Deprecate rather than disallow raw newlines in strings.

Closes #1302

doc-src/SASS_CHANGELOG.md

@@ -187,8 +187,9 @@ adding multiple suffixes to the same parent selector. For example:
   !global` will assign to `$var` globally. This behavior can be detected using
   `feature-exists(global-variable-shadowing)`.
 
-* SassScript strings in SCSS may no longer include unescaped newlines. As doing
-  so produced invalid CSS, there will be no deprecation period for this change.
+* Unescaped newlines in SassScript strings in SCSS are deprecated, since they're
+  invalid according to the CSS string grammar. To include a newline in a string,
+  use "\a" or "\a " as in CSS.
 
 ## 3.3.9 (27 June 2014)
 

lib/sass/script/lexer.rb

@@ -106,7 +106,7 @@ module Sass
         private
 
         def string_re(open, close)
-          /#{open}((?:\\.|\#(?!\{)|[^#{close}\n\\#])*)(#{close}|#\{)/m
+          /#{open}((?:\\.|\#(?!\{)|[^#{close}\\#])*)(#{close}|#\{)/m
         end
       end
 
@@ -275,7 +275,16 @@ module Sass
       end
 
       def string(re, open)
+        line, offset = @line, @offset
         return unless scan(STRING_REGULAR_EXPRESSIONS[re][open])
+        if @scanner[0] =~ /([^\\]|^)\n/
+          Sass::Util.sass_warn <<MESSAGE
+DEPRECATION WARNING on line #{line}, column #{offset}#{" of #{@filename}" if @filename}:
+Unescaped multiline strings are deprecated and will be removed in a future version of Sass.
+To include a newline in a string, use "\\a" or "\\a " as in CSS.
+MESSAGE
+        end
+
         if @scanner[2] == '#{' # '
           @scanner.pos -= 2 # Don't actually consume the #{
           @offset -= 2

test/sass/scss/scss_test.rb

@@ -3436,7 +3436,24 @@ bang";
   bar: $var;
 }
 SCSS
-end
+  end
+
+  def test_raw_newline_warning
+    assert_warning(<<MESSAGE.rstrip) {assert_equal(<<CSS, render(<<SCSS))}
+DEPRECATION WARNING on line 2, column 9:
+Unescaped multiline strings are deprecated and will be removed in a future version of Sass.
+To include a newline in a string, use "\\a" or "\\a " as in CSS.
+MESSAGE
+.foo {
+  bar: "baz\\a bang"; }
+CSS
+.foo {
+  $var: "baz
+bang";
+  bar: $var;
+}
+SCSS
+  end
 
   # Regression