Request for Clarification: What is the behavior of relative paths for the purposes of a #line directive?

[otac0n]

Version Used:
All Versions (and all VS versions)

Steps to Reproduce:

1. Create a Source Generator that emits relative paths using a #line directive.
2. Enable outputting generated files (set EmitCompilerGeneratedFiles to true).
3. Using Source Generators as the primary build and also use the generated files as a fall-back.

Diagnostic Id:

n/a

Expected Behavior:

Relative paths should be supported and well defined to avoid leaking full path information to SourceLink. Full paths are generally unhelpful and just leak private information.

Actual Behavior:
The documentation here provides no guidance on relative paths:
https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/preprocessor-directives#error-and-warning-information

It's unclear how to get VS and CSC to agree on what relative paths mean. They currently don't agree depending on whether you are running from command line or from VS. Double-click to navigate to error behavior seems to be inconsistent between versions of Visual Studio.

[otac0n]

Currently, in my project Weave I am using:

    <CompilerVisibleProperty Include="CompilerGeneratedFilesOutputPath" />

...

    private const string GeneratedOutputPath = "build_property.compilergeneratedfilesoutputpath";

...

    var outputPath = context.AnalyzerConfigOptionsProvider
        .Select((p, ct) =>
        {
            p.GlobalOptions.TryGetValue(ProjectDir, out var root);
            p.GlobalOptions.TryGetValue(GeneratedOutputPath, out var generated);
            var assembly = Assembly.GetExecutingAssembly().FullName;
            var generator = typeof(GenerateWeaveSources).FullName;
            return Path.GetFullPath(Path.Combine(root, generated, assembly, generator));
        });

...

    public static string MakeRelative(string root, string path)
    {
        root = Path.GetFullPath(root);
        var fullFilePath = Path.GetFullPath(Path.Combine(root, path));
        var relativeUri = new Uri(root).MakeRelativeUri(new Uri(fullFilePath));
        var relativePath = Uri.UnescapeDataString(relativeUri.ToString()).Replace(Path.AltDirectorySeparatorChar, Path.DirectorySeparatorChar);
        return relativePath;
    }

....

    var relativePath = PathUtils.MakeRelative(outputPath, template.path);

[jaredpar]

Using a relative file path in a #line directive in a generated file has undefined behavior. Normally relative paths are interpreted relative to the path of the current file but given that generated files do not have a defined path this operation has undefined behavior.