encoding/binary: add AppendByteOrder

[dsnet]

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

[rsc]

This proposal has been added to the active column of the proposals project
and will now be reviewed at the weekly proposal review meetings.
— rsc for the proposal review group

[rsc]

Recently we've been doing things like ReadFileFS for FS with a ReadFile method.
How about type AppendByteOrder?
The -able seems out of place in Go.

Otherwise seems reasonable, and it's nice that LittleEndian and BigEndian have unexported types so we can add the methods and use them directly.

[rsc]

Based on the discussion above, this proposal seems like a likely accept.
— rsc for the proposal review group

[rsc]

No change in consensus, so accepted. 🎉
This issue now tracks the work of implementing the proposal.
— rsc for the proposal review group

[gopherbot]

Change https://go.dev/cl/386017 mentions this issue: binary: add AppendByteOrder

[gopherbot]

Change https://go.dev/cl/389636 mentions this issue: api: update next.txt