是否必须在sync.Pool New函数中返回指针类型？

I saw the issue on Github which says sync.Pool should be used only with pointer types, for example:

var TPool = sync.Pool{
    New: func() interface{} {
       return new(T)
    },
}

Does it make sense? What about return T{} and which is the better choice, why?

The whole point of sync.Pool is to avoid (expensive) allocations. Large-ish buffers, etc. You allocate a few buffers and they stay in memory, available for reuse. Hence the use of pointers.

But here you'll be copying the values on every step, defeating the purpose. (Assuming your T is a "normal" struct and not something like SliceHeader)

It is not necessary. In most cases it should be a pointer as you want to share an object, not to make copies.

In some use cases this can be a non pointer type, like an id of some external resource. I can imagine a pool of paths (mounted disk drives) represented with strings where some large file operations are being conducted.