Moving ISuggestionAwareFunction interface to Core.

src/libraries/Microsoft.PowerFx.Core/Functions/ISuggestionAwareFunction.cs

@@ -0,0 +1,14 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT license.
+
+namespace Microsoft.PowerFx.Core.Functions
+{
+    /// <summary>
+    /// Interface used for TexlFunctions to override the default behavior for intellisense suggestions.
+    /// </summary>
+    internal interface ISuggestionAwareFunction
+    {
+        // Returns true if it's valid to suggest ThisItem for this function as an argument.
+        bool CanSuggestThisItem { get; }
+    }
+}

src/libraries/Microsoft.PowerFx.Core/Texl/Builtins/Collect.cs

@@ -23,7 +23,7 @@
 namespace Microsoft.PowerFx.Core.Texl.Builtins
 {
     // Collect(collection:*[...], item1:![...]|*[...], ...)
-    internal class CollectFunction : BuiltinFunction
+    internal class CollectFunction : BuiltinFunction, ISuggestionAwareFunction
     {
         public override bool AffectsCollectionSchemas => true;