Some idea how compass could use @extend instead of mixins with backward compatibility

1-source.scss

$exend-compass-mixins: true;

.my-box {
  background: red;
  @include box-shadow(black, 0, 1px, 5px);
  @include clearfix;
}

.my-other-box {
  background: green;
  @include box-shadow(red, 2px, 0, 10px);
  @include clearfix;
}

@include extended-compass-mixins;

2-intermediate-step.scss

// this is just to clarify what happens between step 1 and 3; this file would not exist

.my-box {
  background: red;
  @extend .c1;
  @extend .c2;
}

.my-other-box {
  background: green;
  @extend .c2;
  @extend .c3;
}

.c1 {
  @include box-shadow(black, 0, 1px, 5px);
}

.c2 {
  @include clearfix;
}

.c3 {
  @include box-shadow(red, 2px, 0, 10px);
}

3-result.css

.my-box {
  background: red;
}

.my-other-box {
  background: green;
}

.c1, .my-box {
  -moz-box-shadow: black 0 1px 5px 0;
  -webkit-box-shadow: black 0 1px 5px 0;
  -o-box-shadow: black 0 1px 5px 0;
  box-shadow: black 0 1px 5px 0;
}

.c2, .my-box, .my-other-box {
  overflow: hidden;
  *zoom: 1;
}

.c3, .my-other-box {
  -moz-box-shadow: red 2px 0 10px 0;
  -webkit-box-shadow: red 2px 0 10px 0;
  -o-box-shadow: red 2px 0 10px 0;
  box-shadow: red 2px 0 10px 0;
}