Define and implement a stable JSON schema for output.

[plemarquand]

Description

Experimental event streaming support was added in #286. Issues are reported as individual events with richly structured JSON that describe the failure. This is great for tools that need to work with and transform raw issues, but many tools will just want to use the human readable failure string, i.e: Expectation failed: 1 == 2.

Swift-testing has done a lot of work to build up a nice message, and it would be nice to avoid reimplementing this in other tools (and keeping it up to date).

Feature: Add a new message field to the issueRecorded event that provides the human readable failure string. Bonus points if there is a markdown version of the string, as many tools can render markdown directly.

Expected behavior

No response

Actual behavior

No response

Steps to reproduce

No response

swift-testing version/commit hash

No response

Swift & OS version (output of swift --version && uname -a)

No response

[plemarquand]

As an example @grynspan suggested something like:

 {
  "eventKind": "issueRecorded",
  "message": {
    "symbol": "failure",
    "string": "Issue exploded! Oh no!",
    "markdown": "Issue **exploded**! [Oh no!](ohno.gif)"
  },
  "sourceLocation": { fileID: "Module/File.swift", line: 123, column: 456 },
  "instant": { "seconds": 12345, "attoseconds": 123450000000000 },
  ...
}

[grynspan]

As of right now, the experimental event stream just encodes the fields of the events themselves. In order to make a stable API/ABI, we likely need to change that to a formal JSON schema with dedicated and intentional content, not just "whatever JSONEncoder produced."

[grynspan]

Tracked internally as rdar://126858814.