Generated by SassMeister.com.

SassMeister-input-HTML.html

<div class="container">
  <div class="inputGroup">
    <input type="text" class="inputField inputGroup__item">
    <div class="inputGroup__btn">
      <button class="btn">Button</button>
    </div>
  </div>  
</div>

SassMeister-input.scss

// ----
// Sass (v3.4.12)
// Compass (v1.0.3)
// ----

*,
*:after,
*:before {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}

body {
  font: 400 14px/1 sans-serif;
}

.container {
  margin: 0 auto;
  max-width: 768px;
  padding: 48px;
}
.btn {
  -webkit-appearance: none;
  -moz-appearance: none;
  background: #ff6622;
  color: white;
  border: 1px solid #ff6622;
  font: inherit;
  padding: 9px 12px 7px;
}

.inputField {
  -webkit-appearance: none;
  -moz-appearance: none;
  border: 1px solid #d7d7d7;
  display: block;
  font: inherit;
  padding: 8px 12px;
  width: 100%;
}

.inputGroup {
  display: table;
  width: 100%;
}

.inputGroup__item {
  display: table-cell;
}

.inputGroup__item--main {
  width: 99%;
}

.inputGroup__btn {
  display: table-cell;
  vertical-align: middle;
  width: 1%;
}

SassMeister-output.css

*,
*:after,
*:before {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}

body {
  font: 400 14px/1 sans-serif;
}

.container {
  margin: 0 auto;
  max-width: 768px;
  padding: 48px;
}

.btn {
  -webkit-appearance: none;
  -moz-appearance: none;
  background: #ff6622;
  color: white;
  border: 1px solid #ff6622;
  font: inherit;
  padding: 9px 12px 7px;
}

.inputField {
  -webkit-appearance: none;
  -moz-appearance: none;
  border: 1px solid #d7d7d7;
  display: block;
  font: inherit;
  padding: 8px 12px;
  width: 100%;
}

.inputGroup {
  display: table;
  width: 100%;
}

.inputGroup__item {
  display: table-cell;
}

.inputGroup__item--main {
  width: 99%;
}

.inputGroup__btn {
  display: table-cell;
  vertical-align: middle;
  width: 1%;
}

SassMeister-rendered.html

<div class="container">
  <div class="inputGroup">
    <input type="text" class="inputField inputGroup__item">
    <div class="inputGroup__btn">
      <button class="btn">Button</button>
    </div>
  </div>  
</div>