This gist describes a new feature we're experimenting with for Sass 3.2: placeholder selectors. They do not get generated into your output, but they can be used like a class and extended like one.

0_silent_selector_grid.scss

$gutter: 10px;
$grid-unit: 60px;

%clearfix {
  *zoom: 1;
  &:after {
    content: "\0020";
    display: block;
    height: 0;
    clear: both;
    overflow: hidden;
    visibility: hidden;
  }
}

%column {
  @extend %clearfix;
  float: left;
  margin-right: $gutter;
  &.last {
    margin-right: 0;
  }
}

@for $i from 1 through 9 {
  .span-#{$i} {
    @extend %column;
    width: $grid-unit * $i + $gutter * ($i - 1);
  }
}

1_output.css

.span-1, .span-2, .span-3, .span-4, .span-5, .span-6, .span-7, .span-8, .span-9 { *zoom: 1; }
.span-1:after, .span-2:after, .span-3:after, .span-4:after, .span-5:after, .span-6:after, .span-7:after, .span-8:after, .span-9:after { content: "\0020"; display: block; height: 0; clear: both; overflow: hidden; visibility: hidden; }

.span-1, .span-2, .span-3, .span-4, .span-5, .span-6, .span-7, .span-8, .span-9 { float: left; margin-right: 10px; }
.last.span-1, .last.span-2, .last.span-3, .last.span-4, .last.span-5, .last.span-6, .last.span-7, .last.span-8, .last.span-9 { margin-right: 0; }

.span-1 { width: 60px; }
.span-2 { width: 130px; }
.span-3 { width: 200px; }
.span-4 { width: 270px; }
.span-5 { width: 340px; }
.span-6 { width: 410px; }
.span-7 { width: 480px; }
.span-8 { width: 550px; }
.span-9 { width: 620px; }

@jlong In general, I prefer explicit syntax over symbolic syntax as well. The fact that placeholder selectors are obscure and not self-documenting is certainly their greatest shortcoming. However, in this case, the various proposed explicit syntaxes all have pretty significant downsides relative to placeholder selectors, and I think that those concerns outweigh the explicitness.

Specifically, proposals of the form @silent .foo { ... } require a lot of repetitive silencing and are contrary to the usual semantics of directives. Your @module syntax is as I mentioned complex to wrap one's head around; it's still not clear to me how you'd use it to provide for example a library of extendable classes. The most convincing factor for me was that in the end, all of the other proposals were ultimately just indirect ways of expressing "this selector should be a placeholder," which placeholder selectors express directly.

@jlong, yes, that is what I mean. And it's one of the reasons we didn't end up merging that changeset. A named group of rules is a mixin. but at the point that it's in the document it needs to have a selector to be addressable by extend.