<div>
    <span>some dynamic text</span>
    <input type="text" />
</div>

<div class = "container">
    <div class = "text">Glee is awesome!</div>
    <div class = "inputtext"><input type="text" /></div>
</div>

.container {
    display: table;
    width: 100%; /*how much should both the text and the input take*/
}
.container .text {
    white-space: nowrap;
    display: table-cell;
    width: 1px; /*make sure it's only as wide as needed*/
}
.container .inputtext {
    display: table-cell;
}
.container input {
    width: 100%; /*let the input take all available space*/
}

<div class = "text">Glee is awesome!</div>
<div class = "inputtext"><input type = "text" /></div>

.text {
    float: left;
}
.inputtext {
    overflow: hidden; /*this is the key here*/
}
input {
    width: 100%;
}