Remove semantic model keep-alive code in completion (#73844)

* Remove semantic model keep-alive code in completion

There are quite a few providers that don't request/use the semantic model during their computations, and in the case where they are being used to commit / GetDescription, the model would be obtained/calculated unnecessarilly.

I don't believe the keep alive mechanism here is necessary anymore, as Solution.OnSemanticModelObtained caches the model when it's obtained for the active document in the solution.

Additionally, I switched a couple places in CommitManager to use JTF.Runn instead of WaitAndGetResult

src/EditorFeatures/Core/IntelliSense/AsyncCompletion/CommitManager.cs

@@ -212,7 +212,7 @@ public IEnumerable<char> PotentialCommitCharacters
 
         // This might be an item promoted by us, make sure we restore it to the original state first.
         roslynItem = Helpers.DemoteItem(roslynItem);
-        CompletionChange change;
+        CompletionChange change = null!;
 
         // We met an issue when external code threw an OperationCanceledException and the cancellationToken is not canceled.
         // Catching this scenario for further investigations.
@@ -229,7 +229,10 @@ public IEnumerable<char> PotentialCommitCharacters
             if (_textView is IDebuggerTextView)
                 roslynItem = ImportCompletionItem.MarkItemToAlwaysFullyQualify(roslynItem);
 
-            change = completionService.GetChangeAsync(document, roslynItem, commitCharacter, cancellationToken).WaitAndGetResult(cancellationToken);
+            _threadingContext.JoinableTaskFactory.Run(async () =>
+            {
+                change = await completionService.GetChangeAsync(document, roslynItem, commitCharacter, cancellationToken).ConfigureAwait(true);
+            });
         }
         catch (OperationCanceledException e) when (e.CancellationToken != cancellationToken && FatalError.ReportAndCatch(e))
         {
@@ -319,7 +322,11 @@ public IEnumerable<char> PotentialCommitCharacters
                 var spanToFormat = triggerSnapshotSpan.TranslateTo(subjectBuffer.CurrentSnapshot, SpanTrackingMode.EdgeInclusive);
 
                 // Note: C# always completes synchronously, TypeScript is async
-                var changes = formattingService.GetFormattingChangesAsync(currentDocument, subjectBuffer, spanToFormat.Span.ToTextSpan(), cancellationToken).WaitAndGetResult(cancellationToken);
+                var changes = ImmutableArray<TextChange>.Empty;
+                _threadingContext.JoinableTaskFactory.Run(async () =>
+                {
+                    changes = await formattingService.GetFormattingChangesAsync(currentDocument, subjectBuffer, spanToFormat.Span.ToTextSpan(), cancellationToken).ConfigureAwait(true);
+                });
                 subjectBuffer.ApplyChanges(changes);
             }
         }

src/Features/Core/Portable/Completion/CompletionService.cs

@@ -214,14 +214,14 @@ public virtual TextSpan GetDefaultCompletionListSpan(SourceText text, int caretP
 
         var extensionManager = document.Project.Solution.Workspace.Services.GetRequiredService<IExtensionManager>();
 
-        // We don't need SemanticModel here, just want to make sure it won't get GC'd before CompletionProviders are able to get it.
-        (document, var semanticModel) = await GetDocumentWithFrozenPartialSemanticsAsync(document, cancellationToken).ConfigureAwait(false);
+        document = await GetDocumentWithFrozenPartialSemanticsAsync(document, cancellationToken).ConfigureAwait(false);
+
         var description = await extensionManager.PerformFunctionAsync(
             provider,
             cancellationToken => provider.GetDescriptionAsync(document, item, options, displayOptions, cancellationToken),
             defaultValue: null,
             cancellationToken).ConfigureAwait(false);
-        GC.KeepAlive(semanticModel);
+
         return description;
     }
 
@@ -245,8 +245,7 @@ public virtual TextSpan GetDefaultCompletionListSpan(SourceText text, int caretP
         {
             var extensionManager = document.Project.Solution.Workspace.Services.GetRequiredService<IExtensionManager>();
 
-            // We don't need SemanticModel here, just want to make sure it won't get GC'd before CompletionProviders are able to get it.
-            (document, var semanticModel) = await GetDocumentWithFrozenPartialSemanticsAsync(document, cancellationToken).ConfigureAwait(false);
+            document = await GetDocumentWithFrozenPartialSemanticsAsync(document, cancellationToken).ConfigureAwait(false);
 
             var change = await extensionManager.PerformFunctionAsync(
                 provider,
@@ -256,7 +255,6 @@ public virtual TextSpan GetDefaultCompletionListSpan(SourceText text, int caretP
             if (change == null)
                 return CompletionChange.Create(new TextChange(new TextSpan(), ""));
 
-            GC.KeepAlive(semanticModel);
             Debug.Assert(item.Span == change.TextChange.Span || item.IsComplexTextEdit);
             return change;
         }

src/Features/Core/Portable/Completion/CompletionService_GetCompletions.cs

@@ -68,8 +68,7 @@ public abstract partial class CompletionService
          ImmutableHashSet<string>? roles = null,
          CancellationToken cancellationToken = default)
     {
-        // We don't need SemanticModel here, just want to make sure it won't get GC'd before CompletionProviders are able to get it.
-        (document, var semanticModel) = await GetDocumentWithFrozenPartialSemanticsAsync(document, cancellationToken).ConfigureAwait(false);
+        document = await GetDocumentWithFrozenPartialSemanticsAsync(document, cancellationToken).ConfigureAwait(false);
 
         var text = await document.GetValueTextAsync(cancellationToken).ConfigureAwait(false);
         var completionListSpan = GetDefaultCompletionListSpan(text, caretPosition);
@@ -115,8 +114,6 @@ public abstract partial class CompletionService
         var augmentingContexts = await ComputeNonEmptyCompletionContextsAsync(
             document, caretPosition, trigger, options, completionListSpan, augmentingProviders, sharedContext, cancellationToken).ConfigureAwait(false);
 
-        GC.KeepAlive(semanticModel);
-
         // Providers are ordered, but we processed them in our own order.  Ensure that the
         // groups are properly ordered based on the original providers.
         var completionProviderToIndex = GetCompletionProviderToIndex(providers);
@@ -172,19 +169,20 @@ public abstract partial class CompletionService
     }
 
     /// <summary>
-    /// Returns a document with frozen partial semantic unless we already have a complete compilation available.
+    /// Returns a document with frozen partial semantic model unless caller is test code require full semantics.
     /// Getting full semantic could be costly in certain scenarios and would cause significant delay in completion. 
     /// In most cases we'd still end up with complete document, but we'd consider it an acceptable trade-off even when 
     /// we get into this transient state.
     /// </summary>
-    private async Task<(Document document, SemanticModel? semanticModel)> GetDocumentWithFrozenPartialSemanticsAsync(Document document, CancellationToken cancellationToken)
+    private async Task<Document> GetDocumentWithFrozenPartialSemanticsAsync(Document document, CancellationToken cancellationToken)
     {
         if (_suppressPartialSemantics)
         {
-            return (document, await document.GetRequiredSemanticModelAsync(cancellationToken).ConfigureAwait(false));
+            var _ = await document.GetRequiredSemanticModelAsync(cancellationToken).ConfigureAwait(false);
+            return document;
         }
 
-        return await document.GetFullOrPartialSemanticModelAsync(cancellationToken).ConfigureAwait(false);
+        return document.WithFrozenPartialSemantics(cancellationToken);
     }
 
     private static bool ValidatePossibleTriggerCharacterSet(CompletionTriggerKind completionTriggerKind, IEnumerable<CompletionProvider> triggeredProviders,