Run F# backend against coverage tool

[StachuDotNet]

either as a one-off, or long-term through CI.

Some options to consider:

• altcover 1 2
• dotnet-coverage
• jetbrains dotCover
• coverlet
• codecov

[StachuDotNet]

Currently working through hooking up AltCover.

At first, it seemed that many .net code coverage frameworks seem to assume that you use dotnet test to collect/run your tests.
That said, I've finally gotten a(n empty) code coverage report from AltCover with a dotnet tool install --global altcover.global and altcover -i 'fsharp-backend/src' -o '.altcover'. Currently working my way through the usage docs to figure out why the report is empty. I'm likely just missing some options there.

The results I'm seeing are

Instrumenting files from /home/dark/app/fsharp-backend/src/
Writing files to /home/dark/app/.altcover/

Coverage Report: /home/dark/app/coverage.xml

with coverage.xml being rather dull:

<?xml version="1.0" encoding="utf-8" standalone="yes"?>
<CoverageSession xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <Summary numSequencePoints="0" visitedSequencePoints="0" numBranchPoints="0" visitedBranchPoints="0" sequenceCoverage="0" branchCoverage="0" maxCyclomaticComplexity="0" minCyclomaticComplexity="1" visitedClasses="0" numClasses="0" visitedMethods="0" numMethods="0" minCrapScore="0" maxCrapScore="0" />
  <Modules />
</CoverageSession>

This also copies the fsharp-backend/src folder to .altcover but seems to do no actual 'instrumentation', and the process feels instant.

Going to spend a bit more time with altcover, otherwise may explore other tools.
It may end up being worth allowing dotnet test - there are some options to pursue that path.

[StachuDotNet]

A very quick attempt with Coverlet yields something:

dark@dark-dev:~/app$ coverlet ./fsharp-backend/Build/out/Tests/Debug/net6.0/linux-x64/Tests.dll --target 'dotnet'
Usage: dotnet [options]
Usage: dotnet [path-to-application]
Options:
  -h|--help         Display help.
  --info            Display .NET information.
  --list-sdks       Display the installed SDKs.
  --list-runtimes   Display the installed runtimes.
path-to-application:
  The path to an application .dll file to execute.

Calculating coverage result...
  Generating report '/home/dark/app/coverage.json'
+------------------------+------+--------+--------+
| Module                 | Line | Branch | Method |
+------------------------+------+--------+--------+
| LibService             | 0%   | 0%     | 0%     |
+------------------------+------+--------+--------+
| BwdServer              | 0%   | 0%     | 0%     |
+------------------------+------+--------+--------+
| BackendOnlyStdLib      | 0%   | 0%     | 0%     |
+------------------------+------+--------+--------+
| TestUtils              | 0%   | 0%     | 0%     |
+------------------------+------+--------+--------+
| HttpMiddleware         | 0%   | 0%     | 0%     |
+------------------------+------+--------+--------+
| ApiServer              | 0%   | 0%     | 0%     |
+------------------------+------+--------+--------+
| LibExecution           | 0%   | 0%     | 0%     |
+------------------------+------+--------+--------+
| FuzzTests              | 0%   | 0%     | 0%     |
+------------------------+------+--------+--------+
| LibBinarySerialization | 0%   | 0%     | 0%     |
+------------------------+------+--------+--------+
| QueueWorker            | 0%   | 0%     | 0%     |
+------------------------+------+--------+--------+
| LibExecutionStdLib     | 0%   | 0%     | 0%     |
+------------------------+------+--------+--------+
| Tablecloth             | 0%   | 0%     | 0%     |
+------------------------+------+--------+--------+
| LibRealExecution       | 0%   | 0%     | 0%     |
+------------------------+------+--------+--------+
| LibBackend             | 0%   | 0%     | 0%     |
+------------------------+------+--------+--------+
| Prelude                | 0%   | 0%     | 0%     |
+------------------------+------+--------+--------+

+---------+------+--------+--------+
|         | Line | Branch | Method |
+---------+------+--------+--------+
| Total   | 0%   | 0%     | 0%     |
+---------+------+--------+--------+
| Average | 0%   | 0%     | 0%     |
+---------+------+--------+--------+

[StachuDotNet]

haf/expecto#219 seemingly-useful thread to read after lunch

Ooh looks promising: haf/expecto#219 (comment)

[StachuDotNet]

I'm abandoning my old PR at #3756. Mostly, it became less of a priority since we migrated to the F# backend (though it's still important) and I'd like to reconsider if AltCov is ideal for us - after some work with it, reaching for something more "mainstream" may be good. Regardless, there are notes in that PR description worth reviewing when this issue is revisited.

[StachuDotNet]

Folded into #5246