[Sass] Deprecate parent refs in the middle of selectors.

doc-src/SASS_CHANGELOG.md

@@ -3,6 +3,13 @@
 * Table of contents
 {:toc}
 
+## 2.2.24 (Unreleased)
+
+* Parent references -- the `&` character --
+  may only be placed at the beginning of simple selector sequences in Sass 3.
+  Placing them elsewhere is deprecated in 2.2.24 and will print a warning.
+  For example, `foo &.bar` is allowed, but `foo .bar&` is not.
+
 ## 2.2.23
 
 [Tagged on GitHub](http://github.com/nex3/haml/commit/2.2.23).

lib/sass/tree/rule_node.rb

@@ -202,7 +202,15 @@ module Sass::Tree
       while scanner.rest?
         rules.last << scanner.scan(/[^",&]*/)
         case scanner.scan(/./)
-        when '&'; rules.last << :parent
+        when '&'
+          warn <<END unless rules.last.empty? || rules.last.last =~ /(^|\s)$/
+DEPRECATION WARNING:
+On line #{@line}#{" of '#{@filename}'" if @filename}
+In Sass 3, parent selectors will only be able to appear
+at the beginning of simple selector sequences.
+For example, ".foo &.bar" is allowed but ".bar&" is not.
+END
+          rules.last << :parent
         when ','
           scanner.scan(/\s*/)
           rules << [] if scanner.rest?