Perform code-lens 'find' operations serially.

src/EditorFeatures/Test2/FindReferences/FindReferencesTests.vb

@@ -51,10 +51,11 @@ Namespace Microsoft.CodeAnalysis.Editor.UnitTests.FindReferences
             Await TestStreamingFeature(element, searchSingleFileOnly, uiVisibleOnly, host)
         End Function
 
-        Private Shared Async Function TestStreamingFeature(element As XElement,
-                                                    searchSingleFileOnly As Boolean,
-                                                    uiVisibleOnly As Boolean,
-                                                    host As TestHost) As Task
+        Private Shared Async Function TestStreamingFeature(
+                element As XElement,
+                searchSingleFileOnly As Boolean,
+                uiVisibleOnly As Boolean,
+                host As TestHost) As Task
             ' We don't support testing features that only expect partial results.
             If searchSingleFileOnly OrElse uiVisibleOnly Then
                 Return
@@ -254,7 +255,19 @@ Namespace Microsoft.CodeAnalysis.Editor.UnitTests.FindReferences
                 Optional uiVisibleOnly As Boolean = False,
                 Optional options As FindReferencesSearchOptions = Nothing) As Task
 
+            Await TestAPI(definition, host, parallel:=False, searchSingleFileOnly, uiVisibleOnly, options)
+            Await TestAPI(definition, host, parallel:=True, searchSingleFileOnly, uiVisibleOnly, options)
+        End Function
+
+        Private Async Function TestAPI(
+                definition As XElement,
+                host As TestHost,
+                parallel As Boolean,
+                searchSingleFileOnly As Boolean,
+                uiVisibleOnly As Boolean,
+                options As FindReferencesSearchOptions) As Task
             options = If(options, FindReferencesSearchOptions.Default)
+            options = options.With(parallel:=parallel)
             Using workspace = TestWorkspace.Create(definition, composition:=s_composition.WithTestHostParts(host))
                 workspace.SetTestLogger(AddressOf _outputHelper.WriteLine)
 

src/Features/CSharp/Portable/ImplementInterface/CSharpImplementExplicitlyCodeRefactoringProvider.cs

@@ -53,7 +53,7 @@ protected override async Task UpdateReferencesAsync(
             //
             // This can save a lot of extra time spent finding callers, especially for methods with
             // high fan-out (like IDisposable.Dispose()).
-            var findRefsOptions = FindReferencesSearchOptions.Default.WithCascade(false);
+            var findRefsOptions = FindReferencesSearchOptions.Default.With(cascade: false);
             var references = await SymbolFinder.FindReferencesAsync(
                 implMember, solution, findRefsOptions, cancellationToken).ConfigureAwait(false);
 

src/Features/Core/Portable/CodeLens/CodeLensReferencesService.cs

@@ -26,6 +26,11 @@ internal sealed class CodeLensReferencesService : ICodeLensReferencesService
                 typeQualificationStyle: SymbolDisplayTypeQualificationStyle.NameAndContainingTypesAndNamespaces,
                 memberOptions: SymbolDisplayMemberOptions.IncludeContainingType);
 
+        // Do not perform the finding operation in parallel.  We're running ephemerally in the BG and do not want to
+        // saturate the system with work that then slows the user down.
+        private static readonly FindReferencesSearchOptions s_nonParallelSearch =
+            FindReferencesSearchOptions.Default.With(parallel: false);
+
         private static async Task<T?> FindAsync<T>(Solution solution, DocumentId documentId, SyntaxNode syntaxNode,
             Func<CodeLensFindReferencesProgress, Task<T>> onResults, Func<CodeLensFindReferencesProgress, Task<T>> onCapped,
             int searchCap, CancellationToken cancellationToken) where T : struct
@@ -49,8 +54,8 @@ internal sealed class CodeLensReferencesService : ICodeLensReferencesService
             using var progress = new CodeLensFindReferencesProgress(symbol, syntaxNode, searchCap, cancellationToken);
             try
             {
-                await SymbolFinder.FindReferencesAsync(symbol, solution, progress, null,
-                    progress.CancellationToken).ConfigureAwait(false);
+                await SymbolFinder.FindReferencesAsync(
+                    symbol, solution, progress, documents: null, s_nonParallelSearch, progress.CancellationToken).ConfigureAwait(false);
 
                 return await onResults(progress).ConfigureAwait(false);
             }

src/Features/Core/Portable/InlineTemporary/AbstractInlineTemporaryCodeRefactoringProvider.cs

@@ -30,7 +30,7 @@ protected static async Task<ImmutableArray<ReferenceLocation>> GetReferenceLocat
                 // Do not cascade when finding references to this local.  Cascading can cause us to find linked
                 // references as well which can throw things off for us.  For inline variable, we only care about the
                 // direct real references in this project context.
-                var options = FindReferencesSearchOptions.Default.WithCascade(cascade: false);
+                var options = FindReferencesSearchOptions.Default.With(cascade: false);
 
                 var findReferencesResult = await SymbolFinder.FindReferencesAsync(
                     local, document.Project.Solution, options, cancellationToken).ConfigureAwait(false);

src/Workspaces/Core/Portable/FindSymbols/FindReferences/FindReferencesSearchEngine.cs

@@ -29,6 +29,12 @@ internal partial class FindReferencesSearchEngine
         private readonly CancellationToken _cancellationToken;
         private readonly FindReferencesSearchOptions _options;
 
+        /// <summary>
+        /// Scheduler to run our tasks on.  If we're in <see cref="FindReferencesSearchOptions.Parallel"/> mode, we'll
+        /// run all our tasks concurrently.  Otherwise, we will run them serially.
+        /// </summary>
+        private readonly TaskScheduler _scheduler;
+
         public FindReferencesSearchEngine(
             Solution solution,
             IImmutableSet<Document>? documents,
@@ -45,6 +51,9 @@ public FindReferencesSearchEngine(
             _options = options;
 
             _progressTracker = progress.ProgressTracker;
+
+            var pair = new ConcurrentExclusiveSchedulerPair();
+            _scheduler = _options.Parallel ? pair.ConcurrentScheduler : pair.ExclusiveScheduler;
         }
 
         public async Task FindReferencesAsync(ISymbol symbol)
@@ -87,7 +96,7 @@ private async Task ProcessAsync(ProjectToDocumentMap projectToDocumentMap)
                 using var _ = ArrayBuilder<Task>.GetInstance(out var tasks);
 
                 foreach (var (project, documentMap) in projectToDocumentMap)
-                    tasks.Add(Task.Run(() => ProcessProjectAsync(project, documentMap), _cancellationToken));
+                    tasks.Add(Task.Factory.StartNew(() => ProcessProjectAsync(project, documentMap), _cancellationToken, TaskCreationOptions.None, _scheduler).Unwrap());
 
                 await Task.WhenAll(tasks).ConfigureAwait(false);
             }

src/Workspaces/Core/Portable/FindSymbols/FindReferences/FindReferencesSearchEngine_MapCreation.cs

@@ -34,8 +34,8 @@ private async Task<ProjectToDocumentMap> CreateProjectToDocumentMapAsync(Project
                 {
                     foreach (var (symbol, finder) in projectQueue)
                     {
-                        tasks.Add(Task.Run(() =>
-                            DetermineDocumentsToSearchAsync(project, symbol, finder), _cancellationToken));
+                        tasks.Add(Task.Factory.StartNew(() =>
+                            DetermineDocumentsToSearchAsync(project, symbol, finder), _cancellationToken, TaskCreationOptions.None, _scheduler).Unwrap());
                     }
                 }
 
@@ -139,7 +139,7 @@ private async Task DetermineAllSymbolsCoreAsync(
                 using var _ = ArrayBuilder<Task>.GetInstance(out var finderTasks);
                 foreach (var f in _finders)
                 {
-                    finderTasks.Add(Task.Run(async () =>
+                    finderTasks.Add(Task.Factory.StartNew(async () =>
                     {
                         using var _ = ArrayBuilder<Task>.GetInstance(out var symbolTasks);
 
@@ -159,7 +159,7 @@ private async Task DetermineAllSymbolsCoreAsync(
                         _cancellationToken.ThrowIfCancellationRequested();
 
                         await Task.WhenAll(symbolTasks).ConfigureAwait(false);
-                    }, _cancellationToken));
+                    }, _cancellationToken, TaskCreationOptions.None, _scheduler).Unwrap());
                 }
 
                 await Task.WhenAll(finderTasks).ConfigureAwait(false);
@@ -177,7 +177,8 @@ private void AddSymbolTasks(
                 {
                     Contract.ThrowIfNull(child);
                     _cancellationToken.ThrowIfCancellationRequested();
-                    symbolTasks.Add(Task.Run(() => DetermineAllSymbolsCoreAsync(child, result), _cancellationToken));
+                    symbolTasks.Add(Task.Factory.StartNew(
+                        () => DetermineAllSymbolsCoreAsync(child, result), _cancellationToken, TaskCreationOptions.None, _scheduler).Unwrap());
                 }
             }
         }

src/Workspaces/Core/Portable/FindSymbols/FindReferences/FindReferencesSearchEngine_ProjectProcessing.cs

@@ -30,7 +30,7 @@ private async Task ProcessProjectAsync(
                     foreach (var (document, documentQueue) in documentMap)
                     {
                         if (document.Project == project)
-                            documentTasks.Add(Task.Run(() => ProcessDocumentQueueAsync(document, documentQueue), _cancellationToken));
+                            documentTasks.Add(Task.Factory.StartNew(() => ProcessDocumentQueueAsync(document, documentQueue), _cancellationToken, TaskCreationOptions.None, _scheduler).Unwrap());
                     }
 
                     await Task.WhenAll(documentTasks).ConfigureAwait(false);

src/Workspaces/Core/Portable/FindSymbols/FindReferences/FindReferencesSearchOptions.cs

@@ -2,8 +2,6 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
-#nullable disable
-
 using System.Runtime.Serialization;
 using Microsoft.CodeAnalysis.Shared.Extensions;
 
@@ -15,7 +13,8 @@ internal sealed class FindReferencesSearchOptions
         public static readonly FindReferencesSearchOptions Default =
             new(
                 associatePropertyReferencesWithSpecificAccessor: false,
-                cascade: true);
+                cascade: true,
+                parallel: true);
 
         /// <summary>
         /// When searching for property, associate specific references we find to the relevant
@@ -39,26 +38,52 @@ internal sealed class FindReferencesSearchOptions
         [DataMember(Order = 1)]
         public bool Cascade { get; }
 
+        /// <summary>
+        /// Whether or not we should perform the find operation in parallel.  This can produce results more quickly, but
+        /// at the cost of more system resources.
+        /// </summary>
+        /// <remarks>
+        /// Features that run automatically should consider setting this to <see langword="false"/> to avoid
+        /// unnecessarily impacting the user while they are doing other work.
+        /// </remarks>
+        [DataMember(Order = 2)]
+        public bool Parallel { get; }
+
         public FindReferencesSearchOptions(
             bool associatePropertyReferencesWithSpecificAccessor,
-            bool cascade)
+            bool cascade,
+            bool parallel)
         {
             AssociatePropertyReferencesWithSpecificAccessor = associatePropertyReferencesWithSpecificAccessor;
             Cascade = cascade;
+            Parallel = parallel;
         }
 
-        public FindReferencesSearchOptions WithAssociatePropertyReferencesWithSpecificAccessor(bool associatePropertyReferencesWithSpecificAccessor)
-            => new(associatePropertyReferencesWithSpecificAccessor, Cascade);
+        public FindReferencesSearchOptions With(
+            Optional<bool> associatePropertyReferencesWithSpecificAccessor = default,
+            Optional<bool> cascade = default,
+            Optional<bool> parallel = default)
+        {
+            var newAssociatePropertyReferencesWithSpecificAccessor = associatePropertyReferencesWithSpecificAccessor.HasValue ? associatePropertyReferencesWithSpecificAccessor.Value : AssociatePropertyReferencesWithSpecificAccessor;
+            var newCascade = cascade.HasValue ? cascade.Value : Cascade;
+            var newParallel = parallel.HasValue ? parallel.Value : Parallel;
+
+            if (newAssociatePropertyReferencesWithSpecificAccessor == AssociatePropertyReferencesWithSpecificAccessor &&
+                newCascade == Cascade &&
+                newParallel == Parallel)
+            {
+                return this;
+            }
 
-        public FindReferencesSearchOptions WithCascade(bool cascade)
-            => new(AssociatePropertyReferencesWithSpecificAccessor, cascade);
+            return new FindReferencesSearchOptions(newAssociatePropertyReferencesWithSpecificAccessor, newCascade, newParallel);
+        }
 
         /// <summary>
         /// For IDE features, if the user starts searching on an accessor, then we want to give
         /// results associated with the specific accessor.  Otherwise, if they search on a property,
         /// then associate everything with the property.
         /// </summary>
         public static FindReferencesSearchOptions GetFeatureOptionsForStartingSymbol(ISymbol symbol)
-            => Default.WithAssociatePropertyReferencesWithSpecificAccessor(symbol.IsPropertyAccessor());
+            => Default.With(associatePropertyReferencesWithSpecificAccessor: symbol.IsPropertyAccessor());
     }
 }

src/Workspaces/Core/Portable/FindSymbols/FindReferences/Finders/PropertyAccessorSymbolReferenceFinder.cs

@@ -56,7 +56,7 @@ protected override async Task<ImmutableArray<Document>> DetermineDocumentsToSear
                 // defer to the Property finder to find these docs and combine them with the result.
                 var propertyDocuments = await ReferenceFinders.Property.DetermineDocumentsToSearchAsync(
                     property, project, documents,
-                    options.WithAssociatePropertyReferencesWithSpecificAccessor(false),
+                    options.With(associatePropertyReferencesWithSpecificAccessor: false),
                     cancellationToken).ConfigureAwait(false);
 
                 result = result.AddRange(propertyDocuments);
@@ -77,7 +77,7 @@ protected override async ValueTask<ImmutableArray<FinderLocation>> FindReference
             {
                 var propertyReferences = await ReferenceFinders.Property.FindReferencesInDocumentAsync(
                     property, document, semanticModel,
-                    options.WithAssociatePropertyReferencesWithSpecificAccessor(false),
+                    options.With(associatePropertyReferencesWithSpecificAccessor: false),
                     cancellationToken).ConfigureAwait(false);
 
                 var syntaxFacts = document.GetRequiredLanguageService<ISyntaxFactsService>();

src/Workspaces/Core/Portable/FindSymbols/SymbolFinder_FindReferences_Legacy.cs

@@ -82,7 +82,7 @@ public static async Task<IEnumerable<ReferencedSymbol>> FindReferencesAsync(
                 FindReferencesSearchOptions.Default, cancellationToken).ConfigureAwait(false);
         }
 
-        private static async Task<ImmutableArray<ReferencedSymbol>> FindReferencesAsync(
+        internal static async Task<ImmutableArray<ReferencedSymbol>> FindReferencesAsync(
             ISymbol symbol,
             Solution solution,
             IFindReferencesProgress progress,