Generated by SassMeister.com.

SassMeister-input.scss

// ----
// Sass (v3.4.6)
// Compass (v1.0.1)
// ----

// THE SMART EXTEND
//
// If you have multiple selectors with the same
// mixin config "Smart Extend" will group their
// styles together.
//
// Modified from Lu Nelson's "Self Aware Mixin"
// http://lunelson.roughdraft.io/cc1b431a69360949dc01-the-self-aware-sass-mixin

// global map stores every mixin configuration
$mixin-configurations: ();

// define mixin with any format of arguments
@mixin smart-extend($mixin-configuration) {
  // look up these arguments as a key in the global map
  $id: map-get($mixin-configurations, $mixin-configuration);

  // if they return an 'ID'...
  @if $id { 
    // extend that ID
    @extend %#{$id}; 
    
  } @else {

    // create the ID
    $id: unique-id();

    // merge the ID in to the mixin's map against the ARGS
    @include add-configuration($mixin-configuration, $id);
    
    //create a new placeholder
    @include create-placeholder($id, $mixin-configuration);

    // extend that placeholder
    @extend %#{$id};
  }
}

@mixin add-configuration($mixin-configuration, $id) {
    $mixin-configurations: map-merge($mixin-configurations, ($mixin-configuration: $id)) !global;
}

@mixin create-placeholder($id, $args){
  // create a placeholder named after the ID at root
  @at-root {
    %#{$id} {
      @each $argument, $value in $args {
        #{$argument}: $value;
      }
    }    
  }
}

// USAGE EXAMPLE

@mixin border-radius($radius: 0.5em) {
  // pass self aware mixin an array of arguments
  $args: (  
    -webkit-border-radius: $radius,
    -moz-border-radius: $radius,
    border-radius: $radius
  );
  
  @include smart-extend($args);
}

.shared1 {
  @include border-radius(2px);
}

.shared2 {
  @include border-radius(2px);
}

.unique {
  @include border-radius(5px);
}

SassMeister-output.css

.shared1, .shared2 {
  -webkit-border-radius: 2px;
  -moz-border-radius: 2px;
  border-radius: 2px;
}

.unique {
  -webkit-border-radius: 5px;
  -moz-border-radius: 5px;
  border-radius: 5px;
}