Sass depreciation warning

gistfile2.txt

// warning message is optional
@mixin deprecated($item, $msg: null) {
  @if $msg { @warn "DEPRECATION WARNING: #{$item} -- #{$msg}"}
  @else { @warn "DEPRECATION WARNING: #{$item}" }
}

.button--login {
  @include deprecated(&, 'use "btn" prefix');
  color: blue;
}

.button--ex2 {
  @include deprecated(&);
}

.btn--login {
  color: lightblue;
}

from: http://codepen.io/una/pen/EKZKgm?editors=0101