sizefrom is not documented and error message is incorrect

[chenwng]

Hi,

In one of the PR, sizefrom was added. However it is not mentioned in README at all.
And when I tried to use sizeof for slice without size, an incorrect message was reported, which is caused by this line.

[lunixbochs]

Can you post a reproduction case? A struct with sizefrom in it that triggers that error message.

[chenwng]

Actually the issue I encountered was not because of sizefrom.
I was reading the README and saw that I can use a slice in a struct.
Here is the statement about it.

Var []int struc:"[]int32,little,sizeof=StringField" will pack Var as a slice of little-endian int32, and link it as the size of StringField.

So I tried it in a test with something like F1 []uint8 struc:"[]uint8,sizeof=N"`` in one of my struct. Then I saw the error

struc: field F1 is a slice with no length or sizeof field

I wondered why struc reported such error as I have put sizeof in the tag.
After I read the struc code, I found the statement in README is incorrect and error message is misleading. I should use sizefrom in the tag and if I don't use sizefrom the error message should report like is a slice with no length or *sizefrom* field . And after I changed the tag to sizefrom, it works fine.

        if f.Len == -1 && f.Sizefrom == nil {
            return nil, fmt.Errorf("struc: field `%s` is a slice with no length or sizeof field", field.Name)
        }

If the README can be updated and error message is fixed, it will be clearer.

[lunixbochs]

There is a sizeof tag, but it goes on the size field and not on the array. Sizeof and Sizefrom are inverse of the same thing.