Add deprecation warning for #626.

This paves the way to allow us to implement a new syntax for including a
mixin where the mixin being included is specified by a script
expression.

lib/sass/tree/visitors/perform.rb

@@ -336,6 +336,15 @@ def visit_import(node)
   # Loads a mixin into the environment.
   def visit_mixindef(node)
     env = Sass::Environment.new(@environment, node.options)
+
+    if node.name == "mixin"
+      Sass::Util.sass_warn <<WARNING
+DEPRECATION WARNING on line #{node.line}#{" of #{node.filename}" if node.filename}:
+Naming a mixin "mixin" is disallowed and will be an error in future versions of Sass.
+This conflicts with a new syntax for including mixins when the mixin name is specified dynamically.
+WARNING
+    end
+
     @environment.set_local_mixin(node.name,
       Sass::Callable.new(node.name, node.args, node.splat, env,
                          node.children, node.has_content, "mixin"))

test/sass/scss/scss_test.rb

@@ -1014,6 +1014,16 @@ def test_function_args
 SASS
   end
 
+  def test_disallowed_mixin_names
+    assert_warning(<<WARNING) {render(<<SCSS)}
+DEPRECATION WARNING on line 1 of test_disallowed_mixin_names_inline.scss:
+Naming a mixin "mixin" is disallowed and will be an error in future versions of Sass.
+This conflicts with a new syntax for including mixins when the mixin name is specified dynamically.
+WARNING
+@mixin mixin() {}
+SCSS
+  end
+
   def test_disallowed_function_names
     assert_warning(<<WARNING) {render(<<SCSS)}
 DEPRECATION WARNING on line 1 of test_disallowed_function_names_inline.scss:
@@ -3168,12 +3178,12 @@ def test_selector_script_through_mixin
 .foo {
   content: ".foo"; }
 CSS
-@mixin mixin {
+@mixin a-mixin {
   content: "\#{&}";
 }
 
 .foo {
-  @include mixin;
+  @include a-mixin;
 }
 SCSS
   end
@@ -3183,12 +3193,12 @@ def test_selector_script_through_content
 .foo {
   content: ".foo"; }
 CSS
-@mixin mixin {
+@mixin a_mixin {
   @content;
 }
 
 .foo {
-  @include mixin {
+  @include a_mixin {
     content: "\#{&}";
   }
 }