计算目标点到直线/线段的距离，先计算直线/线段上距离目标点最近的点，目标点与最近点的距离，即目标点到直线/线段的距离。

计算最近点：

    //点到直线的最近点
    public void GetNearPointToStraightByPoint(Vector3 targetP,Vector3 lineStartP,Vector3 lineEndP,ref Vector3 nearP)
    {
        GetNearPointToStraightByDir(targetP,lineStartP,lineEndP - lineStartP,ref nearP);
    }
    public void GetNearPointToStraightByDir(Vector3 targetP,Vector3 lineStartP,Vector3 lineDir,ref Vector3 nearP)
    {
        lineDir.Normalize();
        //Vector3.Dot(l1, l2) = l2在l1方向的投影长度
        //目标点在直线方向的投影长度，是最近点到直线给定起始点的距离，起始点+直线方向*投影长度=最近点坐标
        nearP = lineStartP + lineDir * Vector3.Dot(lineDir,targetP - lineStartP);
    }

    //点到线段的最近点
    public void GetNearPointToSegment(Vector3 targetP,Vector3 lineStartP,Vector3 lineEndP,ref Vector3 nearP)
    {
        Vector3 lineDir = (lineEndP - lineStartP).normalized;
        float lineLength = Vector3.Distance(lineStartP,lineEndP);
        float dotLength = Vector3.Dot(lineDir,targetP - lineStartP);
        //Mathf.Clamp，限制投影长度在线段长度范围内
        nearP = lineStartP + lineDir * Mathf.Clamp(dotLength,0,lineLength);
    }

测试代码：

public class Test : MonoBehaviour
{
    private Transform lineStartPoint;
    private Transform lineEndPoint;
    private Transform targetPoint;
    private Transform nearPoint;

    private Vector3 nearPointPos;

    private void Start()
    {
        lineStartPoint = transform.Find("LineStartPoint");
        lineEndPoint = transform.Find("LineEndPoint");
        targetPoint = transform.Find("TargetPoint");
        nearPoint = transform.Find("NearPoint");
    }

    private void Update()
    {
        //GetNearPointToStraightByPoint(targetPoint.position,lineStartPoint.position,lineEndPoint.position,ref nearPointPos);
        GetNearPointToSegment(targetPoint.position,lineStartPoint.position,lineEndPoint.position,ref nearPointPos);

        nearPoint.position = nearPointPos;

        Debug.DrawLine(lineStartPoint.position,lineEndPoint.position,Color.blue);
        Debug.DrawLine(targetPoint.position,nearPoint.position,Color.red);
    }
}