Diagnostic suppressions from editorconfig are ignored in source-generated syntax trees

[jcouv]

Repro:

1. make a change to C# compiler's syntax.xml to add some new public APIs
2. run eng\generate-compiler-code.cmd followed by build.cmd -runAnalyzers: notice RS0016 errors (public APIs must be declared)
3. modify top-level .editorconfig file to suppress RS0016 (see diff below)
4. repeat step 2: notice that RS0016 errors are still there

I took a quick stab at debugging. A few observations:

• CSharpDiagnosticFilter.GetDiagnosticReport has a parameter SyntaxTreeOptionsProvider syntaxTreeOptions which maps syntax trees with diagnostic options from editorconfig, but the source-generated syntax trees are missing from the map.
• CommonCompiler.RunCore prepares that map (local sourceFileAnalyzerConfigOptions) before we run the generators in CompileAndEmit but that map doesn't get updated to account for source-generated syntax trees

We encountered this problem here: #51299 (review)

diff --git a/.editorconfig b/.editorconfig
index 6d966e98ae2..bbff4905b81 100644
--- a/.editorconfig
+++ b/.editorconfig
@@ -204,6 +204,9 @@ csharp_style_inlined_variable_declaration = true:suggestion
 csharp_style_throw_expression = true:suggestion
 csharp_style_conditional_delegate_call = true:suggestion

+# RS0016: Add public types and members to the declared API
+dotnet_diagnostic.RS0016.severity = none
+
 # Space preferences
 csharp_space_after_cast = false
 csharp_space_after_colon_in_inheritance_clause = true

[chsienki]

@jcouv This is by-design.

.editorconfig files don't apply to generated syntax trees, beause they don't have a path on disk (they might if you specify generatedFilesOut, but its not a given).

Instead you need to use a .globalconfig to apply rules to them. We already have a global config in the repo here:https://github.com/dotnet/roslyn/blob/831dffa401f3d0ac4addbbebe14b94e4d20df14e/global.editorconfig

[jcouv]

It's a bit surprising since the generated syntax trees do have a path (even though they are not materialized on disk).
But if that's expected, then let's close. Thanks!