Change the parser to recognize bracketed lists.

This is the idea: in the same way the old parser looks for parenthesis
delimiters, let's add another case for brackets delimiters.

In case an open bracket is found, the parser will look for a list. There
are three possible results of parsing a list:

First, the parsed expression is not a list at all. libsass parser tends
to avoid wrapping if lists are only one item lists. But bracketed lists
prevent unwrapping so the item becomes the solely item for a new
bracketed list.

Example:
singleton-item --> [ singleton-item ]

Second, the list is indeed a list with some explicit delimiter. This
case is similar to the previous one. Brackets prevent unwrapping so this
time is the list as a whole which become the unique item of the
bracketed list.

Example:
(item1, item2, ...) --> [ (item1, item2, ...) ]
[item1, item2, ...] --> [ [item1, item2, ...] ]

Third, the list is a list with no delimiters. So its items actually
belong to the bracketed list. The parse will fix the delimiter of the
list for them to be brackets.

Example:
Suppose | ... | stands for a list with no delimiters.
|item1, item2, ...|  --> [item1, item2, ...]

Author: delapuente

src/parser.cpp

@@ -966,7 +966,7 @@ namespace Sass {
       else {
         value = &parse_list(DELAYED);
         if (List_Ptr list = SASS_MEMORY_CAST(List, value)) {
-          if (list->length() == 0 && !peek< exactly <'{'> >()) {
+          if (!list->is_bracketed() && list->length() == 0 && !peek< exactly <'{'> >()) {
             css_error("Invalid CSS", " after ", ": expected expression (e.g. 1px, bold), was ");
           }
         }
@@ -1000,8 +1000,13 @@ namespace Sass {
     List_Obj map = SASS_MEMORY_NEW(List, pstate, 0, SASS_HASH);
 
     // it's not a map so return the lexed value as a list value
-    if (!lex_css< exactly<':'> >())
-    { return key; }
+    if (!lex_css< exactly<':'> >()) {
+      List_Obj list = SASS_MEMORY_CAST(List, key);
+      if (list && list->delimiter() == SASS_NO_DELIMITER) {
+        list->delimiter(SASS_PARENTHESIS);
+      }
+      return key;
+    }
 
     Expression_Obj value = parse_space_list();
 
@@ -1051,6 +1056,7 @@ namespace Sass {
           exactly<'}'>,
           exactly<'{'>,
           exactly<')'>,
+          exactly<']'>,
           exactly<':'>,
           end_of_file,
           exactly<ellipsis>,
@@ -1085,6 +1091,7 @@ namespace Sass {
             exactly<'}'>,
             exactly<'{'>,
             exactly<')'>,
+            exactly<']'>,
             exactly<':'>,
             end_of_file,
             exactly<ellipsis>,
@@ -1111,6 +1118,7 @@ namespace Sass {
           exactly<'}'>,
           exactly<'{'>,
           exactly<')'>,
+          exactly<']'>,
           exactly<','>,
           exactly<':'>,
           end_of_file,
@@ -1129,6 +1137,7 @@ namespace Sass {
                exactly<'}'>,
                exactly<'{'>,
                exactly<')'>,
+               exactly<']'>,
                exactly<','>,
                exactly<':'>,
                end_of_file,
@@ -1328,6 +1337,21 @@ namespace Sass {
       // expression can be evaluated
       return &value;
     }
+    else if (lex_css< exactly<'['> >()) {
+      // explicit bracketed
+      Expression_Obj value = parse_list();
+      // lex the expected closing square bracket
+      if (!lex_css< exactly<']'> >()) error("unclosed squared bracket", pstate);
+      // fix delimiter
+      List_Obj list = SASS_MEMORY_CAST(List, value);
+      if (!list || list->delimiter() != SASS_NO_DELIMITER) {
+        List_Ptr outer_list = SASS_MEMORY_NEW(List, pstate, 1, SASS_SPACE, false, SASS_BRACKETS);
+        outer_list->append(&value);
+        return outer_list;
+      }
+      list->delimiter(SASS_BRACKETS);
+      return value;
+    }
     // string may be interpolated
     // if (lex< quoted_string >()) {
     //   return &parse_string();