use .col for a simple flex-grow: 1;. It’s described here https://v4-alpha.getbootstrap.com/layout/grid/#variable-width-content
<link href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-alpha.6/css/bootstrap.min.css" rel="stylesheet" />
<div class="container">
<div class="row">
<div class="col">
I use all the space left
</div>
<div class="col-sm col-sm-auto">
I am a small text on the right
</div>
</div>
</div>