How to test FixAllProvider?

[Denis535]

The Roslyn can batch fix results into one result. But I can't figure out how it works and how to test it.

1. Testing the project and getting diagnostics (document-level and project-level).

            var diagnostics = await RoslynTestingUtils.AnalyzeAsync( Project, Analyzers, default ).ConfigureAwait( false );
            //diagnostics = diagnostics.Distinct().Where( i => i.Location != Location.None ).ToArray();

2. Testing the FixAllProvider.

            var document = Project.GetDocument( diagnostics.First().Location.SourceTree ) ?? throw new Exception( "Document is not found" );
            var fixer = new ExampleCodeFixProvider();
            var fixAllProvider = fixer.GetFixAllProvider();

            var context = new FixAllContext( document, fixer, FixAllScope.Solution, fixer.FixableDiagnosticIds.Join(), fixer.FixableDiagnosticIds, new DiagnosticProvider( diagnostics ), default );
            var action = await fixAllProvider.GetFixAsync( context ).ConfigureAwait( false ) ?? throw new Exception( "FixAction is null" );
            var operations = await action.GetOperationsAsync( default ).ConfigureAwait( false );
            var operation = operations.Cast<ApplyChangesOperation>().Single();
            var newProject = operation.ChangedSolution.Projects.First();

3. The DiagnosticProvider.

    internal sealed class DiagnosticProvider : FixAllContext.DiagnosticProvider {

        private Diagnostic[] Diagnostics { get; }

        public DiagnosticProvider(Diagnostic[] diagnostics) {
            Diagnostics = diagnostics;
        }


        public override Task<IEnumerable<Diagnostic>> GetAllDiagnosticsAsync(Project project, CancellationToken cancellationToken) {
            return Task.FromResult( Diagnostics.AsEnumerable() );
        }

        public override Task<IEnumerable<Diagnostic>> GetProjectDiagnosticsAsync(Project project, CancellationToken cancellationToken) {
            // todo: is it ok that project is unused?
            return Task.FromResult( Diagnostics.Where( i => !i.Location.IsInSource ) );
        }

        public override Task<IEnumerable<Diagnostic>> GetDocumentDiagnosticsAsync(Document document, CancellationToken cancellationToken) {
            return Task.FromResult( Diagnostics.Where( i => i.Location.GetLineSpan().Path == document.Name ) );
        }

    }

There are a couple of things that I don't understand.

1. What document I should pass if all diagnostics are project-level or pointing on different documents?
   Or should I pass only diagnostics pointing on the same document?
   Or even should I pass only diagnostics with the same Id?

new FixAllContext( document, fixer, FixAllScope.Solution, fixer.FixableDiagnosticIds.Join(), fixer.FixableDiagnosticIds, new DiagnosticProvider( diagnostics ), default );

2. Why the FixAllProvider.GetFixAsync(...) returns null?