Input.css.scss

.shureicons-font {
  font-family: 'shureicons';
  -webkit-font-smoothing: subpixel-antialiased;
}

@mixin logo($height) {
  @include box-sizing(border-box);
  display: block;
  height: $height;
  padding-top: $height;
  position: relative;
  text-indent: -9999px;

  b {
    @extend .shureicons-font;
    color: #fff;
    font-weight: normal;
    line-height: 0.2;
    left: 0;
    position: absolute;
    text-indent: 0;
    top: 0;
    @include transition();
  }
}

.header--logo {
  @include logo(90px);
  float: left;
  width: 170px;
}

Output.css

/* line 33, ../../../../app/assets/stylesheets/all/layouts/_base.css.scss */
.shureicons-font, .header--shop-link:after, .header--logo b, .footer--logo b, .footer--social a:before {
  font-family: 'shureicons';
  -webkit-font-smoothing: subpixel-antialiased;
}

Note that the '.header--logo b' gets added to the icon font rule, even though it's been extended from within a mixin.

Check it, yo, this doesn't work if you're extending a selector like %shureicons-font

Actually, this isn't working either:

.buttonery {
cursor: pointer;
font-size: 100%;
padding: .5em 3em .5em 2em;
&:link, &:visited {
text-decoration: none;
}
&:hover, &:active, &:focus {
position: relative;
top: 1px; left: 1px;
}
}

@mixin buttonator($button-color: $blue, $text-color: #fff, $border-radius: .5em) {
@extend .buttonery;
background: $button-color;
@include border-radius($border-radius);
color: $text-color !important;
border: 1px adjust-lightness($button-color, -10%) solid;
@include text-shadow((adjust-lightness($button-color, -10%)) 0 1px 0);
}