@mixin awesomething {
  // Some default styles.
  color: blue;
  @content
}

@function getlinegeometry($fontsize){
  @return $fontsize * 1.1;
}

.my-awesome-thing {
  @include awesomething {
    // Styles that should replace @content
    $size: 1rem;
    font-size: $size;
    line-height: getlinegeometry($size);
  }
}
<html>
<head>
  <meta http-equiv="content-type" content="text/html; charset=utf-8" />
  <link rel="stylesheet" href="style.css" title="" type="" />
</head>

<body>
  <div>
    <h1 class="myawesomething">My Awesome Thing</h1>
    
  </div>
</body>

</html>