Use immutable arrays in more places.

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

@@ -56,7 +56,7 @@ internal partial class FindReferencesSearchEngine
             _progressTracker = new StreamingProgressTracker(progress.ReportProgressAsync);
         }
 
-        public async Task<IEnumerable<ReferencedSymbol>> FindReferencesAsync(
+        public async Task<ImmutableArray<ReferencedSymbol>> FindReferencesAsync(
             SymbolAndProjectId symbolAndProjectId)
         {
             await _progress.OnStartedAsync().ConfigureAwait(false);

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

@@ -37,9 +37,8 @@ protected AbstractMethodOrPropertyOrEventSymbolReferenceFinder()
                 {
                     // We have a normal method.  Find any interface methods that it implicitly or
                     // explicitly implements and cascade down to those.
-                    var interfaceMembersImplementedStream = await SymbolFinder.FindImplementedInterfaceMembersAsync(
+                    var interfaceMembersImplemented = await SymbolFinder.FindImplementedInterfaceMembersAsync(
                         symbolAndProjectId, solution, projects, cancellationToken).ConfigureAwait(false);
-                    var interfaceMembersImplemented = interfaceMembersImplementedStream.ToImmutableArray();
 
                     // Finally, methods can cascade through virtual/override inheritance.  NOTE(cyrusn):
                     // We only need to go up or down one level.  Then, when we're finding references on

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

@@ -336,7 +336,8 @@ public static Task<IEnumerable<ISymbol>> FindSourceDeclarationsAsync(Solution so
         /// <summary>
         /// Find the symbols for declarations made in source with the specified name.
         /// </summary>
-        public static Task<IEnumerable<ISymbol>> FindSourceDeclarationsAsync(Solution solution, string name, bool ignoreCase, SymbolFilter filter, CancellationToken cancellationToken = default(CancellationToken))
+        public static async Task<IEnumerable<ISymbol>> FindSourceDeclarationsAsync(
+            Solution solution, string name, bool ignoreCase, SymbolFilter filter, CancellationToken cancellationToken = default(CancellationToken))
         {
             if (solution == null)
             {
@@ -350,24 +351,25 @@ public static Task<IEnumerable<ISymbol>> FindSourceDeclarationsAsync(Solution so
 
             if (string.IsNullOrWhiteSpace(name))
             {
-                return SpecializedTasks.EmptyEnumerable<ISymbol>();
+                return ImmutableArray<ISymbol>.Empty;
             }
 
             using (Logger.LogBlock(FunctionId.SymbolFinder_Solution_Name_FindSourceDeclarationsAsync, cancellationToken))
             {
-                return FindSourceDeclarationsAsyncImpl(solution, SearchQuery.Create(name, ignoreCase), filter, cancellationToken);
+                return await FindSourceDeclarationsAsyncImpl(
+                    solution, SearchQuery.Create(name, ignoreCase), filter, cancellationToken).ConfigureAwait(false);
             }
         }
 
-        private static async Task<IEnumerable<ISymbol>> FindSourceDeclarationsAsyncImpl(
+        private static async Task<ImmutableArray<ISymbol>> FindSourceDeclarationsAsyncImpl(
             Solution solution, SearchQuery query, SymbolFilter filter, CancellationToken cancellationToken)
         {
             if (query.Name != null && string.IsNullOrWhiteSpace(query.Name))
             {
-                return SpecializedCollections.EmptyEnumerable<ISymbol>();
+                return ImmutableArray<ISymbol>.Empty;
             }
 
-            var result = new List<ISymbol>();
+            var result = ArrayBuilder<ISymbol>.GetInstance();
             foreach (var projectId in solution.ProjectIds)
             {
                 var project = solution.GetProject(projectId);
@@ -375,7 +377,7 @@ public static Task<IEnumerable<ISymbol>> FindSourceDeclarationsAsync(Solution so
                 result.AddRange(symbols);
             }
 
-            return result;
+            return result.ToImmutableAndFree();
         }
 
         /// <summary>
@@ -433,13 +435,15 @@ public static Task<IEnumerable<ISymbol>> FindSourceDeclarationsAsync(Solution so
         /// <summary>
         /// Find the symbols for declarations made in source with a matching name.
         /// </summary>
-        public static Task<IEnumerable<ISymbol>> FindSourceDeclarationsAsync(
+        public static async Task<IEnumerable<ISymbol>> FindSourceDeclarationsAsync(
             Solution solution, Func<string, bool> predicate, SymbolFilter filter, CancellationToken cancellationToken = default(CancellationToken))
         {
-            return FindSourceDeclarationsAsync(solution, SearchQuery.CreateCustom(predicate), filter, cancellationToken);
+            return await FindSourceDeclarationsAsync(
+                solution, SearchQuery.CreateCustom(predicate), filter, cancellationToken).ConfigureAwait(false);
         }
 
-        internal static async Task<IEnumerable<ISymbol>> FindSourceDeclarationsAsync(Solution solution, SearchQuery query, SymbolFilter filter, CancellationToken cancellationToken)
+        internal static async Task<ImmutableArray<ISymbol>> FindSourceDeclarationsAsync(
+            Solution solution, SearchQuery query, SymbolFilter filter, CancellationToken cancellationToken)
         {
             if (solution == null)
             {
@@ -448,12 +452,12 @@ internal static async Task<IEnumerable<ISymbol>> FindSourceDeclarationsAsync(Sol
 
             if (query.Name != null && string.IsNullOrWhiteSpace(query.Name))
             {
-                return SpecializedCollections.EmptyEnumerable<ISymbol>();
+                return ImmutableArray<ISymbol>.Empty;
             }
 
             using (Logger.LogBlock(FunctionId.SymbolFinder_Solution_Predicate_FindSourceDeclarationsAsync, cancellationToken))
             {
-                var result = new List<ISymbol>();
+                var result = ArrayBuilder<ISymbol>.GetInstance();
                 foreach (var projectId in solution.ProjectIds)
                 {
                     var project = solution.GetProject(projectId);
@@ -461,7 +465,7 @@ internal static async Task<IEnumerable<ISymbol>> FindSourceDeclarationsAsync(Sol
                     result.AddRange(symbols);
                 }
 
-                return result;
+                return result.ToImmutableAndFree();
             }
         }
 

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

@@ -78,10 +78,10 @@ public static partial class SymbolFinder
             var result = await FindImplementedInterfaceMembersAsync(
                 SymbolAndProjectId.Create(symbol, projectId: null),
                 solution, projects, cancellationToken).ConfigureAwait(false);
-            return result.Select(s => s.Symbol).ToList();
+            return result.SelectAsArray(s => s.Symbol);
         }
 
-        internal static async Task<IEnumerable<SymbolAndProjectId>> FindImplementedInterfaceMembersAsync(
+        internal static async Task<ImmutableArray<SymbolAndProjectId>> FindImplementedInterfaceMembersAsync(
             SymbolAndProjectId symbolAndProjectId, Solution solution, IImmutableSet<Project> projects = null, CancellationToken cancellationToken = default(CancellationToken))
         {
             // Member can only implement interface members if it is an explicit member, or if it is
@@ -92,7 +92,7 @@ public static partial class SymbolFinder
                 var explicitImplementations = symbol.ExplicitInterfaceImplementations();
                 if (explicitImplementations.Length > 0)
                 {
-                    return explicitImplementations.Select(symbolAndProjectId.WithSymbol);
+                    return explicitImplementations.SelectAsArray(symbolAndProjectId.WithSymbol);
                 }
                 else if (
                     symbol.DeclaredAccessibility == Accessibility.Public && !symbol.IsStatic &&
@@ -116,7 +116,7 @@ public static partial class SymbolFinder
                         containingType, solution, projects, cancellationToken).ConfigureAwait(false);
                     var allTypes = derivedClasses.Concat(containingType);
 
-                    List<SymbolAndProjectId> results = null;
+                    var builder = ArrayBuilder<SymbolAndProjectId>.GetInstance();
 
                     foreach (var type in allTypes.Convert<INamedTypeSymbol, ITypeSymbol>())
                     {
@@ -136,8 +136,7 @@ public static partial class SymbolFinder
                                         if (implementation.Symbol != null &&
                                             SymbolEquivalenceComparer.Instance.Equals(implementation.Symbol.OriginalDefinition, symbol.OriginalDefinition))
                                         {
-                                            results = results ?? new List<SymbolAndProjectId>();
-                                            results.Add(bestMethod);
+                                            builder.Add(bestMethod);
                                         }
                                     }
                                 }
@@ -145,14 +144,14 @@ public static partial class SymbolFinder
                         }
                     }
 
-                    if (results != null)
-                    {
-                        return results.Distinct(SymbolAndProjectIdComparer.SymbolEquivalenceInstance);
-                    }
+                    var result = builder.Distinct(SymbolAndProjectIdComparer.SymbolEquivalenceInstance)
+                                        .ToImmutableArray();
+                    builder.Free();
+                    return result;
                 }
             }
 
-            return SpecializedCollections.EmptyEnumerable<SymbolAndProjectId>();
+            return ImmutableArray<SymbolAndProjectId>.Empty;
         }
 
         private static IEnumerable<SymbolAndProjectId> GetMembers(

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

@@ -90,7 +90,7 @@ public static partial class SymbolFinder
             }
         }
 
-        internal static async Task<IEnumerable<ReferencedSymbol>> FindRenamableReferencesAsync(
+        internal static async Task<ImmutableArray<ReferencedSymbol>> FindRenamableReferencesAsync(
             SymbolAndProjectId symbolAndProjectId,
             Solution solution,
             CancellationToken cancellationToken)