Code coverage stops working if assembly contains interfaces from source generators

_Originally posted as https://github.com/reactiveui/refit/issues/1073_

Describe the bug

In a project using Refit 6.0.8 and coverlet.msbuild 3.0.2, upon updating from Refit 5.2.4, code coverage is no longer being collected for the assembly containing Refit interfaces.

Using coverlet’s troubleshooting guide here, I narrowed my investigation down to this line of interest:

[coverlet] Unable to instrument module: C:\Coding\Projects\MyProject\tests\MyProject.Tests\bin\debug\net5.0\MyProject.dll, pdb without local source files, [C:\Coding\Projects\MyProject\src\MyProject\InterfaceStubGenerator.Core\Refit.Generator.InterfaceStubGenerator\IMyApi.g.cs]

As there’s no file on disk here, that sort of makes sense, but causes an issue as it breaks all code coverage for the project for the sake of the one interface.

I’m guessing this is something to do with the usage of source generators in Refit.

The failure then slipped through several CI jobs for Refit updates until it was noticed due to https://github.com/coverlet-coverage/coverlet/issues/1083.

Steps To Reproduce

Clone https://github.com/martincostello/alexa-london-travel and run ./build.ps1.

The build script will eventually output something like this:

+--------+------+--------+--------+
| Module | Line | Branch | Method |
+--------+------+--------+--------+

+---------+------+--------+--------+
|         | Line | Branch | Method |
+---------+------+--------+--------+
| Total   | 100% | 100%   | 100%   |
+---------+------+--------+--------+
| Average | NaN% | NaN%   | NaN%   |
+---------+------+--------+--------+

Example build: https://github.com/martincostello/alexa-london-travel/runs/1869767942?check_suite_focus=true#step:4:78

This seems to be affecting multiple repos I have using Refit, not just this one.

Here’s another example: https://github.com/martincostello/alexa-london-travel-site/runs/1870035564?check_suite_focus=true#step:4:160

Expected behavior

Assemblies containing Refit interfaces are instrumented correctly.

For the referenced repository, it should output something like this from build.ps1:

+--------------------+-------+--------+--------+
| Module             | Line  | Branch | Method |
+--------------------+-------+--------+--------+
| LondonTravel.Skill | 99.1% | 94.82% | 97.9%  |
+--------------------+-------+--------+--------+

+---------+-------+--------+--------+
|         | Line  | Branch | Method |
+---------+-------+--------+--------+
| Total   | 99.1% | 94.82% | 97.9%  |
+---------+-------+--------+--------+
| Average | 99.1% | 94.82% | 97.9%  |
+---------+-------+--------+--------+

Example build: https://github.com/martincostello/alexa-london-travel/runs/1869465349?check_suite_focus=true#step:4:80

[martincostello]

Thanks Marco ❤️

[MarcoRossignoli]

Will this also fix the coverlet.console? @MarcoRossignoli

Yep the engine in shared between “drivers”