Why big.NewInt(0).Bytes() returns [] instead of [0] in Go?

I find it weird that running big.NewInt(0).Bytes() returns [] instead of [0]. Is it really supposed to work that way?

https://play.golang.org/p/EEaS8sCvhFb

big.Int is a struct. It's idiomatic to make the zero value useful whenever possible. big.Int is no exception: The zero value for an Int represents the value 0.

It's an implementation detail, but the data of the Int is stored in a slice. The zero value for slices is nil, that is: no elements.

So this is very convenient, and very efficient. 0 is probably the most frequent value, and there may be cases where an initial big.Int won't get changed, and so no slice for the internal representation will be allocated.

See related: https://stackoverflow.com/questions/64257065/is-there-another-way-of-testing-if-a-big-int-is-0/64257532#64257532

From the documentation:
> Bytes returns the absolute value of x as a big-endian byte slice.

The package API doesn't define how many bytes long the slice will be. In this case, it's using the smallest number of bytes needed to convey the whole number.

The more likely reason why this happens is an implementation detail: The big.Int maintains the bytes of the number in a slice. nil slices in Go (the zero value of a slice) have length 0. When a big.Int value is initially created, we'd expect it to also have a value of 0. Therefore, it simplifies the implementation if an empty slice internally corresponds to a numerical value of 0, without needing to perform extra checks or padding.