https://css-tricks.com/snippets/sass/bem-mixins/ に乗ってるやつ

bem-mixins.scss

/// Block Element
/// @access public
/// @param {String} $element - Element's name
@mixin element($element) {
    &__#{$element} {
        @content;
    }
}

/// Block Modifier
/// @access public
/// @param {String} $modifier - Modifier's name
@mixin modifier($modifier) {
    &--#{$modifier} {
        @content;
    }
}

/// @alias element
@mixin e($element) {
    @include element($element)  {
        @content;
    }
}

/// @alias modifier
@mixin m($modifier) {
    @include modifier($modifier) {
        @content;
    }
}