标准库中 http.Request 中 WithContext 方法为什么要浅拷贝一次而不是直接赋值?

如题。

// WithContext returns a shallow copy of r with its context changed
// to ctx. The provided ctx must be non-nil.
//
// For outgoing client request, the context controls the entire
// lifetime of a request and its response: obtaining a connection,
// sending the request, and reading the response headers and body.
//
// To create a new request with a context, use NewRequestWithContext.
// To make a deep copy of a request with a new context, use Request.Clone.
func (r *Request) WithContext(ctx context.Context) *Request {
	if ctx == nil {
		panic("nil context")
	}
	r2 := new(Request)
	*r2 = *r
	r2.ctx = ctx
	return r2
}