C# LSP can't provide diagnostics for Razor files in .NET 6 Preview4+

[NTaylorMullen]

Version Used: VSMain

Steps to Reproduce:

1. Download latest .NET 6 Preview4 SDK
2. Use VSMain latest
3. Create a new Blazor Server App .NET 6
4. Open FetchData.razor
5. Make a C# error and wait (none will appear):

Expected Behavior:
You get an error, and one that can be resolved after fixing it.

Actual Behavior:

And any new errors/attempts to fix errors get ignored.

Notes

@jasonmalinowski @CyrusNajmabadi and I investigated the issue and here's what we found:

1. C# LSP creates request handlers which operate on various flavors of the C# solution. For EnC there was recently a change to migrate diagnostic understanding to the source generated enabled solution. These changes weren't reflected on the LSP front when creating RequestContext's / handlers.
2. SourceGenerators generate content in relative file paths whereas LSP speaks absolute FilePaths. Meaning, even if the above was fixed looking up an LSP Uri/filepath wouldn't actually locate a SourceGenerated file due to the path mismatch.
3. (This is longer leaded) We can fix the first two issues via code changes in Roslyn but the third is a fundamental design issue related to the Razor compilers dual generated output modes.

Investigatory Razor issue: https://github.com/dotnet/aspnetcore/issues/32188

[tmat]

Re [2]: suggestions that would address this were made in #51998. A suggestion in a comment on this issue would also imo result in more consistent compiler/msbuild interface regarding the directory to place generated files under

[NTaylorMullen]

Fixed by #53027