A common operation when dealing with binary wire formats is the need to append a fixed-width integer.

Currently, a common way to do it is with:

var arr [4]byte
binary.LittleEndian.PutUint32(arr[:], val)
out = append(out, arr[:]...)

This takes 3 lines and cannot be done as a expression.

I propose the addition of a new interface:

type AppendableByteOrder interface {
	AppendUint16([]byte, uint16) []byte
	AppendUint32([]byte, uint32) []byte
	AppendUint64([]byte, uint64) []byte
	String() string
}

And that we document that LittleEndian and BigEndian both implement ByteOrder and AppendableByteOrder.

Thus, the lines above can be simplified as:

out = binary.LittleEndian.AppendUint32(out, val)

In my work with various binary wire formats (e.g., tar, protobuf, etc.), an API like this would be more convenient than the existing PutUintX APIs.

The AppendableByteOrder interface exists primarily for documentation purposes. It is not strictly needed.

\cc @josharian