CSS文字换行空白问题

需求：左边的类class：a换行后右边去除空白，使右边的类class:b靠过来，限制：class：a不可以写死宽度


html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Documenttitle>
    <style>
        .box {
            display: flex;
            align-items: center;
            width: 300px;
            margin-top: 100px;
            margin-left: 100px;
            background-color: green;
        }

        .a {
            /* white-space: normal; */
            /* white-space: nowrap; */
            /* white-space: pre-line; */
            /* word-break: break-all; */
            background-color: pink;
        }

        .b {
            flex-shrink: 0;
            top: 0;
            right: 0;
            background-color: skyblue;

        }
    style>
head>

<body>
    <div class="box">
        <div class="a">
            知无不言知无不言：21356789798654
        div>
        <div class="b">
            言之有理
        div>
    div>
body>
html>

a、b都设置 flex:1 不可以吗？

<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .box {
            display: flex;
            align-items: center;
            width: 300px;
            margin-top: 100px;
            margin-left: 100px;
            background-color: green;
            text-align: left;
        
        }

        .a {
            background-color: pink;
            white-space: nowrap;
            overflow: hidden;
            text-overflow: ellipsis;
        
            
        }

        .b {
            flex-shrink: 0;
            top: 0;
            right: 0;
            background-color: skyblue;
        
        }

        .a {
            width: unset;
            white-space: pre-wrap;
        
        }

        .b {
            margin-left: auto;
        
        }
    </style>
</head>

<body>
    <div class="box">
        <div class="a">知无不言知无不言知无不言知无不言知无不言知无不言
        </div>
        <div class="b">言之有理</div>
    </div>
</body>

</html>

写最大宽啊

 
<!DOCTYPE html>
<html lang="en">
 
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .box {
            display: flex;
            align-items: center;
            width: 300px;
            margin-top: 100px;
            margin-left: 100px;
            background-color: green;
        }
 
        .a {
            max-width: 150px;
            /* white-space: normal; */
            /* white-space: nowrap; */
            /* white-space: pre-line; */
            word-break: break-all;
            background-color: pink;
        }
 
        .b {
            flex-shrink: 0;
            top: 0;
            right: 0;
            background-color: skyblue;
 
        }
    </style>
</head>
 
<body>
    <div class="box">
        <div class="a">
            知无不言知无不言：21356789798654
        </div>
        <div class="b">
            言之有理
        </div>
    </div>
</body>
</html>