See comments within for the rationale.

This is a regression test for a regression that was only present in development
versions of PR #764 and was never present in master.

Reported on issue #757 along with other issues.

E.g., on Cygwin /etc/passwd does not necessarily exist.

Test case: See the first comment of issue #295.

Before:
num  calls                time                       self            name
-----------------------------------------------------------------------------------
 1)   14         284.67    20.33   82.84%    284.67    20.33   82.84%  _zsh_highlight_highlighter_brackets_paint
 2)   15         313.72    20.91   91.30%     26.62     1.77    7.75%  _zsh_highlight
 3)   14         318.68    22.76   92.74%      7.41     0.53    2.16%  _zsh_highlight_call_widget

After:
num  calls                time                       self            name
-----------------------------------------------------------------------------------
 1)   15          57.25     3.82   45.33%     28.30     1.89   22.41%  _zsh_highlight
 2)   14          81.96     5.85   64.90%     27.02     1.93   21.40%  _zsh_highlight_call_widget
 3)   14          26.31     1.88   20.84%     26.31     1.88   20.84%  _zsh_highlight_highlighter_brackets_paint
 4)    2          10.66     5.33    8.44%      8.97     4.49    7.10%  VCS_INFO_formats
 5)    2          25.92    12.96   20.52%      7.72     3.86    6.11%  VCS_INFO_get_data_git

(That's not in my everyday configuration but in my minimal testing
setup, which runs vcs_info with the default settings.)

Fixes #418 (interoperability issue with other plugins).

Discussed: https://github.com/zsh-users/zsh-syntax-highlighting/pull/706#issuecomment-642072978

Fixes #712.

The second test point passes on 0.7.1, but the third does not.

Fixes #585.

Closes #607 #649 #704

The fix is to exempt such aliases from the empty commands sanity check.

% repeat 3 { zsh -f tests/test-zprof.zsh main | tee … | grep -w _zsh_highlight | head -n1 }
18)    1       26895.86 26895.86  100.00%      6.35     6.35    0.02%  _zsh_highlight
19)    1       27399.11 27399.11  100.00%      5.52     5.52    0.02%  _zsh_highlight
19)    1       27027.58 27027.58  100.00%      5.66     5.66    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.

num  calls                time                       self            name
-----------------------------------------------------------------------------------
 1)    3       25689.17  8563.06   98.15%  18422.01  6140.67   70.38%  _zsh_highlight_main_highlighter_highlight_list
 2) 32390        5706.13     0.18   21.80%   2315.68     0.07    8.85%  _zsh_highlight_main_highlighter_highlight_argument
19)    1       26173.33 26173.33  100.00%      5.27     5.27    0.02%  _zsh_highlight

Interestingly, if I make the change in this diff to
_zsh_highlight_main_highlighter_highlight_double_quote —

>     diff --git a/highlighters/main/main-highlighter.zsh b/highlighters/main/main-highlighter.zsh
>     index da6ab2b..bb17618 100644
>     --- a/highlighters/main/main-highlighter.zsh
>     +++ b/highlighters/main/main-highlighter.zsh
>     @@ -1462,10 +1462,13 @@ _zsh_highlight_main_highlighter_highlight_double_quote()
>        local i j k ret style
>        reply=()
>
>     -  for (( i = $1 + 1 ; i <= $#arg ; i += 1 )) ; do
>     +  (( i = $1 ))
>     +  while (( ++i <= $#arg )); do
>     +    i=${arg[(ib.i.)[\"\`\$\\\\${histchars[1]}]]}
>          (( j = i + start_pos - 1 ))
>          (( k = j + 1 ))
>          case "$arg[$i]" in
>     +      ("")  break;;
>            ('"') break;;
>            ('`') saved_reply=($reply)
>                  _zsh_highlight_main_highlighter_highlight_backtick $i

— it actually makes things measurably slower (!), even on input that has
a large number of pasted double-quoted strings: on «BUFFER=": ${(r.8*1500..foo"bar".):-}"»
the slowdown is (1123.24ms / 1091.06ms = 1.0295).  Therefore, I won't be
committing that change.

% 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

Will be fixed in the next commit.

In those versions of zsh, «[[ -o nosuchoption ]]» is regarded as
a syntax error.  In newer zsh versions, it merely returns non-zero
(specifically, it returns 3, unlike «[[ -o unsetoption ]]» which
returns 1).

Fixes #732.
Fixes #733.

An error message was emitted on versions of zsh that don't have the
WARN_NESTED_VAR option.

Fixes #731.

Fixes #728, the performance regression from 0.7.1.

Fixes #727.

Accidentally lost during a rebase.

As described in the last commit's log message, ${parameter_name_pattern]
explicitly matches positional parameters but ${parameters[$MATCH]}
expands to nothing in that case (when, e.g., [[ $MATCH == '1' ]]; note
this is equality of strings, not integers).

As a side effect, this removes the dependency on the zsh/parameter
module for expanding parameters.

On the test case, the behaviour was as follows:

+highlighters/main/main-highlighter.zsh:733> _zsh_highlight_main_highlighter__try_expand_parameter '$1'
+highlighters/main/main-highlighter.zsh:432> local arg='$1'
+highlighters/main/main-highlighter.zsh:433> unset reply
+highlighters/main/main-highlighter.zsh:439> local -a match mbegin mend
+highlighters/main/main-highlighter.zsh:440> local MATCH
+highlighters/main/main-highlighter.zsh:440> integer MBEGIN MEND
+highlighters/main/main-highlighter.zsh:441> local parameter_name
+highlighters/main/main-highlighter.zsh:442> local -a words
+highlighters/main/main-highlighter.zsh:443> [[ '$' != \$ ]]
+highlighters/main/main-highlighter.zsh:446> [[ 1 == { ]]
+highlighters/main/main-highlighter.zsh:449> parameter_name=1
+highlighters/main/main-highlighter.zsh:451> [[ none == none ]]
+highlighters/main/main-highlighter.zsh:451> zmodload -e zsh/parameter
+highlighters/main/main-highlighter.zsh:452> [[ ${parameter_name} -regex-match ^${~parameter_name_pattern}$ ]]
+highlighters/main/main-highlighter.zsh:453> [[ '' != *special* ]]
+highlighters/main/main-highlighter.zsh:456> case array-special (*array*|*assoc*)
+highlighters/main/main-highlighter.zsh:458> words=( '$1' )
+highlighters/main/main-highlighter.zsh:469> reply=( '$1' )

There are two problems here:

- In terms of _zsh_highlight_main_highlighter__try_expand_parameter's
  pre- and postconditions, the expansion of the word «$1» (line 733)
  included that same word (line 469).

  That happened because word-to-be-expanded is passed to
  _zsh_highlight_main_highlighter__try_expand_parameter as its first
  positional parameter, and in this case the word happened to be «$1».

- Furthermore, the exclusion of special parameters (line 453) false
  negatived.  That happened because $parameter_name_pattern explicitly
  allows positional parameters, but ${parameters[(e)1]} expands to
  nothing.  This will be fixed in the next commit.

Not a regression from 0.7.1.

Before this commit, if the value didn't begin with a dollar sign,
_zsh_highlight_main_highlighter__try_expand_parameter() would return 1
by accident.¹  Tweak the input validation to make this behaviour
explicit.  No functional change.

¹ Specifically, it would return 1 because ${parameter_name}'s value
would be the empty string and ${parameter_name_pattern} wouldn't match
that.

This is in order to allow it to be reused.

No functional change.