Set $? for _zsh_highlight
+  true && _zsh_highlight
+  zprof
+}
+
+run_test() {
+  # Do not combine the declaration and initialization: «local x="$(false)"» does not set $?.
+  local __tests_tempdir
+  __tests_tempdir="$(mktemp -d)" && [[ -d $__tests_tempdir ]] || {
+    echo >&2 "Bail out! mktemp failed"; return 1
+  }
+  typeset -r __tests_tempdir # don't allow tests to override the variable that we will 'rm -rf' later on
+
+  {
+    (run_test_internal "$__tests_tempdir" "$@")
+  } always {
+    rm -rf -- "$__tests_tempdir"
+  }
+}
+
+run_test