OCSS mixins. Maybe?

gistfile1.sass

@mixin co($name)
  .co-#{$name}
    @content

@mixin el($name)
  &__#{$name}
    @content

@mixin is($name)
  &-is-#{$name}
    @content
    
+co(nav-user)
  +el(name)
    border-left: 1px solid #707487
    color: white
    padding: 1em
    float: left

    +is(active)
      font-weight: bold
  
  +el(search)
    border-left: 1px solid #707487
    background: #4C4F5C
    float: right
    
    +el(input)
      padding: 1em