[osh2oil] Fix crash bug translating empty string.

case $1 in '') caused a problem because we weren't returning the span_id
of the left single quote.

Also, notes about osh2oil translation:

- (( )) translates to sh-expr
- The different RHS styles

core/word.py

@@ -120,17 +120,10 @@ def LeftMostSpanForPart(part):
    return part.token.span_id

  elif part.tag == word_part_e.SingleQuotedPart:
    if part.tokens:
      return part.tokens[0].span_id
    else:
      return const.NO_INTEGER
    return part.spids[0]  # single quote location

  elif part.tag == word_part_e.DoubleQuotedPart:
    if part.parts:
      return LeftMostSpanForPart(part.parts[0])
    else:
      # We need the double quote location
      return const.NO_INTEGER
    return part.spids[0]  # double quote location

  elif part.tag == word_part_e.SimpleVarSub:
    return part.token.span_id

test/osh2oil.sh

@@ -398,6 +398,22 @@ setglobal g = ''
setglobal g = 'x'
OIL

  cat >/dev/null <<TODO_ENABLE
  # empty quoted words
  osh0-oil3 << 'OSH' 3<< 'OIL'
dq=""
OSH
setglobal dq = ""
OIL

  # empty quoted words
  osh0-oil3 << 'OSH' 3<< 'OIL'
sq=''
OSH
setglobal sq = ''
OIL
TODO_ENABLE

  # Local variable
  osh0-oil3 << 'OSH' 3<< 'OIL'
f() {
@@ -821,6 +837,9 @@ case $var in
  foo|bar)
    [ -f foo ] && echo file
    ;;
  '')
    echo empty
    ;;
  *)
    echo default
    ;;
@@ -830,6 +849,9 @@ match $var {
  with foo|bar
    test -f foo && echo file
    
  with ''
    echo empty
    
  with *
    echo default
    

tools/osh2oil.py

@@ -123,6 +123,36 @@ def PrintAsOil(arena, node, debug_spans):
# ${x:-default}  ->  @-(x or 'default')

def _GetRhsStyle(w):
  """
  Determine what style an assignment should use. '' or "", or an expression.

  SQ      foo=         setglobal foo = ''
  SQ      foo=''       setglobal foo = ''
  DQ      foo=""       setglobal foo = ""  # Or we could normalize it if no subs?
  DQ      foo=""       setglobal foo = ""  # Or we could normalize it if no subs?

  # Need these too.
  # Or honestly should C strings be the default?  And then raw strings are
  # optional?  Because most usages of \n and \0 can turn into Oil?
  # Yeah I want the default to be statically parseable, so we subvert the \t
  # and \n of command line tools?
  # As long as we are fully analyzing the strings, we might as well go all the
  # way!
  # I think I need a PartialStaticEval() to paper over this.
  #
  # The main issue is regex and globs, because they use escape for a different
  # purpose.  I think just do
  # grep r'foo\tbar' or something.

  C_SQ    foo=$'\n'          setglobal foo = C'\n'
  C_DQ    foo=$'\n'"$bar"    setglobal foo = C"\n$(bar)"

  Expr    path=${1:-}             setglobal path = $1 or ''
  Expr    host=${2:-$(hostname)}  setglobal host = $2 or $[hostname]

  What's the difference between Expr and Unquoted?  I think they're the same/
  """

  # NOTE: Pattern matching style would be a lot nicer for this...

  # Arith and command sub both retain $() and $[], so they are not pure
@@ -636,13 +666,14 @@ def DoCommand(self, node, local_symbols, at_top_level=False):
      self.f.write('}')

    elif node.tag == command_e.DParen:
      # Just change (( )) to ( )
      # Test it with while loop

      # TODO: (( a == 0 )) is sh-expr ' a == 0 '
      #
      # NOTE: I have a (( n++ )) in one script.  That can be 'set n++' or
      # 'set n += 1'.

      #self.DoArithExpr(node.child, local_symbols)
      #
      # Auto-translation is:
      #
      # sh-expr 'n++'
      raise NotImplementedError('DParen')

    elif node.tag == command_e.DBracket:
@@ -847,6 +878,7 @@ def DoCommand(self, node, local_symbols, at_top_level=False):
      raise AssertionError(node.__class__.__name__)

  def DoWordAsExpr(self, node, local_symbols):
    # TODO: This is wrong!
    style = _GetRhsStyle(node)
    if style == word_style_e.SQ:
      self.f.write("'")
@@ -856,6 +888,11 @@ def DoWordAsExpr(self, node, local_symbols):
      self.f.write('"')
      self.DoWordInCommand(node, local_symbols)
      self.f.write('"')
    # TODO: Put these back
    #elif style == word_style_e.Expr:
    #  pass
    #elif style == word_style_e.Unquoted:
    #  pass
    else:
      # "${foo:-default}" -> foo or 'default'
      # ${foo:-default} -> @split(foo or 'default')