Test composition fails when a part is rejected

[tmat]

PR #60687 added a new TypeScipt External Access language service, which imports implementation interface that's not implemented in the composition. The test composition should just ignore this service.

System.Composition.Hosting.CompositionFailedException : No export was found for the contract 'IVSTypeScriptCommentSelectionServiceImplementation'
-> required by import 'impl' of part 'VSTypeScriptCommentSelectionService'
-> required by import 'value' of part 'Lazy<ILanguageService, LanguageServiceMetadata>'
-> required by import 'item' of part 'Lazy`2[]'
-> required by initial request for contract 'IEnumerable<Lazy<ILanguageService, LanguageServiceMetadata>> { IsImportMany = True }'

Stack trace

...
Microsoft.CodeAnalysis.Host.CodeStyleHostLanguageServices.MefHostExportProvider.Microsoft.CodeAnalysis.Host.Mef.IMefHostExportProvider.GetExports[TExtension,TMetadata]() in /_/src/CodeStyle/Core/CodeFixes/Host/Mef/CodeStyleHostLanguageServices.cs.cs:line 61
   at Microsoft.CodeAnalysis.Host.Mef.MefLanguageServices..ctor(MefWorkspaceServices workspaceServices, String language) in /_/src/Workspaces/SharedUtilitiesAndExtensions/Workspace/Core/Workspace/Mef/MefLanguageServices.cs:line 37
   at Microsoft.CodeAnalysis.Host.Mef.MefWorkspaceServices.<>c.<GetLanguageServices>b__21_0(String languageName, MefWorkspaceServices self) in /_/src/Workspaces/SharedUtilitiesAndExtensions/Workspace/Core/Workspace/Mef/MefWorkspaceServices.cs:line 163
   at System.Collections.Immutable.ImmutableInterlocked.GetOrAdd[TKey,TValue,TArg](ImmutableDictionary`2& location, TKey key, Func`3 valueFactory, TArg factoryArgument)
   at Microsoft.CodeAnalysis.Host.Mef.MefWorkspaceServices.GetLanguageServices(String languageName) in /_/src/Workspaces/SharedUtilitiesAndExtensions/Workspace/Core/Workspace/Mef/MefWorkspaceServices.cs:line 163
   at Microsoft.CodeAnalysis.Host.CodeStyleHostLanguageServices..ctor(HostLanguageServices hostLanguageServices) in /_/src/CodeStyle/Core/CodeFixes/Host/Mef/CodeStyleHostLanguageServices.MefHostExportProvider.cs:line 27
   at Microsoft.CodeAnalysis.Host.CodeStyleHostLanguageServices.Create(HostLanguageServices hostLanguageServices) in /_/src/CodeStyle/Core/CodeFixes/Host/Mef/CodeStyleHostLanguageServices.MefHostExportProvider.cs:line 38
   at System.Runtime.CompilerServices.ConditionalWeakTable`2.GetValue(TKey key, CreateValueCallback createValueCallback)
   at Microsoft.CodeAnalysis.Host.CodeStyleHostLanguageServices.GetRequiredMappedCodeStyleLanguageServices(HostLanguageServices hostLanguageServices) in /_/src/CodeStyle/Core/CodeFixes/Host/Mef/CodeStyleHostLanguageServices.MefHostExportProvider.cs:line 35
   at Microsoft.CodeAnalysis.Host.HostWorkspaceServicesExtensions.GetExtendedLanguageServices(HostWorkspaceServices hostWorkspaceServices, String languageName) in 
...

[sharwell]

@tmat MEF 2 does not support silent part rejection. Parts defined in the Workspaces or Features layer (where VS MEF might not be used) that need to silently exclude themselves must follow this pattern:
#59047

[sharwell]

Specifically, you want this pattern:

roslyn/src/Features/Core/Portable/ExternalAccess/VSTypeScript/VSTypeScriptDiagnosticAnalyzerLanguageService.cs

Lines 17 to 28 in b38725d

	internal readonly IVSTypeScriptDiagnosticAnalyzerImplementation Implementation;
	// 'implementation' is a required import, but MEF 2 does not support silent part rejection when a required
	// import is missing so we combine AllowDefault with a null check in the constructor to defer the exception
	// until the part is instantiated.
	[ImportingConstructor]
	[Obsolete(MefConstruction.ImportingConstructorMessage, error: true)]
	public VSTypeScriptDiagnosticAnalyzerLanguageService(
	[Import(AllowDefault = true)] IVSTypeScriptDiagnosticAnalyzerImplementation? implementation)
	{
	Implementation = implementation ?? throw new ArgumentNullException(nameof(implementation));
	}

[CyrusNajmabadi]

@sharwell moved to 17.13. Close out if you do not think we will do anything here.