Fix marshallward#56: Literal block continues too far after bullet.

This fixes literal block highlighting as explained in the ticket's
example below:

  - A bullet with literal following::

      Correctly highlighted literal

    Incorrectly highlighted as literal

  Correctly unhighlighted.

syntax/rst.vim

@@ -21,9 +21,333 @@ syn cluster rstCruft                contains=rstEmphasis,rstStrongEmphasis,
       \ rstSubstitutionReference, rstInlineInternalTargets,rstFootnoteReference,
       \ rstHyperlinkReference
 
+""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
+" Literal blocks typically look like this:
+"
+"   Some non-literal text followed by double colons::
+"
+"     This part is literal.
+"     And so is this.
+"
+"   Back to non-literal text here (since it lines up below "Some non-literal").
+"
+" Literal mode turns off when the level of indentation decreases back to
+" the original amount of indentation on the line with the double colons.
+" Put another way, literal mode remains as long as there is more
+" leading whitespace than was found at the start of the line with the double
+" colons.
+"
+" When the double colons are on a bulleted or numbered line, however, literal
+" mode stops once the indentation decreases to match the first non-whitespace
+" position *after* the bullet, e.g.::
+"
+"   - A bullet followed by double colons::
+"
+"       This part is literal.
+"       And so is this.
+"
+"     Back to non-literal text here (since it lines up below "A bullet").
+"
+" For this case, the amount of leading whitespace is calculated by treating
+" the bullet character or the line number as if they were composed of spaces.
+" Consider this example:
+"
+"   - This is a bullet.
+"
+"     - Here is a sub-bullet with double colons::
+"
+"         This part is literal.
+"         And so is this.
+"
+"       Back to non-literal text here (since it lines up below "Here is").
+"       This line is indented four spaces from "- This is".
+"
+" In the above example, the leading whitespace threshold is four spaces: two
+" spaces before "- Here is", plus the bullet character ("-") converted to a
+" space, plus the space following the bullet.  Literal mode remains as long as
+" there is more than the threshold four spaces of indentation.
+"
+" A valid bulleted line begins with zero or more whitespace characters, a single
+" bullet character ("-", "+", or "*"), and whitespace.
+"
+" As explained in
+" https://docutils.sourceforge.io/docs/ref/rst/restructuredtext.html#enumerated-lists,
+" a valid numbered line begins with zero or more whitespace characters followed
+" by an enumerator.  An enumerator consists of an enumeration sequence member
+" and formatting, followed by whitespace. The following enumeration sequences
+" are recognized:
+"
+"   arabic numerals: 1, 2, 3, ... (no upper limit).
+"   uppercase alphabet characters: A, B, C, ..., Z.
+"   lower-case alphabet characters: a, b, c, ..., z.
+"   uppercase Roman numerals: I, II, III, IV, ..., MMMMCMXCIX (4999).
+"   lowercase Roman numerals: i, ii, iii, iv, ..., mmmmcmxcix (4999).
+"
+" In addition, the auto-enumerator, "#", may be used to automatically enumerate
+" a list.  The following formatting types are recognized:
+"
+"   suffixed with a period: "1.", "A.", "a.", "I.", "i.".
+"   surrounded by parentheses: "(1)", "(A)", "(a)", "(I)", "(i)".
+"   suffixed with a right-parenthesis: "1)", "A)", "a)", "I)", "i)".
+"
+" Therefore, the threshold leading whitespace, w, could be described by:
+"
+"   let w = whitespace at the start of the line with double colons.
+"   If this line starts with a bullet and whitespace, extend w by one space for
+"   each character in the sequence (bullet + following whitespace).
+"   If this line starts with a enumeration, extend w by one space for each
+"   character in the enumeration (note that this includes the whitespace at the
+"   end of the enumeration).
+"
+" The difficulty lies in trying to count the number of characters in either a
+" (bullet + following whitespace) sequence or an enumeration.  In a syntax
+" region, there doesn't appear to be any way to calculate the length of a
+" captured group (such as '\z1') in the 'start=' section and pass that to the
+" 'end=' section in such a way that the length can be used to require a match
+" for a run of that many space characters.
+"
+" Consider this example:
+"
+"   -   Some bullet::
+"
+"         This part is literal.
+"         And so is this.
+"
+"       Back to non-literal (since it lines up with "Some bullet").
+"
+" There is no indentation before "-   Some bullet::".  The equivalent amount of
+" indentation before "Some numbered line" is four spaces (one for the "-" and
+" three for the three following spaces).  This enumeration could be matched with
+" a capturing pattern such as '\z(-\s\+\)' (a hyphen bullet, a period, and one
+" or more whitespace characters); but there seems to be no direct way to convert
+" this captured text into a requirement to match four whitespace characters in
+" the 'end=' pattern.
+"
+" To work around this limitation, consider that if a bullet is present, it must
+" be followed by at least one space; if the bullet is not present, then the
+" space won't be present, either.  Consider the following pattern with three
+" capturing groups, where the entire pattern is optional::
+"
+"   \(\z(-\)\z( \)\z(\s*)\)\?
+"
+" If no bullet is present, the above pattern fails to match, and all three
+" captured groups are empty.  If the bullet is present with following
+" whitespace, then \z1 holds the bullet, \z2 holds a single space, and \z3 holds
+" any additional trailing whitespace.  Consider using the following pattern in
+" 'end='::
+"
+"   \z2\z2\z3
+"
+" The above pattern will match a run of whitespace with the length we desire.
+" When no bullet is present, it matches the empty string.  When a bullet is
+" present, we know \z2 will be a single space, in which case the pattern matches
+" a space for the bullet (the first \z2), the following space (the second \z2),
+" and the remaining spaces (the \z3).
+"
+" A similar technique allows matching a enumeration.  In the above technique, we
+" were matching a single bullet character, and we used \z2 in the bullet's
+" position to effectively convert the bullet to a space.  For enumerations, we
+" can match a fixed number, n, of non-whitespace characters, and convert them
+" into n space characters by repeating \z2 that many times.  For example::
+"
+"   1. Some numbered line::
+"
+"        This part is literal.
+"        And so is this.
+"
+"      Back to non-literal (since it lines up with "Some numbered").
+"
+" The following pattern matches optional single-digit enumerations like the one
+" above::
+"
+"   \(\z(\d\.\)\z( \)\z(\s*)\)\?
+"
+" If the above matches, we know that \z1 will be two characters (a digit and a
+" period).  To replace that two-character sequence with two spaces in the
+" 'end=' pattern, we use two copies of \z2 for these non-whitespace characters
+" and an additional \z2 for the immediately following space, plus \z3 to match
+" any additional whitespace::
+"
+"   \z2\z2\z2\z3
+"
+" By carefully matching runs of known-length non-whitespace characters,
+" we can convert these runs into runs of equal numbers of space characters by
+" repeating the group which captured a single space the desired number of times.
+"
+" Bullets and enumeration patterns can then be grouped by the number of
+" non-whitespace characters before the single following space.  Consider the
+" samples below:
+"
+"   1-character sequences:
+"
+"   - Bullets::
+"
+"       -
+"       +
+"       *
+"
+"   2-character sequences::
+"
+"     #.
+"     #)
+"     1.
+"     1)
+"     a)
+"
+"   3-character sequences::
+"
+"     12.
+"     ab)
+"     (a)
+"     (#)
+"
+"   4-character sequences::
+"
+"     123.
+"     abc)
+"     (ab)
+"
+" Overall, then, a pattern to match the start of a literal block with an
+" optional bullet or enumeration could be structured as the following captured
+" groups::
+"
+"   \z(any leading whitespace)
+"   (
+"     (
+"       (one-character sequence)   \z(one space) |
+"       (two-character sequence)   \z(one space) |
+"       (three-character sequence) \z(one space) |
+"       (four-character sequence)  \z(one space) |
+"       (five-character sequence)  \z(one space) |
+"       (six-character sequence)   \z(one space) |
+"       (seven-character sequence) \z(one space)
+"     )
+"     \z(any extra whitespace)
+"   )\?
+"   any characters
+"   ::
+"   end-of-line
+"
+" This uses all nine capture groups, \z1 through \z9, to support the largest
+" number of digits in enumerations (up to six digits and a period, for example).
+"
+" In the above, \z1 holds the leading whitespace, and \z9 holds any extra
+" whitespace.  Each of the remaining capture groups will hold a single space if
+" their corresponding n-character sequence matched, and empty otherwise.  The
+" group can be repeated n times to convert the n-character sequence into n
+" spaces.  For example, \z3\z3\z3 will be three spaces if a three-character
+" sequence is matched, and empty otherwise.  Further, \z2\z2\z3\z3\z3 will be
+" two spaces for a 2-character match, three spaces for a three-character match,
+" and empty otherwise.
+"
+" Therefore, the threshold whitespace pattern is given by the concatenation of
+" all of the below patterns::
+"
+"   \z1
+"   \z2\z2
+"   \z3\z3\z3
+"   \z4\z4\z4\z4
+"   \z5\z5\z5\z5\z5
+"   \z6\z6\z6\z6\z6\z6
+"   \z7\z7\z7\z7\z7\z7\z7
+"   \z8\z8\z8\z8\z8\z8\z8\z8
+"   \z9
+
+" Return a pattern for the given number of digits (in enumerations).
+function! s:digits_pat(num_digits)
+    if a:num_digits == 1
+        " A lone "digit" includes numbers, letters and '#'.
+        return '[0-9a-zA-Z#]'
+    endif
+    " Multiple "digits" comprise numbers and letters (but not '#').
+    return '[0-9a-zA-Z]\{' . a:num_digits . '}'
+endfunction
+
+" Build up s:pat, a pattern to detect the start of rstLiteral.
+
+" Begin group for everything before double-colon:
+let s:pat = '\('
+
+" \z1 captures optional leading whitespace:
+let s:pat .= '^\z(\s*\)'
+
+" Begin optional capture of bullet or enumeration:
+let s:pat .= '\('
+
+" n=1: A bullet character and \z2 capturing a space:
+let s:pat .= '[-+*]\z( \)'
+
+" s:n is the number of non-whitespace characters in the enumeration.
+" Iterate from 2 through 7 (capturing into \z3 through \z8).
+let s:n = 2
+while s:n <= 7
+    let s:pat .= '\|\('
+
+    " Handle 'dddd.' and 'dddd)':
+    let s:pat .= s:digits_pat(s:n - 1) . '[).]'
+
+    " If enough characters, handle the '(ddd)' case:
+    if s:n >= 3
+        let s:pat .= '\|(' . s:digits_pat(s:n - 2) . ')'
+    endif
+
+    let s:pat .= '\)'
+
+    " \z(n+1) captures a space:
+    let s:pat .= '\z( \)'
+
+    let s:n += 1
+endwhile
+
+" End optional capture of bullet or enumeration:
+let s:pat .= '\)\?'
+
+" Trailing whitespace in \z9
+let s:pat .= '\z( *\)'
+
+" Finish with arbitrary characters, then close the group:
+let s:pat .= '.*\)'
+
+" Use a zero-width look-behind for everything before double-colon:
+let s:pat .= '\@<='
+
+" End with double-colon at the end of the line followed by a blank line:
+let s:pat .= '::\n\s*\n'
+
+" Now build up the corresponding end pattern, s:end_pat.
+
+" Match leading whitespace:
+let s:end_pat = '^\(\z1'
+
+" Then match the remaining characters of threshold whitespace for each
+" capture group:
+let s:end_pat .= '\z2\z2'
+let s:end_pat .= '\z3\z3\z3'
+let s:end_pat .= '\z4\z4\z4\z4'
+let s:end_pat .= '\z5\z5\z5\z5\z5'
+let s:end_pat .= '\z6\z6\z6\z6\z6\z6'
+let s:end_pat .= '\z7\z7\z7\z7\z7\z7\z7'
+let s:end_pat .= '\z8\z8\z8\z8\z8\z8\z8\z8'
+
+" Require at least one more space to stay in literal block; then negate the
+" match to exit literal mode:
+let s:end_pat .= '\s\+\)\@!'
+
+" Build up 'syn' command:
+let s:syn = "syn region  rstLiteralBlock         matchgroup=rstDelimiter"
+let s:syn .= " start='" . s:pat . "'"
+let s:syn .= " skip='^\s*$'"
+let s:syn .= " end='" . s:end_pat . "'"
+let s:syn .= " contains=@NoSpell"
+
+" Change to 0 to compare with original:
+if 1
+execute s:syn
+else
 syn region  rstLiteralBlock         matchgroup=rstDelimiter
       \ start='\(^\z(\s*\).*\)\@<=::\n\s*\n' skip='^\s*$' end='^\(\z1\s\+\)\@!'
       \ contains=@NoSpell
+endif
 
 syn region  rstQuotedLiteralBlock   matchgroup=rstDelimiter
       \ start="::\_s*\n\ze\z([!\"#$%&'()*+,-./:;<=>?@[\]^_`{|}~]\)"