'main': Highlight backslash escape sequences outside quotes.

Fixes zsh-users#631.

WIP: test expectations should be updated (12 failures including cthulhu)

Author: danielshahaf

docs/highlighters/main.md

@@ -53,6 +53,7 @@ This highlighter defines the following styles:
 * `dollar-quoted-argument-unclosed` - unclosed dollar-quoted arguments (`` $'foo ``)
 * `rc-quote` - two single quotes inside single quotes when the `RC_QUOTES` option is set (`` 'foo''bar' ``)
 * `dollar-double-quoted-argument` - parameter expansion inside double quotes (`$foo` inside `""`)
+* `back-un-quoted-argument` -  backslash escape sequences outside quotes (`\"` in `print -r \"bar\"`)
 * `back-double-quoted-argument` -  backslash escape sequences inside double-quoted arguments (`\"` in `"foo\"bar"`)
 * `back-dollar-quoted-argument` -  backslash escape sequences inside dollar-quoted arguments (`\x` in `$'\x48'`)
 * `assign` - parameter assignments (`x=foo` and `x=( )`)

highlighters/main/main-highlighter.zsh

@@ -53,7 +53,8 @@
 : ${ZSH_HIGHLIGHT_STYLES[dollar-quoted-argument]:=fg=yellow}
 : ${ZSH_HIGHLIGHT_STYLES[rc-quote]:=fg=cyan}
 : ${ZSH_HIGHLIGHT_STYLES[dollar-double-quoted-argument]:=fg=cyan}
-: ${ZSH_HIGHLIGHT_STYLES[back-double-quoted-argument]:=fg=cyan}
+: ${ZSH_HIGHLIGHT_STYLES[back-un-quoted-argument]:=fg=cyan}
+: ${ZSH_HIGHLIGHT_STYLES[back-double-quoted-argument]:=fg=cyan,underline}
 : ${ZSH_HIGHLIGHT_STYLES[back-dollar-quoted-argument]:=fg=cyan}
 : ${ZSH_HIGHLIGHT_STYLES[assign]:=none}
 : ${ZSH_HIGHLIGHT_STYLES[redirection]:=fg=yellow}
@@ -1202,7 +1203,13 @@ _zsh_highlight_main_highlighter_highlight_argument()
 
   for (( ; i <= $#arg ; i += 1 )); do
     case "$arg[$i]" in
-      "\\") (( i += 1 )); continue;;
+      "\\") 
+        highlights+=(
+          $(( start_pos + i - 1 )) $((start_pos + i + 1)) back-double-quoted-argument
+        )
+        (( i += 1 ))
+        continue
+        ;;
       "'")
         _zsh_highlight_main_highlighter_highlight_single_quote $i
         (( i = REPLY ))

highlighters/main/test-data/backslash-continuation2.zsh

@@ -34,4 +34,5 @@ expected_region_highlight=(
   '1 4 builtin' # echo
   '6 18 default "issue #705"' # foo\\\nbar"baz"
   '14 18 double-quoted-argument "issue #705"' # "baz"
+  # TODO: when fixing this, see if we can highlight the backslash-newline sequence as a comment.
 )