This feature advertises support for local socket communication, instead of stdio.

Example:

```json
{
  "name": "LocalSocket"
}
```

When local socket communication is advertised to an engine supporting the feature, the engine will cease stdio communication and launch the plugin again with the `--local-socket` command line argument. The second argument is either a path to a Unix domain socket on Linux, Android, macOS, and other Unix-like operating systems, or the name of a named pipe (without the `\\.\pipe\` prefix) on Windows.

In either case, during startup, the plugin is expected to establish two separate connections to the socket, in this order:

1. The input stream connection, used to send messages from the engine to the plugin
2. The output stream connection, used to send messages from the plugin to the engine

The connections are separate in order to facilitate ownership of the streams by separate threads. After these connections are both established, the engine will remove the socket, and will not accept further connections.

If local socket communication fails to initialize, the engine will abort, stop the plugin, and start it again with the stdio mode, even if the plugin supports local sockets. Whether local socket mode initialized successfully, and therefore the plugin is allowed to use stdio, can be observed when
[`EngineInterface::is_using_stdio()`](https://docs.rs/nu-plugin/latest/nu_plugin/struct.EngineInterface.html#method.is_using_stdio) returns `false` for Rust plugins.

## Input messages

These are messages sent from the engine to the plugin. [`Hello`](#hello) and [`Stream messages`](#stream-messages) are also included.

### `Call`

The body of this message is a 2-tuple (array): (`id`, `call`). The engine sends unique IDs for each plugin call it makes. The ID is needed to send the [`CallResponse`](#callresponse).

#### `Metadata` plugin call

Ask the plugin to send metadata about itself. Takes no arguments. Returns [`Metadata`](#metadata-plugin-call-response) or [`Error`](#error-plugin-call-response)

Example:

```json
{
  "Call": [0, "Metadata"]
}
```

#### `Signature` plugin call

Ask the plugin to send its command signatures. Takes no arguments. Returns [`Signature`](#signature-plugin-call-response) or [`Error`](#error-plugin-call-response)

Example:

```json
{
  "Call": [0, "Signature"]
}
```

#### `Run` plugin call

Tell the plugin to run a command. The argument is the following map:

| Field     | Type                                        | Description                                           |
| --------- | ------------------------------------------- | ----------------------------------------------------- |
| **name**  | string                                      | The name of the command to run                        |
| **call**  | [`EvaluatedCall`](#evaluatedcall)           | Information about the invocation, including arguments |
| **input** | [`PipelineDataHeader`](#pipelinedataheader) | Pipeline input to the command                         |

<a name="evaluatedcall"></a>

`EvaluatedCall` is a map:

| Field          | Type                                              | Description                                             |
| -------------- | ------------------------------------------------- | ------------------------------------------------------- |
| **head**       | [`Span`](#span)                                   | The position of the beginning of the command execution. |
| **positional** | [`Value`](#value) array                           | Positional arguments.                                   |
| **named**      | 2-tuple (string, [`Value`](#value) or null) array | Named arguments, such as switches.                      |

Named arguments are always sent by their long name, never their short name.

Returns [`PipelineData`](#pipelinedata-plugin-call-response) or [`Error`](#error-plugin-call-response).

Example:

```json
{
  "Call": [
    0,
    {
      "Run": {
        "name": "inc",